SKELETAL REARRANGEMENT OF 8-METHYLENEBICYCLO[4.2.0]OCTAN-2-ONES WITH MERCURY(II) PERCHLORATE

Skeletal rearrangement of C6-substituted 8-methylenebicyclo[4.2.0]octa n-2-ones 1a-d and 16 with mercury(II) perchlorate was examined, and th e remarkable substituent effect on the rearrangement was elucidated. R eactions of the ketones la and Ib having methyl and ethyl groups with Hg(ClO4)(2) . 3H(2)O gave the bicyclo[2.2.2]octanones 2a,b and 3a,b al ong with a small amount of the bicyclo[3.3.1]nonanones 4a,b, respectiv ely. The yield of the bicyclo[3.3.1] ketone 4c increased in the reacti on of the isopropyl derivative Ic. The tert-butyl derivative Id afford ed the bicyclo[3.3.1]nonanone 4d as a major product together with a mi xture of the bicyclo[2.2.2]octanones 2d and 3d. The remarkable substit uent effect is explained based on the stability of the conformers 7 an d 10 of the postulated intermediate 6. The intermediacy of 6 was verif ied by reaction of 6b prepared independently under similar conditions. Similar treatment of the tricyclic ketone 16 yielded the bicyclo[3.3. 1]nonanone 19 as a major product along with a small amount of the bicy clo[2.2.2] ketones 17 and 18. The structures of products were determin ed by NMR spectral data including 2D C-13-INADEQUATE spectrum of 19 an d the X-ray crystallographic analysis of 4d. (+/-)-Desdimethyldihydroc lovene (22) was easily synthesized from 19 via radical deoxygenation f ollowed by transformation of the tertiary hydroxyl group to the methyl group.