Ethyl and methyl thioxoacetates, dienophilic thioaldehydes formed from sulphenyl chlorides by 1,2-elimination

Abstract

Ethoxycarbonyl- and methoxycarbonyl-methanesulphenyl chlorides react at room temperature with triethylamine to form ethyl and methyl thioxoacetates, respectively. Generation of the transient thioladehyde, ethyl thioxoacetate (2a), in the presence of the conjugated dienes 2,3-dimethylbuta-1,3-diene, thebaine (7), cyclohexa-1,3-diene, anthracene, and 9,10-dimethylanthracene gave adducts formed by cycloaddition of the thial group to the diene systems. The cycloadduct (8), derived from thebaine (7) at room temperature, isomerised at 111 °C to give the thermodynamically more stable adduct (9) by dissociation and recombination. The cycloadducts of anthracene and 9,10-dimethyl-anthracene likewise dissociated at 111 °C thereby providing a convenient, ‘clean’ source of ethyl thioxoacetate. When the anthracene adduct was heated with (–)-β-pinene, two ‘ene’ products were obtained, the major product (17; R = CO₂Et) arising by C–S and the minor product (18; R = CO₂Et) by C–C bond formation.