Electro-organic reactions.: Part 49.: The synthesis and stereoselective electrochemical hydroxylation of 2,3-dihydro-4H-furo[2,3-d]pyrido[1,2-a]pyrimidin-4-ones

Attempted electrochemical (anodic) hydroxylation of pyridopyrimidine derivatives in the pyridine ring, using trifluoroacetate as nucleophile, fails because enol forms predominate which undergo anodic C-O coupling. Substitution, aimed at precluding enolisation, led to an alternative tautomerisation to 8-methyl-3-alkylmethylene-4H-pyrido [1,2-a]pyrimidine-2,4-diones which in acidic solution are converted into the title compounds. The 8-methyl-2,3-dihydro-4H-furo[2,3-d]pyrido[1,2-a]pyrimidine-4-ones are novel and have been fully characterised by spectroscopy and X-ray crystallography. They are electroactive and are oxidised anodically in trifluoroacetic acid-dichloromethane-triethylammonium trifluoroacetate solutions to give the corresponding 3-hydroxy derivatives in good yield. Several examples of this allylic substitution reaction are presented; for cases disubstituted at the 2-position (R-1,R-2) diastereoselective hydroxylation with d.e. ca. 50% (3:1) is observed in which preferential nucleophilic attack on the stabilised intermediate allylic cation occurs at the least hindered face. The stereochemistry of the 3-hydroxy derivatives was assigned by NOE experiments.