Synthesis and lithiation of γ,γ-difunctionalised ketene dithioacetals. Access to a new synthetic equivalent of a β-hydroxy-β-lithioacrylate. X-Ray molecular structure of 2-(1,3-dithian-2-ylidenemethyl)-1,3-dithiane

Abstract

A series of γ,γ-dithioalkyl or dithioaryl ketene dithioacetals (3a), (3b), and (3c) has been prepared. Attempts to generate allylic anions under a variety of conditions from 1,1,3,3-tetrakis(phenylthio)propene (3a) and 1,1,3,3-tetrakis(methylthio)propene (3b) failed but 2-(1,3-dithian-2-ylidenemethyl)-1,3-dithiane (3c) is readily deprotonated with lithium di-isopropylamide to give anion (4c). This species can function as an equivalent of β-hydroxy-β-lithioacrylate and this equivalence has been illustrated by a synthesis of (±)-dihydrokawain (14). The use of compound (3c) as a reagent in synthesis is, however, limited in some cases by the nature of the conditions required for dithioacetal–ketone conversion. The X-ray crystal structure of compound (3c) has been determined.