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] cycloaddition 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 1 d , 1 i , and 1 k 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 ( 1 k ) 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 13 C NMR spectroscopy.