NOVEL CONCEPTS IN DIRECTED BIARYL SYNTHESIS .38. SYNTHESIS AND STRUCTURE OF A PROTECTED LACTOLATE-BRIDGED BIARYL WITH RELEVANCE TO THE ATROPISOMER-SELECTIVE RING-OPENING OF BIARYL LACTONES

The preparation and crystal structure of the O-methyl-protected analog 5 of a lactolate-type intermediate 3 in the atropisomer-selective rin g opening of configuratively unstable lactone-bridged biaryls 1 is des cribed. Thus, 5a could be prepared by oxidation of the cyclic ether 7a to give the substituted pyrylium salt 6a and subsequent reaction with methanolate. Attempts to similarly prepare the more strained higher h omologous 5, i.e. with a higher steric hindrance next to the biaryl ax is, failed. Structural information on 5a could be obtained through X-r ay crystallography, but no decoalescence was observed in the low-tempe rature NMR spectrum.