合成路线1

A new, more efficient synthesis has been reported for the intermediate (XVIII). The saccharin derivative (IV) was prepared by two alternative routes. Thus, 2-sulfobenzoic acid cyclic anhydride (XII) was treated with two equivalents of 2-(trifluoromethyl)aniline (II) to give the anilinium salt (XIII), which was cyclized with POCl3 to produce (IV). Also, starting from benzenesulfonyl chloride (XIV), the corresponding sulfonamide (II) was obtained by reaction with aniline (II). Ortho lithiation with n-butyllithium at low temperature, followed by CO2 quenching, afforded carboxylic acid (XVI), which upon treatment with Ac2O and methanesulfonic acid cyclized to the saccharin (IV). Then, the saccharin ring was opened by treatment with NaOMe, and subsequent treatment with methyl bromoacetate furnished diester (XVII). Finally, Claisen cyclization promoted by TiCl4 formed the key intermediate (XVIII).