- [(Trimethylsilyl)methylene]cyclopropanedicarboxylates 8a-c are readily available from the cyclopropanation of the silylallenes 7a-c with dimethyl diazomalonate. The free-radical mediated methylenecyclopentane annulations of unactivated and electron-rich alkenes with 8a-c proceed in high yield. The product vinylsilans are set up for diverse secondary conversions, making annulations with 8a-c highly versatile in the synthesis of cyclopentanes. 1994, American Chemical Society. All rights reserved.