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