l-Menthyl (R)-2-phenyl-4-oxo-1,3-dioxine-2-carboxylate: Diastereofacial selectivity for conjugate addition and its explanation

Article abstract of DOI:10.1016/S0040-4020(01)96260-X

l-Menthyl (R)- and (S)-2-phenyl-4-oxo-1,3-dioxine-2-carboxylates are synthesized from l-menthyl phenylglyoxylate by reacting with formylketene followed by fractional crystallization. Conjugate addition to the (R)-2-phenyl-4-oxo-1,3-dioxine-2-carboxylate by RMgBr/CuI has revealed that the reagent attacks from the phenyl side which takes the quasi-equatorial orientation in the sofa comformation of the dioxinone ring with complete diastereoselection. This finding is contrast to the reverse diastereoselectivity in the same reaction of 2-tert-butyl-6-methyl-1,3-dioxin-4-one and its 2-methyl derivative. A possible reason to account for this difference is proposed.