NOVELTY - Synthesis of oseltamivir phosphate involves adding ethyl 3-amino-4-acetamido-5-(pentan-3-yloxy)benzoate with oxygen-containing solvent, and reacting with phosphoric acid. The ethyl 3-(tert-butylamino)-4-acetamido-5-(pentan-3-yloxy)benzoate is obtained by carrying out tert-butylamine selective ring opening reaction to ethyl 3-acetamido-5-(pentan-3-yloxy)-4-(sulfonyloxy)benzoate. The ethyl 3-amino-4-acetamido-5-(pentan-3-yloxy)benzoate is obtained by carrying out tert-butyl group removal reaction to ethyl 4-acetamido-3-methyl-5-(pentan-3-yloxy)benzoate under the action of an acid.    USE - Synthesis of oseltamivir phosphate (claimed).    ADVANTAGE - The method efficiently provides oseltamivir phosphate with high yield.    DETAILED DESCRIPTION - Synthesis of oseltamivir phosphate involves adding ethyl 3-amino-4-acetamido-5-(pentan-3-yloxy)benzoate with oxygen-containing solvent, and reacting with phosphoric acid. The ethyl 3-(tert-butylamino)-4-acetamido-5-(pentan-3-yloxy)benzoate is obtained by carrying out tert-butylamine selective ring opening reaction to ethyl 3-acetamido-5-(pentan-3-yloxy)-4-(sulfonyloxy)benzoate. The ethyl 3-amino-4-acetamido-5-(pentan-3-yloxy)benzoate is obtained by carrying out tert-butyl group removal reaction to ethyl 4-acetamido-3-methyl-5-(pentan-3-yloxy)benzoate under the action of an acid. The ethyl 4-acetamido-3-methyl-5-(pentan-3-yloxy)benzoate is obtained by performing ring intramolecular substitution reaction to sulfur-containing compound of formula (I) under the action of strong base. The sulfur-containing compound (I) is obtained by carrying out acylation reaction ethyl 3-acetamido-4-hydroxy-5-(pentan-3-yloxy)benzoate with a sulfonyl chloride in a solvent in presence of acid binding agent, and lower catalytic 4-dimethylaminopyridine.The ethyl 3-acetamido-4-hydroxy-5-(pentan-3-yloxy)benzoate is obtained by reacting (3R,4R,5S)-4,5-epoxy-3-(1-ethyl-propoxy)-1-cyclohexen-1-carboxylate with acetonitrile by epoxy ring-opening reaction in presence of Lewis acid catalyst.