l-Cysteine, a Versatile Source of Sulfenic Acids. Synthesis of Enantiopure Alliin Analogues

l-Cysteine is a stimulating starting product for the generation of transient sulfenic acids, such as
4, 6, 9, and 15, which add to suitable acceptors, allowing formation of sulfoxides showing a
biologically active residue. These sulfoxides are easily isolated in enantiomerically pure form. For
instance, N-(tert-butoxycarbonyl)-l-cysteine methyl ester (1a) furnished in few steps sulfenic acid
9a, which was readily converted into (R,SS)-(2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethylsulfinyl)ethene (22), the methyl ester of Boc-protected nor-alliin. Moreover, the addition of 9a
to 2-methyl-1-buten-3-yne has led to a sulfur epimeric and separable mixture of (R)-2-(2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethylsulfinyl)-3-methyl-buta-1,3-dienes 10a and 11a, still
possessing a “masked” sulfenic acid function, producible from their cysteine moieties once the dienes
have been converted into the desired derivatives.