A practical, enantioselective synthetic route to a key precursor to the tetracycline antibiotics

A practical, enantioselective synthetic route to a key precursor to the tetracycline antibiotics is reported. The route proceeds in nine steps (21% yield) from the commercial substance methyl 3-hydroxy-5-isoxazolecarboxylate. Key steps in the route involve enantioselective addition of divinylzinc to 3-benzyloxy-5-isoxazolecarboxaldehyde and an endo-selective intramolecular furan Diels-Alder cycloaddition reaction. The route described has provided more than 40 g of chromatographically pure 1 with 93% ee.