STEREOCONTROL IN INTERMOLECULAR DIRHODIUM(II)-CATALYZED CARBONYL YLIDE FORMATION AND REACTIONS - DIOXOLANES AND DIHYDROFURANS

Ethyl diazoacetate undergoes dirhodium(II)-catalyzed reactions with ar yl aldehydes to form 1,3-dioxolanes as mixtures of diastereoisomers in good yields. Carbonyl ylides are reaction intermediates. Catalyst dep endent diastereocontrol is observed for reactions with p-nitrobenzalde hyde, but not for those with p-anisaldehyde or benzaldehyde, so that a t least with transformations involving p-nitrobenzaldehyde a metal-sta bilized ylide is responsible for product formation, Higher yields are obtained with catalysis by dirhodium(II) carboxamidates than with the carboxylates. Diastereoselectivity in 1,3-dioxolane formation that occ urs through the ''free'' ylide can be effectively controlled so that o nly one diastereomer is produced through the use of 2,6-di-tert-butyl- 4-methylphenyl diazoacetate (BDA) or dicyclohexylmethyl diazoacetate ( DCM). The thermodynamically least stable all-cia trisubstituted 1,3-di oxolane is the primary product from p-nitrobenzaldehyde ''cycloadditio n'' to the metal-stabilized ylide. Reactions that take place in the pr esence of p-anisaldehyde and dimethyl acetylenedicarboxylate (DMAD) re sult in the formation of one 2,5-dihydrofuran-2-carboxylate stereoisom er in good yield. In contrast, with p-nitrobenzaldehyde and DMAD both dihydrofuran stereoisomers are produced along with, mainly, the dioxol ane derived from the metal-stabilized ylide; there is in this case com petition between addition reactions of the ''free'' ylide and the meta l-associated ylide.