The protected methyl 4-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylate is synthesized by two convergent routes from dimethyl-5-(2-formylphenyl)-2-methyl-4,5-dihydrooxazole-4,4-dicarboxylate via cleavage of the oxazolyl moiety, the starting dihydrooxazole is prepared on a large scale at 70°C by 1,3-dipolar cycloaddition using a solvent-free process assisted by focused microwave technology.