Stereochemistry of Seven-Membered Heterocycles: XLII. A Theoretical Study of Stereochemistry of H Complexes Formed by Conformationally Nonuniform 2-R-1,3-Dioxacyclohept-5-enes with Some Proton Donors

Abstract

According to semiempirical AM1 calculations, the stability of the boat and twist forms of 2-R-1,3- dioxacyclohept-5-enes depends on the size of substituents at the acetal carbon atom. The twist form gives diastereomeric H complexes with chloroform and methanol of the cis and trans structure, containing monocentered hydrogen bonds, whereas the chair conformation preferably forms complexes with a two-centered hydrogen bond. Based on theoretical data (θO H, ΔH, geometry of complexes), the specific features of H complexes of the conformers in electrophilic addition and cycloaddition were revealed. Considerable preferableness of the exo attack of the diastereotopic double bond in the H complex having the chair form is due to the steric accessibility of the exo side, whereas in the complexes of the twist form the facial selectivity is appreciably lower.