PHOTOADDITIONS AND DIALKYLCUPRATE ADDITIONS TO 2-TERT-BUTYL-2,6-DIMETHYL-1,3-DIOXIN-4-ONE AND RELATED HETEROCYCLES - EXPERIMENTAL, AB-INITIO THEORETICAL, AND X-RAY STRUCTURAL STUDIES OF FACIAL SELECTIVITY AND ENONE PYRAMIDALIZATION

Preparation and ground-state reactions of 1,3-dioxinone(1), alpha,beta -unsaturated delta-lactone 4, and dihydropyranone 5 are reported. Thes e three substrates have identical alkyl substituents but differ in the number and placement of oxygen atoms in the heterocycle. Reaction of 1 or 4 with (n-Bu)(2)CuLi leads to exclusive addition on the top face (side opposite the tert-butyl group) of the substrate while addition t o 5 gives a 50:50 mixture of diastereomers. Pyramidalizations of the e none portions of 1, 4, and 5 (along with 2-cyclohexenone) have been pr edicted using ab initio Hartree-Fock (HF) methods with a split-valence plus polarization basis set, 6-31G, and with the inclusion of electr on correlation by Moller-Plesset perturbation theory (MP2). These pred ictions have been compared with the results of X-ray crystal structure determinations on related heterocycles which were present in the Camb ridge Crystallographic Database. Both theoretical methods indicate tha t the extent of pyramidalization decreases in the following order: 1,3 -dioxinones > alpha,beta-unsaturated delta-lactones > dihydropyranones > 2-cyclohexenones. This trend suggests that the facial selectivity o bserved in the ground-state reactions of 1, 4, and 5 is related to the extent of pyramidalization in the enone portion of these substrates. Photoaddition reactions of 1 with various cycloalkenes are also report ed. The exclusive or major product in these reactions results from att ack on the bottom face of 1. Thus, substrate 1 allows exclusive entry of ground-state reactants on the top face, but excited-state reactions occur exclusively or primarily on the bottom face.