The invention discloses a preparation method of a baloxavir intermediate (R)-7-(benzyloxy)-3, 4, 12, 12a-tetrahydro-1H-[1, 4]oxazine[3, 4-c]pyrido[2, 1-f][1, 2, 4]-
triazine-6, 8-
diketone, and belongsto the field of drugs. According to the method, 3-benzyloxy-1-Boc amino-4-carbonyl-1, 4-
dihydropyridine-2-
carboxylic acid methyl ester and a chiral substrate L-
serine ester are taken as raw materialsto carry out
condensation reaction,
substitution reaction and cyclization
decarboxylation reaction to synthesize a baloxavir key intermediate (R)-7-(benzyloxy)-3, 4, 12, 12a-tetrahydro-1H-[1, 4]oxazine[3, 4-c]pyrido[2, 1-f][1, 2, 4]-
triazine-6, 8-
diketone and creatively develop the synthetic
route for synthesizing the baloxavir intermediate (R)-7-(benzyloxy)-3, 4, 12, 12a-tetrahydro-1H-[1, 4]oxazine[3, 4-c]pyrido[2, 1-f][1, 2, 4]-
triazine-6, 8-
diketone. Compared to the traditional
route, the preparation method has the advantages of the high yield, low cost, no need of resolution, the high chiral purity, convenience in industrialization and the like.