PHOTOTRANSFORMATIONS OF BRIDGEHEAD-DISUBSTITUTED DIBENZOBARRELENES - INTERESTING REARRANGEMENTS OF DIBENZOSEMIBULLVALENE INTERMEDIATES DERIVED FROM 9-(HYDROXYALKYL)-10-METHOXY-SUBSTITUTED DIBENZOBARRELENES

The photochemistry of -(hydroxymethyl)-10-methoxy-9,10-etheno-anthrace ne (7a) and (1-hydroxyethyl)-10-methoxy-9,10-ethono-anthracene (7b) ha s been studied through steady-state photolysis, product analysis, and laser flash photolysis. Irradiation of 7a in benzene, methanol, or ace tone gave 69-72% yields of a dibenzopentalene ketone 11a, arising thro ugh a dibenzosemibullvalene precursor. Irradiation of 7b, which exists in equilibrium with its cyclic form 7b', gave a mixture of the dibenz opentalene ketone 11b (41%) and the dibenzopentalenopyran derivative, 16b' (26%). The photochemistry of 11,12-dibenzoyl-9, 10-dihydro-9, 10- dimethoxy-9, 10-ethenoanthracene (17) has been reinvestigated. Irradia tion of 17 in benzene and methanol gives a 90% yield of an isomeric pe ntacyclic product 24, formed through the rearrangement of a dibenzosem ibullvalene precursor. Irradiation of 17 in aqueous methanol gives a m ixture of the dibenzopentalene ketone 22 (40%) and a pentacyclic metha nol adduct 21 (34%). The structures of 7b', 11a,b, 16b', 21, and 24 we re confirmed through X-ray crystallographic analysis. The 308 nm laser flash photolysis of 7a,b in benzene results in the formation of their triplets (phi T = 0.55-0.76). These triplets possess short lifetimes (0.45-0.75 ps) and are quenched by oxygen, 2,2,6,6-tetramethylpiperidi nyl-1-oxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HTE MPO), ferrocene, and beta-carotene at rates in the range (0.26-4.7) x 10(9) M(-1) s(-1).