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The Photochemistry of 7,8-Dimethylene1,3,5-~yclooctatrienes. The Synthesis of Bicyclo[6.2.0]deca-l,3,5,7-tetraenel J. A. Elix, M. V. Sargent, and F. Sondheimer2 Contribution f r o m the University Chemical Laboratory, Cambridge, England. Received August 11, 1969 Abstract: Ultraviolet irradiation of 7,8-dimethylene-l,3,5-cyclooctatriene(1) led to bicyclo[6.2.0]deca-l,3,5,7-tetraene (4). Irradiation of 7-methylene-8-bromomethylene-1,3,5-cyclooctatriene (2) also gave 4, as well as 5- (or 6-) methylene-6- (or -5) bromomethylenetricyclo[5.1 .0.04*8]oct-2-ene (10). Similar irradiation of 7-methylene-8chloromethylene-l,3,5-cyclooctatriene(3) yielded 5- (or 6-) methylene-6-(or -5-) chloromethylenetricyclo[5.1.0.0428]oct-2-ene (11) as well as 4- (or 5-) methylene-5- (or -4-) chloromethylene-cis-bicyclo[4.2.0]octa-2,7-diene (12). The structures 4, 11, and 12 were confirmed by catalytic hydrogenation. Diels-Alder reaction of 11 with dimethyl acetylenedicarboxylate led to dimethyl 2,3-benzotricyclo[5.1.0.04~8]octa-2,5-diene-10,1 1-dicarboxylate(19), a benz-
fused derivative of semibullvalene (20). Similar Diels-Alder reaction of 12 with dimethyl acetylenedicarboxylate vielded a ca. 1: 1 mixture of dimethyl 2.3-benzo-cis-bicyclo[4.2.0locta-2,4,7-triene-lO,ll-dicarboxylate (22) and dimethyl benzocyclooctatetraene-lO,li-dicarboxylate(23). Heating the mixture of 22 and 23 at 120" resulted in the complete conversion of 22 to 23.
T
he syntheses of 7,8-dimethylene-1,3,5-cyclooctatriene (l), the 9-bromo derivative (2), and the 9-chloro derivative (3) were reported in the previous paper.' The results obtained by irradiating these substances are now d e ~ c r i b e d . ~It was anticipated that the cyclobutene derivatives 4, 5, and 6, respectively, would
l,R=H 2, R- Br
3,R=C1
4,R=H 5,R=Br 6,R=C1
7
be obtained, since the photoinduced cyclization of conjugated dienes to cyclobutenes is a well-established rea ~ t i o n and , ~ the exocyclic double bonds in the starting materials had been found to be the most reactive in thermal cycloaddition reactions. Substances 4, 5, and 6, containing fused cyclooctatetraene and cyclobutene rings, are of interest per se, and are also potential precursors of the fully conjugated bicyclo[6.2.0]deca1,3,5,7,9-~entaene(7). The last-mentioned substance is an isomer of naphthalene and azulene, and might represent an aromatic system.jI6 I n practice, we have found that irradiation of 1 indeed leads to 4, the exocyclic double bonds being more photolabile than the endocyclic polyene system. On the other hand, introduction of the 9-chloro group deactivates the exocyclic diene grouping, and irradiation of 3 gave products de(1) Unsaturated Eight-Membered Ring Compounds. VIII. For Part VII, see J. A. Elix, M. V. Sargent, and F. Sondheimer, J . Am. Chem. SOC.,92,962 (1970). (2) Author to whom inquiries should be addressed at the Chemistry Department, University College, London W.C.1, England. (3) For preliminary communications, see J. A. Elix, M. V. Sargent, and F. Sondheimer, J . Am. Chem. SOC.,89,180,5081 (1967). (4) See R. Srinivasan, ibid., 84, 4141 (1962); I
