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

Abstract

Sterically highly crowded dienes may undergo RCM reactions to furnish heterocyclic bispiranes in high yields. The diene substrates were C1-bridged bis[(2R)-5-alkenyl-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazines]. The carbonyl or hydroxymethyl bridges were prepared in stereoselective reactions. A free hydroxy group in the present series was incompatible with the RCM reaction. The ring closure proceeded readily after O-methylation. The course of the stereochemical transformations has been verified by X-ray analyses.