Preparation and Acetolysis of 7-Norbornadienylmethyl and (7-Methyl-7-norbornadienyl)methyl Brosylates. An Intramolecular Retro-Diels-Alder Reaction following Laticyclic Participation

Article abstract of DOI:10.1021/jo00149a020

The title compounds, 19-and 20-OBs, respectively, have been prepared and their acetolysis rates and products determined.At 99.2 deg the acetolysis of 19-OBs (k_t = 3.63 * 1E-6 s^-1) produces 40 percent unrearranged acetate, 19-OAc, by direct displacement and 60 percent tetracyclo<3.3.0.0^3,8.0^4,6>oct-7-yl acetate.Under identical conditions 20-OBs is 97 times as reactive (k_t = 3.53 * 1E-4 s^-1) and yields 56 percent of approximately equal amounts of syn- and anti-1-methyltetracyclo<3.3.0.0^3,8.0^4,6>oct-7-yl acetates and total of 44 percent of a mixture of endo-1-methyltricyclo<3.3.0.0^2,7>oct-3-en-6-yl acetate (28-OAc) and its retro-Diels-Alder product, 3-(1-methyl-2,4-cyclopentadien-1-yl)-trans-1-propenyl acetate (29-OAc).Comparison of these results with those of other norbornenyl and norbornadienyl derivatives suggests that unsymmetrical (2⁰+2⁰+1⁺) laticyclic stabilization is enhanced relative to (2⁰+1⁺) pericyclic stabilization in the acetolysis of 19- and 20-OBs.Such stabilization is appreciably greater in 20-OBs than in 19-OBs.The intramolecular retro-Diels-Alder reaction that converts 28- to 29-OAc under acetolytic conditions is discussed, and its comparative rarity is emphasized by citing other norbornadienyl derivatives known to behave in this manner.