CRYSTALLOGRAPHIC DETERMINATION OF THE PREFERRED GEOMETRY FOR INTRAMOLECULAR HYDROGEN-ATOM ABSTRACTION IN 1,8-CYCLOHEXADECANEDIONE

The two carbonyl groups in 1, 8-cyclohexadecanedione (la) are separated by eight methylene groups on one side and six on the other, thus giving rise to two nonequivalent γ-hydrogen atoms for abstraction via the Norrish type II reaction. X-ray crystallography reveals that, in the solid state, the γ-hydrogen atom on the longer methylene chain is more favorably situated for abstraction than that on the shorter chain, both in terms of abstraction distance (2.72 Å vs 2.99 Å) and angular relationship. In accord with this analysis, photolysis of diketone la in the solid state leads exclusively to a single cyclobutanol diastereomer resulting from abstraction of the stereoelectronically favored hydrogen atom. In contrast, irradiation of compound 1a in solution affords a complex mixture of all six possible type II cyclization and cleavage products resulting from nonselective abstraction of both γ-hydrogens. The solid state results are interpreted in terms of the crystallographically determined conformation of diketone la, which favors stereoselective cyclization over cleavage. © 1992, Taylor & Francis Group, LLC. All rights reserved.