Spirocyclobutanes via Metal Carbene Facilitated Diastereoselective [2+2]Cycloaddition Reactions of exo-Methylene Oxacyclopentylidene Complexes of Chromium and Tungsten

Upon reaction with enol ethers α-exo-methylene-2-oxacyclopentylidene complexes of chromium and tungsten 3 and 4 undergo [2+2]cycloaddition under mild thermal conditions to give spirocyclobutanes 5–12 in good yields as single diastereomers. The relative stereochemistry (as established by X-ray analysis for 9 and 10 to be 5R∗,2′S∗) results from a combined re-si approach of both π-systems. The propensity for [2+2]cycloaddition depends on both the substitution pattern and the ring size of the enol ether; 1,1-disubstituted enol ethers and enamines undergo a formal conjugate addition resulting in side-chain elongation.