# Enantiopure *β*3-Trifluoromethyl-*β*3-homoalanine Derivatives: Coupling with Boc-Protected Amino Acids and Conformational Studies of Peptides in Solid State

Nathalie Saraiva Rosa, Fabienne Grellepois

► To cite this version:

Nathalie Saraiva Rosa, Fabienne Grellepois. Enantiopure *β*3-Trifluoromethyl-*β*3-homoalanine Derivatives: Coupling with Boc-Protected Amino Acids and Conformational Studies of Peptides in Solid State. Synthesis: Journal of Synthetic Organic Chemistry, 2022, 54 (13), pp.3025-3046. 10.1055/s-0041-1737396. hal-03722849

HAL Id: hal-03722849

https://hal.univ-reims.fr/hal-03722849

Submitted on 22 Feb 2023

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<figure><img src="image_1.png" /></figure>

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# Enantiopure β³-trifluoromethyl-β³-homoalanine derivatives: Coupling with Boc-protected amino acids and conformational studies of peptides in solid state

Nathalie Saraiva Rosaª

Fabienne Grellepois*ª

ª Université de Reims Champagne Ardenne, Institut de Chimie Moléculaire de Reims CNRS UMR 7312, UFR des Sciences Exactes et Naturelles, BP 1039, 51687 Reims cedex 2

fabienne.grellepois@univ-reims.fr

<figure><img src="image_2.png" /></figure>

(R) and (S)-β³-CF₃-β³-homoalanine :
gram-scale synthesis
N- and/or C-protected derivatives

48 original α/β- and hetero β-di-, tri- and tetrapeptides

<figure><img src="image_3.png" /></figure>

**Abstract:** The use of enantiopure β³-trifluoromethyl-β³-alkyl β-amino acids for the design of peptides would contribute to drastically enhance the peptide stability *in vivo*. Moreover, the steric hindrance generated by the substituents on the tetrasubstituted carbon adjacent to the nitrogen function coupled to the electron-withdrawing effect of the trifluoromethyl group are more likely to influence the 3D conformation of the peptide. Herein, we describe a short, scalable and robust method to synthesize N- and/or C-protected enantiopure (R)- and (S)-β³-trifluoromethyl-β³-methyl β-amino acid derivatives and liquid-phase coupling methods suitable with Boc protected amino acids to incorporate them in short α/β- and β-peptides. Conformational studies of some of these original peptides *via* X-ray diffraction analysis highlighted intraresidue C6 hydrogen bonds into trifluoromethylated amino acids.

**Key words** fluorine, amino acids, peptides, coupling, conformational analysis.

β-Aminoacids are highly promising building blocks in medicinal chemistry as bioactive peptides containing these units do not present the significant disadvantages of α-peptides, i.e. the high conformational freedom of short fragments and the low proteolytic stability *in vivo*.¹ β-Amino acids are the subunits of β-peptides² (made up exclusively of β-amino acids) and α/β-peptides³ (sequence of β- and natural α-amino acids) that have been the subject of numerous research to determine their preferred secondary structures in solid state and solution. Experimental and molecular dynamic studies have both highlighted the profound influence of the nature of the amino acids, the site of the attachment of the side chain and the configuration of the chiral centers.²,³ The effect of geminal disubstitution of β-amino acids on the conformation of peptides is intriguing.⁴ Focusing on β³,³-amino acids (disubstitution on the carbon adjacent to the nitrogen), the influence of achiral 1-amino-cyclohexane acetic acid (β³,³-Ac₆c) has been mostly investigated.⁵ The presence of geminal dissubstituents restricts the torsion angles φ (about N-Cβ bond) and θ (about Cα-Cβ bond) and β³,³-Ac₆c residue adopts almost exclusively *gauche* conformations about Cα-Cβ bonds.⁵b-d Despite that no intramolecular hydrogen bonds were observed in many small α/β-peptides,⁵a,b C6 hydrogen-bonded turns were detected in

free amino acid and homo-β-dipeptide,⁵b C11 α/β-turns, expanded version of the type II β-turn, were highlighted in two α/β-hybrid dipeptides⁵b,c while longer tetra- and pentapeptide folded in mixed C11/C9 helical structure.⁵d Conformations of peptides containing chiral β³,³-amino acids have been rarely documented.⁴b,⁶ Homo-β-hexapeptide consisting of amino acids derived from sugar appears to prefer C8 conformation in solution.⁶

A broad variety of fluorinated amino acids have been reported but most of them are derived from α-amino acids and contained a fluorinated side chain or a fluorinated substituent instead of the α-proton.⁷ Their incorporation into peptides and proteins may promote the stabilization of particular secondary structure, increase their metabolic stability and enhance their hydrophobicity and, as a result, improve their biological activities.⁷f,⁸ Moreover, fluorinated amino acids can serve as probes for ¹⁹F NMR studies.⁸ The influence of fluorinated β-amino acids⁷a,c,f-1,9 on the conformation of peptides has been mainly investigated with a backbone-bound fluorine atom on the carbon adjacent to the CO function (β²-fluoro β-amino acids).¹⁰ The effect of the trifluoromethyl group¹¹ has been studied only with mono substituted β-alanine derivative and was the subject of two publications.¹² In the first one,¹²a a β-turn-like conformation backbone was obtained with α/β-dipeptides containing (S)-2-(aminomethyl)-3,3,3-trifluoropropanoic acid [(S)-β²-trifluoromethyl-β-alanine] through no intramolecular H-bond was observed. In the second one,¹²b two residues of (R)-3-amino-4,4,4-trifluorobutanoic acid [(R)-β³-trifluoromethyl-β-alanine] have significantly stabilized the 14-helix structure of a β-hexapeptide thanks to the highly withdrawing fluorinated groups which enhanced the hydrogen-bond donating property of the neighboring amide NH and thus strengthened intramolecular H-bonds.

Only a few stereoselective synthesis of more substituted, linear, β³-trifluoromethyl β³-alkyl(aryl) β-amino acids derivatives have
been reported to date,¹³ and, thus, synthetic methods for obtaining them in enantiopure form are limited.¹³a-c Our main contribution in this area was the development of the highly diastereoselective Reformatsky reaction on stable precursors of aliphatic and aromatic trifluoromethyl (S<sub>S</sub>)-N-tert-butanesulfinyl ketoimines.¹³b Their incorporation into peptides was rarely reported.¹³b The strongly electron-withdrawing and bulky trifluoromethyl group¹⁴ decreases the nucleophilicity¹⁵ but also greatly increases the steric hindrance of adjacent amine function of β³-trifluoromethyl β³-alkyl(aryl) β-amino acid derivatives. Elongation at the N-terminus of β³-trifluoromethyl-β³-homoalanine containing peptide was thus performed using highly reactive amino acid chloride, intrinsically limiting this coupling to Fmoc-protected amino acids.¹³b While this methodology enables to prepare the Fmoc-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe hetero-tripeptide, attempts to perform the reaction of Fmoc-L-Ala-Cl with dipeptide H-(R)-β³-Tfm-β³-hAla-Phe-OEt containing a bulkier side chain unfortunately failed.¹³b

In this publication, we wish to disclose the scalability of the diastereoselective Reformatsky reaction we reported a few years ago¹³b for the multigram synthesis of enantiopure (R)- as well as of (S)-β³-trifluoromethyl-β³-methyl β-amino acids precursors (named β³-trifluoromethyl-β³-homoalanine or β³-Tfm-β³-hAla precursors). Having in hand various N- and C-protected derivatives of these amino acids, we desired to study the preparation of short hetero-peptides using a coupling method suitable with Boc strategy, the most convenient approach for liquid phase reactions. Finally, our aim was also to explore the effect of geminal trifluoromethyl and methyl substituents on carbon adjacent the nitrogen function of β-aminoacids on the conformations of short peptides in solid state.

**Synthesis of enantiopure (R)- and (S)-β³-trifluoromethyl-β³-homoalanine derivatives.** According to the procedure previously described,¹³b N-tert-butanesulfinyl trifluoromethyl hemiaminals (S<sub>S</sub>)-1¹³b and (R<sub>S</sub>)-1 could be easily prepared by reacting over a hundred millimoles of the commercially available 1,1,1-trifluoroacetone with each enantiomer of Ellman chiral auxiliary¹⁶ in n-hexane in the presence of an excess of Ti(OEt)<sub>4</sub> at room temperature for days (Scheme 1). Reaction of several grams of these bench stable hemiaminals (S<sub>S</sub>)-1 or (R<sub>S</sub>)-1 under Reformatsky conditions in methyl-THF¹³b allowed to release *in situ* the corresponding N-tert-butanesulfinyl trifluoromethylketimine. Addition of the metal enolate on this highly electrophilic intermediate led to the N-protected amino ester (S<sub>S</sub>,R)-2 or (R<sub>S</sub>,S)-2 with excellent diastereoselectivity (the minor isomer could not be detected in the ¹⁹F NMR spectra of the crude reaction mixture). After purification by chromatography on silica gel, enantiomeric N-protected amino esters (S<sub>S</sub>,R)-2¹³b and (R<sub>S</sub>,S)-2 were isolated in good to excellent yields (92% and 68% yields, respectively) (Scheme 1). The observed diastereoselectivity is consistent with a Zimmerman-Traxler-type six-membered transition state.¹³b

<figure><img src="image_4.png" /><figcaption>**Scheme 1** Two-steps multigrams scale synthesis of enantiopure N-tert-butanesulfinyl β³-trifluoromethyl-β³-homoalanine methyl ester (S<sub>S,R</sub>)-2 and (R<sub>S,S</sub>)-2 from commercially available 1,1,1-trifluoroacetone.</figcaption></figure>

Basic treatment of amino methyl esters (S<sub>S,R</sub>)-2 and (R<sub>S,S</sub>)-2 gave quantitatively N-tert-butanesulfinyl protected β³-trifluoromethyl-β³-homoalanine (S<sub>S,R</sub>)-3¹³b and (R<sub>S,S</sub>)-3 (Scheme 2). Ellman auxiliary of (S<sub>S,R</sub>)-2, (R<sub>S,S</sub>)-2 and (S<sub>S,R</sub>)-3 was classically removed under acidic treatment. β³-Trifluoromethyl-β³-homoalanine methyl ester hydrochloride (R)-4 and (S)-4 and β³-trifluoromethyl-β³-homoalanine hydrochloride (R)-5 were thus obtained in excellent yields (Scheme 2). These reactions allowed the multigram synthesis of the desired enantiopure N-protected (S<sub>S,R</sub>)-3, (R<sub>S,S</sub>)-3 or C-protected (R)-4, (S)-4 as well as of the fully deprotected (R)-5 β³-trifluoromethyl-β³-homoalanine derivatives in only three or four steps from the commercially available 1,1,1-trifluoroacetone.

<figure><img src="image_5.png" /><figcaption>**Scheme 2** Synthesis of enantiopure N-tert-butanesulfinyl β³-trifluoromethyl-β³-homoalanine (S<sub>S,R</sub>)-3 and (R<sub>S,S</sub>)-3, β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride (R)-4 and (S)-4 and β³-trifluoromethyl-β³-homoalanine hydrochloride (R)-5.</figcaption></figure>

Attempts were then performed to introduce acetyl (Ac), benzyloxycarbonyl (Cbz) and tert-butoxycarbonyl (Boc)
protecting groups on the amine function of the (R)-β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride (R)-4. Despite the poor nucleophilicity and the steric hindrance of the amine function, Schotten-Baumann-type reaction on millimoles scale of (R)-β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride (R)-4 with acetyl chloride or benzylchloroformate led to the desired N-Ac amino ester (R)-6a and N-Cbz amino ester (R)-6b, which were isolated in very good yields (72% and 88% yields, respectively) (Scheme 3). Saponification of these N-protected amino esters (R)-6a and (R)-6b gave easily the corresponding amino acids (R)-7a and (R)-7b (Scheme 3). Boc-protection of the amino ester (R)-4 was more troublesome even using experimental conditions developed for tetrasubstituted α-trifluoromethyl α-amino acids.17 Reaction of (R)-β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride (R)-4 with an excess of di-tert-butyl carbonate in THF, in the presence of Et₃N and DMAP, afforded only the urea-type derivative having a C₂-symmetry (R,R)-8 which was isolated in 38% yield (Scheme 3). The structure of (R,R)-8 was confirmed by X-ray diffraction.18 No traces of the formation of other product could be detected by ¹⁹F or ¹H NMR, during the course of the reaction and in the crude reaction mixture. No further investigations were performed to obtain Boc-protected (R)-β³-trifluoromethyl-β³-homoalanine methyl ester as the tert-butanesulfinyl group can be considered as a Boc protecting group surrogate as both of them are stable to base and are cleaved under acidic condition.16,19 N-tert-Butanesulfinylamino acids (Ss,R)-3, (Rs,S)-3, as N-acetyl and N-benzyloxycarbonyl amino acids (R)-7a, (R)-7b, could be further used in peptide coupling at the C-terminus of the fluorinated amino acid fragment.

<figure><img src="image_6.png" /><figcaption>**Scheme 3** N-protection of (R)-4 with Ac and Cbz protecting groups and attempt for the introduction of Boc group.</figcaption></figure>

Having in hands enantiopure (R)-β³-trifluoromethyl-β³-homoalanine hydrochloride (R)-5 as well as N- and O-protected derivatives (Ss,R)-3, (Rs,S)-3, (R)-4, (S)-4, (R)-7a and (R)-7b we next attempted the synthesis of short peptides containing enantiopure β³-trifluoromethyl-β³-homoalanine fragment in various position.

**Synthesis of β/α- and β-dipeptides containing (R)- or (S)-β³-trifluoromethyl-β³-homoalanine derivatives at the N-terminal position by elongation at the C-terminus of N-protected β³-trifluoromethyl β³-amino acid (Ss,R)-3, (Rs,S)-3, (R)-7a and (R)-7b.** To prepare β/α- and β-heteropeptides whom are the precursors of tripeptides with the β³-trifluoromethyl-β³-homoalanine fragment in central position, the peptide-bond formation at the C-terminus position of the trifluororomethylated β-amino acid derivatives was investigated. Considering the poor nucleophilicity and the steric hindrance of the amine adjacent to the trifluoromethyl group, direct coupling at the C-terminus of the unprotected (R)-β³-trifluoromethyl-β³-homoalanine hydrochloride (R)-5 with L-alanine methyl ester hydrochloride, a strategy validated with tetrasubstituted α-trifluoromethylalanine,20 was first studied. Unfortunately, the use of EDCI, HOBT and DIPEA in DMF led to a very complex mixture. However, we have previously reported that N-tert-butanesulfinyl-β³-trifluoromethyl-β³-homoalanine (Ss,R)-3 reacted easily with this amino ester under these classical coupling conditions and gave the corresponding dipeptide 9a which was isolated in 93% yield.13b Pleasingly, similar yields could be obtained using CH₂Cl₂ as solvent (89%) but above all with acetonitrile21 (87%), a friendlier alternative to DMF22 (Scheme 4). In order to evaluate the scope of this latter condition, various N-protected derivatives of (R)-β³-trifluoromethyl-β³-homoalanine (Ss,R)-3, (R)-7a and (R)-7b or N-tert-butanesulfinyl-β³-trifluoromethyl-β³-homoalanine (Rs,S)-3 reacted with α-amino acid methyl ester having side chains with variable steric demands (L-alanine, L-leucine, L-phenylalanine and the α,α-disubstituted amino ester, α-methylalanine). The corresponding hybrid β/α-dipeptides 9a-1 were isolated in good to very good yields, ranging from 70 to 99% (Scheme 4). Reaction of (Rs,S)-3 with L-proline methyl ester led to the dipeptide 9m which was isolated in 86% yield as a 85:15 mixture of trans:cis rotamers (Scheme 4).23,24 Reaction of (Ss,R)-3 or (Rs,S)-3 with methyl ester derivatives of L-β³-homoalanine, L-β³-homoleucine and/or L-β³-homophenylalanine hydrochloride provided hetero β-dipeptides 10a-d, isolated in good yields ranging from 76% and 88% (Scheme 4).
<figure><img src="image_7.png" /></figure>

or

<figure><img src="image_8.png" /><figcaption>(R<sub>S</sub>,S)-3</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>n = 0 **9a-n**
n = 1 **10a-d**</figcaption></figure>

<caption>R = (S)-S(O)tbu (S<sub>S</sub>,R)-3
R = Ac (R)-7a
R = Cbz (R)-7b</caption>

<figure><img src="image_10.png" /><figcaption>coupling with α-aa (n = 0)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>coupling with β<sup>3</sup>-aa (n=1)</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Scheme 4** C-Coupling of N-protected β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-homoalanine derivatives (S<sub>S</sub>,R)-3, (R<sub>S</sub>,S)-3, (R)-7a and (R)-7b with α- and β-amino esters.</figcaption></figure>

**Synthesis of various dipeptides containing (R)- or (S)-β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-homoalanine derivatives at the C-terminal position by elongation at the N-terminus of β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-amino methyl esters (R)-4 and (S)-4 – Towards α/β-tripeptide containing trifluoromethylated β-amino acid fragments at each end.** As mentioned in the introduction, the N-coupling of β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-alkyl β-amino acid derivatives is the challenging step for their incorporation into peptides and has been reported only once using highly reactive amino acid chloride.<sup>13b</sup> This methodology is thus limited to Fmoc-protected amino acids and requires the prior synthesis of the acid chlorid. In the literature, the closest example describing the N-coupling of hindered trifluoromethylated amino acid whose conditions are compatible with Boc-protected amino acid concerns a tetrasubstituted α-trifluoromethyl-α-amino acid derivative, the α-trifluoromethyl alanine.<sup>25</sup> The mixed anhydride approach was used but the target peptide was only obtained in moderate yield (39%). In order to develop easy reaction conditions compatible with Boc strategy for the N-coupling of these β<sup>3</sup>-trifluoromethylated β<sup>3,3</sup>-amino acid derivatives, we investigated the use of HATU as this reagent can promote amidation reaction with electron deficient and sterically hindered amines.<sup>26</sup> N-Coupling of Boc-L-alanine with (R)-4 was successfully achieved

in the presence of an excess of HATU and DIPEA in DMF after a long reaction time (98% conversion of the starting material after 6 days of reaction at room temperature, reaction monitored by ¹⁹F NMR) (Scheme 5). α/β-Dipeptide **11a** was thus isolated in very good yield (93%) (Scheme 5). Replacement of DMF by MeCN was detrimental for the reaction. A 50% maximal conversion of (R)-4 into peptide **11a** was obtained after 3 days of stirring at room temperature without further evolution of the reaction mixture (39% yield of **11a**). Attempts under microwaves heating only led, at best, to 25% conversion (T=60°C, P<sub>max</sub>=40W, maximum conversion into **11a** obtained after 10 minutes, longer reaction time didn't improve the conversion into **11a** and led to the formation of side-products). Reaction of (S)-4 with Boc-L-alanine under the best reaction conditions (HATU and DIPEA in DMF) led to 90% conversion of (S)-4 after 7 days of reaction and **11b** was isolated in 81% yield (Scheme 5). The reaction of (R)-4 or (S)-4 with Boc-L-β<sup>3</sup>-homoalanine was much more sluggish. Only 53% and 44% conversion were obtained after 7 and 8 days of reaction respectively, which explains the low yields for β-heterodipeptides **12a** and **12b** (respectively 26% and 35%) (Scheme 5).
<figure><img src="image_13.png" /><figcaption>coupling with Boc-L-alanine (n=0)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>coupling with Boc-L-β3-homoalanine (n=1)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Scheme 5** N-Coupling of β³-trifluoromethyl-β³-homoalanine methyl esters (R)-4 and (S)-4 with Boc-L-alanine and Boc-L-β³-homoalanine.</figcaption></figure>

Boc-protecting group of α/β-dipeptide **11a** was then cleaved under acidic conditions giving the dipeptide **13** which was used in the next step without further purification (Scheme 6). Reaction of dipeptide **13** with *N*-tert-butanesulfinyl protected β³-trifluoromethyl-β³-homoalanine (*S*<sub>5</sub>,*R*)-3 in the presence of EDCI, HOBT and DIPEA in MeCN led to the heterotripeptide **14**. Original β/α/β-tripeptide **14** containing two trifluoromethylated β-amino acid fragments was isolated with a global yield of 52% from dipeptide **11a** (Scheme 6).

<figure><img src="image_16.png" /><figcaption>**Scheme 6** Synthesis of β/α/β-tripeptide **14** containing β³-trifluoromethyl-β³-homoalanine unit at each end.</figcaption></figure>

**Synthesis of α/β/α-tripeptides containing the (R)- and (S)-β³-trifluoromethyl-β³-homoalanine subunit in central position by elongation at the N-terminus of β³-trifluoromethyl-β³-amino acid containing dipeptides 9a-c,j,l and 10b-d** - Towards α/β-tetrapeptide containing a repetitive sequence (R)-β³-trifluoromethyl-β³-homoalanine-L-alanine. *N*-tert-Butanesulfinyl-protecting group of β/α-dipeptides **9a-c,j,l** and β-dipeptides **10b-d** was removed under classical acidic conditions giving the

corresponding β/α-dipeptides **15a-e** and β-dipeptides **16a-c**. These dipeptides **15a-e** and **16a-c** were obtained in yields between 87% and 98% and were used in the next step without further purification (Scheme 7). β/α-Dipeptides **15a-c** containing α-amino methyl ester substituted with alkyl or benzyl side chains reacted on the *N*-terminal position with Boc-L-alanine in the presence of HATU and DIPEA in DMF. After purification, the corresponding α/β/α-tripeptides **17a-c** were obtained in good yields (between 72% and 90% yields, for two steps from *N*-tert-butanesulfinyl protected dipeptides **9a-c**) (Scheme 7). The reaction of dipeptide **15a,b** with Boc-L-leucine was more troublesome as only average 50% conversion into tripeptides **17d,e** was observed after 5 days of stirring. Dipeptides **17a,b** were thus isolated in 38% and 48% yields respectively (from dipeptides **9a,b**). Diastereomeric dipeptides **9j,l** reacted similarly to **9a,c** leading to the dipeptides **17f,g** in 74% and 70% yields respectively. These few examples show that, with these reaction conditions, the hindrance of the side chain of the α-amino acid of the dipeptide has no influence on the coupling of the amine function of (R)-β³-trifluoromethyl-β³-homoalanine fragment; However, the steric hindrance of the side chain of the Boc-protected amino acid has a great impact on its success. Moreover, the configuration of the β³-trifluoromethyl-β³-homoalanine fragment does not appears to exert much influence on the *N*-terminal coupling. As we previously observed, coupling of β-dipeptides **16a-c** with Boc-L-β³-homoalanine under these reaction conditions was more troublesome and the β-tripeptides **18a-c** were isolated in yields ranging only from 48% to 56% (yields for two steps from *N*-protected β-dipeptides **10b-d**) (Scheme 7). Finally, as *C*-terminal amide can have a deep influence on the conformation characteristics of small peptides,²⁷ α/β-tripeptide methyl esters **17a-e**, containing (R)-β³-trifluoromethyl-β³-homoalanine fragment in central position, were then treated with a solution of methylamine in methanol to form the corresponding α/β-tripeptide methyl amides **19a-e**, which were isolated in very good to excellent yields (Scheme7).
<figure><img src="image_17.png" /></figure>

$$
(S_S), R^1 = CF_3, R^2 = \begin{array}{l} \text{Me} \\ \text{n} = 0, \text{R}^3 = \text{Me}, \text{i-Bu}, \text{Bn} \end{array} \quad \begin{array}{l} \text{9a-c} \\ \text{n} = 1, \text{R}^3 = \text{Bn} \end{array}
$$

$$
(R_S), R^1 = \begin{array}{l} \text{Me}, \text{R}^2 = \text{CF}_3 \\ \text{n} = 0, \text{R}^3 = \text{Me}, \text{Bn} \end{array} \quad \begin{array}{l} \text{9j,l} \\ \text{n} = 1, \text{R}^3 = \text{Me}, \text{i-Pr} \end{array}
$$

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

n=0 15a-e
n=1 16a-c

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

access to N-Boc α/β-tripeptides

<figure><img src="image_24.png" /></figure>

R = OMe **17a** 72% (from **9a**)

R = NHMe 19a 100%

<figure><img src="image_25.png" /></figure>

R = OMe **17b** 77% (from **9b**)

R = NHMe 19b 87%

<figure><img src="image_26.png" /></figure>

R = OMe **17c** 90% (from **9c**)

R = NHMe **19c** 93%

<figure><img src="image_27.png" /></figure>

R = OMe **17d** 38% (from **9a**)

R = NHMe **19d** 91%

<figure><img src="image_28.png" /></figure>

R = OMe **17e** 48% (from **9b**)

R = NHMe **19e** 86%

<figure><img src="image_29.png" /></figure>

17f 74% (from 9j)

<figure><img src="image_30.png" /></figure>

17g 70% (from 9l)

access to N-Boc β-tripeptides

<figure><img src="image_31.png" /></figure>

18a 56% (from 10b)

<figure><img src="image_32.png" /></figure>

18b 48% (from 10c)

<figure><img src="image_33.png" /></figure>

18c 52% (from 10d)

<caption>Scheme 7 Synthesis of Boc-protected heterotripeptides **17a-g**, **18a-c** and **19a-e** containing (R)- or (S)-β³-trifluoromethyl-β³-homoalanine fragment in central position.</caption>

Treatment of the Boc-protected α/β/α-tripeptide **17a** under acidic conditions gave the tripeptide **20** which was used in the next step without further purification (Scheme 8). Elongation of the peptide **20** was carried out with *N*-tert-butanesulfinyl protected β³-trifluoromethyl-β³-homoalanine (*S*,*R*)-3, EDCI, HOBT and DIEPA in MeCN. Heterotetrapeptide **21**, containing two (*R*)-β³-trifluoromethyl-β³-homoalanine-L-alanine repetitive fragments, was thus isolated in 71% yield (Scheme 8).

<figure><img src="image_34.png" /></figure>

17a

1. HCl (2N in Et₂O)

MeOH, r.t., 5h

2. Na₂CO₃

<figure><img src="image_35.png" /></figure>

20 73%

(*S*,*R*)-3, EDCI, HOBT, DIPEA

MeCN, r.t., 39h

<figure><img src="image_36.png" /></figure>

21 71%

<caption>**Scheme 8** Synthesis of α/β-tetrapeptide **21** containing two (R)-β³-trifluoromethyl-β³-homoalanine-L-alanine repetitive fragments.</caption>
Having prepared nearly 50 original short α/β- and hetero β-peptides containing (R)- or (S)-β³-trifluoromethyl β³-homoalanine residue, we next envisaged to investigate their conformation in the solid state by X-ray diffraction as the influence of chiral trifluoromethylated β³,³-amino acid has never been explored.

**Crystal structure investigations.** Single crystals were obtained for the β/α-dipeptides **9c,d,h**, the α/β-dipeptide **11a**, the β/α/β-tripeptide **14** as well as for the hetero β-peptides **10b,d** and **12a** having all C-terminal ester function.¹⁸ Figure 1 illustrates the observed conformations. Backbone torsion angles and hydrogen-bond parameters are listed in Tables 1 and 2, respectively.

<figure><img src="image_37.png" /><figcaption>**Figure 1** Solid state molecular conformations of α/β-dipeptides **9c,d,h, 11a** and tripeptide **14** and of hetero β-dipeptides **10b-d** and **12a**. Intramolecular hydrogen bonds are shown in purple dashed lines. All hydrogen atoms except NH and H₂O are removed for the sake of clarity.</figcaption></figure>

In all crystals structures of peptides, geminal di-substitution of (R)- and (S)-β³-trifluoromethyl-β³-homoalanine residue

imposes restrictions on the torsion angle θ (about the Cα-Cβ bond), β³,³-amino acid residue adopts *gauche* conformation with θ torsion values comprised between 41° and 78° with either positive (+) or negative (-) signs (Table 1).

<table><caption>**Table 1** Torsional angles for α/β-peptides **9c,d,h, 11a, 14** and β-peptides **10b,d, 12a** determined in crystal structures.</caption><thead><tr><th>residue</th><th>φ [°]</th><th>θ [°]</th><th>ψ [°]</th><th>ω [°]</th></tr></thead><tbody><tr><td>(S)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Phe-OMe 9c</td><td></td><td></td><td></td><td></td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>-154.8</td><td>-49.5</td><td>-127.7</td><td>-177.7</td></tr><tr><td>Phe</td><td>-80.5</td><td>164.5</td><td></td><td></td></tr><tr><td>(S)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Aib-OMe 9d</td><td></td><td></td><td></td><td></td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>-146.1</td><td>-41.4</td><td>-132.8</td><td>-147.0</td></tr><tr><td>Aib</td><td>-51.9</td><td></td><td>-43.5</td><td></td></tr><tr><td>Ac-(R)-β³-Tfm-β³-hAla-Aib-OMe 9h</td><td></td><td></td><td></td><td></td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>-167.6</td><td>72.8</td><td>124.7</td><td>-177.9</td></tr><tr><td>Aib</td><td>55.1</td><td></td><td>-151.4</td><td></td></tr><tr><td>Boc-Ala-(R)-β³-Tfm-β³-hAla-OMe 11a</td><td></td><td></td><td></td><td></td></tr><tr><td>Ala</td><td>-96.3</td><td></td><td>141.6</td><td>-175.7</td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>33.9</td><td>68.4</td><td>-59.4</td><td></td></tr><tr><td>(S)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Ala-(R)-β³-Tfm-β³-hAla-OMe 14</td><td></td><td></td><td></td><td></td></tr><tr><td>(R)-β³-Tfm-β³-hAla (1)</td><td>-111.2</td><td>-47.1</td><td>-137.4</td><td>-176.8</td></tr><tr><td>Ala</td><td>-73.8</td><td></td><td>-35.6</td><td>-177.6</td></tr><tr><td>(R)-β³-Tfm-β³-hAla (2)</td><td>-159.9</td><td>-49.2</td><td>-135.6</td><td></td></tr><tr><td>(S)-tBuS(O)-(R)-β³-Tfm-β³-hAla-βhPhe-OMe 10b</td><td></td><td></td><td></td><td></td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>-112.4</td><td>-50.4</td><td>-119.5</td><td>178.6</td></tr><tr><td>βhPhe</td><td>89.8</td><td>65.7</td><td>-124.8</td><td></td></tr><tr><td>(R)-tBuS(O)-(S)-β³-Tfm-β³-hAla-βhLeu-OMe 10d</td><td></td><td></td><td></td><td></td></tr><tr><td>(S)-β³-Tfm-β³-hAla</td><td>98.6</td><td>54.8</td><td>125.7</td><td>-175.0</td></tr><tr><td>βhLeu</td><td>-81.9</td><td>171.0</td><td>153.0</td><td></td></tr><tr><td>Boc-βhAla-(R)-β³-Tfm-β³-hAla-OMe 12a</td><td></td><td></td><td></td><td></td></tr><tr><td>βhAla</td><td>-122.4</td><td>54.0</td><td>-126.9</td><td>-178.2</td></tr><tr><td>(R)-β³-Tfm-β³-hAla</td><td>22.6</td><td>50.6</td><td>-165.1</td><td></td></tr></tbody></table>

Intra-residue hydrogen bonds²⁸ between the sulfinamido NH and CO groups are observed in dipeptides **9c,d, 10b,d** and tripeptide **14** [Δ(N...O) = 2.69-2.81 Å, Δ(H...O) = 1.90-2.14 Å, ≪(N-H...O) = 137.8-149.5°] (Table 2). In all of them, *gauche* conformation along the Cα-Cβ bond is associated with a tendency of φ (about N-Cβ bond) and/or ψ (about Cα-CO bond) to prefer fully extended conformations²,⁵,⁹,¹⁰ and with all three dihedral angles having the same sign (Table 1).²,⁵,⁹ No intramolecular hydrogen bond was observed in β/α-dipeptide **9h**, N-acetyl analog of β/α-sulfinamidopeptide **9d** [intramolecular C6 H-bond for **9d**: Δ(Nβ³,³...Oα) = 2.69 Å, Δ(Hβ³,³...Oα) = 1.92 Å, ≪(N-Hβ³,³...Oα) = 144.3° (table 2); data for **9h**: Δ(Nβ³,³...Oα) = 2.95 Å, Δ(Hβ³,³...Oα) = 2.64 Å, ≪(N-Hβ³,³...Oα) = 102.2°].¹⁸,²⁸ Hydrogen-bonded C6 conformation¹⁹,³,⁵,⁵,⁸,¹⁰ for sulfinamidopeptides **9c,d, 10b,d** and **14** might result from a better hydrogen-bond donor character of sulfinamide NH other than amide³¹ or from minimization of steric hindrance and/or of electrostatic repulsion between the Ellman auxiliary and the trifluoromethyl group which imposed a spatial arrangement favorable for the formation of intra-residue H-bond. Nevertheless, it is interesting to note that both residues of (R)-β³-trifluoromethyl-β³-homoalanine in β/α/β-tripeptide **14** adopts C6 hydrogen-bonded conformation. Moreover, various intermolecular hydrogen-bonding interactions have also been observed in the crystals, including N-H···O=S hydrogen bonds (Table 2).
<table><caption>**Table 2** Hydrogen bond parameters<sup>28a</sup> for α/β-peptides **9c,d,h, 11a, 14** and β-peptides **10b,d, 12a** determined in crystal structures.</caption><thead><tr><th>Type</th><th>Donor<br>(D)</th><th>Acceptor<br>(A)</th><th>D-A<br>(Å)</th><th>H-A<br>(Å)</th><th>≤D-H-A<br>(°)</th></tr></thead><tbody><tr><td colspan="6"><strong>(S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Phe-OMe 9c</strong></td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>)</td><td>CO(β<sup>3,3</sup>)</td><td>2.79</td><td>2.14</td><td>137.8</td></tr><tr><td>intermol.</td><td>NH(α)</td><td>SO<sup>a</sup></td><td>2.88</td><td>2.04</td><td>169.6</td></tr><tr><td colspan="6"><strong>(S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Aib-OMe 9d</strong></td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>)</td><td>CO(β<sup>3,3</sup>)</td><td>2.69</td><td>1.92</td><td>144.3</td></tr><tr><td>intermol.</td><td>NH(α)</td><td>SO<sup>b</sup></td><td>2.82</td><td>1.96</td><td>172.9</td></tr><tr><td colspan="6"><strong>Ac-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Aib-OMe 9h</strong></td></tr><tr><td>intermol.</td><td>NH(β<sup>3,3</sup>)</td><td>CO(α)<sup>c</sup></td><td>2.97</td><td>2.10</td><td>171.2</td></tr><tr><td>intermol.</td><td>NH(α)</td><td>CO(Ac)<sup>d</sup></td><td>2.87</td><td>2.05</td><td>168.5</td></tr><tr><td colspan="6"><strong>Boc-Ala-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OMe 11a</strong></td></tr><tr><td>intermol.</td><td>H<sub>2</sub>O</td><td>CO(Boc)</td><td>2.78</td><td>1.74</td><td>172.5</td></tr><tr><td>intermol.</td><td>H<sub>2</sub>O</td><td>CO(α)<sup>e</sup></td><td>2.78</td><td>1.79</td><td>162.4</td></tr><tr><td>intermol.</td><td>NH(α)</td><td>H<sub>2</sub>O<sup>f</sup></td><td>2.81</td><td>2.05</td><td>177.7</td></tr><tr><td>intermol.</td><td>NH(β<sup>3,3</sup>)</td><td>CO(α)<sup>g</sup></td><td>2.93</td><td>2.19</td><td>148.5</td></tr><tr><td colspan="6"><strong>(S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Ala-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OMe 14</strong></td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>(1))</td><td>CO(β<sup>3,3</sup>(1))</td><td>2.75</td><td>1.90</td><td>149.5</td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>(2))</td><td>CO(β<sup>3,3</sup>(2))</td><td>2.80</td><td>2.03</td><td>138.9</td></tr><tr><td>intermol.</td><td>NH(α)</td><td>SO<sup>h</sup></td><td>2.76</td><td>1.79</td><td>178.5</td></tr><tr><td colspan="6"><strong>(S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-βhPhe-OMe 10b</strong></td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>)</td><td>CO(β<sup>3,3</sup>)</td><td>2.80</td><td>2.02</td><td>145.3</td></tr><tr><td>intermol.</td><td>NH(β)</td><td>SO<sup>i</sup></td><td>2.80</td><td>1.96</td><td>156.4</td></tr><tr><td colspan="6"><strong>(R<sub>5</sub>)-tBuS(O)-(S)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-βhLeu-OMe 10d</strong></td></tr><tr><td>intramol. (C6)</td><td>NH(β<sup>3,3</sup>)</td><td>CO(β<sup>3,3</sup>)</td><td>2.81</td><td>2.02</td><td>145.0</td></tr><tr><td>intermol.</td><td>NH(β)</td><td>SO<sup>j</sup></td><td>2.83</td><td>1.93</td><td>177.4</td></tr><tr><td colspan="6"><strong>Boc-βhAla-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OMe 12a</strong></td></tr><tr><td>intermol.</td><td>NH(β)</td><td>CO(β)<sup>k</sup></td><td>3.04</td><td>2.26</td><td>152.3</td></tr><tr><td>intermol.</td><td>NH(β<sup>3,3</sup>)</td><td>CO(Boc)<sup>l</sup></td><td>2.08</td><td>2.90</td><td>173.7</td></tr></tbody></table>

symmetry operation: $^a$: 1-x, 1/2+y, 2-z; $^b$: 1-x, -1/2+y, -z; $^c$: -1+x, -y, z; $^d$: 1-x, 1/2+y, 3/2-z; $^e$: -y, -1+x, -1/4+z; $^f$: y, -x, 1/4+z; $^g$: -y, +x, -1/4+z; $^h$: -1/2+x, 5/2-y, 1-z; $^i$: 2-x, -1/2+y, 1-z; $^j$: 1/2+x, 3/2-y, 1-z; $^k$: -1/2+x, 1/2-y, 1-z.

In summary, the diastereoselective Reformatsky reaction on chiral N-tert-butanesulfinyltrifluoromethylketimines was the key reaction of a short and robust synthesis enabling the multigram synthesis of N- and/or C-protected (R)- and (S)-β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-homoalanine derivatives. C-Coupling of these residues was performed with EDCI in the presence of HOBt and DIPEA in acetonitrile instead of the more conventional solvents, DMF or CH<sub>2</sub>Cl<sub>2</sub>. Despite the hindrance and the low nucleophilicity of α-trifluoromethyl amine moiety, N-coupling of these β<sup>3,3</sup>-amino acid derivatives was successfully achieved with HATU in the presence of DIPEA in DMF, practical reaction conditions and, above all, suitable with Boc-protected amino acids. These methods allowed the preparation of a small library of original di-, tri- and tetra- α/β- and hetero-β-peptides containing one or two (R)- or (S)-β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-homoalanine residues in various positions. Investigation of the conformations of some of these peptides by X-ray diffraction analysis highlighted the presence of an intra-residue C6-hydrogen bond within the trifluoromethylated β-amino acid. All the results described herein should contribute to the development of novel peptides containing original trifluoromethyl substituted β-amino acids.

The experimental section has no title; please leave this line here.

CH<sub>2</sub>Cl<sub>2</sub> and MeCN were dried using a Pure Solv solvent drying system over aluminum oxide under argon atmosphere. n-Hexane (97% extra dry over molecular sieves, Acroseal®) and DMF (99.8% extra dry over molecular sieves, water <0.005%, Acroseal®) were purchased from Acros Organics. Ti(OEt)<sub>4</sub> was distilled prior to use. Diisopropylethylamine (DIPEA) was first distilled from ninhydrin and then from potassium hydroxide at atmospheric pressure and was stored

under potassium hydroxide. Zn was activated by stirring with a 10% HCl (v/v) solution and successive washing with distilled H<sub>2</sub>O, 95% EtOH and Et<sub>2</sub>O. Zn was then dried thoroughly in vacuum oven at 70°C for at least 24h (activated Zn can be stored under these conditions for 10 days). Thin-layer chromatography using precoated aluminum backed plates (Merck Kieselgel 60F254) were visualized by UV light and/or by phosphomolybdic acid, p-anisaldehyde, or ninhydrin solutions followed by heating. Silica gel (40–63 µm; Macherey-Nagel GmbH & Co KG) was used for flash chromatography. NMR spectra were recorded in CDCl<sub>3</sub>, D<sub>2</sub>O or CD<sub>3</sub>CN with 250, 500, or 600 MHz spectrometers. Chemical shifts (δ) were reported in ppm relative to TMS for <sup>1</sup>H and <sup>13</sup>C(<sup>1</sup>H) NMR spectra and to CFCI<sub>3</sub> for <sup>19</sup>F NMR spectra. In the <sup>13</sup>C(<sup>1</sup>H) NMR data (<sup>13</sup>C NMR or J-MOD), reported signal multiplicities are related to C-F coupling. The following abbreviations were used to indicate the multiplicities: s (singlet), br s (broad singlet), br d (broad doublet), d (doublet), t (triplet), q (quartet), quint (quintuplet), sext (sexcuplet), hept (heptuplet), and m (multiplet). HRMS were recorded on an ESI-Q-TOF mass spectrometer using an electrospray source in positive mode. Melting points (mp) were determined on a Tottoli apparatus and were uncorrected. Optical rotations were measured at room temperature (ca. 20 °C). X-ray data were collected on a Bruker D8 venture diffractometer equipped with a PHOTON detector. Data collection were obtained using APEX 3 software. Olex 2 software was used to solve and to refine the structures. The structure were solved with the XT structure solution program using Intrinsic Phasing and refined with the XL refinement package using Least Squares minimisation. Hydrogen atoms were added at their geometrically ideal positions and their ADP were refined isotropically (see Table 3 for crystal data and tructure refinements for peptides **9c,d,h, 10b,d, 11a, 12a and 14**).

The synthesis on smaller scale of (S<sub>5</sub>)-1, (S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OMe (S<sub>5</sub>,R)-2 and of (S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OH (S<sub>5</sub>,R)-3 [(S<sub>5</sub>)-1: 8,1g, 70%; (S<sub>5</sub>,R)-2: 648 mg, 86%, (S<sub>5</sub>,R)-3: 274 mg, 99%], the preparation of dipeptide (S<sub>5</sub>)-tBuS(O)-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Ala-OMe 9a using DMF as solvent (742 mg, 93%) and the synthesis of H-(R)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-Ala-OMe 15a (569 mg, 87%) have been previously described.<sup>13b</sup>

## Procedures

(R<sub>5</sub>)-(-)-N-(2-Ethoxy-1,1,1-trifluoropropan-2-yl)-tert-butylsulfinamide (R<sub>5</sub>-1). A solution of 1,1,1-trifluoroacetone (12.5 g, 111.6 mmol), (R)-tert-butanesulfinamide (16.5 g, 136.5 mmol, 1.2 equiv), and Ti(OEt)<sub>4</sub> (47 mL, 224.1 mmol, 2 equiv) in n-hexane (150 mL) was stirred at r.t. and under Ar for 7 days. The reaction mixture was then quenched with H<sub>2</sub>O and after 5 min of stirring was filtered on a pad of Dicalite 4158 (Et<sub>2</sub>O). The organic layer of the filtrate was separated, washed with brine, dried (MgSO<sub>4</sub>), filtered and concentrated under reduced pressure. Purification of the residue by chromatography on silica gel (petroleum ether/Et<sub>2</sub>O 6:4) afforded hemiaminal (R<sub>5</sub>-1) (18.7 g, 64%, dr=100:0) as a white solid.

M<sub>p</sub> 55°C; [α]<sub>D</sub><sup>20</sup>-75 (c 1.04, CHCl<sub>3</sub>).

IR (KBr): 3160, 2982, 2585, 2165, 1664, 1374, 1329, 1124, 1019, 941, 897, 849, 771 cm<sup>-1</sup>.

<sup>1</sup>H NMR (600 MHz, CDCl<sub>3</sub>): δ = 3.89 (br s, 1H), 3.69 (quint, J=7.0 Hz, 1H), 3.57 (m, 1H), 1.65 (s, 3H), 1.21 (s, 9H), 1.19 (t, J=7.0 Hz, 3H).

<sup>13</sup>C NMR (150.9 MHz, CDCl<sub>3</sub>): δ = 123.5 (q, J=286.0 Hz), 87.8 (q, J=30.0 Hz), 58.5, 56.6, 22.5, 16.5, 15.3.

<sup>19</sup>F NMR (235.3 MHz, CDCl<sub>3</sub>): δ=-84.8 (s).

HRMS-ESI: m/z [M+Na]<sup>+</sup> calcd for C<sub>9</sub>H<sub>18</sub>F<sub>3</sub>NNaO<sub>2</sub>S: 284.0908; found: 284.0908.

(R<sub>5</sub>,S)-(-)-N-tert-butanesulfinyl-β<sup>3</sup>-trifluoromethyl-β<sup>3</sup>-homoalanine methyl ester [(R<sub>5</sub>)-tBuS(O)-(S)-β<sup>3</sup>-Tfm-β<sup>3</sup>-hAla-OMe] (R<sub>5</sub>,S)-2. A three-necked flask containing activated Zn (21.9 g, 334.1 mmol, 10 equiv) and CuCl (3.29 g, 33.3 mmol, 1 equiv) was heated under Ar with a heat gun for 5 min. The flask was allowed to cool to r.t., 2-Me-THF (160 mL) was added (formation of a black slurry), and the suspension was heated to reflux under vigorous stirring for 30 min under Ar. The heating bath was then removed, methyl bromoacetate (7.9 mL, 83.5 mmol, 2.5 equiv) was added dropwise (caution: exothermic reaction) while
maintaining the vigorous stirring, and the reaction was stirred for an additional 30 min at room temperature and 45 min at 50 °C. The reaction was then cooled to 0 °C, a solution of hemiaminal (Rs)-1 (8.65 g, 33.1 mmol) in 2-Me-THF (125 mL) was added, and the resulting reaction mixture was stirred at 0 °C under Ar. After 23 h of stirring at this temperature, the reaction mixture was filtered on a pad of Dicalite 4158 (Et₂O). The filtrate was washed with an aq. sol. of HCl 10% (v/v), a sat. aq. sol. of NaHCO₃ and brine. The organic layer was dried (Na₂SO₄), filtered and concentrated under reduced pressure. Purification of the residue by chromatography on silica gel (petroleum ether/EtOAc from 8:2 to 3:7) afforded N-tert-butanesulfinylamino ester (Rs,S)-2 (6.52 g, 68%) as a yellow oil.

[α]D²⁰-78 (c 0.98, CHCl₃)

IR (film): 3456, 3265, 2959, 1730, 1634, 1462, 1362, 1235, 1166, 1074 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ = 5.61 (br s, 1H), 3.71 (s, 3H), 2.76 (d, *J*=15.5 Hz, 1H), 2.69 (d, *J*=15.5 Hz, 1H), 1.67 (s, 3H), 1.24 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ = 170.6; 125.8 (q, *J*=285.0 Hz), 59.1 (q, *J*=28.0 Hz), 56.5, 52.5, 40.4, 22.7, 19.6.

¹⁹F NMR (235.3 MHz, CDCl₃): δ = -81.0 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₀H₁₈F₃NNaO₃S: 312.0857; found: 312.0851.

(Rs,S)-(-)-N-tert-butanesulfinyl-β³-trifluoromethyl-β³-homoalanine [(Rs)-tBuS(O)-(-)-β³-Tfm-β³-hAla-OH] (Rs,S)-3. A solution of β-amino ester (Rs,S)-2 (2.03 g, 7.0 mmol) and LiOH.H₂O (590 mg, 14.0 mmol, 2.0 equiv) in a mixture of MeOH and H₂O (3:1, 24 mL) was stirred 2 h at r.t. The residue was diluted with AcOEt and acidified with HCl 1M (*pH* =1). The organic layer was separated and the aqueous layer was extracted three times with AcOEt. The organic layers were combined, dried (Na₂SO₄) and concentrated under reduced pressure to afford N-tert-butanesulfinylamino acid (Rs,S)-3 (1.93 g, 99%) as a white solid.

Mp 137°C; [α]D²⁰-60 (c 1.00, CHCl₃)

IR (KBr plates): 3256, 2993, 2965, 2928, 2725, 2603, 2554, 2355, 1714, 1467, 1419, 1285, 1239, 1155, 1027, 939, 774, 650 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=11.21 (br s, 1H), 5.88 (s, 1H), 2.74 (d, *J*=15.5 Hz, 1H), 2.68 (d, *J*=15.5 Hz, 1H), 1.69 (s, 3H, CH₃), 1.26 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=171.4, 125.8 (q, *J*=284.5 Hz), 59.3 (q, *J*=28.0 Hz), 57.3, 39.7, 22.7, 19.3

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-80.4 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₉H₁₆F₃NNaO₃S: 298.0701; found: 298.0696.

(R)-(-)-β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride [H-(R)-β³-Tfm-β³-hAla-OMe.HCl] (R)-4. A solution of N-tert-butanesulfinyl β-amino ester (Ss,R)-2 (2.30 g, 7.95 mmol) in MeOH (20 mL) reacted with HCl (2 N in Et₂O, 10 mL). After 3 h of stirring at r.t. under Ar, the reaction mixture was concentrated under reduced pressure and the residue was triturated with Et₂O to afford β-amino ester hydrochloride (R)-4 (1.62 g, 92%) as a pale yellow hygroscopic solid.

[α]D²⁰-0.5 (c 1.00, MeOH)

IR (KBr plate): 3423, 3168, 3069, 2793, 2574, 2023, 1738, 1575, 1537, 1444, 1370, 1257, 1185, 1123 cm⁻¹.

¹H NMR (500 MHz, D₂O): δ=3.79 (s, 3H), 3.12 (d, *J*=17.0 Hz, 1H), 3.07 (d, *J*=17.0 Hz, 1H), 1.67 (s, 3H).

¹³C NMR (125.8 MHz, D₂O): δ=170.2, 124.1 (q, *J*=283.0 Hz), 57.1 (q, *J*=30.0

Hz), 53.0, 35.9, 17.4.

¹⁹F NMR (235.3 MHz, D₂O): δ=-80.0 (s).

HRMS-ESI: *m/z* [M+H]⁺ calcd for C₆H₁₁F₃NO₂: 186.0742; found: 186.0745.

(S)-(+)-β³-trifluoromethyl-β³-homoalanine methyl ester hydrochloride [H-(S)-β³-Tfm-β³-hAla-OMe.HCl] (S)-4. A solution of N-tert-butanesulfinyl β-amino ester (Rs,S)-2 (1.52 g, 5.24 mmol) in MeOH (16 mL) reacted with HCl (2 N in Et₂O, 8 mL). After 2 h of stirring at r.t. under Ar, the reaction mixture was concentrated under reduced pressure and the residue was triturated with Et₂O to afford β-amino ester hydrochloride (S)-4 (979 g, 84%) as a yellowish hygroscopic solid.

[α]D²⁰+0.5 (c 0.98, MeOH).

IR (KBr plate) 3420, 3165, 2959, 2855, 2569, 2356, 2037, 1737, 1614, 1532, 1445, 1374, 1322, 1279, 1188, 1068, 1007, 961, 898, 856, 773, 647, 584, 550, 444 cm⁻¹.

¹H NMR (500 MHz, D₂O): δ=3.77 (s, 3H), 3.11 (d, *J*=17.0 Hz, 1H), 3.05 (d, *J*=17.0 Hz, 1H), 1.66 (s, 3H).

¹³C NMR (125.8 MHz, D₂O): δ=170.2, 124.1 (q, *J*=283.0 Hz), 57.1 (q, *J*=30.0 Hz), 53.0, 35.9, 17.4.

¹⁹F NMR (235.3 MHz, D₂O): δ=-80.0 (s).

HRMS-ESI: *m/z* [M+H]⁺ calcd for C₆H₁₁F₃NO₂: 186.0742; found: 186.0740.

(R)-(+)-β³-trifluoromethyl-β³-homoalanine hydrochloride [H-(R)-β³-Tfm-β³-hAla-OH.HCl] (R)-5. N-tert-Butanesulfinyl β-amino acid (Ss,R)-3 (2.61 g, 9.46 mmol) reacted with HCl (2 N in Et₂O, 53 mL). After 23 h of stirring at r.t. under Ar, the reaction mixture was concentrated under reduced pressure and the residue was triturated with Et₂O to afford β-amino acid hydrochloride (R)-5 (1.93 g, 98%) as a white solid.

[α]D²⁰+5 (c 0.94, H₂O).

IR (KBr plate): 3490, 3423, 3124, 2969, 2643, 2574, 2016, 1734, 1621, 1536, 1417, 1193, 1135, 853, 820, 532 cm⁻¹.

¹H NMR (500 MHz, D₂O): δ=3.05 (d, *J*=17.0 Hz, 1H), 2.97 (d, *J*=17.0 Hz, 1H), 1.67 (s, 3H, CH₃).

¹³C NMR (125.8 MHz, D₂O): δ=172.1, 124.3 (q, *J*=283.0 Hz), 57.3 (q, *J*=30.0 Hz), 35.9, 17.4.

¹⁹F NMR (235.3 MHz, D₂O): δ=-80.4 (s).

HRMS-ESI: *m/z* [M+H]⁺ calcd for C₅H₉F₃NO₂: 172.0585; found: 172.0584.

(R)-(-)-N-acetyl-β³-trifluoromethyl-β³-homoalanine methyl ester [Ac-(R)-β³-Tfm-β³-hAla-OMe] (R)-6a. A solution of β-amino methyl ester hydrochloride (R)-4 (400 mg, 1.81 mmol) and K₂CO₃ (2.50 g, 18.1 mmol, 10 equiv) in a mixture of THF and H₂O (3:2, 10 mL) was stirred under Ar at 0 °C for 10 min. Acetyl chloride (129 µL, 1.81 mmol, 1 equiv) was then added and after 10 min of stirring at this temperature, the reaction mixture was allowed to warm at room temperature. After 19 h of stirring, the mixture was cooled to 0 °C and another amount of K₂CO₃ (2.50 g, 18.1 mmol, 10 equiv) and AcCl (129 µL, 1.81 mmol, 1 equiv) were added following the same procedure. After 21 h of stirring (reaction monitored by ¹⁹F NMR), the reaction was quenched with water (10 mL) and was extracted three times with AcOEt. The combined organic layers were washed with HCl 10% (v/v), a sat. aq. sol. of NaHCO₃ and brine. The organic layer was dried (Na₂SO₄), filtered and concentrated under
reduced pressure. Purification of the residue by chromatography on silica gel (petroleum ether/AcOEt 7:3) afforded the *N*-acetyl aminoester **(R)-6a** (297 mg, 72%) as a beige solid.

Mp 49°C; [α]<sub>D<sup>20</sup></sub> -15 (c 0.31, CHCl<sub>3</sub>) [lit. enantiomer S: [α]<sup>20</sup><sub>D</sub>=+2.4 (c=0.3 in chloroform)<sup>13a</sup>].

IR (KBr plate): 3287, 3091, 3007, 2959, 2855, 1750, 1667, 1573, 1441, 1380, 1287, 1169, 1097, 1013, 732, 661 cm<sup>-1</sup>.

¹H NMR (600 MHz, CDCl<sub>3</sub>): δ=6.26 (br s, 1H), 3.67 (s, 3H), 3.31 (d, *J*=15.5 Hz, 1H), 2.64 (d, *J*=15.5 Hz, 1H), 1.98 (s, 3H), 1.64 (s, 3H).

¹³C NMR (150.9 MHz, CDCl<sub>3</sub>): δ=170.4, 170.1, 125.9 (q, *J*=286.0 Hz), 57.8 (q, *J*=28.5 Hz), 52.1, 36.8, 24.5, 20.1.

¹⁹F NMR (235.3 MHz, CDCl<sub>3</sub>): δ=-80.0 (s).

HRMS-ESI: *m/z* [M+Na]<sup>+</sup> calcd for C<sub>8</sub>H<sub>12</sub>F<sub>3</sub>NNaO<sub>4</sub>: 250.0667; found: 250.0670.

(R)-(-)-*N*-benzyloxycarbonyl-β³-trifluoromethyl-β³-homoalanine methyl ester [Cbz-(R)-β³-Tfm-β³-hAla-OMe] (R)-6b. A solution of β-amino methyl ester hydrochloride (R)-4 (420 mg, 1.90 mmol) and K₂CO₃ (2.56 g, 18.5 mmol, 9.7 equiv) in a mixture of THF and H₂O (3:2, 10 mL) was stirred under Ar at 0 °C for 10 min. Benzyl chloroformate (270 µL, 1.90 mmol, 1 equiv) was then added and after 10 min of stirring at this temperature, the reaction mixture was allowed to warm at room temperature. After 23h of stirring (reaction monitored by ¹⁹F NMR), the reaction was quenched with water (10 mL) and was extracted three times with AcOEt. The combined organic layers were washed with an aq. sol. of HCl 10% (v/v), a sat. aq. sol. of NaHCO₃ and brine. The organic layer was dried (Na₂SO₄), filtered and concentrated under reduced pressure. Purification of the residue by chromatography on silica gel (petroleum ether/AcOEt 9:1) afforded the *N*-benzyloxycarbonyl aminoester (R)-6b (534 mg, 88%) as a colourless oil.

[α]<sup>b<sup>20</sup></sup>-5 (c 0.99, CHCl<sub>3</sub>).

IR (KBr plate): 3355, 3067, 3034, 2956, 1744, 1534, 1458, 1386, 1260, 1175, 1068, 745, 699 cm<sup>-1</sup>.

¹H NMR (600 MHz, CDCl<sub>3</sub>): δ=7.39-7.29 (m, 5H), 5.56 (br s, 1H), 5.09 (d, *J*=12.5 Hz, 1H), 5.07 (d, *J*=12.5 Hz, 1H), 3.67 (s, 3H), 3.27 (d, *J*=15.5 Hz, 1H), 2.63 (d, *J*=15.5 Hz, 1H), 1.66 (s, 3H).

¹³C NMR (150.9 MHz, CDCl<sub>3</sub>): δ=169.7, 154.5, 136.2, 128.6, 128.3, 128.2, 125.9 (q, *J*=286.0 Hz), 66.9, 57.3 (q, *J*=29.0 Hz), 52.2, 37.2, 20.0.

¹⁹F NMR (235.3 MHz, CDCl<sub>3</sub>): δ=-80.5 (s).

HRMS-ESI: *m/z* [M+Na]<sup>+</sup> calcd for C<sub>14</sub>H<sub>16</sub>F<sub>3</sub>NNaO<sub>4</sub>: 342.0929; found: 342.0925.

(R)-(+)-*N*-acetyl-β³-trifluoromethyl-β³-homoalanine [Ac-(R)-β³-Tfm-β³-hAla-OH] (R)-7a. A solution of β-amino ester (R)-6a (433 mg, 1.90 mmol) and LiOH.H₂O (161 mg, 3.84 mmol, 2.0 equiv) in a mixture of MeOH and H₂O (3:1, 8 mL) was stirred 30 h at r.t. The residue was diluted with AcOEt and acidified with HCl 1M (pH =1). The organic layer was separated and the aqueous layer was extracted three times with AcOEt. The organic layers were combined, dried (Na₂SO₄) and concentrated under reduced pressure to afford *N*-acetylamino acid (R)-7a (391 mg, 96%) as a white solid.

Mp 100°C; [α]<sub>D<sup>20</sup></sub> +8 (c 1.00, H₂O).

IR (KBr plate): 3328, 3111, 2906, 2638, 2548, 1714, 1647, 1578, 1268, 1163, 1098, 919, 720, 648 cm<sup>-1</sup>.

¹H NMR (600 MHz, CD₃CN): δ=9.33 (br s, 1H), 6.65 (br s, 1H), 3.56 (d, *J*=16.0 Hz, 1H), 2.62 (d, *J*=16.0 Hz, 1H), 1.89 (s, 3H), 1.56 (s, 3H).

¹³C NMR (150.9 MHz, CD₃CN): δ=171.9, 170.8, 127.2 (q, *J*=285.5 Hz), 58.3 (q, *J*=28.5 Hz), 36.1, 24.0, 20.1.

¹⁹F NMR (235.3 MHz, CD₃CN): δ=-80.4 (s).

HRMS-ESI: *m/z* [M+Na]<sup>+</sup> calcd for C<sub>7</sub>H<sub>10</sub>F<sub>3</sub>NNaO<sub>3</sub>: 236.0510; found: 236.0503.

(R)-(-)-*N*-benzyloxycarbonyl-β³-trifluoromethyl-β³-homoalanine [Cbz-(R)-β³-Tfm-β³-hAla-OH] (R)-7b. A solution of β-amino ester (R)-6b (493 mg, 1.54 mmol) and LiOH.H₂O (131 mg, 3.11 mmol, 2.0 equiv) in a mixture of MeOH and H₂O (3:1, 8 mL) was stirred 30 h at r.t. The residue was diluted with AcOEt and acidified with HCl 1M (pH =1). The organic layer was separated and the aqueous layer was extracted three times with AcOEt. The organic layers were combined, dried (Na₂SO₄) and concentrated under reduced pressure to afford *N*-benzyloxycarbonylamino acid (R)-7b (444 mg, 94%) as a pale yellow oil.

[α]<sup>D<sup>20</sup></sup>-5 (c 1.05, CHCl<sub>3</sub>).

IR (film): 3330, 3036, 2958, 2679, 1737, 1532, 1459, 1413, 1183, 1138, 1096, 1070, 974, 745, 699, 632 cm<sup>-1</sup>.

¹H NMR (600 MHz, CDCl<sub>3</sub>): δ=9.47 (br s, 1H), 7.40-7.30 (m, 5H), 5.50 (br s, 1H), 5.09 (m, 2H), 3.43 (d, *J*=16.0 Hz, 1H), 2.68 (d, *J*=16.0 Hz, 1H), 1.64 (s, 3H).

¹³C NMR (150.9 MHz, CDCl<sub>3</sub>): δ=174.3, 154.9, 135.9, 128.7, 128.4, 128.2, 125.8 (q, *J*=284.0 Hz), 67.2, 57.2 (q, *J*=29.0 Hz), 36.7, 20.1.

¹⁹F NMR (235.3 MHz, CDCl<sub>3</sub>): δ=-80.3 (s).

HRMS-ESI: *m/z* [M+Na]<sup>+</sup> calcd for C<sub>13</sub>H<sub>14</sub>F<sub>3</sub>NNaO<sub>4</sub>: 328.0773; found: 328.0778.

(3R,3'R)-(-)-dimethyl 3,3'-(carbonylbis(azanediyl))bis(4,4,4-trifluoro-3-methylbutanoate) (R,R)-8. Triethylamine (106 µL, 0.78 mmol, 1.2 equiv) and DMAP (16 mg, 0.13 mmol, 0.2 equiv) were added at r.t. and under Ar to a solution of β-amino ester hydrochloride (R)-4 (145 mg, 0.65 mmol) in THF (2 mL). After 5 min of stirring, Boc₂O (285 mg, 1.31 mmol, 2.0 equiv) was added. After 4 days of stirring (reaction monitored by ¹⁹F NMR), the reaction mixture was diluted with AcOEt and washed with an aq. sol. of HCl 10% (v/v), a sat. aq. sol. of NaHCO₃ and brine. The organic layer was dried (Na₂SO₄), filtered and concentrated under reduced pressure. Purification of the residue by chromatography (petroleum ether/AcOEt 8:2 containing 0.01% Et₃N) afforded the urea (R,R)-8 (49 mg, 38%) as a white solid. An analytical sample of (R,R)-8 was crystallized from Et₂O-PE by slow evaporation.

Mp 119-120°C; [α]<sub>D<sup>20</sup></sub>-3 (c 1.00, CHCl<sub>3</sub>).

¹H NMR (500 MHz, CDCl<sub>3</sub>): δ=5.38 (s, 2H), 3.68 (s, 6H), 3.24 (d, *J*=15.5 Hz, 2H), 2.65 (d, *J*=15.5 Hz, 2H), 1.62 (s, 6H).

¹³C NMR (125.8 MHz, CDCl<sub>3</sub>): δ=170.3, 154.9, 126.1 (q, *J*=286.5 Hz), 57.6 (q, *J*=28.5 Hz), 52.1, 37.5, 20.5.

¹⁹F NMR (235.3 MHz, CDCl<sub>3</sub>): δ=-80.1 (s).

HRMS-ESI: *m/z* [M+Na]<sup>+</sup> calcd for C<sub>13</sub>H<sub>18</sub>F<sub>6</sub>N<sub>2</sub>NaO<sub>5</sub>: 419.1018; found: 419.1026.

General procedure for the coupling reaction with EDCI/HOBt (general procedure A): A solution of β³,³-amino acid (S<sub>s</sub>,R)-3, (R<sub>s</sub>,S)-3, (R)-7a or (R)-7b, amino ester or C-protected peptide, EDCI.HCl (2.0-2.2 equiv),
HOBt.xH₂O (2.0-2.4 equiv, calculated on anhydrous HOBt) and DIPEA (4.0-5.0 equiv) in acetonitrile was stirred at r.t. and under Ar. The reaction was diluted with AcOEt, washed with HCl 10% (v/v), a sat. aq. sol. of NaHCO₃, water and brine. The organic layer was dried (Na₂SO₄), filtered, concentrated under reduced pressure and, if necessary, the residue was purified by chromatography on silica gel.

**General procedure for the coupling reaction with HATU (general procedure B):** A solution of β³,³-amino ester **(R)-4**, **(S)-4** or dipeptide **15a-e**, **16a-c**, N-Boc protected amino acid (1.5 equiv), HATU (2-2.1 equiv), DIPEA (4.0-5.0 equiv) in DMF was stirred at r.t. and under Ar. The reaction was diluted with AcOEt, washed with HCl 10% (v/v), a sat. aq. sol. of NaHCO₃, water and brine. The organic layer was dried (Na₂SO₄), filtered, concentrated under reduced pressure and the residue was purified by chromatography on silica gel.

**General procedure for the cleavage of Boc or tert-butanesulfinyl groups (general procedure C):** A solution of N-tert-butanesulfinyl peptides **9a-c,j,l, 10b-d** or N-Boc peptides **11a, 17a** in MeOH reacted with HCl (2 N in Et₂O). The reaction mixture was diluted with an aq. sol. of HCl 5% (v/v) and was washed with Et₂O and *n*-hexane. Na₂CO₃ solid was added to the aqueous layer until pH = 9 and was then extracted with AcOEt. The combined organic layers were dried (Na₂SO₄), filtered and concentrated under reduced pressure to afford deprotected peptide, used in the next step without further purification.

**General procedure for the amidation of tripeptides (general procedure D):** MeNH₂ (2 N in MeOH, 5.0-15.3 equiv) was added portionwise at r.t. and under Ar to a solution of tripeptide methyl ester **17a-e** in MeOH. After completion of the reaction, the mixture was concentrated under reduced pressure to afford the *C*-amide peptide.

(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Ala-OMe **9a**.¹³b According to the general procedure A, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (1.52 g, 5.52 mmol) reacted with H-Ala-OMe.HCl (1.58 g, 11.28 mmol, 2.0 equiv), EDCI.HCl (2.34 g, 12.22 mmol, 2.2 equiv), HOBt.xH₂O (1.79 g, 13.26 mmol, 2.4 equiv) and DIPEA (3.85 mL, 22.10 mmol, 4.0 equiv) in CH₃CN (30 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 30:1) afforded the dipeptide **9a** as a yellow oil (1.73 g, 87%).

[α]<sub>D</sub>²⁰ +49 (c 1.10, CHCl₃).

IR (film): 3262, 3075, 2986, 2958, 2876, 2615, 1747, 1660, 1555, 1459, 1156, 1054, 757 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.94 (br s, 1H), 6.11 (s, 1H), 4.53 (quint, *J*=7.0 Hz, 1H), 3.72 (s, 3H), 2.62 (d, *J*=14.5 Hz, 1H), 2.57 (d, *J*=14.5 Hz, 1H), 1.64 (s, 3H), 1.39 (d, *J*=7.0 Hz, 3H), 1.23 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.0, 168.6, 125.9 (q, *J*=285.0 Hz), 59.7 (q, *J*=28.0 Hz), 56.5, 52.6, 48.3, 41.9, 22.7, 19.8, 18.1.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.9 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₃H₂₃F₃N₂NaO₄S: 383.1228; found: 383.1235.

(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Leu-OMe **9b**. According to the general procedure A, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (507 mg, 1.84 mmol) reacted with H-Leu-OMe.HCl (669 mg, 3.68 mmol, 2.0 equiv), EDCI.HCl (778 mg, 4.06 mmol, 2.2 equiv), HOBt.xH₂O (547 mg, 4.05 mmol, 2.2 equiv) and DIPEA (1.30 mL, 7.46 mmol, 4.1 equiv) in CH₃CN (10 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 30:1) afforded the dipeptide **9b** as a white solid (625 mg, 84%).

Mp 104°C; [α]<sub>D</sub>²⁰ +35 (c 0.85, CHCl₃).

IR (KBr plate): 3254, 3075, 2963, 2875, 1748, 1666, 1570, 1470, 1434, 1276, 1160, 1046, 869, 797, 740 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.71 (d, *J*=8.0 Hz, 1H), 6.02 (s, 1H), 4.58 (m, 1H), 3.71 (s, 3H), 2.64 (d, *J*=14.5 Hz, 1H), 2.58 (d, *J*=14.5 Hz, 1H), 1.65 (s, 3H), 1.64-1.55 (m, 3H), 1.24 (s, 9H), 0.912 (d, *J*=6.0 Hz, 3H), 0.909 (d, *J*=6.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.1, 168.8, 125.9 (q, *J*=285.0 Hz), 59.7 (q, *J*=27.5 Hz), 56.5, 52.4, 51.0, 42.2, 41.2, 25.0, 22.9, 22.7, 21.9, 19.9.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.9 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₆H₂₉F₃N₂NaO₄S: 425.1698; found: 425.1701.

(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Phe-OMe **9c**. According to the general procedure A, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (1.54 g, 5.58 mmol) reacted with H-Phe-OMe.HCl (2.41 g, 11.19 mmol, 2.0 equiv), EDCI.HCl (2.36 g, 12.30 mmol, 2.2 equiv), HOBt.xH₂O (1.67 g, 12.35 mmol, 2.2 equiv) and DIPEA (3.90 mL, 22.39 mmol, 4.0 equiv) in CH₃CN (30 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 13:1) afforded the dipeptide **9c** as a white solid (2.37 g, 97%). An analytical sample of **9c** was crystallized from CH₂Cl₂-*n*-hexane by slow evaporation.

Mp 119°C; [α]<sub>D</sub>²⁰⁺=107 (c 1.01, CHCl₃).

IR (KBr plate): 3257, 3079, 2956, 1755, 1662, 1566, 1414, 1374, 1275, 1219, 1169, 1129, 1093, 1051, 987, 702 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.30-7.27 (m, 2H), 7.25-7.20 (m, 1H), 7.10-7.07 (m, 2H), 6.55 (d, *J*=7.5 Hz, 1H), 6.11 (s, 1H), 4.84 (dt, *J*=7.5 Hz, *J*=6.0 Hz, 1H), 3.72 (s, 3H), 3.17 (dd, *J*=14.0 Hz, *J*=6.0 Hz, 1H), 3.09 (dd, *J*=14.0 Hz, *J*=6.0 Hz, 1H), 2.59 (d, *J*=14.5 Hz, 1H), 2.52 (d, *J*=14.5 Hz, 1H), 1.60 (s, 3H), 1.23 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=171.7, 168.8, 135.8, 129.3, 128.8, 127.3, 125.9 (q, *J*=285.0 Hz), 59.7 (q, *J*=27.5 Hz), 56.5, 53.4, 52.6, 41.9, 37.5, 22.7, 19.8.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.9 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₉H₂₇F₃N₂NaO₄S: 459.1541; found: 459.1547.

(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Aib-OMe **9d**. According to the general procedure A, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (103 mg, 0.37 mmol) reacted with H-Aib-OMe.HCl (117 mg, 0.76 mmol, 2.0 equiv), EDCI.HCl (160 mg, 0.84 mmol, 2.2 equiv), HOBt.xH₂O (120 mg, 0.88 mmol, 2.4 equiv) and DIPEA (270 µL, 1.52 mmol, 4.1 equiv) in CH₃CN (2 mL) for 20h. After work-up, the dipeptide **9d** (137 mg, 98%) was obtained as a white solid. An analytical sample of **9d** was crystallized from CH₂Cl₂-*n*-hexane by slow evaporation.

Mp 120°C; [α]<sub>D</sub>²⁰⁻2 (c 1.00, CHCl₃).

IR (KBr plate): 3234, 3055, 2992, 1743, 1659, 1555, 1471, 1433, 1382, 1332, 1284, 1181, 1096, 1050, 929, 609, 564 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.24 (s, 1H), 5.66 (s, 1H), 3.70 (s, 3H), 2.57 (br s, 2H), 1.60 (s, 3H), 1.56 (s, 3H), 1.47 (s, 3H), 1.25 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=175.2, 167.8, 125.6 (q, *J*=285.0 Hz), 60.3 (q, *J*=28.0 Hz), 56.9, 56.7, 52.8, 43.9, 25.6, 24.1, 22.7, 20.7.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.2 (s).
HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₅F₃N₂NaO₄S: 397.1385; found: 397.1378.

**Ac-(R)-β³-Tfm-β³-hAla-Ala-OMe 9e.** According to the general procedure A, Ac-(R)-β³-Tfm-β³-hAla-OH (R)-7a (113 mg, 0.53 mmol) reacted with H-Ala-OMe.HCl (158 mg, 1.13 mmol, 2.1 equiv), EDCI.HCl (230 mg, 1.20 mmol, 2.3 equiv), HOBT.xH₂O (174 mg, 1.29 mmol, 2.4 equiv) and DIPEA (370 µL, 2.12 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 30:1) afforded the dipeptide **9e** as a yellow oil (122 mg, 77%).

[α]D²⁰ +32 (c 1.01, CHCl₃).

IR (film): 3309, 3078, 3000, 2955, 2852, 1746, 1682, 1537, 1456, 1376, 1303, 1159, 1102, 1058, 983, 757, 662 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.84 (d, J=7.5 Hz, 1H), 6.42 (s, 1H), 4.50 (quint, J=7.5 Hz, 1H), 3.70 (s, 3H), 3.48 (d, J=13.5 Hz, 1H), 2.37 (d, J=13.5 Hz, 1H), 2.03 (s, 3H), 1.58 (s, 3H), 1.36 (d, J=7.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.1, 172.0, 168.5, 126.0 (q, J=286.5 Hz), 58.5 (q, J=29.0 Hz), 52.5, 48.2, 38.5, 24.8, 20.7, 17.9.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.2 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₁H₁₇F₃N₂NaO₄: 321.1038; found: 321.1042.

**Cbz-(R)-β³-Tfm-β³-hAla-Ala-OMe 9f.** According to the general procedure A, Cbz-(R)-β³-Tfm-β³-hAla-OH (R)-7b (111 mg, 0.36 mmol) reacted with H-Ala-OMe.HCl (101 mg, 0.73 mmol, 2.0 equiv), EDCI.HCl (154 mg, 0.81 mmol, 2.2 equiv), HOBT.xH₂O (108 mg, 0.80 mmol, 2.2 equiv) and DIPEA (260 µL, 1.46 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. Purification on SiO₂ (petroleum ether/AcOEt 1:1 to 0:100) afforded the dipeptide **9f** as a pale yellow oil (99 mg, 70%).

[α]D²⁰ +34 (c 0.86, CHCl₃).

IR (film): 3327, 3067, 3035, 2997, 2954, 1740, 1662, 1538, 1456, 1384, 1166, 1098, 1067, 971, 748, 698 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.37-7.30 (m, 5H), 6.82 (d, J=7.5 Hz, 1H), 5.61 (s, 1H), 5.13 (d, J=12.0 Hz, 1H), 5.09 (d, J=12.0 Hz, 1H), 4.53 (quint, J=7.5 Hz, 1H), 3.69 (s, 3H), 3.45 (d, J=13.5 Hz, 1H), 2.40 (d, J=13.5 Hz, 1H), 1.57 (s, 3H), 1.33 (d, J=7.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.1, 167.9, 155.5, 136.0, 128.6, 128.4, 128.3, 126.1 (q, J=287.0 Hz), 67.3, 57.8 (q, J=28.5 Hz), 52.5, 48.1, 38.2, 20.6, 18.1.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-80.0 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₇H₂₁F₃N₂NaO₅: 413.1300; found: 413.1292.

**Ac-(R)-β³-Tfm-β³-hAla-Leu-OMe 9g.** According to the general procedure A, Ac-(R)-β³-Tfm-β³-hAla-OH (R)-7a (154 mg, 0.72 mmol) reacted with H-Leu-OMe.HCl (262 mg, 1.44 mmol, 2.0 equiv), EDCI.HCl (304 mg, 1.59 mmol, 2.2 equiv), HOBT.H₂O (216 mg, 1.60 mmol, 2.2 equiv) and DIPEA (510 µL, 2.90 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. After work-up, the dipeptide **9g** (244 mg, 99%) was obtained as a white solid.

Mp 95°C; [α]D²⁰ +25 (c 1.00, CHCl₃).

IR (KBr plate): 3373, 3310, 3088, 2963, 2874, 1748, 1679, 1563, 1450, 1375, 1308, 1272, 1210, 1167, 1100, 1035, 985, 736, 650 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.70 (br s, 1H), 6.24 (br s, 1H), 4.52 (td, J=9.0 Hz, J=4.5 Hz, 1H), 3.69 (s, 3H), 3.57 (d, J=13.0 Hz, 1H), 2.37 (d, J=13.0 Hz, 1H), 2.04 (s, 3H), 1.67-1.59 (m, 2H), 1.57 (s, 3H), 1.52-1.47 (m, 1H), 0.92 (d, J=5.0 Hz, 3H), 0.91 (d, J=5.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.2, 172.2, 168.7, 126.0 (q, J=286.5 Hz), 58.5 (q, J=29.0 Hz), 52.3, 51.0, 41.0, 38.4, 25.0, 24.9, 22.9, 21.8, 20.8.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.3 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₃F₃N₂NaO₄: 363.1508; found: 363.1504.

**Ac-(R)-β³-Tfm-β³-hAla-Aib-OMe 9h.** According to the general procedure A, Ac-(R)-β³-Tfm-β³-hAla-OH (R)-7a (101 mg, 0.47 mmol) reacted with H-Aib-OMe.HCl (145 mg, 0.94 mmol, 2.0 equiv), EDCI.HCl (201 mg, 1.05 mmol, 2.2 equiv), HOBT.H₂O (142 mg, 1.05 mmol, 2.2 equiv) and DIPEA (330 µL, 1.89 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. After work-up, the dipeptide **9h** (129 mg, 88%) was obtained as a beige solid. An analytical sample of **9h** was crystallized from EtOAc by slow evaporation.

Mp 143°C; [α]D²⁰ +39 (c 1.02, CHCl₃).

IR (KBr plate): 3371, 3308, 3061, 2992, 2956, 1735, 1678, 1545, 1443, 1384, 1289, 1212, 1167, 1098, 1028, 974, 920, 760, 648, 605 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.77 (s, 1H), 6.75 (s, 1H), 3.70 (s, 3H), 3.19 (d, J=14.0 Hz, 1H), 2.34 (d, J=14.0 Hz, 1H), 2.01 (s, 3H), 1.62 (s, 3H), 1.49 (s, 3H), 1.45 (s, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.7, 171.3, 168.3, 126.1 (q, J=286.5 Hz), 58.5 (q, J=28.5 Hz), 56.4, 52.6, 39.1, 24.9, 24.8, 24.6, 20.6.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-78.8 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₂H₁₉F₃N₂NaO₄: 335.1195; found: 335.1199.

**Cbz-(R)-β³-Tfm-β³-hAla-Aib-OMe 9i.** According to the general procedure A, Cbz-(R)-β³-Tfm-β³-hAla-OH (R)-7b (106 mg, 0.35 mmol) reacted with H-Aib-OMe.HCl (107 mg, 0.70 mmol, 2.0 equiv), EDCI.HCl (147 mg, 0.77 mmol, 2.2 equiv), HOBT.xH₂O (105 mg, 0.77 mmol, 2.2 equiv) and DIPEA (250 µL, 1.41 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. After work-up, the dipeptide **9i** (124 mg, 89%) was obtained as a beige oil.

[α]D²⁰ +30 (c 1.01, CHCl₃).

IR (film): 3338, 3063, 3034, 2992, 2952, 1737, 1663, 1527, 1458, 1388, 1287, 1254, 1161, 1098, 1067, 1028, 971, 752, 698 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=7.40-7.29 (m, 5H), 6.63 (s, 1H), 5.80 (s, 1H), 5.11-5.04 (m, 2H), 3.68 (s, 3H), 3.24 (d, J=13.5 Hz, 1H), 2.36 (d, J=13.5 Hz, 1H), 1.60 (s, 3H), 1.45 (s, 3H), 1.38 (s, 3H).

¹³C NMR (125.8 MHz, CDCl₃): δ=174.6, 167.8, 155.3, 136.0, 128.7, 128.4, 128.2, 126.1 (q, J=286.5 Hz), 67.1, 57.9 (q, J=28.5 Hz), 56.4, 52.6, 38.7, 24.7, 24.6, 20.6.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₈H₂₃F₃N₂NaO₅: 427.1457; found: 427.1464.

(R₃)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Ala-OMe 9j. According to the general procedure A, (R₃)-tBuS(O)-(S)-β³-Tfm-β³-hAla-OH (R₃,S)-3 (816 mg, 2.97
mmol) reacted with H-Ala-OMe.HCl (831 mg, 5.96 mmol, 2.0 equiv), EDCI.HCl (1.26 g, 6.57 mmol, 2.2 equiv), HOBT.xH₂O (882 mg, 6.53 mmol, 2.2 equiv) and DIPEA (2.60 mL, 14.93 mmol, 5.0 equiv) in CH₃CN (15 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 13:1) afforded the dipeptide **9j** (807 mg, 76%) as a yellow oil.

$$[\alpha]_D^{20} + 1 \ (c \ 0.60, \mathrm{CHCl}_3).$$

IR (film): 3260, 3075, 2986, 2959, 2877, 2613, 1746, 1660, 1557, 1459, 1384, 1331, 1275, 1159, 1101, 1054, 981, 916, 852, 757, 689, 665 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.44 (br d, *J*=7.0 Hz, 1H), 5.89 (s, 1H), 4.54 (quint, *J*=7.0 Hz, 1H), 3.72 (s, 3H), 2.69 (d, *J*=14.0 Hz, 1H), 2.63 (d, *J*=14.0 Hz, 1H), 1.61 (s, 3H), 1.40 (d, *J*=7.5 Hz, 3H), 1.26 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.0, 168.3, 125.5 (q, *J*=285.0 Hz), 60.5 (q, *J*=28.0 Hz), 56.8, 52.7, 48.4, 43.8, 22.7, 20.9, 17.4.

$$^{19}F \text{ NMR (235.3 MHz, CDCl}_3): \delta=-79.1 \text{ (s).}$$

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₃H₂₃F₃N₂NaO₄S: 383.1228; found: 383.1218.

(R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Leu-OMe **9k**. According to the general procedure A, (R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-OH (R<sub>S,S</sub>)-3 (207 mg, 0.75 mmol) reacted with H-Leu-OMe.HCl (278 mg, 1.53 mmol, 2.0 equiv), EDCI.HCl (317 mg, 1.66 mmol, 2.2 equiv), HOBT.xH₂O (225 mg, 1.67 mmol, 2.2 equiv) and DIPEA (530 µL, 3.04 mmol, 4.0 equiv) in CH₃CN (4 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 13:1) afforded the dipeptide **9k** (212 mg, 70%) as a white solid.

$$\text{Mp } 146^{\circ}\text{C; } [\alpha]_D^{20} + 15 \ (c \ 1.00, \mathrm{CHCl}_3).$$

IR (KBr plate): 3250, 3089, 2962, 2874, 1755, 1653, 1567, 1471, 1436, 1373, 1335, 1273, 1199, 1156, 1056, 988, 920, 764, 723 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.46 (d, *J*=8.0 Hz, 1H), 5.83 (s, 1H), 4.53 (ddd, *J*=10.0 Hz, *J*=8.0 Hz, *J*=5.0 Hz, 1H), 3.70 (s, 3H), 2.71 (d, *J*=14.0 Hz, 1H), 2.64 (d, *J*=14.0 Hz, 1H), 1.70 (m, 1H), 1.60 (s, 3H), 1.59 (m, 2H), 1.26 (s, 9H), 0.93 (d, *J*=6.5 Hz, 3H), 0.90 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.2; 168.5; 125.4 (q, *J*=284.5 Hz), 60.6 (q, *J*=28.0 Hz), 56.9, 52.5, 51.4, 44.1, 40.4, 25.1, 23.0, 22.7, 21.7, 21.1.

$$^{19}F \text{ NMR (235.3 MHz, CDCl}_3): \delta=-79.3 \text{ ppm (s).}$$

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₆H₂₉F₃N₂NaO₄S: 425.1698; found: 425.1697.

(R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Phe-OMe **9l**. According to the general procedure A, (R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-OH (R<sub>S,S</sub>)-3 (809 mg, 2.94 mmol) reacted with H-Phe-OMe.HCl (1.27 g, 5.88 mmol, 2.0 equiv), EDCI.HCl (1.24 g, 6.47 mmol, 2.2 equiv), HOBT.xH₂O (875 mg, 6.47 mmol, 2.2 equiv) and DIPEA (2.60 mL, 14.93 mmol, 5.1 equiv) in CH₃CN (15 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 13:1) afforded the dipeptide **9l** (1.12 g, 87%) as a white solid.

$$\text{Mp } 143^{\circ}\text{C; } [\alpha]_D^{20} + 31 \ (c \ 1.00, \mathrm{CHCl}_3).$$

IR (KBr plate): 3277, 3205, 3083, 3032, 2958, 2869, 1754, 1655, 1560, 1440, 1371, 1330, 1271, 1219, 1186, 1159, 1125, 1096, 1054, 984, 919, 759, 702 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.37 (m, 1H), 7.28 (t, *J*=7.5 Hz, 2H), 7.22 (t, *J*=7.5 Hz, 1H), 7.17 (d, *J*=7.5 Hz, 2H), 5.93 (s, 1H), 4.78 (td, *J*=8.0 Hz, *J*=5.0 Hz, 1H), 3.71 (s, 3H), 3.12 (dd, *J*=14.0 Hz, *J*=5.0 Hz, 1H), 2.99 (dd, *J*=14.0 Hz, *J*=8.0 Hz, 1H), 2.64 (d, *J*=14.0 Hz, 1H), 2.56 (d, *J*=14.0 Hz, 1H), 1.58 (s, 3H), 1.26 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.7, 168.5, 136.1, 129.3, 128.7, 127.2, 125.5 (q, *J*=285.0 Hz), 60.4 (q, *J*=28.5 Hz), 56.8, 54.1, 52.6, 43.6, 37.5, 22.7, 20.8.

$$^{19}F \text{ NMR (235.3 MHz, CDCl}_3): \delta=-79.2 \text{ (s).}$$

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₉H₂₇F₃N₂NaO₄S: 459.1541; found: 459.1523.

(R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Pro-OMe **9m**. According to the general procedure A, (R<sub>S</sub>)-tBuS(O)-(S)-β³-Tfm-β³-hAla-OH (R<sub>S,S</sub>)-3 (123 mg, 0.45 mmol) reacted with H-Pro-OMe.HCl (152 mg, 0.92 mmol, 2.0 equiv), EDCI.HCl (196 mg, 1.02 mmol, 2.3 equiv), HOBT.xH₂O (135 mg, 1.00 mmol, 2.2 equiv) and DIPEA (390 µL, 2.24 mmol, 5.0 equiv) in CH₃CN (2 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 15:1) afforded the dipeptide **9m** (148 mg, 86%, mixture of *trans*/*cis* rotamers: 85:15) as a colourless oil.

$$[\alpha]_D^{20} - 120 \ (c \ 1.00, \mathrm{CHCl}_3).$$

IR (film): 3462, 3197, 2959, 2882, 1745, 1633, 1451, 1365, 1323, 1277, 1187, 1157, 1123, 1097, 1064, 963, 916, 866, 756 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=7.23 (s, 1H *cis*), 6.74 (s, 1H *trans*), 4.51 (dd, *J*=8.5 Hz, *J*=3.5 Hz, 1H *trans*), 4.37 (dd, *J*=8.0 Hz, *J*=3.0 Hz, 1H *cis*), 3.73 (s, 3H *cis*), 3.70 (s, 3H *trans*), 3.69-3.63 (m, 2H *cis* and *trans*), 3.63-3.50 (m, 2H *cis* and *trans*), 2.77 (d, *J*=15.0 Hz, 1H *trans*), 2.62 (d, *J*=15.0 Hz, 1H *trans*), 2.60 (d, *J*=16.0 Hz, 1H *cis*), 2.53 (d, *J*=16.0 Hz, 1H *cis*), 2.16 (m, 3H *cis* and *trans*), 2.09-1.95 (m, 3H *cis* and *trans*), 1.92 (m, 2H *cis*), 1.70 (s, 3H *trans*), 1.63 (s, 3H *cis*), 1.24 (s, 9H *cis*), 1.21 (s, 9H *trans*).

¹³C NMR (125.8 MHz, CDCl₃): δ=172.1 (*trans*), 172.0 (*cis*), 168.8 (*cis*), 168.6 (*trans*), 126.2 (q, *J*=285.5 Hz, *trans*), 126.0 (q, *J*=285.0 Hz, *cis*), 59.70 (q, *J*=27.5 Hz, *cis*), 59.67 (*cis*), 59.60 (q, *J*=27.5 Hz, *trans*), 58.9 (*trans*), 56.4 (*trans*), 56.3 (*cis*), 52.9 (*cis*), 52.5 (*trans*), 47.6 (*trans*), 46.6 (*cis*), 38.7 (*trans*), 38.6 (*cis*), 31.4 (*cis*), 29.2 (*trans*), 24.7 (*trans*), 22.83 (*cis*), 22.80 (*trans*), 22.5 (*cis*), 20.27 (*trans*), 20.26 (*cis*).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.6 (s, major *trans* rotamer), -79.8 (s, minor *cis* rotamer).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₅H₂₅F₃N₂NaO₄S: 409.1385; found: 409.1391.

(S<sub>S</sub>)-tBuS(O)-(R)-β³-Tfm-β³-hAla-β-hAla-OMe **10a**. According to the general procedure A, (S<sub>S</sub>)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S<sub>S,R</sub>)-3 (404 mg, 1.47 mmol) reacted with H-β-hAla-OMe.HCl (451 mg, 2.94 mmol, 2.0 equiv), EDCI.HCl (622 mg, 3.25 mmol, 2.2 equiv), HOBT.xH₂O (439 mg, 3.25 mmol, 2.2 equiv) and DIPEA (1.05 mL, 6.03 mmol, 4.1 equiv) in CH₃CN (6 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 20:1) afforded the dipeptide **10a** (448 mg, 81%) as a white solid.

$$\text{Mp } 145^{\circ}\text{C; } [\alpha]_D^{20} + 27 \ (c \ 1.02, \mathrm{CHCl}_3).$$

IR (KBr plate): 3270, 3156, 3088, 2983, 1737, 1651, 1563, 1463, 1437, 1371, 1303, 1195, 1153, 1126, 1096, 1053, 964, 920, 710, 616 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.64 (d, *J*=8.0 Hz, 1H), 6.42 (s, 1H), 4.31 (m, 1H), 3.67 (s, 3H), 2.55 (d, *J*=14.5 Hz, 1H), 2.52 (m, 2H), 2.45 (d, *J*=14.5 Hz, 1H), 1.63 (s, 3H), 1.24 (s, 9H), 1.21 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.1, 168.4, 126.0 (q, *J*=285.0 Hz), 59.7 (q, *J*=27.5 Hz), 56.5, 51.8, 42.4, 42.2, 39.3, 22.8, 20.0, 19.9.

$$^{19}F \text{ NMR (235.3 MHz, CDCl}_3): \delta=-79.7 \text{ (s).}$$

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₅F₃N₂NaO₄S: 397.1385; found: 397.1390.
(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-β-hPhe-OMe 10b. According to the general procedure A, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (404 mg, 1.47 mmol) reacted with H-β-hPhe-OMe.HCl (517 mg, 2.25 mmol, 1.5 equiv), EDCI.HCl (629 mg, 3.28 mmol, 2.2 equiv), HOBT.xH₂O (437 mg, 3.23 mmol, 2.2 equiv) and DIPEA (1.02 mL, 5.87 mmol, 4.0 equiv) in CH₃CN (6 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 20:1) afforded the dipeptide 10b (583 mg, 88%) as a white solid. An analytical sample of 10b was crystallized from CH₂Cl₂-n-hexane by slow evaporation.

Mp 163°C; [α]D²⁰ +35 (c1.00, CHCl₃).

IR (KBr plate): 3755, 3423, 3283, 2959, 2373, 2339, 1748, 1655, 1557, 1431, 1368, 1268, 1197, 1151, 1091, 1055, 992, 920, 759, 702, 569 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.28 (t, *J*=7.5 Hz, 2H), 7.22 (t, *J*=7.5 Hz, 1H), 7.15 (d, *J*=7.5 Hz, 2H), 6.58 (d, *J*=8.0 Hz, 1H), 6.32 (s, 1H), 4.47 (m, 1H), 3.67 (s, 3H), 2.94 (dd, *J*=14.0 Hz, *J*=7.0 Hz, 1H), 2.85 (dd, *J*=14.0 Hz, *J*=8.0 Hz, 1H), 2.54 (d, *J*=14.5 Hz, 1H), 2.53 (dd, *J*=16.5 Hz, *J*=5.5 Hz, 1H), 2.48 (dd, *J*=16.5 Hz, *J*=5.5 Hz, 1H), 2.44 (d, *J*=14.5 Hz, 1H), 1.60 (s, 3H), 1.24 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.3, 168.5, 137.3, 129.2, 128.8, 127.0, 126.0 (q, *J*=285.0 Hz), 59.7 (q, *J*=27.5 Hz), 56.5, 51.9, 47.4, 42.5, 39.6, 36.4, 22.8, 20.0.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₀H₂₉F₃N₂NaO₄S: 473.1698; found: 473.1700.

(R₅)-tBuS(O)-(S)-β³-Tfm-β³-hAla-β-hAla-OMe 10c. According to the general procedure A, (R₅)-tBuS(O)-(S)-β³-Tfm-β²-hAla-OH (R₅,S)-3 (101 mg, 0.37 mmol) reacted with H-β-hAla-OMe.HCl (114 mg, 0.74 mmol, 2.0 equiv), EDCI.HCl (158 mg, 0.83 mmol, 2.3 equiv), HOBT.xH₂O (110 mg, 0.82 mmol, 2.2 equiv) and DIPEA (260 µL, 1.49 mmol, 4.1 equiv) in CH₃CN (2 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 20:1) afforded the dipeptide 10c (108 mg, 79%) as a colourless oil.

[α]D²⁰ -58 (c 0.95, CHCl₃).

IR (KBr plate): 3270, 3210, 3161, 3095, 2982, 1742, 1651, 1566, 1463, 1432, 1376, 1298, 1269, 1216, 1180, 1124, 1098, 1054, 1015, 969, 922, 724, 620, 573 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.87 (d, *J*=8.5 Hz, 1H), 6.23 (s, 1H), 4.33 (m, 1H), 3.66 (s, 3H), 2.61-2.54 (m, 2H), 2.50 (d, *J*=14.5 Hz, 1H), 2.44 (dd, *J*=15.5 Hz, *J*=6.0 Hz, 1H), 1.61 (s, 3H), 1.26 (s, 9H), 1.20 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.3, 168.0, 125.9 (q, *J*=285.0 Hz), 59.9 (q, *J*=27.5 Hz), 56.6, 51.9, 43.1, 42.4, 39.5, 22.8, 20.5, 20.0.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₅F₃N₂NaO₄S: 397.1385; found: 397.1380.

(R₅)-tBuS(O)-(S)-β³-Tfm-β³-hAla-β-hLeu-OMe 10d. According to the general procedure A, (R₅)-tBuS(O)-(S)-β³-Tfm-β²-hAla-OH (R₅,S)-3 (103 mg, 0.37 mmol) reacted with H-β-hLeu-OMe.HCl (147 mg, 0.75 mmol, 2.0 equiv), EDCI.HCl (159 mg, 0.83 mmol, 2.2 equiv), HOBT.xH₂O (112 mg, 0.83 mmol, 2.2 equiv) and DIPEA (260 µL, 1.49 mmol, 4.0 equiv) in CH₃CN (2 mL) for 20h. Purification on SiO₂ (CH₂Cl₂/MeOH 20:1) afforded the dipeptide 10d (118 mg, 76%) as a white solid. An analytical sample of 10d was crystallized from CH₂Cl₂-cyclohexane by slow evaporation.

Mp 163°C; [α]D²⁰ -55 (c 1.00, CHCl₃).

IR (KBr plate): 3421, 3269, 3210, 3171, 3091, 2963, 2875, 1743, 1652, 1564, 1468, 1433, 1372, 1307, 1273, 1215, 1194, 1155, 1125, 1098, 1057, 1022, 986, 919, 767, 714, 617 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.74 (d, *J*=9.0 Hz, 1H), 6.22 (s, 1H), 4.30 (m, 1H), 3.65 (s, 3H), 2.61 (d, *J*=14.5 Hz, 1H), 2.60 (dd, *J*=15.0 Hz, *J*=4.5 Hz, 1H), 2.52 (d, *J*=14.5 Hz, 1H), 2.39 (dd, *J*=15.0 Hz, *J*=6.0 Hz, 1H), 1.61 (s, 3H), 1.60 (m, 1H), 1.49 (ddd, *J*=14.0 Hz, *J*=9.5 Hz, *J*=5.5 Hz, 1H), 1.27 (s, 9H), 1.26 (m, 1H), 0.892 (d, *J*=6.5 Hz, 3H), 0.888 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.5, 168.0, 125.8 (q, *J*=285.0 Hz), 60.0 (q, *J*=28.0 Hz), 56.7, 51.9, 44.7, 43.3, 43.2, 38.7, 25.0, 23.0, 22.8, 22.1, 20.7.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₇H₃₁F₃N₂NaO₄S: 439.1854; found: 439.1861.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-OMe 11a.** According to the general procedure B, H-(R)-β³-Tfm-β³-hAla-OMe.HCl (R)-4 (748 mg, 3.38 mmol) reacted with Boc-Ala-OH (960 mg, 5.08 mmol, 1.5 equiv), HATU (2.59 g, 6.80 mmol, 2.0 equiv) and DIPEA (2.9 mL, 16.65 mmol, 5.0 equiv) in DMF (15 mL) for 6 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the dipeptide 11a (1.12 g, 93%) as a white solid. An analytical sample of 11a was crystallized from Et₂O-n-hexane by slow evaporation.

Mp 75°C; [α]D²⁰ -41 (c 0.50, CHCl₃).

IR (KBr plate): 3514, 3438, 3333, 3252, 3087, 2984, 1721, 1682, 1566, 1447, 1372, 1312, 1254, 1167, 1101, 1020, 864, 765, 666, 621, 547 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.10 (br s, 1H), 4.97 (br s, 1H), 4.13 (m, 1H), 3.67 (s, 3H), 3.36 (m, 1H), 2.63 (d, *J*=15.5 Hz, 1H), 1.65 (s, 3H), 1.44 (s, 9H), 1.32 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.6, 169.7, 156.0, 125.9 (q, *J*=286.0 Hz), 80.5, 57.7 (q, *J*=27.5 Hz), 52.1, 50.8, 36.9, 28.4, 19.9, 17.4.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-80.2 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₃F₃N₂O₅+Na⁺: 379.1457; found: 379.1465.

**Boc-Ala-(S)-β³-Tfm-β³-hAla-OMe 11b.** According to the general procedure B, H-(S)-β³-Tfm-β³-hAla-OMe.HCl (S)-4 (213 mg, 0.96 mmol) reacted with Boc-Ala-OH (275 mg, 1.45 mmol, 1.5 equiv), HATU (734 mg, 1.93 mmol, 2.0 equiv) and DIPEA (840 µL, 4.82 mmol, 5.0 equiv) in DMF (5 mL) for 7 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the dipeptide 11b (276 mg, 81%) as a yellow oil.

[α]D²⁰ -22 (c 0.87, CHCl₃).

IR (film): 3318, 3087, 2982, 1742, 1686, 1527, 1452, 1370, 1290, 1250, 1167, 1098, 859, 758, 658 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.06 (br s, 1H), 5.12 (br s, 1H), 4.13 (br s, 1H), 3.65 (s, 3H), 3.36 (d, *J*=14.0 Hz, 1H), 2.66 (d, *J*=15.5 Hz, 1H), 1.64 (s, 3H), 1.41 (s, 9H), 1.30 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.7, 169.7, 156.0, 125.9 (q, *J*=286.0 Hz), 80.4, 57.8 (q, *J*=28.5 Hz), 52.1, 50.6, 36.6, 28.3, 20.1, 17.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.8 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₄H₂₃F₃N₂NaO₅: 379.1457; found: 379.1452.
**Boc-β-hAla-(R)-β³-Tfm-β³-hAla-OMe** 12a. According to the general procedure B, H-(R)-β³-Tfm-β³-hAla-OMe.HCl (R)-4 (107 mg, 0.48 mmol) reacted with Boc-β-hAla-OH (150 mg, 0.74 mmol, 1.5 equiv), HATU (373 mg, 0.98 mmol, 2.0 equiv) and DIPEA (430 µL, 2.47 mmol, 5.1 equiv) in DMF (3 mL) for 7 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the dipeptide 12a (47 mg, 26%) as a white solid. An analytical sample of 12a was crystallized from CHCl₃-pentane by vapour diffusion.

Mp 98°C; [α]D²⁰⁻¹⁰ (c 0.55, CHCl₃).

IR (KBr plate): 3355, 3311, 3100, 2984, 2367, 1756, 1679, 1567, 1531, 1456, 1376, 1281, 1164, 1097, 1060, 957, 902, 657 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=6.27 (s, 1H), 5.16 (m, 1H), 3.96 (m, 1H), 3.69 (s, 3H), 3.43 (d, *J*=16.0 Hz, 1H), 2.64 (d, *J*=16.0 Hz, 1H), 2.39 (d, *J*=5.5 Hz, 2H), 1.65 (s, 3H), 1.43 (s, 9H), 1.21 (d, *J*=7.0 Hz, 3H).

¹³C NMR (125.8 MHz, CDCl₃): δ=171.0, 170.0, 155.5, 125.9 (q, *J*=286.0 Hz), 79.5, 57.8 (q, *J*=28.5 Hz), 52.2, 44.3, 43.9, 36.6, 28.5, 20.4, 20.3.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₅H₂₅F₃N₂NaO₅: 393.1613; found: 393.1611.

**Boc-β-hAla-(S)-β³-Tfm-β³-hAla-OMe** 12b. According to the general procedure B, H-(S)-β³-Tfm-β³-hAla-OMe.HCl (S)-4 (116 mg, 0.52 mmol) reacted with Boc-β-HAla-OH (160 mg, 0.79 mmol, 1.5 equiv), HATU (398 mg, 1.05 mmol, 2.0 equiv) and DIPEA (460 µL, 2.64 mmol, 5.1 equiv) in DMF (2 mL) for 8 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the dipeptide 12b (68 mg, 35%) as a white solid.

Mp 143°C. [α]D²⁰⁻¹⁰ (c 1.00, CHCl₃).

IR (KBr plate): 3293, 3232, 3090, 2981, 2935, 1737, 1674, 1627, 1564, 1534, 1442, 1367, 1285, 1252, 1176, 1099, 1063, 1013, 939, 906, 853, 782, 664, 579, 509, 463, 425, 404 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=6.41 (br s, 1H), 5.16 (br s, 1H), 3.96 (sext, *J*=7.0 Hz, 1H), 3.69 (s, 3H), 3.34 (d, *J*=15.5 Hz, 1H), 2.68 (d, *J*=15.5 Hz, 1H), 2.39 (m, 2H), 1.66 (s, 3H), 1.42 (s, 9H), 1.21 (d, *J*=6.5 Hz, 3H).

¹³C NMR (125.8 MHz, CDCl₃): δ=171.0, 170.0, 155.5, 125.9 (q, *J*=286.0 Hz), 79.5, 57.9 (q, *J*=28.5 Hz), 52.2, 44.3, 44.0, 36.8, 28.5, 20.5, 20.2.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.6 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₅H₂₅F₃N₂NaO₅: 393.1613; found: 393.1608.

**H-Ala-(R)-β³-Tfm-β³-hAla-OMe** 13. According to the general procedure C, Boc-Ala-(R)-β³-Tfm-β³-hAla-OMe 11a (537 mg, 1.51 mmol) reacted with HCl (2N in Et₂O, 8.4 mL) in MeOH (16.8 mL) for 23h leading to the deprotected dipeptide 13 (310 mg, 80%) as an orange oil.

¹H NMR (250 MHz, CDCl₃): δ=7.98 (br s, 1H), 3.67 (s, 3H), 3.43 (m, 2H), 2.70 (m, 1H), 1.66 (s, 5H), 1.31 (d, *J*=7.0 Hz, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-80.1 (s).

**(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Ala-(R)-β³-Tfm-β³-hAla-OMe** 14. According to the general procedure A, H-Ala-(R)-β³-Tfm-β³-hAla-OMe 13 (204 mg, 0.80 mmol) reacted with (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (269 mg, 0.98 mmol, 1.2 equiv), EDCI.HCl (307 mg, 1.60 mmol, 2.0 equiv), HOBt.xH₂O (216 mg, 1.60 mmol, 2.2 equiv) and DIPEA (560 µL, 3.22 mmol, 4.0 equiv) in CH₃CN (4 mL) for 42h. Purification on SiO₂ (petroleum ether/EtOAc 1:1 to 1:4) afforded the tripeptide 14 (264 mg, 65%) as a white solid. An analytical sample of 14 was crystallized from

MeOH by slow evaporation. An analytical sample of 14 was crystallized from MeOH by slow evaporation.

Mp 151°C; [α]D²⁰⁻¹⁰ (c 1.00, CHCl₃).

IR (KBr plate): 3342, 3223, 3051, 2981, 2376, 1732, 1707, 1655, 1531, 1437, 1368, 1332, 1290, 1238, 1187, 1160, 1124, 1100, 1053, 1017, 969, 921, 742, 693, 625, 577, 550 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.00 (s, 1H), 6.93 (m, 1H), 6.08 (s, 1H), 4.45 (quint, *J*=7.0 Hz, 1H), 3.66 (s, 3H), 3.37 (d, *J*=15.5 Hz, 1H), 2.66 (d, *J*=15.5 Hz, 1H), 2.65 (d, *J*=14.5 Hz, 1H), 2.60 (d, *J*=14.5 Hz, 1H), 1.65 (s, 3H), 1.61 (s, 3H), 1.35 (d, *J*=7.0 Hz, 3H), 1.24 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=171.6, 169.7, 169.3, 125.9 (q, *J*=285.0 Hz), 125.8 (q, *J*=286.0 Hz), 59.8 (q, *J*=28.0 Hz), 57.9 (q, *J*=29.0 Hz), 56.6, 52.1, 49.9, 41.9, 36.6, 22.7, 19.9, 19.8, 17.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s, 3F), -79.9 (s, 3F).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₈H₂₉F₆N₃NaO₅S: 536.1630; found: 536.1611.

**H-(R)-β³-Tfm-β³-hAla-Leu-OMe** 15b. According to the general procedure C, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Leu-OMe 9b (625 mg, 1.55 mmol) reacted with HCl (2N in Et₂O, 10 mL) in MeOH (20 mL) for 15 min leading to the deprotected dipeptide 15b (455 mg, 98%) as a pale yellow oil.

¹H NMR (600 MHz, CDCl₃): δ=8.18 (d, *J*=8.0 Hz, 1H), 4.56 (m, 1H), 3.69 (s, 3H), 2.48 (d, *J*=15.0 Hz, 1H), 2.40 (d, *J*=15.0 Hz, 1H), 1.84 (s, 2H), 1.63 (m, 1H), 1.61 (m, 1H), 1.54 (m, 1H), 1.38 (s, 3H), 0.91 (d, *J*=6.0 Hz, 3H), 0.90 (d, *J*=6.0 Hz, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-84.0 (s).

**H-(R)-β³-Tfm-β³-hAla-Phe-OMe** 15c. According to the general procedure C, (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Phe-OMe 9c (2.37 g, 5.42 mmol) reacted with HCl (2N in Et₂O, 33 mL) in MeOH (66 mL) for 2h leading to the deprotected dipeptide 15c (1.61 g, 91%) as a white solid.

¹H NMR (600 MHz, CDCl₃): δ=8.20 (d, *J*=7.0 Hz, 1H), 7.28 (t, *J*=7.5 Hz, 2H), 7.24 (t, *J*=7.5 Hz, 1H), 7.12 (d, *J*=7.5 Hz, 2H), 4.88 (q, *J*=7.5 Hz, 1H), 3.73 (s, 3H), 3.15 (dd, *J*=14.0 Hz, *J*=5.5 Hz, 1H), 3.07 (dd, *J*=14.0 Hz, *J*=7.0 Hz, 1H), 2.42 (d, *J*=15.0 Hz, 1H), 2.38 (d, *J*=15.0 Hz, 1H), 1.68 (s, 2H), 1.33 (s, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-84.0 (s).

**H-(S)-β³-Tfm-β³-hAla-Ala-OMe** 15d. According to the general procedure C, (R₅)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Ala-OMe 9j (807 mg, 2.24 mmol) reacted with HCl (2N in Et₂O, 14 mL) in MeOH (20 mL) for 3h leading to the deprotected dipeptide 15d (527 mg, 92%) as a yellow oil.

¹H NMR (600 MHz, CDCl₃): δ=7.94 (d, *J*=7.0 Hz, 1H), 4.59 (quint, *J*=7.0 Hz, 1H), 3.74 (s, 3H), 2.52 (d, *J*=15.0 Hz, 1H), 2.43 (d, *J*=15.0 Hz, 1H), 2.07 (br s, 2H), 1.42 (d, *J*=7.0 Hz, 3H), 1.34 (s, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-83.6 (s).

**H-(S)-β³-Tfm-β³-hAla-Phe-OMe** 15e. According to the general procedure C, (R₅)-tBuS(O)-(S)-β³-Tfm-β³-hAla-Phe-OMe 9l (532 mg, 1.22 mmol) reacted with HCl (2N in Et₂O, 8 mL) in MeOH (16 mL) for 2h leading to the deprotected dipeptide 15e (379 mg, 94%) as a yellow oil.

¹H NMR (600 MHz, CDCl₃): δ=7.94 (d, *J*=7.5 Hz, 1H), 7.30-7.27 (m, 2H), 7.23 (m, 1H), 7.17 (d, *J*=7.0 Hz, 2H), 4.87 (td, *J*=7.5 Hz, *J*=5.5 Hz, 1H), 3.73 (s, 3H), 3.21 (dd, *J*=14.0 Hz, *J*=5.5 Hz, 1H), 3.03 (dd, *J*=14.0 Hz, *J*=8.0 Hz,
1H), 2.46 (d, J=15.5 Hz, 1H), 2.36 (d, J=15.5 Hz, 1H), 1.81 (s, 2H), 1.05 (s, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-83.8 (s).

**H-(R)-β³-Tfm-β³-hAla-β-hPhe-OMe 16a.** According to the general procedure *C*, (R₅)-tBuS(O)-(*R*)-β³-Tfm-β³-hAla-β-hPhe-OMe 10b (272 mg, 0.60 mmol) reacted with HCl (2N in Et₂O, 5 mL) in MeOH (10 mL) for 2h30 leading to the deprotected dipeptide **16a** (204 mg, 98%) as an orange oil.

¹H NMR (500 MHz, CDCl₃): δ=7.70 (br d, J=8.5 Hz, 1H), 7.29 (t, J=7.5 Hz, 2H), 7.22 (t, J=7.5 Hz, 1H), 7.18 (d, J=7.5 Hz, 2H), 4.51 (m, 1H), 3.68 (s, 3H), 2.94 (dd, J=13.5 Hz, J=7.0 Hz, 1H), 2.86 (dd, J=13.5 Hz, J=7.5 Hz, 1H), 2.57 (dd, J=16.0 Hz, J=5.0 Hz, 1H), 2.49 (dd, J=16.0 Hz, J=6.0 Hz, 1H), 2.39 (d, J=15.0 Hz, 1H), 2.33 (d, J=15.0 Hz, 1H), 1.76 (s, 2H), 1.24 (s, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-83.1 (s).

**H-(S)-β³-Tfm-β³-hAla-β-hAla-OMe 16b.** According to the general procedure *C*, (R₅)-tBuS(O)-(*S*)-β³-Tfm-β³-hAla-β-hAla-OMe 10c (249 mg, 0.67 mmol) reacted with HCl (2N in Et₂O, 5 mL) in MeOH (10 mL) for 3h30 leading to the deprotected dipeptide **16b** (172 mg, 96%) as a yellow oil.

¹H NMR (600 MHz, CDCl₃): δ=7.65 (br d, J=7.0 Hz, 1H), 4.35 (m, 1H), 3.67 (s, 3H), 2.52 (d, J=5.5 Hz, 2H), 2.43 (d, J=15.0 Hz, 1H), 2.37 (d, J=15.0 Hz, 1H), 2.06 (s, 2H), 1.32 (s, 3H), 1.23 (d, J=7.0 Hz, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-83.3 (s).

**H-(S)-β³-Tfm-β³-hAla-β-hLeu-OMe 16c.** According to the general procedure *C*, (R₅)-tBuS(O)-(*S*)-β³-Tfm-β³-hAla-β-hLeu-OMe 10d (255 mg, 0.61 mmol) reacted with HCl (2N in Et₂O, 5 mL) in MeOH (10 mL) for 2h30 leading to the deprotected dipeptide **16c** (179 mg, 93%) as a yellow oil.

¹H NMR (600 MHz, CDCl₃): δ=7.52 (s, 1H), 4.30 (m, 1H), 3.65 (s, 3H), 2.54 (dd, J=15.5 Hz, J=5.5 Hz, 1H), 2.48 (dd, J=15.5 Hz, J=5.5 Hz, 1H), 2.44 (d, J=15.0 Hz, 1H), 2.37 (d, J=15.0 Hz, 1H), 1.85 (s, 2H), 1.61 (m, 1H), 1.51 (ddd, J=14.0 Hz, J=9.5 Hz, J=5.5 Hz, 1H), 1.31 (s, 3H), 1.30 (m, 1H), 0.90 (d, J=6.5 Hz, 6H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-83.2 (s).

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe 17a.** According to the general procedure *B*, *H-(R)-β³-Tfm-β³-hAla-Ala-OMe 15a* (113 mg, 0.44 mmol) reacted with Boc-Ala-OH (126 mg, 0.67 mmol, 1.5 equiv), HATU (336 mg, 0.88 mmol, 2.0 equiv) and DIPEA (385 µL, 2.21 mmol, 5.0 equiv) in DMF (3 mL) for 7 days. Purification on SiO₂ (petroleum ether/EtOAc 2:3) afforded the tripeptide **17a** (156 mg, 83%) as a pale yellow oil.

[α]D²⁰⁻⁴⁸ (c 1.00, CHCl₃).

IR (KBr plate): 3612, 3486, 3278, 3099, 2986, 1755, 1690, 1659, 1584, 1555, 1456, 1389, 1301, 1258, 1177, 1099, 1071, 1026, 988, 962, 889, 852, 780, 673, 628, 568, 519, 486, 454 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=7.13 (br s, 1H), 6.93 (d, J=7.5 Hz, 1H), 5.46 (br s, 1H), 4.46 (m, 1H), 4.15 (br s, 1H), 3.67 (s, 3H), 3.60 (d, J=13.0 Hz, 1H), 2.32 (d, J=13.0 Hz, 1H), 1.51 (s, 3H), 1.36 (s, 9H), 1.31 (d, J=7.0 Hz, 3H), 1.27 (d, J=7.0 Hz, 3H).

¹³C NMR (125.8 MHz, CDCl₃): δ=174.1, 173.8, 168.8, 156.2, 126.0 (q, J=286.5 Hz), 80.2, 58.2 (q, J=29.0 Hz), 52.4, 50.6, 48.1, 37.9, 28.2, 20.3, 17.7, 17.0.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₇H₂₈F₃N₃NaO₆: 450.1828; found: 450.1833.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Leu-OMe 17b.** According to the general procedure *B*, *H-(R)-β³-Tfm-β³-hAla-Leu-OMe 15b* (413 mg, 1.39 mmol) reacted with Boc-Ala-OH (394 mg, 2.08 mmol, 1.5 equiv), HATU (1.04 g, 2.77 mmol, 2.0 equiv) and DIPEA (1.20 mL, 6.89 mmol, 5.0 equiv) in DMF (10 mL) for 5 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the tripeptide **17b** (516 mg, 79%) as a pale yellow oil.

[α]D²⁰⁻³⁷ (c 1.01, CHCl₃).

IR (film): 3316, 3079, 2962, 2875, 2252, 1681, 1529, 1452, 1369, 1327, 1248, 1164, 1100, 1067, 1026, 986, 916, 858, 758, 666 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.87 (br s, 1H), 6.62 (br s, 1H), 5.49 (br s, 1H), 4.56 (m, 1H), 4.21 (m, 1H), 3.76 (d, J=13.0 Hz, 1H), 3.72 (s, 3H), 2.35 (d, J=13.0 Hz, 1H), 1.54 (s, 3H), 1.62 (m, 1H), 1.56 (m, 1H), 1.51 (m, 1H), 1.43 (s, 9H), 1.33 (d, J=7.0 Hz, 3H), 0.92 (d, J=6.0 Hz, 3H), 0.91 (d, J=6.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.3, 174.2, 168.4, 156.6, 126.0 (q, J=286.5 Hz), 80.5, 58.2 (q, J=29.0 Hz), 52.5, 50.8, 50.6, 41.2, 37.9, 28.4, 25.0, 23.0, 21.8, 20.8, 16.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.5 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₀H₃₄F₃N₃NaO₆: 492.2297; found: 492.2311.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Phe-OMe 17c.** According to the general procedure *B*, *H-(R)-β³-Tfm-β³-hAla-Phe-OMe 15c* (336 mg, 1.01 mmol) reacted with Boc-Ala-OH (289 mg, 1.53 mmol, 1.5 equiv), HATU (773 mg, 2.03 mmol, 2.0 equiv) and DIPEA (700 µL, 4.02 mmol, 4.0 equiv) in DMF (6 mL) for 5 days. Purification on SiO₂ (petroleum ether/EtOAc 3:7) afforded the tripeptide **17c** (508 mg, 99%) as a colourless oil.

[α]D²⁰⁻¹¹ (c 0.80, CHCl₃).

IR (film): 3313, 3068, 2980, 2638, 2451, 2289, 1953, 1671, 1530, 1451, 1369, 1249, 1164, 1128, 1100, 1067, 1027, 992, 913, 857, 757, 701, 668, 407 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.29 (t, J=7.5 Hz, 2H), 7.24 (t, J=7.5 Hz, 1H), 7.11 (d, J=7.5 Hz, 2H), 6.92 (br s, 1H), 6.70 (d, J=8.0 Hz, 1H), 5.36 (br d, J=7.0 Hz, 1H), 4.81 (m, 1H), 4.16 (m, 1H), 3.71 (s, 3H), 3.67 (d, J=13.0 Hz, 1H), 3.09 (dd, J=14.0 Hz, J=5.5 Hz, 1H), 2.98 (dd, J=14.0 Hz, J=7.5 Hz, 1H), 2.29 (d, J=13.0 Hz, 1H), 1.50 (s, 3H), 1.43 (s, 9H), 1.22 (d, J=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.1, 172.6, 168.2, 156.4, 135.7, 129.2, 128.8, 127.4, 125.9 (q, J=286.5 Hz), 80.5, 58.1 (q, J=29.0 Hz), 53.4, 52.5, 50.5, 37.9, 28.4, 20.6, 16.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.5 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₃H₃₂F₃N₃NaO₆: 526.2141; found: 526.2134.

**Boc-Leu-(R)-β³-Tfm-β³-hAla-Ala-OMe 17d.** According to the general procedure *B*, *H-(R)-β³-Tfm-β³-hAla-Ala-OMe 15a* (512 mg, 2.00 mmol) reacted with Boc-Leu-OH (699 mg, 3.02 mmol, 1.5 equiv), HATU (1.53 g, 4.02 mmol, 2.0 equiv) and DIPEA (1.75 mL, 10.05 mmol, 5.1 equiv) in DMF (12 mL) for 5 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the tripeptide **17d** (413 mg, 44%) as a pale yellow oil.

[α]D²⁰⁻⁴⁶ (c 1.02, CHCl₃).

IR (KBr plate): 3371, 3324, 3284, 3064, 2965, 2874, 2370, 1745, 1702,
1650, 1549, 1457, 1388, 1367, 1286, 1245, 1171, 1103, 1048, 983, 853, 789, 749, 693, 631, 603, 472, 446 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.87 (s, 1H), 6.78 (br s, 1H), 5.26 (br d, *J*=7.0 Hz, 1H), 4.55 (m, 1H), 4.10 (m, 1H), 3.74 (s, 3H), 3.71 (d, *J*=13.0 Hz, 1H), 2.36 (d, *J*=13.0 Hz, 1H), 1.75-1.65 (m, 2H), 1.54 (s, 3H), 1.46 (m, 1H), 1.42 (br s, 9H), 1.37 (d, *J*=7.0 Hz, 3H), 0.93 (d, *J*=6.5 Hz, 3H), 0.92 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.0, 173.9, 168.0, 156.6, 126.0 (q, *J*=286.5 Hz), 80.5, 58.2 (q, *J*=29.0 Hz), 53.5, 52.7, 48.2, 39.6, 38.1, 28.4, 24.8, 23.1, 22.1, 20.7, 18.1.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s).

HRMS-ESI : *m/z* [M+Na]⁺ calcd for C₂₀H₃₄F₃N₃NaO₆: 492.2297; found : 492.2318.

**Boc-Leu-(R)-β³-Tfm-β³-hAla-Leu-OMe 17e.** According to the *general procedure B*, H-(R)-β³-Tfm-β³-hAla-Leu-OMe **15b** (426 mg, 1.43 mmol) reacted with Boc-Leu-OH (496 mg, 2.14 mmol, 1.5 equiv), HATU (1.09 g, 2.88 mmol, 2.0 equiv) and DIPEA (1.25 mL, 7.18 mmol, 5.0 equiv) in DMF (10 mL) for 5 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the tripeptide **17e** (361 mg, 49%) as a colourless oil.

[α]D²⁰⁻³⁹ (c 1.00, CHCl₃).

IR (film): 3312, 3077, 2960, 2873, 2726, 2401, 2288, 1668, 1536, 1460, 1370, 1158, 1102, 1048, 1022, 987, 922, 873, 851, 757, 665, 616, 410 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.88 (br s, 1H), 6.66 (br d, *J*=7.5 Hz, 1H), 5.32 (br d, *J*=7.5 Hz, 1H), 4.57 (m, 1H), 4.12 (m, 1H), 3.71-3.68 (m, 4H), 2.35 (d, *J*=13.0 Hz, 1H), 1.54 (s, 3H), 1.75-1.47 (m, 6H), 1.42 (s, 9H), 0.94-0.89 (m, 12H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.1, 173.1, 168.3, 156.7, 126.0 (q, *J*=286.5 Hz), 80.4, 58.2 (q, *J*=28.5 Hz), 53.4, 52.5, 50.8, 41.3, 39.6, 38.0, 28.4, 25.0, 24.8, 23.2, 23.0, 22.0, 21.8, 20.7.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s).

HRMS-ESI : *m/z* [M+Na]⁺ calcd for C₂₃H₄₀F₃N₃NaO₆: 534.2767; found : 534.2761.

**Boc-Ala-(S)-β³-Tfm-β³-hAla-Ala-OMe 17f.** According to the *general procedure B*, H-(S)-β³-Tfm-β³-hAla-Ala-OMe **15d** (416 mg, 1.63 mmol) reacted with Boc-Ala-OH (463 mg, 2.45 mmol, 1.5 equiv), HATU (1.24 g, 3.26 mmol, 2.0 equiv) and DIPEA (1.20 mL, 6.89 mmol, 4.2 equiv) in DMF (7 mL) for 7 days. Purification on SiO₂ (petroleum ether/EtOAc 3:7) afforded the tripeptide **17f** (559 mg, 80%) as a colourless oil.

[α]D²⁰⁻³⁶ (c 1.01, CHCl₃).

IR (film): 3329, 3083, 2985, 2369, 1748, 1690, 1534, 1457, 1374, 1293, 1252, 1217, 1163, 1099, 1062, 1024, 982, 855, 783, 760, 654, 615, 559, 450 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.42 (br s, 1H), 6.74 (m, 1H), 5.05 (d, *J*=7.0 Hz, 1H), 4.55 (quint, *J*=7.0 Hz, 1H), 4.11 (m, 1H), 3.74 (s, 3H), 3.19 (br d, *J*=13.5 Hz, 1H), 2.53 (d, *J*=13.5 Hz, 1H), 1.66 (s, 3H), 1.42 (s, 9H), 1.36 (d, *J*=7.0 Hz, 3H), 1.32 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.6, 173.4, 168.1, 156.0, 126.0 (q, *J*=288.0 Hz), 80.6, 58.7 (q, *J*=29.0 Hz), 52.7, 50.9, 48.2, 39.3, 28.3, 20.7, 18.0, 17.4.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.0 (s).

HRMS-ESI : *m/z* [M+Na]⁺ calcd for C₁₇H₂₈F₃N₃NaO₆: 450.1828; found : 450.1824.

**Boc-Ala-(S)-β³-Tfm-β³-hAla-Phe-OMe 17g** According to the *general procedure B*, H-(S)-β³-Tfm-β³-hAla-Phe-OMe **15e** (155 mg, 0.47 mmol) reacted with Boc-Ala-OH (136 mg, 0.72 mmol, 1.5 equiv), HATU (357 mg, 0.94 mmol, 2.0 equiv) and DIPEA (330 µL, 1.89 mmol, 4.1 equiv) in DMF (3 mL) for 5 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the tripeptide **17g** (173 mg, 74%) as a colourless oil.

[α]D²⁰⁻²⁹ (c 1.01, CHCl₃).

IR (KBr plate): 3324, 3072, 2982, 2371, 1745, 1688, 1528, 1452, 1372, 1252, 1221, 1167, 1099, 1066, 1027, 858, 749, 701, 664, 605, 449 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.41 (br s, 1H), 7.29 (t, *J*=7.5 Hz, 2H), 7.24 (t, *J*=7.5 Hz, 1H), 7.12 (d, *J*=7.5 Hz, 2H), 6.61 (br s, 1H), 4.99 (br d, *J*=6.5 Hz, 1H), 4.84 (q, *J*=6.5 Hz, 1H), 4.02 (m, 1H), 3.70 (s, 3H), 3.20 (d, *J*=14.0 Hz, 1H), 3.07 (m, 2H), 2.52 (d, *J*=14.0 Hz, 1H), 1.61 (s, 3H), 1.44 (s, 9H), 1.24 (br d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=173.5, 172.0, 168.2, 156.0, 135.9, 129.2, 128.7, 127.3, 126.0 (q, *J*=286.5 Hz), 80.5, 58.6 (q, *J*=28.5 Hz), 53.4, 52.5, 50.6, 39.2, 37.8, 28.3, 20.6, 17.3.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.1 (s).

HRMS-ESI : *m/z* [M+Na]⁺ calcd for C₂₃H₃₂F₃N₃NaO₆: 526.2141; found : 526.2147.

**Boc-β-hAla-(R)-β³-Tfm-β³-hAla-β-hPhe-OMe 18a.** According to the *general procedure B*, H-(R)-β³-Tfm-β³-hAla-β-hPhe-OMe **16a** (103 mg, 0.30 mmol) reacted with Boc-β-hAla-OH (93 mg, 0.46 mmol, 1.5 equiv), HATU (227 mg, 0.60 mmol, 2.0 equiv) and DIPEA (210 µL, 1.21 mmol, 4.1 equiv) in DMF (2 mL) for 3 days. Purification on SiO₂ (petroleum ether/EtOAc 3:7) afforded the tripeptide **18a** (90 mg, 57%) as a colourless oil.

[α]D²⁰⁻⁹ (c 0.90, CHCl₃).

IR (KBr plate): 3325, 3078, 2978, 2934, 1737, 1687, 1531, 1451, 1370, 1308, 1256, 1163, 1099, 1057, 849, 748, 701 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.29 (t, *J*=7.5 Hz, 2H), 7.22 (t, *J*=7.5 Hz, 1H), 7.17 (d, *J*=7.5 Hz, 2H), 7.03 (s, 1H), 6.63 (d, *J*=8.5 Hz, 1H), 5.21 (br s, 1H), 4.48 (m, 1H), 3.96 (m, 1H), 3.67 (s, 3H), 3.14 (br d, *J*=14.0 Hz, 1H), 2.89 (dd, *J*=13.5 Hz, *J*=7.5 Hz, 1H), 2.83 (dd, *J*=13.5 Hz, *J*=7.5 Hz, 1H), 2.51 (dd, *J*=16.0 Hz, *J*=5.0 Hz, 1H), 2.43-2.39 (m, 2H), 2.34 (m, 1H), 2.31 (d, *J*=14.0 Hz, 1H), 1.58 (s, 3H), 1.42 (s, 9H), 1.20 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.1, 171.9, 168.1, 155.6, 137.4, 129.3, 128.8, 127.0, 126.1 (q, *J*=287.0 Hz), 79.6, 58.4 (q, *J*=28.5 Hz), 51.9, 47.6, 44.3, 44.2, 40.1, 39.4, 37.4, 28.5, 20.7, 20.4.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-78.6 (s).

HRMS-ESI : *m/z* [M+Na]⁺ calcd for C₂₅H₃₆F₃N₃NaO₆: 554.2454; found : 554.2437.

**Boc-β-hAla-(S)-β³-Tfm-β³-hAla-β-hAla-OMe 18b.** According to the *general procedure B*, H-(S)-β³-Tfm-β³-hAla-β-hAla-OMe **16b** (105 mg, 0.39 mmol) reacted with Boc-β-hAla-OH (121 mg, 0.60 mmol, 1.5 equiv), HATU (299 mg, 0.79 mmol, 2.0 equiv) and DIPEA (270 µL, 1.55 mmol, 4.0 equiv) in DMF (2 mL) for 7 days. Purification on SiO₂ (petroleum ether/EtOAc 3:7) afforded the tripeptide **18b** (89 mg, 50%) as a white solid.

M.p 128°C; [α]D²⁰⁻²⁸ (c 0.90, CHCl₃).
IR (KBr plate): 3385, 3294, 3096, 2980, 2938, 1746, 1713, 1650, 1561, 1257, 1173, 1099 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.33 (s, 1H), 6.63 (d, *J*=8.0 Hz, 1H), 5.21 (br s, 1H), 4.31 (m, 1H), 3.99 (m, 1H), 3.68 (s, 3H), 2.90 (d, *J*=14.0 Hz, 1H), 2.53 (dd, *J*=16.0 Hz, *J*=5.5 Hz, 1H), 2.50 (dd, *J*=16.0 Hz, *J*=5.5 Hz, 1H), 2.45 (d, *J*=14.0 Hz, 2H), 2.34 (dd, *J*=14.5 Hz, *J*=6.0 Hz, 1H), 1.68 (s, 3H), 1.41 (s, 9H), 1.20 (d, *J*=6.5 Hz, 3H), 1.19 (d, *J*=6.5 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.1, 171.5, 168.1, 155.6, 126.1 (q, *J*=287.0 Hz), 79.5, 58.6 (q, *J*=28.5 Hz), 51.9, 44.3, 44.2, 42.3, 40.1, 39.7, 28.5, 20.7, 20.5, 19.9.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-78.3 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₉H₃₂F₃N₃NaO₆: 478.2141; found: 478.2125.

**Boc-β-hAla-(S)-β³-Tfm-β³-hAla-β-hLeu-OMe 18c.** According to the general procedure *B*, *H-(S)-β³-Tfm-β³-hAla-β-hLeu-OMe 16c* (105 mg, 0.34 mmol) reacted with Boc-β-hAla-OH (103 mg, 0.50 mmol, 1.5 equiv), HATU (268 mg, 0.71 mmol, 2.1 equiv) and DIPEA (235 µL, 1.35 mmol, 4.0 equiv) in DMF (2 mL) for 3 days. Purification on SiO₂ (petroleum ether/EtOAc 1:1) afforded the tripeptide **18c** (92 mg, 56%) as a white solid.

Mp 143°C; [α]D²⁰⁻³⁰ (c 0.90, CHCl₃).

IR (KBr plate): 3325, 3078, 2978, 1737, 1687, 1531, 1451, 1370, 1256, 1163, 1099, 1057, 748, 701 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.40 (s, 1H), 6.50 (br d, *J*=8.0 Hz, 1H), 5.18 (br s, 1H), 4.29 (m, 1H), 4.00 (m, 1H), 3.67 (s, 3H), 2.87 (d, *J*=14.0 Hz, 1H), 2.55 (dd, *J*=15.5 Hz, *J*=5.0 Hz, 1H), 2.48-2.44 (m, 3H), 2.32 (dd, *J*=14.5 Hz, *J*=6.0 Hz, 1H), 1.68 (s, 3H), 1.57 (m, 1H), 1.47 (ddd, *J*=14.0 Hz, *J*=9.5 Hz, *J*=5.5 Hz, 1H), 1.42 (s, 9H), 1.26 (ddd, *J*=14.0 Hz, *J*=8.5 Hz, *J*=5.5 Hz, 1H), 1.20 (d, *J*=6.5 Hz, 3H), 0.89 (d, *J*=6.5 Hz, 6H).

¹³C NMR (150.9 MHz, CDCl₃): δ=172.3, 171.4, 168.2, 155.6, 126.1 (q, *J*=287.0 Hz), 79.5, 58.6 (q, *J*=28.5 Hz), 51.9, 44.5, 44.3, 44.2, 43.1, 40.3, 38.8, 28.5, 25.0, 23.0, 22.0, 20.7, 20.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-78.2 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₂H₃₈F₃N₃NaO₆: 520.2610; found: 520.2605.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Ala-NHMe 19a.** According to the general procedure *D*, reaction of Boc-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe **17a** (104 mg, 0.24 mmol) and MeNH₂ (2N in MeOH, 605 µL, 1.21 mmol, 5.0 equiv) in MeOH (750 µL) for 4d gave the dipeptide **19a** (103 mg, 100%) as a colourless oil.

[α]D²⁰⁻⁵⁴ (c 1.03, CHCl₃).

IR (KBr plate): 3313, 3087, 2983, 2940, 1686, 1536, 1455, 1373, 1291, 1251, 1166, 1099, 1066, 1026, 858, 757, 663, 529, 452 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=6.91 (br s, 1H), 6.66 (m, 1H), 6.13 (m, 1H), 5.77 (m, 1H), 4.34 (quint, *J*=7.0 Hz, 1H), 4.20 (m, 1H), 3.65 (d, *J*=13.5 Hz, 1H), 2.82 (d, *J*=5.0 Hz, 3H), 2.37 (d, *J*=13.5 Hz, 1H), 1.55 (s, 3H), 1.45 (s, 9H), 1.36 (d, *J*=7.0 Hz, 3H), 1.34 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.5, 173.1, 168.4, 156.6, 126.0 (q, *J*=286.5 Hz), 80.4, 58.2 (q, *J*=29.0 Hz), 50.8, 49.0, 38.3, 28.4, 26.4, 20.6, 18.2, 16.8.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₁₇H₂₉F₃N₄NaO₅: 449.1988; found: 449.1976.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Leu-NHMe 19b.** According to the general procedure *D*, reaction of Boc-Ala-(R)-β³-Tfm-β³-hAla-Leu-OMe **17b** (108 mg, 0.23 mmol) and MeNH₂ (2N in MeOH, 3x575 µL, 3x1.15 mmol, 3x5.0 equiv at t = 0, 5d and 6d) in MeOH (650 µL) for 7d gave the dipeptide **19b** (93 mg, 87%) as a white foamy solid.

[α]D²⁰⁻⁵¹ (c 1.02, CHCl₃).

IR (KBr plate): 3311, 3088, 2965, 2878, 2815, 2378, 1653, 1540, 1456, 1372, 1286, 1254, 1168, 1100, 1069, 1024, 955, 858, 758, 652, 623, 471 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=6.95 (br s, 1H), 6.71 (m, 1H), 6.33 (m, 1H), 5.89 (br d, *J*=6.5 Hz, 1H), 4.32 (td, *J*=8.5 Hz, *J*=5.5 Hz, 1H), 4.22 (m, 1H), 3.66 (d, *J*=13.0 Hz, 1H), 2.80 (d, *J*=4.5 Hz, 3H), 2.35 (d, *J*=13.0 Hz, 1H), 1.63-1.58 (m, 2H), 1.54 (s, 3H), 1.50-1.46 (m, 1H), 1.44 (s, 9H), 1.36 (d, *J*=7.0 Hz, 3H), 0.92 (d, *J*=6.0 Hz, 3H), 0.89 (d, *J*=6.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.8, 173.0, 168.6, 156.8, 126.0 (q, *J*=286.5 Hz), 80.4, 58.2 (q, *J*=29.0 Hz), 52.0, 50.8, 41.1, 38.2, 28.4, 26.4, 24.9, 23.0, 22.1, 20.7, 16.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₀H₃₅F₃N₄NaO₅: 491.2457; found: 491.2456.

**Boc-Ala-(R)-β³-Tfm-β³-hAla-Phe-NHMe 19c.** According to the general procedure *D*, Boc-Ala-(R)-β³-Tfm-β³-hAla-Phe-OMe **17c** (254 mg, 0.50 mmol) reacted with MeNH₂ (2N in MeOH, 1.30 mL, 2.60 mmol, 5.2 equiv) in MeOH (1 mL) for 30h. Purification of the residue on SiO₂ (petroleum ether/EtOAc 1:4) afforded the dipeptide **19c** (237 mg, 93%) as a white solid.

Mp 101°C; [α]D²⁰⁻⁴⁷ (c 1.00, CHCl₃).

IR (KBr plate): 3312, 3086, 3032, 2981, 2939, 2812, 1655, 1540, 1454, 1372, 1288, 1252, 1167, 1100, 1069, 1025, 964, 909, 858, 752, 700, 650, 623, 526, 487, 454 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.27 (t, *J*= 7.0 Hz, 2H), 7.22 (t, *J*= 7.0 Hz, 1H), 7.15 (d, *J*= 7.0 Hz, 2H), 7.09 (br s, 1H), 6.96 (m, 1H), 6.31 (m, 1H), 5.75 (br s, 1H), 4.57 (q, *J*=7.5 Hz, 1H), 4.22 (m, 1H), 3.51 (d, *J*=13.5 Hz, 1H), 3.01 (dd, *J*=13.5 Hz, *J*=7.5 Hz, 1H), 2.95 (dd, *J*=13.5 Hz, *J*=7.5 Hz, 1H), 2.69 (d, *J*=5.0 Hz, 3H), 2.30 (d, *J*=13.5 Hz, 1H), 1.49 (s, 3H), 1.42 (s, 9H), 1.34 (d, *J*=7.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.5, 171.8, 168.4, 156.6, 136.6, 129.2, 128.8, 127.2, 125.9 (q, *J*=286.5 Hz), 80.4, 58.1 (q, *J*=29.0 Hz), 54.8, 50.7, 38.5, 38.2, 28.4, 26.3, 20.4, 16.9.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.8 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₃H₃₃F₃N₄NaO₅: 525.2301; found: 525.2311.

**Boc-Leu-(R)-β³-Tfm-β³-hAla-Ala-NHMe 19d.** According to the general procedure *D*, reaction of Boc-Leu-(R)-β³-Tfm-β³-hAla-Ala-OMe **17d** (105 mg, 0.22 mmol) and MeNH₂ (2N in MeOH, 2x560 µL, 2x1.12 mmol, 2x5.0 equiv at t = 0 and 6d) in MeOH (650 µL) for 7d gave the dipeptide **19d** (96 mg, 91%) as a colourless oil.

[α]D²⁰⁻⁵⁸ (c 1.02, CHCl₃).

IR (KBr plate): 3314, 3083, 2964, 2876, 2815, 1684, 1539, 1457, 1373,
1289, 1252, 1167, 1101, 1049, 1022, 922, 874, 758, 664, 531, 461 cm⁻¹.

¹H NMR (500 MHz, CDCl₃): δ=6.93 (br s, 1H), 6.64 (m, 1H), 6.21 (m, 1H), 5.49 (m, 1H), 4.38 (m, 1H), 4.09 (m, 1H), 3.57 (d, *J*=13.5 Hz, 1H), 2.82 (br s, 3H), 2.39 (d, *J*=13.5 Hz, 1H), 1.75-1.68 (m, 2H), 1.57 (s, 3H), 1.51 (m, 1H), 1.44 (s, 9H), 1.35 (d, *J*=7.0 Hz, 3H), 0.96 (d, *J*=6.0 Hz, 3H), 0.93 (d, *J*=6.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.3, 173.0, 168.3, 156.7, 126.0 (q, *J*=286.0 Hz), 80.4, 58.2 (q, *J*=29.0 Hz), 53.7, 49.0, 39.8, 38.5, 28.4, 26.5, 24.8, 23.2, 22.0, 20.5, 18.2.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.7 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₀H₃₅F₃N₄O₅+Na⁺: 491.2457; found: 491.2465.

**Boc-Leu-(R)-β³-Tfm-β³-hAla-Leu-NHMe 19e.** According to the general procedure *D*, reaction of Boc-Leu-(R)-β³-Tfm-β³-hAla-Leu-OMe 17e (103 mg, 0.20 mmol) and MeNH₂ (2N in MeOH, 3x510 µL, 3x1.02 mmol, 3x5.1 equiv at t = 0, 22h and 31h) in MeOH (600 µL) for 4d gave the dipeptide **19e** (89 mg, 86%) as a colourless oil.

[α]₂₀°-71 (c 1.00, CHCl₃).

IR (KBr plate): 3311, 3083, 2962, 2876, 2375, 1688, 1658, 1539, 1462, 1387, 1372, 1288, 1253, 1168, 1101, 1049, 1022, 876, 852, 756, 678, 654, 619, 463 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.07 (s, 1H), 6.82 (br s, 1H), 6.52 (br s, 1H), 5.62 (br s, 1H), 4.36 (m, 1H), 4.11 (m, 1H), 3.54 (d, *J*=13.5 Hz, 1H), 2.78 (d, *J*=4.5 Hz, 3H), 2.38 (d, *J*=13.5 Hz, 1H), 1.71-1.65 (m, 2H), 1.64-1.57 (m, 2H), 1.55 (s, 3H), 1.54-1.48 (m, 2H), 1.43 (s, 9H), 0.94 (d, *J*=6.5 Hz, 3H), 0.92 (d, *J*=6.0 Hz, 3H), 0.91 (d, *J*=6.5 Hz, 3H), 0.88 (d, *J*=6.0 Hz, 3H).

¹³C NMR (150.9 MHz, CDCl₃): δ=174.5, 172.9, 168.6, 156.8, 126.0 (q, *J* = 286.5 Hz), 80.4, 58.2 (q, *J*=29.0 Hz), 53.6, 52.0, 41.0, 39.8, 38.5, 28.4, 26.4, 24.9, 24.8, 23.3, 23.0, 22.1, 21.9, 20.5.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.6 (s).

HRMS-ESI: *m/z* [M+Na]⁺ calcd for C₂₃H₄₁F₃N₄NaO₅: 533.2927; found: 533.2924.

**H-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe 20.** According to the general procedure *C*, Boc-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe 17a (214 mg, 0.50 mmol) reacted with HCl (2N in Et₂O, 3 mL) in MeOH (6 mL) for 5h leading to the deprotected tripeptide **20** (120 mg, 73%) as a yellow oil.

¹H NMR (500 MHz, CDCl₃): δ=7.99 (s, 1H), 7.01 (br d, *J*=7.0 Hz, 1H), 4.40 (quint, *J*=7.5 Hz, 1H), 3.70 (d, *J*=13.0 Hz, 1H), 3.65 (s, 2H), 3.41 (q, *J*=7.0 Hz, 1H), 2.34 (d, *J*=13.0 Hz, 1H), 2.11 (s, 2H), 1.52 (s, 3H), 1.30 (d, *J*=7.5 Hz, 3H), 1.27 (d, *J*=7.0 Hz, 3H).

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-80.5 (s).

**(S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe 21.** According to the general procedure *A*, H-Ala-(R)-β³-Tfm-β³-hAla-Ala-OMe 20 (92 mg, 0.28 mmol) reacted with (S₅)-tBuS(O)-(R)-β³-Tfm-β³-hAla-OH (S₅,R)-3 (155 mg, 0.56 mmol, 2.0 equiv), EDCI.HCl (117 mg, 0.62 mmol, 2.2 equiv), HOBt.xH₂O (84 mg, 0.62 mmol, 2.2 equiv) and DIPEA (200 µL, 1.15 mmol, 4.1 equiv) in CH₃CN (2 mL) for 39h. Purification on SiO₂ (CH₂Cl₂/MeOH 9:1) afforded the tetrapeptide **21** (116 mg, 71%) as a white solid.

Mp 173°C; [α]₂₀°-30 (c 0.91, CHCl₃).

IR (KBr plate): 3300, 3093, 2990, 2960, 1757, 1706, 1657, 1560, 1456, 1385, 1268, 1224, 1157, 1054, 704 cm⁻¹.

¹H NMR (600 MHz, CDCl₃): δ=7.40 (d, *J*=8.0 Hz, 1H), 6.87 (s, 1H), 6.66 (d, *J*=7.0 Hz, 1H), 5.74 (s, 1H), 4.63 (dq, *J*=8.0 Hz, *J*=7.0 Hz, 1H), 4.46 (quint, *J*=7.0 Hz, 1H), 3.80 (d, *J*=13.0 Hz, 1H), 3.73 (s, 3H), 2.64 (d, *J*=14.5 Hz, 1H), 2.54 (d, *J*=14.5 Hz, 1H), 2.32 (d, *J*=13.0 Hz, 1H), 1.69 (s, 3H), 1.52 (s, 3H), 1.36 (d, *J*=7.5 Hz, 3H), 1.31 (d, *J*=7.0 Hz, 3H), 1.23 (s, 9H).

¹³C NMR (150.9 MHz, CDCl₃): δ=175.1, 173.4, 169.0, 168.2, 126.1 (q, *J*=285.0 Hz), 125.9 (q, *J*=286.5 Hz), 60.0 (q, *J*=27.5 Hz), 58.4 (q, *J*=29.0 Hz), 56.7, 52.6, 49.4, 48.3, 41.8, 37.8, 22.7, 20.6, 19.3, 17.6, 16.8.

¹⁹F NMR (235.3 MHz, CDCl₃): δ=-79.4 (s, CF₃), -80.4 (s, CF₃).

HRMS-ESI: *m/z* [M+H]⁺ calcd for C₂₁H₃₅F₆N₄O₆S: 585.2182; found: 585.2191.
<table><caption>Table 3 Crystal data and structure refinements for peptides 9c,d,h, 10b,d, 11a, 12a and 14</caption><thead><tr><th>peptide</th><th>9c</th><th>9d</th><th>9h</th><th>10b</th><th>10d</th><th>11a</th><th>12a</th><th>14</th></tr></thead><tbody><tr><td>empirical formula</td><td>C<sub>19</sub>H<sub>27</sub>F<sub>3</sub>N<sub>2</sub>O<sub>4</sub>S</td><td>C<sub>34</sub>H<sub>25</sub>F<sub>3</sub>N<sub>2</sub>O<sub>4</sub>S</td><td>C<sub>12</sub>H<sub>19</sub>F<sub>3</sub>N<sub>2</sub>O<sub>4</sub></td><td>C<sub>20</sub>H<sub>29</sub>F<sub>3</sub>N<sub>2</sub>O<sub>4</sub>S</td><td>C<sub>17</sub>H<sub>31</sub>F<sub>3</sub>N<sub>2</sub>O<sub>4</sub>S</td><td>C<sub>14</sub>H<sub>25</sub>F<sub>3</sub>N<sub>2</sub>O<sub>6</sub></td><td>C<sub>15</sub>H<sub>25</sub>F<sub>3</sub>N<sub>2</sub>O<sub>5</sub></td><td>C<sub>18</sub>H<sub>29</sub>F<sub>6</sub>N<sub>3</sub>O<sub>5</sub>S</td></tr><tr><td>formula weight</td><td>436.49</td><td>374.42</td><td>312.29</td><td>450.51</td><td>416.50</td><td>374.36</td><td>370.37</td><td>513.50</td></tr><tr><td>crystal system</td><td>monoclinic</td><td>monoclinic</td><td>orthorhombic</td><td>monoclinic</td><td>orthorhombic</td><td>tetragonal</td><td>orthorhombic</td><td>orthorhombic</td></tr><tr><td>crystal size (mm<sup>3</sup>)</td><td>0.2x0.15x0.12</td><td>0.2x0.15x0.1</td><td>0.24x0.2x0.15</td><td>0.2x0.16x0.14</td><td>0.2x0.18x0.16</td><td>0.25x0.15x0.10</td><td>0.16x0.14x0.12</td><td>0.24x0.18x0.16</td></tr><tr><td>crystallizing solvent</td><td>CH<sub>2</sub>Cl<sub>2</sub>/n-hexane</td><td>CH<sub>2</sub>Cl<sub>2</sub>/n-hexane</td><td>EtOAc</td><td>CH<sub>2</sub>Cl<sub>2</sub>/n-hexane</td><td>CH<sub>2</sub>Cl<sub>2</sub>/cyclohexane</td><td>Et<sub>2</sub>O/n-hexane</td><td>CHCl<sub>3</sub>/pentaene</td><td>MeOH</td></tr><tr><td>space group</td><td>P2<sub>1</sub></td><td>P2<sub>1</sub></td><td>P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub></td><td>P2<sub>1</sub></td><td>P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub></td><td>P4<sub>3</sub></td><td>P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub></td><td>P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub></td></tr><tr><td>a (Å)</td><td>9.218(5)</td><td>8.882(5)</td><td>6.259(5)</td><td>10.5533(3)</td><td>9.2664(2)</td><td>9.797(5)</td><td>9.6769(2)</td><td>9.742(5)</td></tr><tr><td>b (Å)</td><td>11.973(5)</td><td>11.622(5)</td><td>13.764(5)</td><td>9.0676(3)</td><td>11.3591(3)</td><td>9.797(5)</td><td>11.4809(3)</td><td>10.698(5)</td></tr><tr><td>c (Å)</td><td>9.579(5)</td><td>9.576(5)</td><td>17.824(5)</td><td>12.0900(4)</td><td>21.2498(6)</td><td>18.900(5)</td><td>17.0959(4)</td><td>23.475(5)</td></tr><tr><td>β (°)</td><td>91.032(5)</td><td>109.976(5)</td><td></td><td>101.3260(10)</td><td></td><td></td><td></td><td></td></tr><tr><td>volume (Å<sup>3</sup>)</td><td>1057.0(9)</td><td>929.0(8)</td><td>1535.5(14)</td><td>1134.40(6)</td><td>2236.71(10)</td><td>1814.0(14)</td><td>1899.35(8)</td><td>2446.6(18)</td></tr><tr><td>Z</td><td>2</td><td>2</td><td>4</td><td>2</td><td>4</td><td>4</td><td>4</td><td>4</td></tr><tr><td>ρ<sub>calc</sub> (g.cm<sup>-3</sup>)</td><td>1.371</td><td>1.338</td><td>1.351</td><td>1.319</td><td>1.237</td><td>1.371</td><td>1.295</td><td>1.394</td></tr><tr><td>F (000)</td><td>460.0</td><td>396.0</td><td>656.0</td><td>476.0</td><td>888.0</td><td>792.0</td><td>784.0</td><td>1072.0</td></tr><tr><td>radiation</td><td>CuKα</td><td>CuKα</td><td>CuKα</td><td>CuKα</td><td>CuKα</td><td>CuKα</td><td>CuKα</td><td>CuKα</td></tr><tr><td>T (K)</td><td>100(2)</td><td>104(4)</td><td>100(2)</td><td>104(2)</td><td>104(2)</td><td>100(2)</td><td>100(2)</td><td>100(2)</td></tr><tr><td>2Θ max (°)</td><td>144.42</td><td>144.6</td><td>144.48</td><td>149.12</td><td>149.26</td><td>144.4</td><td>149.24</td><td>145.328</td></tr><tr><td>measured reflns</td><td>37228</td><td>29987</td><td>28331</td><td>24944</td><td>48077</td><td>30403</td><td>46605</td><td>42555</td></tr><tr><td>unique reflns</td><td>4152</td><td>3661</td><td>3023</td><td>4585</td><td>4576</td><td>3567</td><td>3883</td><td>4824</td></tr><tr><td>R<sub>int</sub></td><td>0.0318</td><td>0.0355</td><td>0.0284</td><td>0.0417</td><td>0.0556</td><td>0.0294</td><td>0.0505</td><td>0.0558</td></tr><tr><td>final R<sub>1</sub> (all data)</td><td>0.0220</td><td>0.0242</td><td>0.0225</td><td>0.0280</td><td>0.0302</td><td>0.0371</td><td>0.0324</td><td>0.0545</td></tr><tr><td>final wR<sub>2</sub> (all data)</td><td>0.0561</td><td>0.0633</td><td>0.0587</td><td>0.0627</td><td>0.0650</td><td>0.0998</td><td>0.0671</td><td>0.1338</td></tr></tbody></table>

## Conflicts of interest

The authors declare non conflicts of interest.

## Funding Information

We gratefully thank the Agence Nationale de la Recherche for financial support (TFM4Asym project ANR JCJC: ANR-13-JS07-0011-01), CNRS and the Conseil Regional de Champagne- Ardenne, and the EU-program FEDER for the facilities of the Analytical Platform (PIAneT CPER project).

## Acknowledgment

We thank A. Martinez for her technical assistance with NMR, Dr S. Chevreux for X-ray crystallographic structure determination, Dr C. Kowandy for her assistance with X-ray crystallographic structure analysis and S. Lanthony for HRMS analyses. We also thank the French Fluorine Network (GIS CNRS).

## Supporting Information

Supporting information for this article is available online at https://doi.org/10.1055/s-0041-1737396.

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