Highly stereoselective synthesis of bicyclo[n.3.0]alkanes by titanium tetrachloride promoted [3+2] cycloaddition of allylsilanes and 1-acetylcycloalkenes

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-allylcyclohexane 4 (Hosomi-Sakurai product). Here we report that systematic variation of the substituents at the silicon atom of 1 allows suppression of the classical Hosomi-Sakurai reaction in favor of the [3+2] cycloaddition. Cycloaddition of the allylsilanes 1d, 1i, and 1k with 1-acetylcycloalkenes 10, containing a 5-, 6-, 7-, 8-, or 12-membered ring, gives rise to the corresponding silylbicyclo[n.3.0]alkanes 11-13. The cycloaddition of allyltriisopropylsilane (1k) and 1-acetyl-2-methylcycloalkenes 15 provides silylbicyclo[n.3.0]alkanes 16 with two contiguous quaternary carbon centers. The stereochemistry of the silylbicyclo[n.3.0]alkanes 11 a-c and 14 is unambiguously determined by X-ray analysis and C-13 NMR spectroscopy.