Dienophilic thioaldehydes and dithioesters formed by base-catalysed cleavage of alkyl phthalimidosulfanylacetates

Treatment of ethyl and methyl phthalimidosulfanylacetate 1a and 1b with triethylamine at room temperature generates [eqn. (1)] the transient thioaldehydes, ethyl and methyl thioxoacetate 2, which have been trapped in situ with 2,3-dimethylbuta-1,3-diene to yield the Diels-Alder cycloadducts 8a and 8b in high yield. The cycloadducts 9, 10 and 11a and 12a of ethyl thioxoacetate 2a and anthracene, cyclohexa-1,3-diene and cyclopentadiene, respectively, were obtained similarly. The endo-11a and exo-12a cycloadducts of cyclopentadiene, formed in the ratio endo : exo = 7:3, dissociate reversibly when heated in toluene under reflux, to give the same, equilibrium mixture, endo:exo = 3:7, and thereby can serve as 'clean' auxiliary precursors of ethyl thioxoacetate 2a. Thus, when mixtures of the cycloadducts 11a and 12a were heated in turn with dimethylbutadiene and (E,E)-1,4-diphenylbuta-1,3-diene, the corresponding cycloadducts 8a and 13 of the dienes were obtained, Unexpectedly, treatment of the thioxoacetate precursor la with triethylamine(1 mol equiv.) and 4-dimethylaminopyridine (DMAP) (0.1 mol equiv,) in the presence of dimethylbutadiene gave the thioxoacetate adduct 8a and, as the major product, the cycloadduct 15a of the dithioester 16. In the presence of cyclopentadiene, relatively less of the corresponding dithioester adducts 17 was formed, but the amount increased when DMAP alone was used to effect elimination, The dienophilic dithioester 16, formed from the precursor ta and DMAP in the absence of any diene, was isolated and used to prepare the cycloadducts 15a, 17 and 19 in good yield. The cyclopentadiene adducts 17, like those of ethyl thioxoacetate, dissociated thermally and can serve as auxiliary precursors of the dithioester 16, e.g. in the preparation of the dimethylbutadiene adduct 15a.