Diastereoselective Access to anti-β-Hydroxy Sulfoxides from Chiral Epoxides and Prochiral Sulfenate Anions: Mechanistic Insights, Scope, and Limitation
This work was supported by the Japanese Society for the Promotion of Science via KAKENHI grants JP18H04391, JP21H04703, and JP23H04888, the Nagase Science and Technology Foundation, and the Novartis Foundation for the Promotion of Science (Japan).
Reported herein is a stereoselective route to anti-β-hydroxy sulfoxides through the reaction of epoxides with sulfenate anions. Extensive experimental/computational studies revealed the dual special roles of MgBr2·OEt2, serving to generate the bromohydrin alkoxide intermediate, which undergoes nucleophilic attack on the prochiral sulfenate in a diastereoselective manner. The present study has opened a general stereoselective synthetic route to anti-β-hydroxy sulfoxides.
9 The relative stereochemistry of the major diastereomer of 7a was determined by comparison with an authentic sample of anti-7a prepared by another method. For details, see the Supporting Information and Ref. 4d.
10 The use of other Lewis acids such as ZnBr2, AlBr3, InBr3, ScBr3, Sc(OTf)3, In(OTf)3, or Zn(OTf)2 met with failure. For details, see the Supporting Information.
11(RS*,2S*)-1-Phenyl-3-(phenylsulfinyl)propan-2-ol (anti-7a); Typical Procedure
A solution of sulfoxide 1a (0.800 mmol, 2.0 equiv) in THF (3 mL) was added to a solution of LiTMP (0.800 mmol, 2.0 equiv) in THF (3 mL) at –78 °C, and the mixture was stirred at –78 °C for 20 min to give the corresponding sulfenate anion PhS(O)Li (2a). MgBr2.OEt2 (207 mg, 0.800 mmol, 2.0 equiv) and LiI (11.0 mg, 0.0800 mmol, 0.2 equiv) were then added at –78 °C, and the mixture was stirred for 20 min. A solution of epoxide 6 (0.400 mmol, 1.0 equiv) in THF (2 mL) was then at –78 °C. The cooling bath was removed, the temperature was raised to RT, and the mixture was stirred for 9 h. The mixture was then poured into 1 M aq HCl (40 mL) at 0 °C, and the products were extracted with CH2Cl2 (3 × 50 mL). The combined organic layers were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography (silica gel, hexane–EtOAc) to give a diastereomeric mixture of 7a [yield: 90 mg (86%), dr = 9:1 (assessed by 1H NMR peak integration of the mixture)]. Further purification by flash chromatography [silica gel, hexane–EtOAc (2:3)] and crystallization from CH2Cl2–hexane (1:5) gave the major diastereomer anti-7a as a white powder; mp 88–90 °C; Rf = 0.4 (hexane–EtOAc, 1:1).
IR (neat): 3630, 3030, 2927, 2825, 1580, 1477, 1452, 1059, 1050, 765 cm–1. 1H NMR (600 MHz, CDCl3): δ = 7.56–7.54 (m, 2 H), 7.53–7.50 (m, 3 H), 7.25 (t, J = 7.8 Hz, 2 H), 7.21–7.19 (m, 1 H), 7.09 (d, J = 7.2 Hz, 2 H), 4.44–4.40 (m, 1 H), 3.64 (d, J = 3.0 Hz, 1 H), 3.03 (dd, J = 13.8, 9.6 Hz, 1 H), 2.87 (dd, J = 13.8, 7.2 Hz, 1 H), 2.78 (dd, J = 13.8, 6.6 Hz, 1 H), 2.70 (dd, J = 13.2, 1.2 Hz, 1 H). 13C NMR (150 MHz, CDCl3): δ = 142.9, 137.0, 131.1, 129.5, 129.4, 128.7, 126.9, 124.1, 67.8, 60.7, 43.4. HRMS (ESI): m/z [M + H]+ calcd for C15H17O2S: 261.0944; found: 261.0945.
12 For details of the TLC analyses, see the Supporting Information.
The few examples that have so far been reported on the diastereoselective alkylation of prochiral sulfenate anions using a chiral electrophile are limited to the pioneering studies by Schwan and co-workers, who used enantiopure chiral β-aminoalkyl iodides to obtain the corresponding diastereomeric β-amino sulfoxides, for selected examples, see:
13a
Schwan AL,
Verdu MJ,
Singh SP,
O’Donnell JS,
Ahmadi AN.
J. Org. Chem. 2009; 74: 6851
15 We deduced that the deprotonation of the halohydrins 8a and 8b to form the corresponding magnesium alkoxide occurred due to the co-existence of 2,2,6,6-tetramethylpiperidine (HTMP), generated together with the formation of the sulfenate species 2a.
16 To simplify the computational calculation, we employed MgBr2·OMe2 instead of MgBr2·OEt2.
17 The relative configuration of 17b was determined based on single-crystal X-ray diffraction analyses. For details, see the Supporting Information.
18 The relative stereochemistry was not determined.
19 The poor diastereoselectivity might be attributable to the following reason. The key interactions between the leaving bromo atom and the magnesium atom were inhibited by the presence of the strongly coordinative sulfinyl group initially introduced by the reaction, which might coordinate with the magnesium atom. This could also account for the low diastereoselectivity in the reaction of compound 6 shown next.
20 The relative configuration of 19b was determined by single-crystal X-ray diffraction analyses. For details, see the Supporting Information. CCDC 2280543, 2280545, 2280547, and 2280548 contain the supplementary crystallographic data for compounds anti-7d, anti-7f, anti-17b and 19b. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures17b
