A new phototransformation of methoxycarbonyl-substituted (E,Z,E)-1,3,5-hexatrienes: easy access to ring-annelated 8-oxabicyclo[3.2.1]octa-2,6-diene derivatives.

On attempting photochemically induced electrocyclizations of the previously reported 1,6-bismethoxycarbonyl- or 1,6-bistrimethylsilyl-substituted ring-attached (E,Z,E)-1,3,5-hexatrienes 4 b, 4 c and 5 b, 5 c, equilibrium mixtures of the starting materials and their diastereomers, the corresponding (E,Z,Z)-hexatrienes 4 b, 4 c and 5 b, 5 c were obtained. The desired trans-disubstituted ring-annelated cyclohexadienes 9 b and 10 b were formed by subsequent thermal 6pi-electrocyclization of the (E,Z,Z)-hexatrienes 4 b and 5 b in good yields (77-83 %). Upon treating the bissilyl-substituted hexatrienes (E,Z,E)-5 c or (E,Z,Z)-5 c under the same conditions, 6pi-electrocyclizations did occur, but the primary products immediately isomerized to a large extent, and mixtures of the cyclohexane-annelated cyclohexadienes 10 c-12 c along with the dehydrogenation products 13, 14 c were formed. When the bismethoxycarbonyl-substituted hexatriene (E,Z,E)- 5 b was irradiated for an extended period of time (4.5 h), the gradual formation of the oxabicyclo[3.2.1]octa-2,6-diene 17 b by a formal intramolecular hetero-Diels-Alder reaction was observed and 17 b could be isolated in up to 69 % yield. To explore the scope of this new photochemical reaction, the new ring-attached (E,Z,E)-hexatrienes 4 a, 5 a and 6 b were synthesized by twofold Heck reactions from 1,2-dibromocycloalkenes 1-3 (59-66 %). While irridiation of the cyclopentene-attached 1,3,5-hexatrienes only led to decomposition, the cyclohexene- and cycloheptene-attached hexatrienes gave the hetero-Diels-Alder products or other photoproducts depending on the size of the cycloalkene moiety and the nature of the alkoxycarbonyl substituents at the vinyl termini. The photoreaction products 17 b and 18 b which are bicyclic acetals, underwent cleavage upon treatment with a Lewis acid such as Me(3)SiOTf to give the ring-annelated methoxycarbonyl-substituted troponecarboxylates 21 b and 22 b.