A new efficient enantioselective synthesis of (+)-cis-2-methyl-4-propyl-1,3-oxathiane, a valuable ingredient for the aroma of passion fruit.

A new simple route to (+)-cis-2-methyl-4-propyl-1,3-oxathiane, the main component responsible of the passion fruit aroma is shown. Such a procedure consists in the sequence of the asymmetric conjugated addition of a thiol to trans-2-hexenal/cleavage of sulfide moiety, which allows to prepare, in only two steps, its immediate precursor, (R)-3-mercaptohexan-1-ol. Various procedures have been investigated for both the steps, using benzyl and tert-butyl mercaptan as thiols. The best results have been achieved in the addition of benzyl thiol to trans-2-hexenal mediated by a proline-derived organocatalyst, followed by debenzylation with sodium naphthalenide, affording (R)-3-mercaptohexan-1-ol with 84% e.e. and a yield much higher (32%) than that previously reported (8.5%).