The transient dienophile diethyl thioxomalonate and its S-oxide (sulphine) formed by retro-Diels–Alder cleavage reactions

Abstract

Diels–Alder adducts 3 and 4 of diethyl thioxomalonate 1, formed from diethyl oxomalonate (mesoxalate) and phosphorus pentasulphide, with cyclopentadiene and anthracene, dissociate at 111 °C to release the reactive thioketone 1. This was trapped in situ with 2,3-dimethylbuta-1,3-diene to give the corresponding cycloadduct 2. Similarly, the thioketone 1 reacted with 2-trimethylsiloxybuta-1,3-diene 8 to form only the 5-trimethylsiloxythiine 13, and with β-pinene 16 to form only one ‘ene’ product 17b, with C–S bond formation. The S-oxide 19 of the anthracene adduct 4 and the exo-S-oxide 27 of the cyclopentadiene adduct 3 likewise dissociated thermally to release the corresponding sulphine, diethyl thioxomalonate S-oxide 21, which was trapped in situ as the dimethylbutadiene adduct 23. The exo-sulphoxide 27, when heated alone, gave the sultene 29, presumably via[2,3]-sigmatropic rearrangement of the transient endo-sulphoxide 28. The sulphine 21, generated by flash vacuum pyrolysis of the anthracene adduct 19, reacted at low temperature with cyclopentadiene to give the exo-sulphoxide 27, and with water to give diethyl malonate.