Crystal structures of methyl 3-(4-isopropylphenyl)-1-methyl-1,2,3,3a,4,9b-hexahydrothiochromeno[4,3-b]pyrrole-3a-carboxylate, methyl 1-methyl-3-(o-tolyl)-1,2,3,3a,4,9b-hexahydrothiochromeno[4,3-b]pyrrole-3a-carboxylate and methyl 1-methyl-3-(o-tolyl)-3,3a,4,9b-tetrahydro-1H-thiochromeno[4,3-c]isoxaz