STEREOCHEMICAL CONSEQUENCES OF 6-SUBSTITUTION AND 8-SUBSTITUTION IN REACTIONS OF BICYCLO[4.2.0]OCTAN-7-ONES

Reduction (zinc/copper couple) of each epimer of 8-chloro-8-methylbicy clo[4.2.0]oct-2-en-7-one (6 and 7) gives the same product mixture cont aining 95% of the 8-endo-methyl compound 8. Ketalization of 8 catalyze d by TMSOTf gives the endo-methyl cyclic ethylene ketal 10 at -78-degr ees-C and the exo-methyl epimer 11 (90:10) at 25-degrees-C, in contras t to beta-naphthalenesulfonic acid-catalyzed reactions. Wittig olefina tion of both the functionalized 8-endo-and 8-exo-methyl ketones gives only the 8-endo-methyl olefin, whereas introduction of a 6-methoxy or 6-methyl substituent favors the 8-exo configuration. A rationale for t hese observations is proposed.