# Crystallization-Induced Diastereoisomer Transformation of Dihydroartemisinic Aldehyde with the Betti Base

Andrea Zanetti, Pauline Chaumont-Olive, Geoffrey Schwertz, Marllon Nascimento de Oliveira, Mario Andrés Gómez Fernández, Zacharias Amara, Janine Cossy

► To cite this version:

Andrea Zanetti, Pauline Chaumont-Olive, Geoffrey Schwertz, Marllon Nascimento de Oliveira, Mario Andrés Gómez Fernández, et al.. Crystallization-Induced Diastereoisomer Transformation of Dihydroartemisinic Aldehyde with the Betti Base. Organic Process Research and Development, 2020, 24 (5), pp.850-855. 10.1021/acs.oprd.9b00481. hal-03197851

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# Crystallization-Induced Diastereoisomer Transformation of Dihydroartemisinic Aldehyde with the Betti Base

Andrea Zanetti,§ Pauline Chaumont-Olive,§ Geoffrey Schwertz, Marllon Nascimento de Oliveira, Mario Andrés Gomez Fernandez, Zacharias Amara,* and Janine Cossy*

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Cite This: *Org. Process Res. Dev.* 2020, 24, 850–855

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## ACCESS

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Supporting Information

**ABSTRACT:** Artemisinin is an important drug to fight malaria. It is produced either by extraction or via a semisynthetic route involving enzyme engineering. A key intermediate to produce artemisinin by the enzymatic route is dihydroartemisinic aldehyde (DHAAI). However, control of the absolute configuration of the stereocenter α to the aldehyde is highly challenging. Herein we report a protocol that allows the diastereomeric enrichment of a mixture of (11R)/(11S)-DHAAI to the desired (11R)-DHAAI by utilizing a crystallization-induced diastereomer transformation induced by the Betti base. In addition, the Betti base can be quantitatively recovered and reused after the reaction.

**KEYWORDS:** *artemisinin*, *dihydroartemisinic aldehyde*, *CIDT*, *Betti base*, *deracemization*

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## INTRODUCTION

Nearly half of the world population is at risk of malaria. In 2017, around 435 000 deaths, mostly located in sub-Saharan African regions, were attributed to this disease.¹ The fight against malaria therefore requires the efficient production of antimalarial drugs at a very low cost. Among these, artemisinin is considered to be one of the most potent drugs on the market, and this natural product is mainly obtained by extraction from *Artemisia annua*. However, the world market price is highly volatile, ranging from US $250 to $1100 per kilogram.² To complement the natural production and narrow the market price fluctuation, a semisynthetic process based on isoprenoid fermentation has been developed.³ This manufacturing route uses bioengineered microorganisms to produce advanced biosynthetic intermediates such as amorphadiene,⁴ artemisinic acid,³ and more recently dihydroartemisinic aldehyde (DHAAI)⁵ (Scheme 1). The last of these is a key product because it can be obtained directly by fermentation or by means of cost-competitive synthetic chemistry.⁶ However, one of the main drawbacks of these strategies is the control of the (11R)-configuration of DHAAI, which is relatively unstable and this compound is usually obtained as a mixture of two diastereomers (Scheme 1).⁵

Optical resolution of racemates via the formation of diastereomers using enantiopure auxiliaries and their separation by crystallization is the preferred large-scale purification method to access highly diastereo- and enantioenriched compounds in pharmaceutical companies.⁷ One practical way to realize a diastereo- and enantioenrichment is to use a crystallization-induced diastereomer transformation (CIDT), in which an epimerizable compound is converted into a crystalline diastereomer by condensation with a chiral auxiliary.⁸ A particularly useful method to realize CIDT relies on the use of the so-called “Betti base”, which was first reported in 1900.⁹ In 2003, Košmrlj and Weigel demonstrated that racemic 2-

ethylhexanal can be transformed to (R)-2-ethylhexanal by CIDT using trans-(1R,2R)-1-amino-6-nitroindan-2-ol.¹⁰ Additional examples have recently been reported in the literature,¹⁰⁻¹³ and Fülöp et al. showed that treatment of a racemic α-substituted aldehyde with the Betti base leads to the formation of a naphthoxazine ring in which the 1- and 3-substituents are in a trans relationship¹⁴ and that the (S)-Betti base induces the formation of the (1'R,3'S)-naphthoxazine. The epimerization of the stereocenter α to the aldehyde presumably occurs through an imine/enamine equilibrium (Scheme 2).

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## RESULTS AND DISCUSSION

In this study, gram-scale quantities of a mixture of (11R)- and (11S)-DHAAI with a diastereomeric ratio (dr) of 65:35 were prepared using a non-stereoselective reduction of artemisinic acid followed by oxidation.¹⁵ In order to improve this ratio, a mixture of (11R)- and (11S)-DHAAI was treated with the (S)-Betti base, which was identified as the correct enantiomer to access the desired diastereoenriched (11R)-DHAAI. Treatment of a 65:35 mixture of DHAAI diastereomers with (S)-1-(α-aminobenzyl)-2-naphthol [(S)-2] in methanol for 4 h at room temperature (rt) led to the formation of a mixture of naphtho[1,2-e][1,3]oxazines (1'R,3'S,11R)-3 and (1'R,3'S,11S)-3 in a ratio of 72:28.¹⁶ When the reaction was performed on a 0.545 mmol scale, the yield of (1'R,3'S,11R)-3/((1'R,3'S,11S)-3 was 63%, and when the reaction was performed on a larger scale (3.694 mmol), the yield of (1'R,3'S,11R)-3/((1'R,3'S,11S)-3 was increased to 83%. The structures of these isomers were established by ¹H and ¹³C NMR analysis, while

**Special Issue:** A Taste of Current French Organic Chemistry

Received: November 7, 2019

Published: January 14, 2020
<figure><img src="image_9.png" /><figcaption>Scheme 2. Formation of One Naphthoxazine Diastereomer by Imine and Enamine Equilibrium</figcaption></figure>

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nuclear Overhauser effect (NOE) experiments confirmed that for each isomer, the hydrogen atoms of the 1,3-oxazine ring at C1' and C3' are in a trans relationship. The independent preparation of (1'S,3'R,11R)-3 by reaction of (11R)-DHAAI with (S)-2 confirmed that the major isomer of the 72:28 mixture is the desired (11R)-isomer (Scheme 3).

When the 72:28 mixture of (1'R,3'S,11R)-3 and (1'R,3'S,11S)-3 was heated in refluxing methanol for an 4 h, no change in the ratio was observed. However, when the same mixture was treated with acetic acid (2.5 mol %) for 3 h in methanol and heated at 65 °C, the (1'R,3'S,11S)-3 diastereomer was obtained by filtration with a dr of 99:1 in 63% yield. In order to improve the yield of (11R)-DHAAI, a screening of solvents, temperatures, and times was performed, and the best conditions were to carry out the reaction at 65 °C in acetonitrile for 92 h in the presence of AcOH (2.5 mol %). Under these conditions, (1'R,3'S,11R)-3 was isolated in 85% yield with a dr of 95:5 in favor of the (11R)-isomer. Careful monitoring of the crude reaction mixture over time by ¹H NMR spectroscopy showed that the diastereomeric excess (de) increased with time in favor of the (11R)-isomer (Scheme 3).

The crucial step was the hydrolysis of dihydronaphthoxazine 3 to isolate pure **DHAAI** and the recovery of the Betti base. Accordingly, treatment of (1'S,3'S,11R)-3 with Dowex 50WX8-100 (2 g/mmol) in the presence of a 2% TsOH aqueous solution (0.2 mL/mmol) in THF/EtOAc (1:1) at rt for 14 h led to the isolation of the desired (11R)-**DHAAI** epimer in quantitative yield with a dr of 95:5 (Scheme 4). A comparison of the [α]~D~ of

**DHAAI** obtained by CIDT with the [α]~D~ of **DHAAI** prepared by reduction of dihydroartemisinic acid (dr > 99:1) followed by an oxidation step (Py·SO~3~, Et~3~N, DMSO)~17~ allowed us to attribute the *R* absolute configuration of the C11 stereogenic center of **DHAAI** obtained by CIDT [(α]~D~<sup>25</sup> +13, *c* 0.98, CHCl~3~ from a sample obtained by CIDT with a dr of 95:5) versus (α]~D~<sup>25</sup> +16, *c* 0.98, CHCl~3~ from a sample obtained by reduction and oxidation of pure dihydroartemisinic acid with an 11R/11S ratio of 99:1)]. It is worth mentioning that the Betti base can be easily recovered at the end of the process in quantitative yield~18~ and can be recycled in a new CIDT cycle.~19

■ CONCLUSION

We developed a simple proof-of-concept protocol for the epimerization of the stereogenic center *α* to the aldehyde of **DHAAI** via a CIDT method utilizing the Betti base, which allowed the isolation of (11R)-**DHAAI** with a dr of 95:5 in a nonoptimized overall yield of 71% (three steps). The chiral auxiliary can be easily recovered and reused in another CIDT cycle.¹⁸,¹⁹ This process can potentially offer an alternative to the current route to artemisinin involving hydrogenation, which utilizes an expensive optically active organometallic rhodium catalyst, to control the (11R) stereogenic center.³ CIDT could be an efficient route for the preparation of artemisinin on scale and open more economically viable alternatives to low-cost artemisinin manufacturing.
<figure><img src="image_11.png" /><figcaption>Scheme 3. Synthesis of a 72:28 Mixture of Naphthozazines 3 and Its Diastereoenrichment<sup>a</sup></figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Scheme 4. Synthesis of Naphthozazine 3 and Isolation of Diastereoenriched DHAAl Using the Betti Base [((S)-2]</figcaption></figure>

## EXPERIMENTAL SECTION

**General Information.** Reagents were purchased from Aldrich as reagent grade and used without further purification. Dowex 50WX8-100 was also purchased from Aldrich.

Reactions were performed in oven-dried glassware under an argon atmosphere. The solvent compositions are reported individually in parentheses. Analytical thin-layer chromatography (TLC) was performed on aluminum sheets coated with silica gel 60 F<sub>254</sub>

<footnote><sup>a</sup>A mixture of naphthozazines 3 (11R/11S = 72:28) (30 mg, 0.133 mmol, 1 equiv) was suspended in CH<sub>3</sub>CN (0.4 mL). AcOH (2 µL from a 10% solution in MeCN, 2.5 mol %) was added, and the suspension was stirred at 65 °C for the indicated times. The white suspension was solubilized in CHCl<sub>3</sub> (1 mL), and the solution was concentrated under reduced pressure. The diastereomeric excess (de) of 3 was measured by ¹H NMR spectroscopy of the crude mixture by integration of the aminal protons.</footnote>
(Merck, Macherey-Nagel) or with silica gel 60 RP-18 F₂₅₄s (Merck, Macherey-Nagel). Visualization was achieved using an alkaline aqueous solution of potassium permanganate. Evaporation of solvents in vacuo was performed at 25–35 °C under 9–10 mbar. Reported yields refer to spectroscopically and chromatographically pure compounds that were dried under high vacuum (0.1–0.05 mbar) before analytical characterization. ¹H and ¹³C NMR spectra were recorded on a Bruker AV 400 spectrometer at 400 MHz (¹H) or 101 MHz (¹³C). Chemical shifts (δ) are reported in parts per million upfield using the residual deuterated solvent signals as internal references (CDCl₃: δ<sub>H</sub> = 7.26 ppm, δ<sub>C</sub> = 77.16 ppm). For ¹H NMR spectra, coupling constants (J) are given in hertz, and the resonance multiplicity is described as s (singlet), d (doublet), t (triplet), pent (pentuplet), m (multiplet), or br (broad). All spectra were recorded at 298 K. Infrared (IR) spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer and are reported as wavenumbers ν̃ in reciprocal centimeters. High-resolution mass spectrometry (HRMS) and analyses were performed by the Laboratoire de Spectrométrie de Masse at Sorbonne Université, Paris. Melting points were determined using a Büchi melting point apparatus in open capillaries. The enantiomeric ratios were determined by supercritical fluid chromatography (SFC) on a chiral stationary phase using a Minigram Berger SFC-Mettler Toledo apparatus; iPrOH was used as the polar eluent. Nomenclature follows the suggestions proposed by the software ChemDraw Professional 16.0.

(1S,3R)-3-((R)-1-((1R,4R,4aS,8aS)-4,7-Dimethyl-1,2,3,4,4a,5,6,8a-octahydronaphthalen-1-yl)ethyl)-1-phenyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine [(1'R,3'S,11R)-3]. Diastereoenriched dihydroartemisinin aldehyde (11R/11S = 99:1, 67 mg, 0.304 mmol, 1 equiv) was added to a suspension of (S)-Betti base (75 mg, 0.301 mmol, 0.99 equiv) in MeOH (1.3 mL), and the suspension was stirred at 23 °C. After 4 h, the white solid was collected by vacuum filtration, washed with ice-cooled MeOH (2 × 0.4 mL), and dried under reduced pressure to afford the corresponding naphthoxazine as a white solid (79 mg, 58%). Mp: 145–146 °C (MeOH). [α]<sup>25</sup><sub>D</sub> -24 (c 0.92, CHCl<sub>3</sub>). IR (ATR): ν̃ = 2907, 1621, 1598, 1514, 1448, 1389, 1233, 967, 918, 809, 743, 699 cm<sup>-1</sup>. ¹H NMR (400 MHz, CDCl<sub>3</sub>): δ 7.78 (m, 1H), 7.74 (d, J = 9.0 Hz, 1H), 7.43 (m, 1H), 7.34–7.20 (m, 7H), 7.12 (d, J = 8.9 Hz, 1H), 5.58 (s, 1H), 5.22 (br s, 1H), 4.82 (d, J = 3.0 Hz, 1H), 2.50 (m, NH, 1H), 2.46 (br s, 1H), 2.03 (m, 1H), 1.97–1.73 (m, 3H), 1.61 (br s, 3H), 1.54–1.16 (m, 5H), 1.02 (d, J = 6.9 Hz, 3H), 0.83 (d, J = 6.2 Hz, 3H), 0.76–0.64 (m, 3H). ¹³C NMR (101 MHz, CDCl<sub>3</sub>): δ 153.5, 142.9, 135.4, 132.1, 129.4 (2C), 129.2, 128.60, 128.59, 128.0 (2C), 127.2, 126.7, 123.2, 123.0, 120.7, 119.5, 115.0, 83.7, 53.7, 42.8, 42.1, 37.4, 37.2, 35.4, 27.8, 26.8, 26.0, 25.5, 24.0, 19.9, 9.8. HR-ESI-MS: *m/z* calcd for C₃₂H₃₇NOH⁺, 452.2948; found, 452.2939 [M + H]⁺.

(1S,3R)-3-((R:S = 72:28)-1-((1R,4R,4aS,8aS)-4,7-Dimethyl-1,2,3,4,4a,5,6,8a-octahydronaphthalen-1-yl)-ethyl)-1-phenyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine [72:28 (1'R,3'S,11R)-3/(1'R,3'S,11S)-3]. Dihydroartemisinic aldehyde (11R/11S = 65:35, 814 mg, 3.694 mmol, 1 equiv) was added to a suspension of (S)-Betti base (875 mg, 3.509 mmol, 0.95 equiv) in MeOH (16 mL), and the suspension was stirred at 23 °C for 4 h. The white solid was collected by vacuum filtration, washed with ice-cooled MeOH (2 × 2 mL), and dried under reduced pressure to afford the corresponding naphthoxazines in an 11R/11S ratio of 72:28 as a white solid (1.317 g, 83%). ¹H NMR (400 MHz, CDCl₃): δ 7.78

(m, 1H), [7.74 (d, J = 8.8 Hz, major) and 7.69 (m, minor), 1H], 7.34–7.18 (m, 7H), 7.12 (d, J = 8.9 Hz, 1H), 5.58 (s, 1H), [5.22 (br s, 0.72H) and 4.91 (br s, 0.28H)], [4.82 (d, J = 3.0 Hz, 0.72H) and 4.79 (m, 0.28H)], 2.50 (m, NH, 1H), 2.46 (br s, 1H), 2.04 (m, 1H), 1.97–1.73 (m, 3H), [1.61 (br s, major) and 1.45 (br s, minor), 3H], 1.56–1.16 (m, 5H), 1.02 (d, J = 6.9 Hz, 3H), [0.85 (d, J = 6.6 Hz, minor) and 0.83 (d, J = 6.2 Hz, major), 3H], 0.73–0.64 (m, 3H). ¹³C NMR (101 MHz, CDCl₃): δ [153.5 (major), 153.2 (minor)], [143.0 (minor), 142.9 (major)], [135.3 (major), 134.7 (minor)], [132.1 (major), 131.99 (minor)], [129.39 (major, 2C), 129.3 (minor, 2C)], [129.2 (major), 129.0 (minor)], [128.7 (minor), 128.60 (major)], [128.59 (major), 128.5 (minor)], [128.3 (minor, 2C), 128.0 (major, 2C)], [127.2 (major), 127.1 (minor)], [126.7 (major), 126.6 (minor)], [123.2 (major), 123.1 (minor)], 123.0, [120.8 (minor), 120.7 (major)], [119.6 (minor), 119.5 (major)], [115.0 (major), 114.9 (minor)], [83.7 (major), 82.8 (minor)], [54.4 (minor), 53.7 (major)], 42.8, [42.14 (major), 42.0 (minor)], [39.0 (minor), 37.2 (major)], [38.4 (minor), 37.4 (major)], [35.9 (minor), 35.4 (major)], [28.0 (minor), 27.8 (major)], [26.8 (major), 26.6 (minor)], [26.4 (minor), 26.0 (major)], [25.8 (minor), 25.5 (major)], [24.00 (major), 23.8 (minor)], [20.0 (minor), 19.9 (major)], [11.3 (minor), 9.8 (major)].

(1S,3R)-3-((R)-1-((1R,4R,4aS,8aS)-4,7-Dimethyl-1,2,3,4,4a,5,6,8a-octahydronaphthalen-1-yl)ethyl)-1-phenyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine [95:5 (1'R,3'S,11R)-3/(1'R,3'S,11S)-3]. A mixture of naphthoxazines (11R/11S = 72:28, 150 mg, 0.332 mmol, 1 equiv) was suspended in CH₃CN (2 mL). AcOH (10% in CH₃CN, 9 µL, 2.5 mol %) was added, and the suspension was stirred at 65 °C for 92 h. The white suspension was solubilized in CHCl₃ (5 mL), washed with a saturated aqueous solution of NaHCO₃ (2 mL) and brine (2 mL), dried over Na₂SO₄, filtered, and concentrated under reduced pressure to afford a white solid. The solid was suspended in methanol (1 mL), filtered, and washed with ice-cooled methanol (0.5 mL). The residue was dried under reduced pressure to afford the naphthoxazines in an 11R/11S ratio of 95:5 as a white solid (85%, 127 mg). Mp: 149–153 °C (MeOH). [α]<sup>25</sup><sub>D</sub> -19 (c 0.68, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ 7.78–7.74 (m, 2H), 7.45–7.19 (m, 8H), 7.12 (d, J = 8.9 Hz, 1H), 5.58 (s, 1H), [5.22 (br s, 0.95H) and 4.91 (br s, 0.05H)], [4.82 (d, J = 3.0 Hz, 0.95H) and 4.79 (d, J = 3.0 Hz, 0.05H)], 2.50 (m, NH, 1H), 2.46 (br s, 1H), 2.08–1.73 (m, 4H), 1.61 (br s, 3H), 1.56–1.18 (m, 5H), 1.02 (d, J = 6.9 Hz, 3H), [0.85 (d, J = 6.6 Hz, minor) and 0.83 (d, J = 6.2 Hz, major), 3H], 0.75–0.64 (m, 3H). ¹³C NMR (101 MHz, CDCl₃): δ 153.5, 142.9, 135.4, 132.1, 129.4 (2C), 129.2, 128.60, 128.59, 128.0 (2C), 127.2, 126.7, 123.2, 123.0, 120.7, 119.5, 115.0, 83.7, 53.7, 42.8, 42.1, 37.4, 37.2, 35.4, 27.8, 26.8, 26.0, 25.5, 24.0, 19.9, 9.8.

(R)-2-((1R,4R,4aS,8aS)-4,7-Dimethyl-1,2,3,4,4a,5,6,8a-octahydronaphthalen-1-yl)propanal [95:5 (11R)-1/(11S)-1]. The naphthoxazines (11R/11S = 95:5, 127 mg, 0.281 mmol) were dissolved in a 1:1 THF/EtOAc solution (1.12 mL). The solution was treated with Dowex 50WX8-100 (0.562 g, 2 g/mmol) and a 2% TsOH aqueous solution (0.056 mL, 0.2 mL/mmol). The suspension was stirred at 23 °C. After 14 h, the suspension was filtered under vacuum, and the brown residue was washed with Et₂O (1.12 mL × 3, 6 mL/g of resin). The organic layer was cooled with a water/ice bath and washed with a saturated aqueous solution of Na₂CO₃ (2 × 3 mL). The organic layer was washed with water (2 × 3 mL) and brine (2 × 3
mL), dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The crude residue was filtered on silica gel (85:15 hexanes/EtOAc) to afford dihydroartemisinic aldehyde as a colorless oil (62 mg, quantitative yield). R_f = 0.78 (SiO₂; 9:1 petroleum ether/EtOAc). [α]D²⁵ +13 (c 0.98, CHCl₃). IR (ATR): ν̃ = 2912, 2868, 1706, 1449, 1377, 1289, 1264, 1165, 1110, 1078, 1032, 990 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ [9.63 (d, J = 4.0 Hz, 0.05H) and 9.57 (d, J = 4.0 Hz, 0.95H)], [5.27 (br s, 0.05H) and 5.13 (br s, 0.95H)], 2.48 (br s, 1H), 2.36 (m, 1H), 2.00–1.76 (m, 3H), 1.64 (br s, 3H), 1.62–1.22 (m, 6H), 1.12 (dddₐpp, JAB = 3.3, 13.0, 25.0 Hz, 1H), 1.06 (d, J = 7.0 Hz, 3H), 0.95 (dddₐpp, JAB = 3.2, 12.1, 26.0 Hz, 1H), 0.87 (d, J = 6.5 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃): δ 206.2, 136.2, 119.7, 48.6, 42.0, 41.6, 36.7, 35.4, 27.8, 27.5, 26.7, 25.9, 24.0, 19.8, 11.9. t<sub>R</sub>: 2.84 and 3.07 min (93:7) (SFC, OD-H column, 100 bar, 4 mL/min, isocratic gradient 99:1 CO₂/iPrOH). HR-ESI-MS: m/z calcd for C₁₅H₂₄OH⁺, 221.1900; found, 221.1899 [M + H]⁺.

**Synthesis of (R)-2-[(1R,4R,4aS,8aS)-4,7-Dimethyl-1,2,3,4,4a,5,6,8a-octahydronaphthalen-1-yl]propanal (DHAAI) from Dihydroartemisinic Alcohol.** Dihydroartemisinic alcohol (dr > 98:2, 99 mg, 0.450 mmol, 1 equiv), obtained by LiAlH₄ reduction of dihydroartemisinic acid, was dissolved in CH₂Cl₂ (2 mL) and DMSO (0.5 mL). The mixture was cooled to -10 °C with an ice/NaCl bath and slowly treated with Et₃N (0.25 mL, 1.80 mmol, 4 equiv). Py·SO₃ (179 mg, 1.124 mmol, 2.5 equiv) was added in three portions over 20 min. The mixture was allowed to warm to 23 °C and stirred for 16 h. GC/MS analysis showed full conversion of dihydroartemisinic alcohol to **DHAAI**. The mixture was treated with citric acid (aq, 10%, 2 mL) and stirred for 10 min. The layers were then separated, and the organic layer was washed with citric acid (aq, 10%, 2 mL), a saturated aqueous solution of NaHCO₃ (2 mL), and brine (4 mL), dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The yellow residue was filtered on silica gel (85:15 hexanes/EtOAc) to afford **DHAAI** as a colorless oil (81 mg, 82%). It is worth mentioning that when the reaction was performed on 300 mg of dihydroartemisinic alcohol, **DHAAI** was obtained in 97% yield. R_f = 0.78 (SiO₂; 9:1 petroleum ether/ EtOAc). [α]D²⁵ +16 (c 0.98, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ 9.57 (d, J = 4.0 Hz, 1H), 5.13 (br s, 1H), 2.48 (br s, 1H), 2.36 (m, 1H), 2.00–1.76 (m, 3H), 1.64 (br s, 3H), 1.66–1.22 (m, 6H), 1.12 (dddₐpp, JAB = 3.2, 12.6, 25.1 Hz, 1H), 1.06 (d, J = 7.0 Hz, 3H), 0.95 (dddₐpp, JAB = 3.2, 12.2, 26.1 Hz, 1H), 0.87 (d, J = 6.5 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃): δ 206.2, 136.2, 119.7, 48.6, 42.0, 41.6, 36.7, 35.4, 27.8, 27.5, 26.7, 25.9, 24.0, 19.8, 11.9. t<sub>R</sub>: 2.84 min (SFC, OD-H column, 100 bar, 4 mL/min, isocratic gradient 99:1 CO₂/iPrOH).

**Recovery of the Betti Base.** The solid residue, Dowex/Betti base (562 mg), was suspended in THF (1.23 mL), and then MeOH (0.15 mL) was added, followed by the addition of a 28% NH₄OH aqueous solution (0.31 mL). The mixture was stirred for 45 min at rt. After filtration, the filtrate was washed with THF (3 × 1 mL), and the solvents were evaporated under reduced pressure. The Betti base was recovered in quantitative yield as a white solid (70 mg, 0.281 mmol). Mp: 130–131 °C (lit.¹ 133–134 °C). [α]D²⁵ +93.0 (c 0.42, CHCl₃) [lit.²⁰ +94.1 (c 1.0, CHCl₃)].

■ ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.oprd.9b00481.

Stability of **DHAAI** and naphthoxazines under basic and acidic conditions, ¹H and ¹³C NMR spectra, and SFC chromatograms (PDF)

■ AUTHOR INFORMATION

Corresponding Authors

Zacharias Amara — Conservatoire national des arts et métiers, HESAM Université, Paris, France; ● orcid.org/0000-0001-6824-1116; Phone: +33 1 40 27 26 14; Email: zacharias.amara@lecnam.net

Janine Cossy — ESPCI Paris/CNRS/PSL Research University, Paris, France; ● orcid.org/0000-0001-8746-9239; Phone: +33 1 40 79 44 29; Email: janine.cossy@espci.fr; Fax: +33 1 40 79 44 54

Other Authors

Andrea Zanetti — ESPCI Paris/CNRS/PSL Research University, Paris, France

Pauline Chaumont-Olive — ESPCI Paris/CNRS/PSL Research University, Paris, France

Geoffrey Schwertz — ESPCI Paris/CNRS/PSL Research University, Paris, France

Marlon Nascimento de Oliveira — Conservatoire national des arts et métiers, HESAM Université, Paris, France

Mario Andrés Gomez Fernandez — Conservatoire national des arts et métiers, HESAM Université, Paris, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.oprd.9b00481

Author Contributions

A.Z. and P.C.-O. contributed equally.

Funding

This work was supported by the Bill & Melinda Gates Foundation [OPP1190174].

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Dr. Kai Rossen and Dr. Trevor Laird for their advice and fruitful discussions and Dr. Fabienne Dioury for synthesizing gram-scale quantities of the key biosynthetic precursors required for this study.

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