Cobaloxime 1,3-dienyl complex, 2-pyridine bis(dimethylglyoximato)cobalt(III)-(3E)-1,3-pentadiene, participates in exo selective hetero Diels-Alder reactions with a variety of alkyl and aryl aldehydes. The cobaloxime-substituted cycloadduct products of these reactions can be demetalated to provide diastereomerically pure dihydropyrans with concomitant cobalt recovery. One of the cobaloxime-substituted dihydropyrans has been characterized by X-ray crystallography to prove the exo selective nature of the Diels-Alder cycloaddition.