Studies on the synthesis of novel carbohydrates with sulphur in the ring. Part II. Analogues of derivatives of unsaturated deoxy-ulopyranosidonic acids via Diels–Alder reactions with methyl cyanodithioformate
3,6-Dihydro-2-methylthio-2H-thiopyran-2-carbonitrile (5) and its 3-methoxy-derivative (11) were obtained directly from the Diels–Alder reactions of methyl cyanodithioformate (4) with buta-1,3-diene and trans-1-methoxy-buta-1,3-diene, respectively. The regioselectivity of the reagent (4) is opposite to that of carbonyl dienophiles with 1-alkoxybuta-1,3-dienes. The structure of compound (11) was established by 1H n.m.r. studies involving compounds (5) and (11) and their selectively oxidized and/or reduced derivatives. The results indicated that (11) exists predominantly in the 0H2 conformation in chloroform.