合成路线1

Formylation of 1,3-dimethoxybenzene (A) with DMF in the presence of POCl3 gives 2,4-dimethoxybenzaldehyde. Selective ether cleavage of this product with AlCl3 produces the corresponding salicylaldehyde (I), which is alkylated with ethyl chloroacetate to give (II). Ring closure of (II) under phase-transfer conditions leads to a benzofuran derivative from which the corresponding dihydrobenzofuran (III) is obtained by catalytic hydrogenation over Pd/C. Use of excess AlCl3 at room temperature during the Friedel-Crafts acylation step cleaves the methyl ether, and treatment of the crude product with HBr in acetic acid gives (IV) in 93% overall yield. The oximation reaction requires prolonged heating with excess hydroxylamine hydrochloride, but, after esterification of the crude oxime, a 90% yield (V) is obtained. In the cyclization leading to isoxazole (VI), the presence of pyridine increases the reaction rate and reduces formation of an oxazole, isomeric with (VI), which apparently occurs from a Beckman rearrangement with intramolecular capture by the phenol. Treatment of the crude oxime acetate with K2CO3 in DMF at 60 C gives isoxazole (VI) cleanly, in high yield. The remarkable step in the synthesis is the facile chlorination of (VI) with trichloroisocyanuric acid, utilizing all three chlorines from the reagent, leading to the corresponding 8-chloro derivative in nearly quantitative yield. Ester hydrolysis using KOH in water then gives A-56234.