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4 - Synthetic Diels–Alder routes to rings B and C

Published online by Cambridge University Press:  07 September 2010

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Summary

Additions to 1,4-naphthoquinones

1-Vinylcyclohexene, 195, has been reacted with 1,4-naphtho-quinone, 196, to give 1,2,3,4,6,6a,12a,12b-octahydroBAQ, 197, in high yield (Backer and Bij, 1943; Azerbaev, 1945), Scheme XXIV. On air oxidation 197 was converted into 1,2,3,4-tetrahydroBAQ, 198 (Fieser and Hershberg, 1937b), which was reduced to 1,2,3,4-tetrahydroBA. With the exception of two other examples (Inbasekaran et al., 1980; Carothers and Coffman, 1932), no further work with the use of vinylcyclohexenes in BA synthesis has been reported.

Rather, the addition of styrenes, 199, to 196, has been studied and well developed. When the reactions were carried out in refluxing toluene containing chloranil and trichloroacetic acid, the following nine BAQs, 200, were obtained:4-Br-, 1-, 2-, 3-, 4-Cl-, 4-F-, 4-methoxy-, 2-methyl-, and 1,4-dimethyl- (Manning et al., 1977). Subsequently, 3,4-dimethoxy- BAQ, 201, was synthesized (Manning and Wilbur, 1980) by a similar route. When 5-hydroxy-1,4-naphthoquinone, 202, and 5-methoxy-1,4- naphthoquinone, 203, were used, mixtures of 8-hydroxy- and 11-hydroxyBAQs, 204 and 206, and 8-methoxy- and 11-methoxyBAQs, 205 and 207, resulted (Manning, 1979). By the use of 6-hydroxy- and 6-methoxy-1,4-naphthoquinones, 208 and 209, there were obtained mixtures of 9-hydroxy- and 10-hydroxyBAQs, 210 and 212, and 9-methoxy- and 10-methoxyBAQs, 211 and 213, respectively (Manning, 1979; Manning et al., 1979; Manning, 1981). The conversion of many of the BAQs into BAs and 7,12-DMBAs by conventional methods has been described (Muschik et al., 1979). However, difficulties were encountered in the synthesis of BAs bearing the 1-methoxy group because of steric factors.

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Publisher: Cambridge University Press
Print publication year: 1988

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