CYCLOADDITION REACTIONS OF PYRIDINIUM AND RELATED AZOMETHINE YLIDES

The Rh(H)-catalyzed reaction of alpha-diazoacetophenone in the presence of 2-(methylthio)pyridine and dimethyl acetylenedicarboxylate gave 3-benzoyl-1,2-dicarbomethoxy-3,5-dihydro-5-(methylthio)-indolizine. The formation of the cycloadduct proceeds via a pyridinium ylide formed by attack of the nitrogen lone pair on the electrophilic keto carbenoid. A related cyclization occurred using 1-diazo-3-[(2-pyridyl)thio]-2-propanone. The Rh(II)-catalyzed reaction of 1-(3'-diazoacetonyl)-2-pyridone with DMAD was also found to give cycloadducts derived from an azomethine ylide. The initial reaction involves generation of the expected carbonyl ylide dipole by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the amide group. A subsequent proton exchange generates the thermodynamically more stable azomethine ylide which is trapped by DMAD. Azomethine ylide cycloadducts derived from keto carbenoid cyclization onto a thiobenzoxazole are also formed. When 1-diazo-3-[(2-benzoxazolyl)thio]-2-propanone was used, the initially formed cycloadduct underwent a subsequent 1,3-sigmatropic thio shift. An analogous cyclization occurred using 2-(4-diazo-3-oxobutyl)benzoxazole. In addition to undergoing dipolar cycloaddition, the highly stabilized dipole formed from this benzoxazole loses a proton to produce a cyclic ketene N,O-acetal. This compound reacts further with DMAD to eventually produce a novel phenolic lactam whose structure was established by an X-ray crystal structure analysis.