HETEROATOMIC INFLUENCES ON THE PI-FACIAL SELECTIVITY OF DIELS-ALDER CYCLOADDITIONS TO DISPIRO[4.0.4.4]TETRADECA-11,13-DIENES

Derivatives of the title structure bearing one or two heteroatoms adjacent to the spirocyclic carbons have been reacted principally with N-phenylmaleimide and N-methyltriazolinedione. The distributions of [4 + 2] cycloadducts have been quantified and, where mixtures have resulted, the stereoisomeric adducts have been isolated in a pure state following chromatographic separation. The stereochemical assignments follow principally from NOE measurements and X-ray crystallographic determinations. A relative reactivity ordering was established for the triazolinedione cycloadditions since this dienophile reacted with all of the available dienes. The stereoselectivities observed for these reactions suggested the intermediacy of aziridinium imide intermediates, and calculations of the AM1 type were carried out in order to examine the steric, electronic, and electrostatic properties of the reactants. Our analysis shows that electrostatic effects dominate in the syn dioxa system, whereas steric factors must be accorded proper consideration when accounting for the pi-facial selectivity exhibited by the oxa/thia and dithia compounds.