Preparation of Polycyclic Systems by Sequential 5-E xo-D igonal Radical Cyclization, 1,5-Hydrogen Transfer from Silicon, and 5-E ndo-T rigonal Cyclization

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-, α-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-Bu4NF and to BF3·OEt2, reagents that cleave the newly formed carbon−silicon bond.