Abstract
Two alternative directions for thermal transformation of 6โaminoโ4โoxopyrano[3,4โd] [1,2,3]thiadiazoles 1, leading either to 6โhydroxyโ4โoxoโ[1,2,3]thiadiazolo[4,5โc]pyridines 2 or 2โcyanoโ2โ(1,2,3โthiadiazolโ5โyl)acetamide 4b, were observed. The first one represents a new, Dimrothโtype rearrangement and proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate 7 (sโcis) to 7 (sโtrans) and its recyclization onto the amido group to form the pyridinโ2โone cycle.The first step of the rearrangement has a calculated [B3LYP/6โ31G(d)] activation barrier of 24โ34 kcal/mol, the involvement of amines reduces it by ca. 5 kcal/mol [PCMโB3LYP/6โ31G(d), DMSO]. In contrast, the recyclization step onto the amido group is calculated to occur essentially barrierless (ฮ__E__ < 2 kcal/mol). The influence of substituents, in particular of those capable of intramolecular hydrogen bonding, on the preferred reaction path was also studied. The alternative ring opening of the pyrane cycle was calculated to be at least 5 kcal/mol [B3LYP/6โ31G(d)] less favorable than the Dimroth rearrangement. (ยฉ WileyโVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)