Regioselective and regiospecific Di-π-methane rearrangements. Photoisomerization of 9-substituted 9,10-Dihydro-9,10-ethenoanthracenes

Abstract

The di-p-methane photoisomerization of a series of 9-substituted dimethyl 9,10-dihydro-9,10- ethenoanthracene-ll,12-dicarboxylates has been investigated. The bromo compound (13) rearranges regiospecifically to give 4b-bromo-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[cd] pentalene-8c,8d- dicarboxylate (19) whereas the amine (16) rearranges regiospecifically via the 8b-amino compound which further rearranges via an imine to give the keto diester (12). Rearrangement of the acetamido compound (17) gave the 4b-isomer (22) (60%), the keto diester (12) (25 %) and a cyclooctatetraene (28) (15%), while the methoxy and ethoxy compounds (14) and (15) gave the 4b-isomers (20) and (21) (85%) and the keto diester (11) derived from the 8b-isomer. Both the 4b- and 8b-isomers (23) and (27), (65 %) and (35 %), were isolated when the methyl ethenoanthracene (18) was photolysed. The rearrangement of these compounds is explained in terms of the electronegativity and hydrogen bonding ability of the 9-substituent. The more electronegative the substituent the more the 4b-isomer is favoured while hydrogen bonding favours the 8b-isomer. Photoisomerization of the monoesters (29), (31) and (32) confirms the importance of odd-electron stabilization by an adjacent methoxy- carbonyl group in determining the direction of the rearrangement.