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
作者:Michael G. Organ、Robert D. J. Froese、John D. Goddard、Nicholas J. Taylor、Gordon L. Lange
DOI:10.1021/ja00087a018
日期:1994.4
Preparation and ground-state reactions of 1,3-dioxinone(1), alpha,beta-unsaturated delta-lactone 4, and dihydropyranone 5 are reported. These 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 to 5 gives a 50:50 mixture of diastereomers. Pyramidalizations of the enone portions of 1, 4, and 5 (along with 2-cyclohexenone) have been predicted using ab initio Hartree-Fock (HF) methods with a split-valence plus polarization basis set, 6-31G*, and with the inclusion of electron correlation by Moller-Plesset perturbation theory (MP2). These predictions have been compared with the results of X-ray crystal structure determinations on related heterocycles which were present in the Cambridge Crystallographic Database. Both theoretical methods indicate that 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 observed 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 reported. The exclusive or major product in these reactions results from attack 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.