C([C:3]1(CC)[C@H:6]([O:7][C:8]2[CH:13]=[CH:12][C:11]([CH2:14]Br)=[CH:10][CH:9]=2)[N:5](C(N[C@H](CCC)C2C=CC(C)=CC=2)=O)[C:4]1=[O:30])C.[P:33]([O:40]CC)([O:37][CH2:38][CH3:39])[O:34][CH2:35][CH3:36].C(OCC)(=O)C>CCCCCC>[CH2:35]([O:34][P:33]([CH2:14][C:11]1[CH:10]=[CH:9][C:8]([O:7][CH:6]2[NH:5][C:4](=[O:30])[CH2:3]2)=[CH:13][CH:12]=1)([O:37][CH2:38][CH3:39])=[O:40])[CH3:36]A mixture of (4S)-3,3-diethyl-1-[(R)-α-n-propyl-(4-methyl)-benzylaminocarbonyl]-4-[4-(bromomethyl)phenoxy]-azetidin-2-one (362 mg, 0.72 mmol) and triethyl phosphite (1.4 mL, 8.2 mmol) was heated for 2 hours at 90° C. The crude product mixture was subjected to flash chromatography on silica gel using initially 25% ethyl acetate in hexane and subsequently 33% ethyl acetate in hexane as the mobile phase. (4S)-3,3-Diethyl-1-[(R)-α-n-propyl(4-methyl)-benzylaminocarbonyl)]-4-[4-(diethoxyphosphinylmethyl)-phenoxy]-azetidin-2-one was obtained as colorless oil; yield 371 mg, 92%; Fast atom bombardment mass spectrum (FAB MS): m/z 559 (M+1).
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