title: Dipolar Cycloaddition Reactions of Dihydropyrimidine-Fused Mesomeric Betaines. An Approach toward Conformationally Restricted Dihydropyrimidine Derivatives(1).
abstract: The bimolecular and intramolecular cycloaddition potential of various 4-aryldihydropyrimidine-fused mesomeric betaines was investigated. Dihydropyrimidine-fused isothiomünchnones and isomünchnones were found to undergo 1,3-dipolar cycloaddition reactions with electron-deficient dipolarophiles such as DMAD, methyl propiolate, methyl vinyl ketone, or N-methylmaleimide. In contrast, cross-conjugated mesomeric thiazinium betaines underwent 1,4-dipolar cycloaddition reaction with electron-rich dipolarophiles such as ynamines or ketene acetals. In general, these cycloadditions show a high degree of regioselectivity, facial selectivity, and exo/endo diastereoselectivity. Intramolecular variations of the above processes involving o-alkenylaryl-tethered dihydropyrimidine-fused isomünchnones lead to polycyclic dihydropyrimidine analogs that closely mimic the proposed receptor-bound conformation of dihydropyridine calcium channel modulators. These cycloadducts are the result of an endo-addition of the pi-bond to the carbonyl ylide dipole embedded in the isomünchnone system. The relative stereochemistry of these cycloadducts was established by single-crystal X-ray analysis.
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70506 Stuttgart, Germany.
authors.forename: C. Oliver
authors.initials: CO
authors.lastname: Kappe
authors.forename: Karl
authors.initials: K
authors.lastname: Peters
authors.forename: Eva-Maria
authors.initials: EM
authors.lastname: Peters