[2-CYCLOADDITION AND [2+2]-CYCLOADDITION REACTION OF 1-SELENO-2-SILYLETHENES TO METHYLENEMALONATE ESTERS - A NOVEL RING CONTRACTION OF CYCLOBUTANE TO CYCLOPROPANE(1])

Reaction of (E)-1-(phenylseleno)-2-silylethenes 1a-c and methylenemalo nate esters 2a,b with Lewis acids (TiCl4-1/2Ti((OPr)-Pr-i)(4), BF3 . E t(2)O, SnCl2 and ZnBr2) gave [2 + 2] cycloadducts in addition to [2 1] cycloadducts. This is a novel mode of reactivity for 1-seleno-2-sil ylethenes. [2 + 1] and [2 + 2] cycloaddition can be controlled by Lewi s acids, the substituents on silicon, temperature, and time. A novel r earrangement of cyclobutane to cyclopropane in the presence of ZnBr2 u nder reversible conditions was also discovered. The stereochemistry of the cyclopropane products was elucidated by NMR.