Cyclization of 1,5-dienes: an efficient synthesis of beta-Georgywood.

In the acid-promoted 1,5-diene cyclization of pseudo- to beta-Georgywood, the cyclization product is obtained with high selectivity in spite of an unfavorable substituent at the C(2)-position of the diene precursor. Preisomerization of the cyclohexene double bond, which occurs in the presence of Brønsted acids, is suppressed with >1 equiv of MXn-type Lewis acids, whereas RAlX2-type Lewis acids such as >2 equiv of MeAlCl2 have the additional benefit of steering the double bond of the cyclized product into the desired beta-position. Mechanistic studies revealed a crucial participation or nonparticipation of the carbonyl group in the cyclization reaction, depending on the acid family employed, and allowed finally the development of a cyclization reaction catalyzed by MeAlCl2 that can be generated in situ from precatalyst AlMe3.1.