Studies in pyrimidine-annulated heterocycles: unusual formation of spiroheterocyclic compounds from acid catalyzed reaction of enol ether

A number of spiroheterocycles are synthesized in moderate yield by treating 5-aryloxymethylene-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones with H2SO4 in chloroform–methanol–water. These spiroheterocycles are also regioselectively synthesized by tri-n-butyltin hydride induced radical cyclization of 5-(o-bromoaryloxymethylene)-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones.