Methyl (6R*,6aR*,12bR*)-2,4-dimethyl-6-(2-methyl­phenyl)-1,3-dioxo-2,3,4,6a,7,12b-hexa­hydro-1H,6H-chromeno[4′,3′:4,5]pyrano[2,3-d]pyrimidine-6a-carboxyl­ate