2-Methyl-4-oxo-6,7,8,9-tetrahydro-thieno[2',3':4,5]pyrimidino-[1,2-a]pyridine-3-carboxylic acid

There are two independent mol-ecules in the asymmetric unit of the title compound, C(12)H(12)N(2)O(3)S. With the exception of the methyl-ene groups, a mean plane fitted through all non-H atoms of each mol-ecule has an r.m.s. deviation of 0.035 Å for one mol-ecule and 0.120 Å for the second. In one of the independent mol-ecules, the methyl-ene groups was refined using a disorder model with an occupancy ratio of 0.53:0.47 (14). Each molecule features an intra-molecular O-H⋯O hydrogen bond, which generates an S(7) ring.