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
G. Bringmann et al., 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, Liebigs Annalen der Chemie, (4), 1994, pp. 439-444
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.