Benzyl mercaptan, reduction

In a study of the reducing action of various thiols at 0.1 M concentration in aqueous solutions of alcohols (Maclaren, 1962), benzyl mercaptan was found to be the most effective. Maximum reduction exceeding 90 % was obtained in 20 % propanol solution, and under these conditions the wool remained intact. If 5 M Nal was incorporated in the solution approximately 65 % of the protein was extracted in 48 hr at 20°C under essentially neutral conditions (Maclaren, unpublished observations, 1962). This protein could be alkylated with iodoacetate and fractioned into low-sulfur and high-sulfur fractions using the methods of Gillespie. 

This has been achieved by (a) reduction with 4 M mercaptoethanol (Thompson and O Donnell, 1961,1962b), (b) using 0.1 M benzyl mercaptan in ethanol-water mixture (Maclaren, 1962), (c) electrolytic reduction at a constant cathode potential, using catalytic concentrations of thioglycolate maintained in the SH form continuously (Leach et al., unpublished observations, 1963), and (d) repeated reduction and alkylation of the SH groups of the wool (O Donnell, 1954). 

The addition of other sulfur nucleophiles was reported by Jacobsen to be catalyzed by the same Cr(salen) complex 1 initially reported for the ARO with TMSNj. Benzyl mercaptan afforded the ring-opened hydroxy sulfides in excellent yield and 59-70% ee [17]. The moderate levels of enantioselectivity were improved by use of the dithiol 5, which afforded mixtures of bishydroxy sulfides in which the ee of the chiral product 6 was substantially enriched (Scheme 5). The sulfide products could be easily elaborated into the free thiols by reductive de-benzylation, providing access to the jl-silyloxy thiol 8 in optically pure form. 

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