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A new “Mitsunobu homocoupling”
reaction using aldol adducts of kojic acid
Hideki Azuma, Yasuhito Morishima, Keijin Nakaguro,
Reiko Kato, Takeshi Nagasaki
Citation

Tetrahedron. 94; 132314

Issue Date

2021-08-13

Type
Textversion
Supplementary

Journal Article
Author
Supplementary data is available at https://doi.org/10.1016/j.tet.2021.132314

data© 2021 Published by Elsevier Ltd. Ltd. This manuscript version is madeavailable under the CC-BY-NC-ND 4.0 License.
Rightshttp://creativecommons.org/licenses/by-nc-nd/4.0/.
This is the accepted manuscript version. The article has been published in final
form at https://doi.org/10.1016/j.tet.2021.132314.

DOI

10.1016/j.tet.2021.132314

Self-Archiving by Author(s)Placed on: Osaka City University

Azuma, H., Morishima, Y., Nakaguro, K., Kato, R., & Nagasaki, T. (2021). A new “Mitsunobu
homocoupling” reaction using aldol adducts of kojic acid. Tetrahedron (Vol. 94, p. 132314). Elsevier
BV. https://doi.org/10.1016/j.tet.2021.132314

A new “Mitsunobu homocoupling” reaction using aldol adducts of kojic acid
Hideki Azuma a,*, Yasuhito Morishima a, Keijin Nakaguro b, Reiko Kato b, Takeshi Nagasaki aaDepartment of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City

University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japanb

Applied Chemistry and Bioengineering, Faculty of Engineering, Osaka City University, Sugimoto 3-

3-138, Sumiyoshi-ku, Osaka, 558-8585, Japan
Corresponding author. Tel: +81-6-6605-2168; Fax: +81-6-6605-2168
Graphical abstract

Highlights• Mitsunobu dehydration of a kojic acid analog with a hydroxymethyl group on the C-6 carbon yields
the homocoupling product.
• The dimerization, the so-called “Mitsunobu homocoupling,” may occur via generation of a quinone
methide-like intermediate.
• Presumably, the enol ether moiety of the intermediate acts as an electron donor and attacks anothermolecule.
E-mail address: azumah@osaka-cu.ac.jp (H. Azuma)

1Abstract
In this study, we attempted to carry out the Mitsunobu 1,4-elimination using the kojic acid analog
3, which carries a hydroxymethyl group on C-6 introduced by aldol condensation, to obtain an
effective Michael acceptor 4. To the ethyl acetate solution of 3 and Ph3P, diisopropyl azodicarboxylate
was added quickly at 0 °C, resulting in the immediate generation of a yellow precipitate. NMR, mass
spectrometry, and X-ray crystal structure analyses allowed to identify the precipitate as adiastereomeric/racemic mixture of dimeric compounds of 4. The dimerization reaction was
hypothesized to occur through two major steps: (i) intramolecular dehydration of 3 by Mitsunobu
reagents and (ii) homocoupling of the dehydration product 4 to form a new trans-carbon–carbon
double bond. Presumably, the enol ether moiety of 4 acts as an electron donor and immediately attacks
to the α,β-unsaturated carbonyl group of another molecule via a reaction that can be dubbed“Mitsunobu homocoupling.”

Keywords: Kojic Acid; Mitsunobu Reaction; Homocoupling; Quinone Methide; Michael Acceptor21. Introduction
Kojic acid (5-hydroxy-2-hydroxymethyl-4H-pyran-4-one, KA, Fig. 1) is a natural compound
synthesized by several species of fungi, such as those belonging to the Aspergillus genus.1 KA displays
inhibitory effect against tyrosinase, which is a key enzyme in the synthesis of melanin in melanocytes.2
Therefore, KA has been used as a skin whitening agent in the cosmetic industry.3(Fig. 1)
KA has an enolic property between C-5 and C-6 carbons, so it can react with various aldehydes by
way of an aldol condensation, so that the corresponding 1-hydroxyalkyl groups are introduced at theC-6 carbon. For example, Ferard and co-workers observed the aldol condensations of KA or 5hydroxy-2-[(tetrahydropyranyloxy)methyl]-4-pyrone (2) with formaldehyde in basic aqueousconditions.4 Ellis et al. also reported the hydroxymethylation of 3-hydroxy-6-methyl-4H-pyran-4-one(allomaltol) in the same conditions.5 On the other hand, Sharma and others demonstrated that the bestbase for the aldol condensations of KA with both aromatic or aliphatic aldehydes is 1,4diazabicyclo[2.2.2]octane (DABCO) in dioxane–H2O (1:1) as solvent.6
The Mitsunobu reaction is a versatile approach to the nucleophilic substitution of primary or
secondary hydroxyl groups with nucleophiles using a triaryl or trialkyl phosphine, such as
triphenylphosphine (Ph3P) or tributylphosphine (n-Bu3P), and a dialkyl azodicarboxylate, such as
diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD).7,8 A possible main
pathway for the Mitsunobu substitution of hydroxyl groups goes as follows: (i) formation of betaine
reacted Ph3P with a dialkyl azodicarboxylate, (ii) deprotonation of the nucleophile by betaine toenhance the nucleophilicity of the former, (iii) and an SN2 reaction between the alkoxyphosphonium
ion generated by reacting the protonated betaine with the alcohol and the deprotonated nucleophile.8
On the other hand, some reports have been published whereby a Mitsunobu 1,2-elimination reaction
had occurred in the absence of a nucleophile. For example, Olpp et al. achieved the anti-selective 1,2dehydration of γ-(α-hydroxyalkyl)butenolides under Mitsunobu reaction condition.9 Yadav and coworkers also reported the observation of a Mitsunobu anti-dehydration reaction that yielded a
dienone.10 Furthermore, Chen and Leu demonstrated the unique 1,2-elimination reaction between 2(butylthio)ethanthionyl and hydroxy moieties from γ,δ-alkynyl alcohols using Ph3P and DIAD.11 In
addition, Mulzer et al. reported a Mitsunobu 1,4-elimination-like reaction consisting of thedecarboxylative dehydration of the threo-β-hydroxy acid analogs achieved by treatment with Ph3P and
DEAD, leading to the formation of the corresponding (Z)-olefins.12
In this study, we attempted to carry out the Mitsunobu 1,4-elimination using the KA analog 3hydroxy-2-(hydroxymethyl)-6-((tetrahydro-2H-pyran-2-yloxy)methyl)-4H-pyran-4-one (3), which
carries a hydroxymethyl group on the C-6 carbon (Scheme 1). Generally, an enolic hydroxy group of
KA is acidic (pKa = 7.9–8.0).13 Therefore, we considered that the enolic hydroxy group of 3 could be
preferentially deprotonated, and the additional primary hydroxy moiety could be modified with Ph3P3

as a removing group under the Mitsunobu reaction condition. In fact, formation of the hydroxy anion
was expected to favor the 1,4-elimination, which would generate the 2-methylene-2H-pyran-3,4-dione
4, a quinone methide-like species that would act as an effective Michael acceptor (Scheme 1). Contrary
to our expectations, however, an unprecedented dimerization of 3 with dehydration occurred
preferentially under the Mitsunobu reaction conditions. In the current study, we also investigated theunique dimerization mechanism of 3.
(Scheme 1)
2. Results and discussion
2.1. Mitsunobu 1,4-elimination of the KA analog 3
The primary hydroxy group of KA was selectively protected with the tetrahydropyranyl (THP)
group implementing the previously described method.14 An additional hydroxymethyl moiety was
introduced at the C-6 carbon of 2 via an aldol condensation with formaldehyde under basic conditions.4
A Mitsunobu 1,4-elimination of the diol 3 was then attempted to try and obtain the quinone methidelike 4. For this purpose, to a CH2Cl2 solution of 3 and Ph3P (1.2 equiv), was added a DIAD solution(40% in toluene, 1.2 equiv) at 0 °C, and the resulting mixture was stirred for 1 h at room temperature.
The mixture was then concentrated in vacuo and ethyl acetate (EtOAc) was added to the residue thus
obtained. A yellow precipitate was generated, which was also insoluble in water; it was isolated byfiltration. The 1H NMR spectrum of the yellow precipitate was similar to that of 2, expect for the
resonance signal due to H-6, which appeared at 7.86 ppm (Fig. 2). On the other hand, the 13C NMR
spectrum of the obtained product was characterized by the presence of 12 different resonance signals(see experimental section). Furthermore, the molecular formula of the yellow precipitate wasestimated to be C24H28O10 based on results from high-resolution electrospray ionization mass
spectrometry (ESI–HRMS) analysis. This molecular formula is characterized by twice as many C, H,
and O atoms as compound 4. Therefore, it was thought that the obtained precipitate was a dimericcompound of 4 and that it was similar to 2. In Fig. 3 is depicted the predicted structure of the said
dimer(s), 5. However, the compound 5 should be a diastereomeric/racemic mixture because thesubstrate 3 has a racemic THP-protecting group. Nevertheless, 1H and 13C NMR spectrometry data
indicated the obtained yellow precipitate to be a single and symmetric molecule. In order to determine
the structure of the yellow compound, a crystal structure analysis was attempted after the racemic THP
groups of 5 were removed under acidic conditions. However, the deprotected derivative of 5 wasinsoluble in water and various organic solvents (i.e., CHCl3, CH2Cl2, EtOAc, MeOH, and EtOH) andonly slightly soluble in dimethyl sulfoxide (DMSO). Thus, no sufficient amounts of high-quality
crystalline material were obtained to perform X-ray crystal structure analysis. Notably, however, thedeprotected derivative of 5 was readily dissolved in a basic water solution. Evidence thus suggested
that the compound had an acidic, KA-like structure. In fact, 1H NMR spectrum of this compound was
4also similar to the spectrum of KA (see experimental section).
(Figure 2)(Figure 3)
Fortunately, the yellow precipitate 5 could be recrystallized from CHCl3. Therefore, an X-ray crystal
structure analysis of the obtained crystalline material was carried out. In Fig. 4 is reported the crystal
structure of the yellow precipitate 5. Although it was a disordered structure over three states of theracemic (R,R- or S,S-) and the meso (S,R-) compounds, the evidence pointed to a dimeric structurewith a trans-configuration, as expected. Furthermore, both two carbonyl groups and the conjugatedtriene were found to be arranged linearly. Both the racemic and meso dimers are symmetric, and two
THP groups of these compounds are far from the trans-type olefin moiety. Therefore, the twodiastereomers of 5 were assumed to be indistinguishable by 1H and 13C NMR spectrometry. Notably,reaction of the diol 3 with 1.3 equiv of DIAD and Ph3P in CH2Cl2 produced 5 as a precipitate after
addition of EtOAc in 32–43% yield. On the other hand, it was difficult to purify the remaining dimers
5 in the filtrate, which included various byproducts, by silica gel chromatography. In order to improve
the yield of this reaction, EtOAc was used as a solvent instead of CH2Cl2, because the dimers 5
displayed poor solubility in EtOAc. In these conditions, a yellow precipitate was generated
immediately after addition of the solution of DIAD to the solution of 3 and Ph3P, and the dimers 5
were readily isolated by filtration (54% yield). Based on the thin-layer chromatography (TLC)
analyses performed on the filtrates, most of the dimers 5 had been removed from the reaction mixture.
The effect of changing the dosages of the Mitsunobu reagents (DIAD and Ph3P) from 1.1 to 1.6 equiv
was also examined. The yield of 5 as a precipitate remained within the 41–54% range. In particular,when 1.3 or 1.4 equiv of the Mitsunobu reagents were utilized, a tendency to high yields (>50%) was
detected. By contrast, the yield of 5 was affected by the rate of the addition of the DIAD solution. In
the typical Mitsunobu reaction, DEAD or DIAD solutions are added to the reaction mixture slowly,
so as to limit byproduct formation. In the present case, however, the yield of 5 decreased substantially(to 19–24%) when the DIAD solutions (1.3–1.4 equiv) were slowly added dropwise to the solution of
3 and Ph3P. Probably, in the case of the slow addition of the DIAD solution, the dehydrated andextremely unstable compound 4, which is quickly generated, would have the opportunity to engage inside-reactions, instead of yielding the dimerization product.
(Figure 4)
2.2. A possible dimerization mechanism of compound 3 under the Mitsunobu condition
Next, we have evaluated the mechanism through which the dimers 5 were generated from compound3 under the Mitsunobu reaction conditions. The dehydrated compound 4 is expected to be produced
as a result of the Mitsunobu 1,4-elimination, because the obtained dimers 5 have a newly formedcarbon–carbon double bond. The quinone methide-like compound 4 could be regarded as an effective
5

Michael acceptor, because the 1,4-adducts of 4 with the nucleophiles could return to the stablearomatic structures. For example, Honda et al. reported that the proton-transfer-induced photocleavage
reaction of 2-acetoxy-3-biphenylmethanol in MeOH gave 2-methoxy-3-biphenylmethanol via the
production of a quinone methide intermediate form.15
On the other hand, the oxygen atom on the dihydropyran ring of 4 would act as an electron donor,
so compound 4 would also behave as a nucleophile. A proposed dimeric mechanism is illustrated in
Scheme 2. Although compound 4 has two enol ether moieties, the nucleophilicity of the C-5 carbonof 4 is thought to be reduced by the electron-withdrawing effect of the neighboring carbonyl group.
In addition, the C-5 carbon of 4 is sterically hindered compared with the methylene moiety on the C2 carbon. Thus, the enol ether moiety of 4 characterized by the lower steric hindrance would act as anelectron donor and attack to the α,β-unsaturated carbonyl group of another molecule. After the
intramolecular proton rearrangements of the dimeric intermediates, a trans-type double bond would
be formed, and two vinyl alcohol moieties would be reproduced.
(Scheme 2)
The yield of a substitution product obtained using a weak nucleophile (pKa > 11) is known to bequite low under normal Mitsunobu conditions, because betaine is unable to deprotonate thenucleophile substantially.8 In addition, a hydrazide adduct is mainly generated no nucleophiles are
utilized in the Mitsunobu reaction. For example, Brown and co-workers reported the direct substitutionof the allylic alcohol group by the reduced azodicarboxylate under nucleophile-excluded Mitsunobu
conditions.16 Frank and Roush showed that substantial amounts of the hydrazide coupling byproductwere produced in the Mitsunobu macrocyclization between an allylic hydroxy moiety and acarboxylate group.17 Therefore, in the present system, significant amounts of the hydrazide adductmay have been generated in competition with the dimerization process of 4. In order to confirm theexistence of the hydrazide coupling byproduct 7, ESI-MS analysis of the filtrate obtained after
separating the dimers 5 (51% yield) was carried out (Fig. 5). Contrary to expectations, no significant
peak attributable to the protonated (or Na+-adducted) molecular ion peak of the hydrazide adduct 7(calcd for C20H31N2O3, 443.2 or C20H30N2O3Na, 465.2) was observed. On the other hand, the substrate
3 was detected based on TLC analysis of the filtrate. In order to determine the amount of remaining
substrate 3, a chromatographic separation was carried out. It was difficult to separate the substrate 3
from the residue of the filtrate, therefore, 3 was isolated in mixture with Ph3PO. Based on1H NMR
analysis, the percentage of the unreacted 3 was estimated to be about 12% of the initial amount.
(Figure 5)
Next, we ascertained whether a group endowed with steric hindrance located on the terminal alkene
carbon could inhibit the proposed dimerization reaction. A 1-hydroxypropyl moiety was introduced at
the C-6 carbon of 2 via an aldol condensation with 1-propanal in the presence of DABCO.6 The same6

Mitsunobu reaction (solvent: EtOAc) was tried using the aldol condensation product 8 as substrate(Scheme 3). Contrary to the case of 3, no precipitate was observed after addition of DIAD to the mixed
solution of 8 and Ph3P. After removing the solvent, a chromatographic separation of the main productwas tried. Separation of the main product from the byproduct Ph3PO was difficult to achieve; therefore,
the yield of the product could not be accurately determined. The 1H NMR and MS analyses indicatedthat the main products of the reaction were probably hydrazide adducts 10 (>50%,
diastereomeric/racemic mixture). The steric hindrance around the methylene moiety of 9 by theexistence of the ethyl group is thought to suppress the dimerization reaction. Interestingly, thehydrazide adducts 10 were observed to be the main reaction product, although no significant amountof the corresponding adduct 7 was observed when 3 was used as substrate of the Mitsunobu reaction.
In the latter reaction, the nucleophilicity of 4 is predicted to be higher than that of the DIAD reductant,so that the dimer(s) 5 would be generated preferentially over adduct 7.
(Scheme 3)
2.3. Mitsunobu dimerization of other kojic acid analogs
We attempted the Mitsunobu dimerization reaction using another KA analog, tert-butyl ((5hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2-yl)methyl)carbamate (15) as substrate, which ischaracterized by the presence of a bulky and achiral tert-butyloxycarbonyl (Boc) group. The synthesisof the N-Boc-KA analog 15 was accomplished in five steps starting from KA (Scheme 4). A primaryhydroxy group of KA was substituted with a chloro group, which was subsequently substituted withan azide moiety.18 A Staudinger reaction between Ph3P and the kojic azide 12, followed by hydrolysis
and acidification, provided the kojic amine HCl salt 13 in 68% yield. In the case of the amino
protection of 13 using Boc2O and triethylamine in CH2Cl2, the N,O-diBoc-compound was mainly
produced rather than the desired N-Boc-compound 14. Therefore, the selective protection of the aminogroup of 13 was implemented according to the literature’s method, which consists of an N-Boc
protection of 4-hydroxybenzylammonium bromide.19 When the HCl salt 13 was made to react with
Boc2O (1.1 equiv) in a methanolic suspension of NaHCO3, a small amount of N,O-diBoc-compound
was also observed to be produced by TLC analysis. However, the diBoc-byproduct was hydrolyzed to
the N-Boc-compound 14 following additional treatment with 1 M NaOH solution;20 notably, 14 was
obtained in 68% yield. A hydroxymethyl moiety was introduced on the C-6 carbon of 14 implementing
the same aldol reaction described above (yield: 76%).
(Scheme 4)
The obtained compound 15 exhibited low solubility in EtOAc, so the Mitsunobu dimerization
reaction of 15 was carried out at room temperature. A yellow precipitate was generated also in thisreaction, and the reaction product was also presumed to be the dimeric compound 16 based on the7results of 1H NMR and MS analyses (yield: 51–57%). No X-ray crystal structure analysis of 16 was
carried out, because 16 was insoluble with various organic solvents (i.e., CHCl3, CH2Cl2, EtOAc,
MeOH, and EtOH) and hardly soluble in DMSO. The cluster ion peak at m/z 1035.4 ([2M+Na]+) was
observed beside the molecular ion peak at m/z 529.2 ([M+Na]+) by the ESI–MS pattern of the DMSO
solution of 16 (Fig. 6). On the other hand, this signal diminished greatly in intensity in the case of the
deprotected compound 17 (analyzed in H2O). Probably, compound 16 forms an aggregate in DMSO
by intermolecular π-π stacking interaction.
(Figure 6)
We also tried the Mitsunobu dimerization reaction using the KA analog, 3-hydroxy-2(hydroxymethyl)-6-((prop-2-ynyloxy)methyl)-4H-pyran-4-one (21). This compound has a propargyl
moiety which facilitates to introduce various functional groups by click reaction. The synthesis of thealkynyl compound 21 was accomplished in four steps from KA (Scheme 5). Firstly, selective
protection of the enolic hydroxy group of KA was carried out using 4-methoxybenzyl chloride(PMBCl) in the presence of K2CO3 according to the previous report.21 Subsequent treatment of 18
with two equimolar amounts of sodium hydride and 3-bromo-1-propyne provided the propargyl ether
19 in 61% yield. The PMB group of 19 was removed by the treatment with trifluoroacetic acid (TFA).
A hydroxymethyl moiety was introduced on the C-6 carbon of 20 by the same aldol reaction describedabove (77%). The obtained compound 21 also showed a low solubility in EtOAc. Therefore, theMitsunobu dimerization reaction of 21 was also carried out at room temperature. Orange precipitate
was generated in this reaction, and the 1H NMR spectrum of the reaction product 22 was similar to
those of compounds 5 and 16 (Supplementary material: S3, S11 and S16). However, the ESI–HRMS
analysis of 22 showed the unknown ion peak at m/z 645.2 between the molecular ion peak of the
dimerized product 22 (m/z 407.1, [M+Na]+) and the cluster ion peak (m/z 791.1, [2M+Na]+, Fig. 7).
From the 1H and 13C NMR analyses, the obtained product was a single and symmetric molecule aswell as compounds 5 and 16 (Supplementary material: S16 and S17). Probably, the unidentified ion
appeared at m/z 645.2 would be a cluster ion of one molecule (M) and its fragment ion ([M–123]+, or[M–146+Na]+), although the structure of the fragment is unknown. Therefore, the obtained product 22
is the dimeric compound as well as compounds 5 and 16 (yields: 41–47%). The obtained 22 was also
hardly soluble in various organic solvents (i.e., CHCl3, CH2Cl2, EtOAc, MeOH, and EtOH), except
for DMSO. It is thought that the poor solubilities of these achiral dimeric products 16 and 22 are dueto the strong intermolecular π-π stacking. As both two dimeric products are symmetric molecules,
there is no effectual method to determine the configurations of the newly formed carbon-carbon double
bonds of them except for X-ray crystal structure analysis. However, the configurations of the olefin
moieties of both two compounds are expected to be a trans-configuration because a trans-type doublebond would be generated preferentially during the expected intramolecular proton rearrangements of8the dimeric intermediates (Scheme 2). Furthermore, the resulting planar structures are advantageous
to construct the strong intermolecular π-π stacking. It is thought that more bulky moiety than the THPprotected group, for example, 1-adamantyl group, is needed for the substrate to enhance the solubilityof the resulting dimeric compounds and to obtain the high-quality crystalline materials.
(Scheme 5)(Figure 7)

3. Conclusions
We attempted to perform a Mitsunobu 1,4-elimination reaction using the KA analog 3 as substrate,
which is characterized by a hydroxymethyl moiety on the C-6 carbon; however, we were unable toisolate the desired quinone methide-like product 4. Alternatively, we identified a novel one-pot
dimerization reaction starting from the diol 3 occurring under the Mitsunobu reaction conditions. This
unique reaction is likely to go through two major steps: (i) intramolecular dehydration of 3 by the
Mitsunobu reagents, and (ii) homocoupling of the dehydration product 4 to form a new trans-carbon–
carbon double bond. Therefore, this reaction can be defined as a “Mitsunobu homocoupling”.
Purification of the Mitsunobu reaction products from the reaction mixture, including the two main
byproducts, phosphine oxides and dialkylhydrazine 1,2-dicarboxylates, is generally hard to achieve.
However, implementation of the hereby developed synthetic approach readily afforded the dimericproducts as a precipitate, upon selection of a suitable solvent. Although the reaction yields were nothigh, our findings are of interest for the unique, transient-metal-free homocoupling reaction under
mild conditions.

4. Experimental
All materials were obtained commercially (guaranteed reagent grade) and used without further
purification. CH2Cl2 was distilled from CaH2 under nitrogen gas atmosphere before use. Super
dehydrated EtOAc and tetrahydrofuran (THF) were purchased from Fujifilm Wako Chemicals.
Column chromatography was performed on silica gel (Wakogel® C-300, Fujifilm Wako Chemicals).
1H NMR and 13C NMR spectra were recorded on Bruker AVANS 300N spectrometer.

4.1. 3-Hydroxy-2-(hydroxymethyl)-6-((tetrahydro-2H-pyran-2-yloxy)methyl)-4H-pyran-4-one(3).
The aldol reaction of 5-hydroxy-2-[(tetrahydropyranyloxy)methyl]-4-pyrone (2)14 with aformaldehyde was carried out referring to Farard’s method.4 Briefly, sodium hydroxide (550 mg, 13.8mmol) was dissolved with distilled water (20 mL). To the just prepared sodium hydroxide solution
was added compound 2 (3.00 g, 13.3 mmol). The pH value of the solution thus obtained was adjusted
9to 10.5 using 1 M NaOH and 1 M HCl. A solution of 37% formaldehyde (1.1 mL, 1.1 equiv) was thenadded slowly to the reaction solution, and the mixture thus obtained was stirred overnight at room
temperature. The resulting mixture was neutralized with 1 M HCl and then extracted with CH2Cl2.
The organic layer was dried over Na2SO4 and concentrated under reduced pressure. To the organic
layer was then added n-hexane, which prompted the precipitation of a crystalline solid, which wasrecrystallized from an EtOAc–n-hexane to afford 3 (1.97 g, 58%) as a colorless solid; mp 71 °C; IR(KBr) 3383, 3251, 1658, 1620, 1584, 1231, 1143 cm-1; 1H NMR (300 MHz, CDCl3); δ 6.55 (s, 1 H),
4.74 (t, J = 3.4 Hz, 1 H), 4.71 (s, 2 H), 4.57 (dd, J = 0.57, 14.7 Hz, 1 H), 4.39 (d, J = 0.57, 14.7 Hz, 1H), 3.77-3.89 (m, 1 H), 3.51-3.61 (m, 1 H), 1.47-1.95 (m, 6 H); HRMS (ESI) calcd for C12H16O6Na,[M+Na]+ 279.0845; found, 279.0852.
4.2.

(E)-6,6'-(Ethene-1,2-diyl)bis(5-hydroxy-2-(((tetrahydro-2H-pyran-2-yl)oxy)methyl)-4H-

pyran-4-one) (5)To a stirred solution of 3 (400 mg, 1.56 mmol) and Ph3P (532 mg, 1.3 equiv) in dry EtOAc (20 mL),diisopropyl azodicarboxylate (40% in toluene, 1.03 g, 1.3 equiv) was added quickly under N2
atmosphere at 0 °C. Upon addition, the color of the solution changed from pale brown to dark brown,
and a yellow precipitate was immediately generated. After completing the addition of DIAD, thereaction mixture was stirred for 1 h at room temperature. The mentioned yellow residue was thencollected by filtration and was washed with EtOAc until the filtrate was colorless. The residue wasdried under reduced pressure to yield the dimeric compounds 5 (202 mg, 54%); mp 325 °C (decomp.);IR (KBr) 3220, 1645, 1618, 1584, 1441, 1243 cm-1; 1H NMR (300 MHz, CDCl3); δ 7.32 (s, 2 H), 6.56
(s, 2 H), 4.78 (t, 2 H, J = 3.0 Hz), 4.66 (dd, 2 H, J = 0.8, 14.9 Hz), 4.47 (dd, 2 H, J = 0.5, 14.9 Hz),3.81-3.95 (m, 2 H), 3.55-3.67 (m, 2 H), 1.51-1.95 (m, 12 H); 13C NMR (75 MHz, DMSO-d6): δ 137.9,
163.7, 144.8, 143.5, 118.1, 111.0, 98.2, 64.7, 61.7, 30.3, 25.2, 19.1; HRMS (ESI) calcd for
C24H28O10Na, [M+Na]+ 499.1580; found, 499.1577.
4.3. (E)-6,6'-(Ethene-1,2-diyl)bis(5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one) (6)The dimers 5 (200 mg, 0.42 mmol) were dissolved in 60 mL of the mixed solvent CHCl3–MeOH(2:1); subsequently, conc. hydrochloric acid (125 μL) was added to the obtained solution. The resulting
mixture was stirred for 1 h at room temperature. It was then concentrated under reduced pressure, andthe resulting residue was dispersed in CH2Cl2 and filtered. The yellow solid thus isolated was washedwith CH2Cl2 and dried to give 6 (129 mg, quantitative); mp 314 °C (decomp.); IR (KBr) 3160, 1646,
1625, 1582, 1444, 1252, 1089 cm-1; 1H NMR (300 MHz, DMSO-d6); δ 9.97 (brs, 2 H), 7.27 (s, 2 H),
6.35 (s, 2 H), 5.78 (t, J = 6.0 Hz, 2 H), 4.39 (d, J = 6.0 Hz, 4 H), the spectrum was similar to that of
KA expect for the resonance signal due to H-6 (7.27 ppm) [lit.22 (KA in DMSO-d6); δ 9.06 (s, 1 H,OH), 8.02 (s, 1 H, H-6), 6.33 (s, 1 H, H-3), 5.67 (s, 1 H, OH), 4.28 (s, 2 H, -CH2-)]; HRMS (ESI) calcd
10

for C14H12O8Na, [M+Na]+ 331.0430; found, 331.0435.
4.4.

3-Hydroxy-2-(1-hydroxypropyl)-6-(((tetrahydro-2H-pyran-2-yl)oxy)methyl)-4H-pyran-4-one (8)The aldol reaction of 2 with 1-propanal was carried out referring to the published literature.6 Briefly,
to a mixture of 2 (1.85 g, 8.18 mmol) and 1-propanal (704 μL, 1.2 equiv) in dioxane:H2O (1:1, 20 mL),DABCO (1.10 g, 1.2 equiv) was added, and the resulting mixture was stirred at room temperature for
2 days. Saturated saline was then added to the reaction mixture, and the resulting mixture was extractedwith EtOAc. The organic layer was dried over Na2SO4 and concentrated under reduced pressure. The
residue thus obtained was purified by silica gel chromatography using n-hexane–EtOAc (1:5) as eluent,and it was recrystallized from EtOAc–n-hexane to afford 8 as a colorless solid (1.20 g, 52%); mp
86 °C: IR (KBr) 3275, 2937, 1653, 1623, 1593, 1457, 1365, 1208, 969 cm-1; 1H-NMR (300 MHz,CDCl3) δ: 6.54 (s, 1 H), 4.85 (t, J = 7.2 Hz, 1 H), 4.73 (dd, J = 3.4, 7.0 Hz, 1 H), 4.56 (dd, J = 0.76,
14.7 Hz, 1 H), 4.39 (dd, J = 0.56, 14.7 Hz, 1 H), 3.77-3.89 (m, 1 H), 3.51-3.61 (m, 1 H), 2.63 (brs, 1
H), 1.49-1.99 (m, 8 H), 0.99 (t, J = 7.4 Hz, 3 H); HRMS (ESI) calcd for C14H20O6Na, [M+Na]+
307.1158; found, 307.1162.
4.5. Diisopropyl 1-(1-(3-hydroxy-4-oxo-6-(((tetrahydro-2H-pyran-2-yl)oxy)methyl)-4H-pyran2-yl)propyl)hydrazine-1,2-dicarboxylate (10)The Mitsunobu reaction was carried out as described for the preparation of the dimers 5, using 7(222 mg, 0.78 mmol), Ph3P (287 mg, 1.4 equiv) and DIAD (40% in toluene, 552 mg, 1.4 equiv) in dry
EtOAc (10 mL). After stirring the reaction solution for 1 h at room temperature, the solvent wasremoved by rotary evaporation. Diethyl ether was then added to the residue, and the resulting mixture
was cooled in and ice-bath. After removing the large precipitate of Ph3PO by filtration, the organic
layer was concentrated under reduced pressure. The residue thus obtained was purified by silica gel
chromatography (CHCl3, then CHCl3:MeOH = 30:1, as eluents) to afford the hydrazide adducts 10(diastereomeric/racemic mixture) as a pearl yellow oil (182 mg, 50%); IR (NaCl) 3280, 1714, 1630,
1112, 1037 cm-1; 1H-NMR (300 MHz, CDCl3) δ: 6.78 (brs, 1 H), 6.66 (brs, 1 H), 6.48 (d, J = 9.3 Hz,
1 H), 5.20-5.44 (m, 1 H), 4.85-5.08 (m, 2 H), 4.66-4.75 (m, 1 H), 4.52 (dd, J = 5.5, 14.4 Hz, 1 H), 4.39
(t, J = 14.4 Hz, 1 H), 3.77-3.89 (m, 1 H), 3.51-3.61 (m, 1 H), 1.48-2.08 (m, 8 H), 1.10-1.42 (m, 12 H),
1.02 (t, J = 7.2 Hz, 3 H); HRMS (ESI) calcd for C22H34N2O9Na, [M+Na]+ 493.2162; found, 493.2163.
4.6. 2-(Aminomethyl)-5-hydroxy-4H-pyran-4-one hydrochloride (13)To a stirred solution of 2-(azidomethyl)-5-hydroxy-4H-pyran-4-one 1218 (3.60 g, 21.5 mmol) in dry
THF (50 mL), Ph3P (8.48 g, 1.5 equiv) was added slowly at 0 °C. The mixture thus obtained wasstirred for 10 min at 0 °C. H2O (1.92 mL) was then added dropwise to the resulting solution, which
11was stirred overnight at 55 °C. The solution was concentrated under reduced pressure and thenacidified with 1 M HCl (60 mL). The water layer was then extracted several times with CH2Cl2 toremove the byproduct Ph3PO. The water layer was concentrated under reduced pressure, and the solid
thus obtained was recrystallized from H2O–ethanol to afford 13 (2.48 g, 65%) as a brown solid; mp
222 °C (decomp.); IR (KBr) 3252, 3069, 3019, 1655, 1627, 1592, 1552, 1526, 1449, 1387, 1217, 934,
897, 876, 768 cm-1; 1H-NMR (300 MHz, D2O) δ: 8.16 (s, 1 H), 6.68 (s, 1 H), 4.22 (s, 2 H); HRMS
(ESI) calcd for C6H8NO3, [M−Cl]+ 142.0504; found, 142.0487.
4.7. tert-Butyl ((5-hydroxy-4-oxo-4H-pyran-2-yl)methyl)carbamate (14)
Boc protection of the amino group of 13 was carried out according to the N-Boc protection methodof 4-hydroxybenzylammonium bromide.19 Briefly, the HCl salt 13 (400 mg, 2.25 mmol) wassuspended in dry MeOH (20 mL) and then NaHCO3 (757 mg, 4 equiv) was subsequently added to thesuspension. Boc2O (540 mg, 1.1 equiv) was added to the mixture, and the resulting mixture was stirred
overnight at room temperature. 1 M NaOH (22.5 mL, 10 equiv) was added to the resulting solution tohydrolyze the carbonate byproduct.20 After stirring for 1 h at room temperature, the pH of the mixture
was adjusted to 4–5 using 1 M HCl; the mixture was then extracted with EtOAc. Subsequently, theorganic layer was dried over Na2SO4 and concentrated under reduced pressure. The residue thus
obtained was crystallized by adding n-hexane to it, and recrystallized from CHCl3–n-hexane to afford
14 (372 mg, 68%) as a colorless solid; mp 113 °C; IR (KBr) 3385, 3279, 1688, 1650, 1625, 1600,
1587, 1520, 1464, 1392, 1368, 1270, 1216, 1170, 940, 927, 884, 727 cm-1; 1H-NMR (300 MHz,CDCl3) δ: 7.82 (s, 1 H), 6.43 (s, 1 H), 5.04 (brs, 1 H), 4.19 (d, J = 6.0 Hz, 2 H), 1.74 (brs, 1 H), 1.46(s, 9 H); HRMS (ESI) calcd for C11H15NO5Na, [M+Na]+ 264.0848; found, 264.0855.
4.8. tert-Butyl ((5-hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2-yl)methyl)carbamate (15)
NaOH (105 mg, 2.63 mmol) was dissolved with distilled water (6 mL), and compound 14 (600 mg,
2.49 mmol) was added to the solution thus obtained. The pH of the resulting solution was adjusted to10.5 using 1 M NaOH and 1 M HCl. A solution of 37% formaldehyde (238 μL, 1.3 equiv) was added
slowly to the solution, and the obtained mixture was stirred overnight at room temperature. The
resulting mixture was acidified with 1 M HCl and then extracted with CH2Cl2. The organic layer wasdried over Na2SO4 and concentrated under reduced pressure. The residue thus obtained was purified
by silica gel chromatography using first CHCl3–MeOH (10:1) and then CHCl3–MeOH (8:1) as eluents;
it was then recrystallized from CHCl3–n-hexane to afford the aldol adduct 15 (516 mg, 76%) as acolorless solid; mp 144 °C; IR (KBr) 3330, 1684, 1618, 1591, 1532, 1286, 1235, 1162, 1011, 963, 864,
763 cm-1; 1H NMR (300 MHz, CDCl3); δ 6.39 (s, 1 H), 5.14 (brs, 1 H), 4.69 (s, 1 H), 4.20 (d, J = 6.1
Hz, 2 H), 1.67 (brs, 2 H), 1.46 (s, 9 H); HRMS (ESI) calcd for C12H17NO6Na, [M+Na]+ 294.0954;
found, 294.0954.
124.9. Di-tert-butyl ((ethene-1,2-diylbis(5-hydroxy-4-oxo-4H-pyran-6,2-diyl))bis(methylene))(E)dicarbamate (16)To a stirred solution of 15 (100 mg, 0.369 mmol) and Ph3P (126 mg, 1.3 equiv) in dry EtOAc (5mL), DIAD (40% in toluene, 242 mg, 1.3 equiv) was added quickly under N2 atmosphere at room
temperature. The mixture thus obtained was then stirred for 1 h at room temperature; a yellow
precipitate appeared as a result, which was collected by filtration. The precipitate was washed with
EtOAc and dried to give the dimeric compound 16 (53 mg, 57%); mp 255 °C (decomp.); IR (KBr)
3365, 3189, 1689, 1639, 1624, 1592, 1518, 1442, 1241, 1168, 983, 951, 774 cm-1; 1H NMR (300 MHz,DMSO-d6); δ, 10.02 (brs, 2 H, exchanged with D2O), 7.58 (t, J = 5.9 Hz, 2 H, exchanged with D2O),
7.27 (s, 2 H), 6.20 (s, 2 H), 4.08 (d, J = 5.9 Hz, 2 H), 1.40 (s, 18 H); HRMS (ESI) calcd for
C24H30N2O10Na, [M+Na]+ 529.1798; found, 529.1787.
4.10.

(E)-6,6'-(Ethene-1,2-diyl)bis(2-(aminomethyl)-5-hydroxy-4H-pyran-4-one)dihydrochloride (17)The dimeric compound 16 (40 mg, 0.079 mmol) was suspended in H2O–MeOH (1:1, 10 mL), anda 4 M solution of HCl (0.8 mL) was added to the said suspension. The mixture thus obtained wasstirred for 4 days at room temperature. It was subsequently concentrated and dried to yield 17 (28 mg,
93%) as a yellow solid; mp 262 °C (decomp.); IR (KBr) 3189, 3064, 3034, 1647, 1620, 1585, 1505,
1469, 1442, 1369, 1340, 1246, 959, 773 cm-1; 1H NMR (300 MHz, D2O); δ 7.57 (s, 2 H), 6.66 (s, 2H), 4.29 (s, 4 H); HRMS (ESI) calcd for C14H15N2O6, [M−2HCl+H]+ 307.0930; found, 307.0928.
4.11. 5-((4-Methoxybenzyl)oxy)-2-((prop-2-yn-1-yloxy)methyl)-4H-pyran-4-one (19)To a stirred solution of 2-(Hydroxymethyl)-5-(4-methoxybenzyloxy)-4H-pyran-4-one 1821 (300 mg,
1.14 mmol) in dry DMF (15 mL), a suspension of sodium hydride (60% dispersion, washed with nhexane before use, 92 mg, 2 equiv) in dry DMF (6 mL) was added at 0 °C. The mixture was stirredfor 30 minutes under N2 atmosphere at 0 °C. 3-Bromo-1-propyne (273 mg, 2 equiv) was added
dropwise and the resulting mixture was stirred for 2 h at room temperature. The solvent wasconcentrated under reduced pressure, and then 1 M citric acid was added to the residue. The aqueous
layer was extracted with CH2Cl2. The organic layer was dried over Na2SO4 and concentrated. The
residue was purified by silica gel chromatography using CHCl3 as eluents to afford 19 as a blown oil(210 mg, 61%); IR (NaCl) 2116, 1651, 1613, 1515, 1250 cm-1; 1H-NMR (300 MHz, CDCl3) δ: 7.52
(s, 1 H), 7.30 (d, J = 8.7 Hz, 2 H), 6.89 (d, J = 8.7 Hz, 2 H), 6.47 (s, 1 H), 5.01 (s, 2 H), 4.37 (s, 2 H),
4.23 (d, J = 2.5 Hz, 2 H), 3.80 (s, 3 H), 2.50 (t, J = 2.5 Hz, 1 H); HRMS (ESI) calcd for C17H16O5Na,[M+Na]+ 323.0895; found, 323.0912.
4.12. 5-Hydroxy-2-((prop-2-yn-1-yloxy)methyl)-4H-pyran-4-one (20)13

To a stirred solution of 19 (155 mg, 0.516 mmol) in dry CH2Cl2 (1.8 mL), TFA (302 μL, 10 equiv)
was added at room temperature. The resulting mixture was stirred for 2 h at room temperature. The
solvent and excess TFA were removed under reduced pressure and the residue was purified by silica
gel chromatography using first CHCl3 and then CHCl3–MeOH (20:1) as eluents to afford 20 as a pale
blown solid (64 mg, 69%); mp 91 °C; IR (KBr) 3230, 2115, 1657, 1630 cm-1; 1H-NMR (300 MHz,CDCl3) δ: 7.85 (s, 1 H), 6.56 (s, 1 H), 4.44 (s, 2 H), 4.27 (d, J = 2.5 Hz, 2 H), 2.53 (t, J = 2.5 Hz, 1 H);
HRMS (ESI) calcd for C9H8O4Na, [M+Na]+ 203.0320; found, 203.0318.
4.13. 3-Hydroxy-2-(hydroxymethyl)-6-((prop-2-ynyloxy)methyl)-4H-pyran-4-one (21).
Sodium hydroxide (23 mg, 0.57 mmol) was dissolved with distilled water (3 mL). Compound 20(101 mg, 0.56 mmol) was added to the solution and dissolved. The pH value of the mixture wasadjusted to 10.5 using 1 M NaOH and 1 M HCl solutions. A solution of 37% formaldehyde (0.17 mL)
was added to the mixture and stirred overnight at room temperature. The resulting mixture wasneutralized with 1 M HCl and then extracted with CH2Cl2. The organic layer was dried with Na2SO4
and concentrated. The residue was purified by column chromatography using CHCl3–MeOH (10:1)
as eluents to give 21 (91 mg, 77%) as a pale blown solid; mp 101 °C; IR (KBr) 3265, 2114, 1656,
1620, 1578, 1334, 1122 cm-1; 1H NMR (300 MHz, CDCl3); δ 9.12 (brs, 1 H), 6.37 (s, 1 H), 5.42 (brs,
1 H), 4.41 (d, J = 2.2 Hz, 2 H), 4.39 (s, 2 H), 4.26 (d, J = 2.4 Hz, 2 H), 3.56 (t, J = 2.4 Hz, 1 H); HRMS
(ESI) calcd for C10H10O5Na, [M+Na]+ 233.0426; found, 233.0432.
4.14.

(E)-6,6'-(Ethene-1,2-diyl)bis(5-hydroxy-2-((prop-2-yn-1-yloxy)methyl)-4H-pyran-4-one)(22)
To a stirred solution of 21 (30.0 mg, 0.143 mmol) and Ph3P (52.5 mg, 1.4 eq) in dry EtOAc (1.8mL), DIAD (40% in toluene, 101 mg, 1.4 equiv) was added quickly under N2 atmosphere at room
temperature. The mixture was stirred for 1 h at room temperature and the precipitate was collected byfiltration. The orange precipitate was washed with EtOAc and dried to give the dimeric compound 22(12.8 mg, 47%); mp 197 °C (decomp.); IR (KBr) 3228, 2117, 1645, 1618, 1583, 1246, 1119 cm-1; 1H
NMR (300 MHz, DMSO-d6); δ, 10.12 (brs, 2 H), 7.25 (s, 2 H), 6.41 (s, 2 H), 4.49 (s, 4 H), 4.29 (d, J= 2.5 Hz, 4 H), 3.54 (t, J = 2.5 Hz, 2 H); 13C NMR (75 MHz, DMSO-d6): δ 173.5, 162.4, 144.5, 143.2,
117.9, 111.2, 79.6, 78.2, 67.1, 57.8; HRMS (ESI) calcd for C20H16O8Na, [M+Na]+ 407.0743; found,407.0750.

5. References[1] T. Yabuta, J. Chem. Soc. Trans. 125 (1924), 575–587. https://doi.org/10.1039/CT9242500575.

14[2] L. Saghaie, M. Pourfarzam, A. Fassihi, B. Sartippour, Res. Pharm. Sci. 8 (2013) 233–242,
http://rps.mui.ac.ir/index.php/jrps/article/view/1510.
[3] M. Saeedi, M. Eslamifar, K. Khezri, Biomed. Pharmacother. 110 (2019) 582–593,
https://doi.org/10.1016/j.biopha.2018.12.006.
[4] J. Farard, G. Lanceart, C. Logé, M. R. Nourrisson, F. Cruzalegui, B. Pfeiffer, M. Duflos, J.
Enzyme Inhib. Med. Chem. 23 (2008) 629–640, https://doi.org/10.1080/14756360802205299.
[5] B. L. Ellis, A. K. Duhme, R. C. Hider, M. B. Hossain, S. Rizvi, D. van der Helm, J. Med. Chem.
39 (1996) 3659–3670, https://doi.org/10.1021/jm960220g.
[6] D. K. Sharma, J. Pandey, A. K. Tamrakar, D. Mukherjee, Eur. J. Med. Chem. 85 (2014) 727–736,
https://doi.org/10.1016/j.ejmech.2014.08.041.
[7] O. Mitsunobu, Synthesis (1981) 1–28, https://doi.org/10.1055/s-1981-29317.
[8] K. C. K. Swamy, N. N. B. Kumar, E. Balaraman, K. V. P. P. Kumar, Chem. Rev. 109 (2009)
2551–2651, https://doi.org/10.1021/cr800278z.
[9] T. Olpp, R. Brückner, Angew. Chem. Int. Ed. 44 (2005) 1553–1557,
https://doi.org/10.1002/anie.200460259.
[10] J. S. Yadav, P. Dutta, J. Org. Chem. 81 (2016) 1786–1797,
https://doi.org/10.1021/acs.joc.5b02426.
[11] (a) C. W. Chen, T. Y. Luh, J. Org. Chem. 73 (2008) 8357–8363,
https://doi.org/10.1021/jo8013885; (b) C. W. Chen, K. Fan, C. H. Chen, S. L. Huang, Y. H. Liu, T. S.
Lim, G. W. Wang, T. Y. Luh, J. Org. Chem. 80 (2015) 8772–8781,
https://doi.org/10.1021/acs.joc.5b01626.
[12] J. Mulzer, A. Pointner, A. Chucholowski, G. Brüntrup, J. Chem. Soc., Chem. Commun. (1979)
52–54, https://doi.org/10.1039/C39790000052.
[13] K. Sander, T. Kottke, L.Weizel, H. Stark, Chem. Pharm. Bull. 58 (2010) 1353–1361.
https://doi.org/10.1248/cpb.58.1353.
[14] M. Miyano, J. R. Deason, A. Nakao, M. A. Stealey, C. I. Villamil, D. D. Sohn, R. A. Mueller, J.
Med. Chem. 31 (1988) 1052–1061. https://doi.org/10.1021/jm00400a030.
[15] (a) T. Honda, Y. Ishida, T. Arai, Chem. Lett. 45 (2016) 1081–1083,
https://doi.org/10.1246/cl.160475; (b) T. Honda, T. Arai, Bull. Chem. Soc. Jpn. 89 (2016) 254–256,
https://doi.org/10.1246/bcsj.20150347.
[16] M. J. Brown, G. J. Clarkson, G. G. Inglis, M. Shipman, Org. Lett. 13 (2011) 1686–1689,
https://doi.org/10.1021/ol200193n.
[17] S. A. Frank, W. R. Roush, J. Org. Chem. 67 (2002) 4316–4324,
https://doi.org/10.1021/jo025580s.
[18] S. M. Meier, M. Novak, W. Kandioller, M. A. Jakupec, V. B. Arion, N. Metzler-Nolte, B. K.
Keppler, C. G. Hartinger, Chemistry 19 (2013) 9297–9307, https://doi.org/10.1002/chem.201300889.
15[19] K. S. Chichak, A. J. Peters, S. J. Cantrill, J. F. Stoddart, J. Org. Chem. 70 (2005) 7956–7962,
https://doi.org/10.1021/jo050969b.
[20] K. Nakamura, T. Nakajima, H. Kayahara, E. Nomura, H. Taniguchi, Tetrahedron Lett. 45 (2004)
495–499, https://doi.org/10.1016/j.tetlet.2003.11.008.
[21] Y. H. Chen, P. J. Lu, C. Hulme, A. Y. Shaw, Med. Chem. Res. 22 (2013), 995-1003.
[22] X. Li, J. H. Jeong, K. T. Lee, J. R. Rho, H. D. Choi, J. S. Kang, B. W. Son, Arch. Pharm. Res.
26(2003) 532-534, https://doi.org/10.1007/BF02976876.

16

Figure 1. Structure of kojic acid (KA).

17

Figure 2.

1H

NMR spectra of the Mitsunobu elimination product (upper panel) and thetetrahydropyranyl (THP)-protected derivative of kojic acid (2, lower panel) in CDCl3.

18

Figure 3.

Predicted structures (diastereomeric/racemic mixture) of the Mitsunobu elimination

products.

19

Figure 4. X-ray diffraction-determined crystal structure of products 5 obtained by the Mitsunobu
elimination reaction of 3.

20

Figure 5. Electrospray ionization mass spectrum of the residue obtained after removing the dimers
5 by filtration. The solvent was removed, and the sample was prepared by dissolving the obtained
residue in MeOH. The peaks at m/z 301.1 and 333.1 were ascribed to the Na+ adducts of Ph3PO or
Ph3PO with MeOH. The peak at m/z 279.1 was ascribed to the H+ adducts of Ph3PO or the Na+ adductof unreacted 3.

21

Figure 6.

Electrospray ionization mass spectra of compound 16 (A, dissolved in DMSO) andcompound 17 (B, dissolved in H2O).

22

Figure 7. High-resolution electrospray ionization mass spectrum of compound 22.

23

Scheme 1. Proposed mechanism of the Mitsunobu 1,4-elimination of KA analog 3.

24

Scheme 2.

Predicted mechanism of the dimerization of 4 generated by the Mitsunobu 1,4-

elimination of 3.

25

Scheme 3. Reagents and conditions: (a) 1-Propanal, DABCO, 1,4-dioxane–H2O (1:1), rt, 2 days,
52%; (b) Ph3P (1.4 equiv), DIAD (1.4 equiv), EtOAc, 0 °C, 1 h, >50%.

26

Scheme 4. Reagents and conditions: (a) SOCl2, CH2Cl2, rt, overnight, 73%; (b) NaN3, DMF, rt, 55%;(c) 1) Ph3P, THF–H2O, 4 h, 55 °C; 2) acidified with 1 M HCl, then washed with CH2Cl2, 65%; (d) 1)
Boc2O, NaHCO3, MeOH, overnight, rt; 2) 1 M NaOH (10 eq), 1 h, rt, 68%; (e) NaOH, HCHO, H 2O,rt, overnight, 76%; (f) Ph3P (1.3 equiv), DIAD (1.3 equiv), EtOAc, rt, 1 h, 51−57%; (g) 4 M HCl,
H2O–MeOH, rt, 4 days, 93%.

27

Scheme 5. Reagents and conditions: (a) PMBCl, K2CO3, DMF, 50 °C, 2 h, 77%; (b) NaH, DMF,
0 °C, then 3-Bromo-1-propyne, 0 °C to rt, 2 h, 61%; (c) TFA, CH2Cl2, rt, 2 h, 69%; (d) NaOH,HCHO, H2O, overnight, rt, 77%; (e) Ph3P, DIAD, EtOAc, rt, 1 h, 41−47%.

28