Preparation of polycyclic systems by sequential 5-exo-digonal radical cyclization, 1,5-hydrogen transfer from silicon, and 5-endo-trigonal cyclization.

Abstract

Radicals of type 1 undergo 5-exo diagonal cyclization, and the resulting vinyl radical abstracts hydrogen from silicon to afford a silicon-centered radical. This radical closes in a 5-endo trigonal manner to generate radicals of type 4, which are reduced (4 --> 5) by stannane, except when the starting acetylene carries a terminal trimethylstannyl group. In this case, radicals 4 expel trimethylstannyl radical to afford vinyl silanes 6. The stereochemical outcome of the radical cascade 1 --> 5 is controlled by the stereochemistry of the oxygen-bearing carbon in 1 (see starred atom). The sequence can be initiated by carbon-, alpha-substituted carbon-, oxyacyl-, and carbamoyl radicals and generates a silicon-containing ring fused onto a carbocycle or heterocycle. Numerous examples are described, as well as a number of transformations of the final cyclization products, especially their response to n-Bu(4)NF and to BF(3).OEt(2), reagents that cleave the newly formed carbon-silicon bond.