Reactions between Triazolinediones and Equilibrating Forms of Cycloheptatriene Derivatives Featuring 7,7-Spiro and 1,7-Fused Heterocyclic Rings

Abstract

Nitrile oxide−azaheptafulvene adducts consist of rapidly equilibrating mixtures of fused (1) and spiro (2) isomers, the relative stabilities of which are nicely reproduced by B3LYP/6-31G* calculations. The reaction between these compounds and MTAD affords only two diastereomeric adducts [9 (dominant) and 10], both deriving from the reaction of MTAD with 1 even in cases in which that isomer could not be detected by NMR spectroscopy. These adducts are formal Diels−Alder adducts deriving from attacks on the two diastereotopic faces of the triene moiety of 1 and involving only the diene system adjacent to the amino substituent (N4−C4a=C5−C6=C7). The structures of the adducts are firmly supported by spectroscopic data and X-ray analysis, and so previous incorrect assignments are revised. The mechanism of the reaction between MTAD and 1 is briefly discussed.