J. Efskind et al., Ru(II)-catalysis in preparation of sterically congested and stereochemically homogeneous heterospiranes, J CHEM S P1, (12), 1999, pp. 1677-1684
Sterically highly crowded dienes may undergo RCM reactions to furnish heter
ocyclic bispiranes in high yields. The diene substrates were C-1-bridged bi
s[(2R)-5-alkenyl-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazines]. The carbo
nyl or hydroxymethyl bridges were prepared in stereoselective reactions. A
free hydroxy group in the present series was incompatible with the RCM reac
tion. The ring closure proceeded readily after O-methylation. The course of
the stereochemical transformations has been verified by X-ray analyses.