TRANSITION-METAL-MEDIATED DIHYDROPYRAN SYNTHESES - EXO SELECTIVE HETERO-DIELS-ALDER REACTIONS OF COLBALOXIME DIENYL COMPLEXES WITH ALDEHYDES

Cobaloxime 1,3-dienyl complex, 2-pyridine imethylglyoximato)cobalt(III )-(3E)-1,3-pentadiene, participates in exo selective hetero Diels-Alde r reactions with a variety of alkyl and aryl aldehydes. The cobaloxime -substituted cycloadduct products of these reactions can be demetalate d to provide diastereomerically pure dihydropyrans with concomitant co balt recovery. One of the cobaloxime-substituted dihydropyrans has bee n characterized by X-ray crystallography to prove the exo selective na ture of the Diels-Alder cycloaddition.