We are working on synthesis of Paclitaxel modified as NHS ester at 2' position to further couple it to an amine. PTX is initially converted to PTX-hemisuccinate using succinic anhydride and which is reacted further with succinimidyl diphenyl phosphate to convert it into PTX-NHS ester. Yield of PTX-NHS is less and with impurities. With your expertise kindly suggest what to be done to improve the reaction output.