Synthesis and reactions of Biginelli-compounds. Part 14. A rhodium-inducedcyclization-cycloaddition sequence for the construction of conformationally rigid calcium channel modulators of the dihydropyrimidine type

Conformationally rigid polyheterocycles 14a,b which mimic the putative rece ptor-bound confirmation of dihydropyridine type calcium channel modulators are prepared in a six-step reaction sequence starting from urea, ethyl acet oacetate and 2-alkenylbenzaldehydes. The key step in the synthesis involves the regio- and diastereoselective intramolecular 1,3-dipolar cycloaddition reaction of a dihydropyrimidine-fused isomunchnone dipole. Deprotection of the CBZ-protected intermediates 13a,b leads to the desired target molecule s 14a,b. Prolonged exposure of these cyclic enamines in solution to the atm osphere results in oxidation to the corresponding alpha-hydroxy imines 16a, b. Catalytic hydrogenation of the related N-benzyl-protected polycycle 19 f urnishes the fully saturated hexahydropy pyrimidine derivative 20. The rela tive stereochemistry in 20 was established by an X-ray crystallographic ana lysis.