Dithiols. Part XXIII. Optically active forms of 2,3-dimercaptopropanol and related thiols

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Abstract
A new synthesis of 2,3-dimercaptopropanol, from 1,2-O-isopropylideneglycerol, by way of benzyl 2,3-epoxy-propyl ether, is described. Following a similar route, (R)-2,3-dimercaptopropanol has been synthesised from (S)-1,2-O-isopropylideneglycerol through (S)-benzyl 2,3-epoxypropyl ether. By other routes, (S)-1,2-O-iso-propylideneglycerol is also converted (i) through (R)-benzyl 2,3-epoxypropyl ether into (S)-2,3-dimercapto-propanol, (ii) through (R)-2,3-isopropylidenedioxypropyl thiolacetate into (R)-3-mercaptopropane-1,2-diol, and (iii) through (R)-benzyl 2,3-epithiopropyl ether into (S)-2-mercaptopropanol. An alternative synthesis of (R)-2,3-dimercaptopropanol failed to give an optically active product because of complete racemisation during the reduction, by lithium aluminium hydride, of (R)-2,3-carbonyldithiopropion-aldehyde. This unusual behaviour of the reagent is attributed to stabilisation of the enolate anion of the aldehyde by resonance involving the sulphur atom at C-2.

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