The synthesis of pyrano[2,3-b]quinoxalines related to molybdopterin

Abstract

Two cobalt complexes, 5 and 7, both containing 11,11a-dihydropyrano[2,3-b]quinoxaline nuclei, were synthesised as model substances for the molybdenum cofactor of the oxomolybdoenzymes. The organic proligands for the ene-1,2-dithiolates, from which these complexes were formed, were 1,3-dithiol-2-ones, the dianionic ligands being liberated by reaction with caesium hydroxide. The pyran ring in the tetracyclic 1,3-dithiol-2-one proligands, 4b and 6, was formed by ring closure of the side-chain alcohol in a 4-(1-hydroxyalkyl)-5-(quinoxalin-2-yl)-1,3-dithiol-2-one onto an aromatic quinoxaline via reaction with a chloroformate, generating 6-alkyloxycarbonyl-2-oxo-5a,6-dihydro-4H-[1,3]dithiolo[4′,5′:4,5]pyrano[2,3-b]quinoxalines which were then reduced with cyanoborohydride to give 6-alkyloxycarbonyl-5a,6,11,11a-tetrahydro-2-oxo-4H-[1,3]dithiolo[4′,5′:4,5]pyrano[2,3-b]quinoxalines – the proligands.