Stereochemistry of seven-membered heterocycles: XL. Diastereoselectivity of reactions of 3,6-dimethoxycarbonyltetrazine with 2-R-1,3-dioxacyclohept-5-enes

Reactions of conformationally nonuniform 2-R-1,3-dioxacyclohept-5-enes (R = Ph, Me, t-Bu), existing in solution in chair and twist conformations, with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate yield mixtures of diastereome ric 4-R-8,11-dimethoxycarbonyl-3,5-dioxa-9,10-diazabicyclo [5.4.0]dodeca-7, 10-dienes. The reaction diastereoselectivity is significantly influenced by the medium. The maximal selectivity for the 4-tert-butyl derivative is att ained in proton-donor chloroform, and the minimal selectivity, in acetone. Single crystal X-ray diffraction study of 4-methyl-8,11-dimethoxycarbonyl-3 ,5-dioxa-9,10-diazabicyclo-[5.4.0]dodeca-7,10-diene showed that the major i somers have the exo configuration, and the seven-membered acetal fragment o ccurs in the chair conformation with equatorial location of the substituent .