Reactions of complex ligands, part 90 - Spirocyclobutanes via metal carbene facilitated diastereoselective [2+2]cycloaddition reactions of exo-methylene oxacyclopentylidene complexes of chromium and tungsten

Upon reaction with enol ethers alpha-exo-methylene-2-oxacyclopentylidene co mplexes 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 las established by X-ray analysis for 9 and 10 to be 5R*,2(l)S*) results from a combined re-si appr oach of both pi-systems. The propensity for [2 + 2]cycloaddition depends on both the substitution pattern and the ring size of the enol ether; 1,1-dis ubstituted enol ethers and enamines undergo a formal conjugate addition res ulting in side-chain elongation. (C) 2000 Elsevier Science Ltd. All rights reserved.