Ru(II)-catalysis in preparation of sterically congested and stereochemically homogeneous heterospiranes

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.