HIGHLY STEREOSELECTIVE SYNTHESIS OF BICYCLO[N.3.0]ALKANES BY TITANIUMTETRACHLORIDE PROMOTED [3-CYCLOADDITION OF ALLYLSILANES AND 1-ACETYLCYCLOALKENES(2])

The titanium tetrachloride promoted reaction of allylsilanes 1 with 1- acetylcyclohexene is shown to afford the silylbicyclo[4.3.0]nonanes 9 ([3+2] cyclo-addition products) along with the 1-acetyl-2-allylcyclohe xane 4 (Hosomi-Sakurai product). Here we report that systematic variat ion of the substituents at the silicon atom of 1 allows suppression of the classical Hosomi-Sakurai reaction in favor of the [3+2] cycloaddi tion. Cycloaddition of the allylsilanes 1d, 1i, and 1k with 1-acetylcy cloalkenes 10, containing a 5-, 6-, 7-, 8-, or 12-membered ring, gives rise to the corresponding silylbicyclo[n.3.0]alkanes 11-13. The cyclo addition of allyltriisopropylsilane (1k) and 1-acetyl-2-methylcycloalk enes 15 provides silylbicyclo[n.3.0]alkanes 16 with two contiguous qua ternary carbon centers. The stereochemistry of the silylbicyclo[n.3.0] alkanes 11 a-c and 14 is unambiguously determined by X-ray analysis an d C-13 NMR spectroscopy.