Synthesis of fused dihydro-pyrimido[4,3-d]coumarins using Biginelli multicomponent reaction as key step

A new efficient approach for the synthesis of functionalized dihydro-pyrimido[4,3-d]coumarins is described. Use of benzyl- or tert-butyl acetoacetate and various salicyl aldehydes in typical Biginelli multicomponent reactions provides esters that can be easily deprotected to yield the corresponding hydroxy acids. Annelation of the latter leads to the target dihydro-pyrimido[4,3-d]coumarins. Formation of some unexpected oxo-bridged tetrahydro-pyrimidines is also described. Spectral properties of the synthesized dihydro-pyrimido[4,3-d]coumarins indicate the necessity to revisit some compounds previously reported as products of direct one-pot Biginelli condensation involving 4-hydroxycoumarin as dicarbonyl component.