Preparation method of key intermediate of saxagliptin

The invention relates to a preparation method of (1S, 3S, 5S)-3-(amidogen carbonyl)-2- azabicyalo [3.1.0] hexane-2-tert-butyl formate. The preparation method comprises the following steps that (1), Boc-L-pyroglutamic acid methyl ester is restored through lithium triethylborohydride and then dewatered through trifluoroacetic anhydride to obtain (S)-1-N-tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole ethyl formate; (2), DIPEA is added in the hydrolysis reaction of (S)-1-N-tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole ethyl formate under a alkaline condition, then salifying is conducted, and (S)-1-N-tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole formic acid N, N-diisopropyl ethylamine salt is obtained; (3), (S)-1-N-tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole formic acid N and N-diisopropyl ethylamine salt obtained in the step (2) are subjected to amidating, and (S)-1-N- tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole formamide is obtained; and (4), (S)-1-N-tert-butyloxycarbonyl-2,3-dihydro-2-pyrrole formamide is catalyzed th