Synthetic and Theoretical Aspects of New Dimroth Rearrangement of6-Aminopyran-2-ones to 6-Hydroxypyridin-2-ones via Carbamoyl Ketenes

Two alternative directions for thermal transformation of 6-amino-4-oxopyrano[3,4-d] [1,2,3]thiadiazoles 1, leading either to 6-hydroxy-4-oxo-[1,2,3]thiadiazolo[4,5-c]pyridines 2 or 2-cyano-2-(1,2,3-thiadiazol-5-yl)acetamide 4b, were observed. The first one represents a new, Dimroth-type rearrangement and proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate 7 (s-cis) to 7 (s-trans) and its recyclization onto the amido group to form the pyridin-2-one cycle.The first step of the rearrangement has a calculated [B3LYP/6-31G(d)] activation barrier of 24–34 kcal/mol, the involvement of amines reduces it by ca. 5 kcal/mol [PCM-B3LYP/6-31G(d), DMSO]. In contrast, the recyclization step onto the amido group is calculated to occur essentially barrierless (ΔE < 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/6-31G(d)] less favorable than the Dimroth rearrangement. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)