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 adja cent to the spirocyclic carbons have been reacted principally with N-p henylmaleimide 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 follow ing chromatographic separation. The stereochemical assignments follow principally from NOE measurements and X-ray crystallographic determina tions. A relative reactivity ordering was established for the triazoli nedione cycloadditions since this dienophile reacted with all of the a vailable dienes. The stereoselectivities observed for these reactions suggested the intermediacy of aziridinium imide intermediates, and cal culations of the AM1 type were carried out in order to examine the ste ric, electronic, and electrostatic properties of the reactants. Our an alysis shows that electrostatic effects dominate in the syn dioxa syst em, whereas steric factors must be accorded proper consideration when accounting for the pi-facial selectivity exhibited by the oxa/thia and dithia compounds.