Kopsia alkaloids

A further use of Barton esters has been described as a path to enol ether radicals. The reaction involves the photochemical decomposition at 355 nm of the derivative (85). As part of an approach to the synthesis of a series of Kopsia alkaloids, the reductive decarboxylation of the derivative (86) was carried out. This involved irradiation of the Barton ester (86a) in the presence of t-BuSH. This affords the product (86b). The photochemical decomposition of the Barton ester (87) provides a path to the silyl derivatives (88). The nature of the trapping agent X is dependent on the conditions under which the reaction is carried out. Thus a variety of derivatives can be obtained using alcohols to afford ethers, or using ethanesulfonyl azide to give azides. The Barton esters (89) undergo the usual photochemical decarboxylation to afford ethenoyloxy radicals. Cyclization within these, in the presence of tributylstannane yields the lactones (90). ... 

Scheme 4.5 Common intermediate for the synthesis of Kopsia alkaloids...

SCHEME 15.25 Collective natural product synthesis of six structurally diverse Strychnos, Aspidosperma, and Kopsia alkaloids. 

Magnus, R, Payne, A. H., and Hobson, L., Synthesis of the Kopsia alkaloids ( )-ll,12-demethox-ylahadinine B, ( )-kopsidasine and ( )-kopsidasine-N-oxide, Tetrahedron Lett., 41, 2077, 2000. 

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