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9 The description of a typical experiment is given below:
The oxalic salt of the corresponding γ-amino-α,β-unsaturated phosphonates was dissolved in dry CH2Cl2. One equivalent of pyridinium hydrobromide perbromide (PHP) was added. After 10 min stirring at r.t., a second equivalent of PHP was added. After 10 min stirring at r.t., the mixture was brought to reflux. The reaction was monitored by 1H NMR and 31P NMR. After completion of the bromination (see Table
[1]
), the mixture was cooled to r.t. and 4 equiv of Et3N in dry CH2Cl2 were added drop by drop. Further stirring for 20 min at r.t. was followed by gentle reflux during 1.5 h. The mixture was cooled, the precipitated salts were filtered off and the filtrate was washed with a mixture of CH2Cl2-Et2O (1:1, v/v). The filtration was repeated until the salts stopped precipitating. The residual red-brown oil was then purified on a silica gel column with ethyl acetate and a gradient (10-50%) of EtOH. The analytical data for 7 with R = benzyl: 1H NMR (300 MHz, CDCl3): δ = 1.35 (6 H, t, J = 6.16 Hz, 2 × OCH2CH3), 4.03 (2 H, d × d, J
H-P = 6.88 Hz, J = 2.48 Hz, CH2CH=Cq), 4.07 (2 H, s, CH2Car), 4.15 (4 H, m, 2 × OCH2CH3), 7.26 (1 H, d × t, J
H-P = 39.35 Hz, J = 7.02 Hz, CH=Cq), 7.36 (3 H, m, ArH), 7.55 (2 H, m, ArH). 13C NMR (75 MHz, CDCl3): δ = 16.26 (J
C-P = 5.77 Hz, 2 × OCH2CH3), 46.03 (J
C-P = 3.46 Hz, CH2CH=Cq), 51.34 (CH2Car), 64.14 (J
C-P = 4.62 Hz, 2 × OCH2CH3), 115.89 (J
C-P = 193.84 Hz, CH=Cq), 129.16 (Car), 129.35 (Car), 129.57 (Car), 130.00 (Car), 130.19 (Car), 131.56 (Cq ar), 144.14 (J
C-P = 13.85 Hz, CH=Cq). 31P NMR (109 MHz, CDCl3): δ = 6.94. IR (NaCl): 3412 (νN-H), 1239 (νP=O), 1608 (νC=C), 978 (νCH=CBr) cm-1. MS (positive mode): m/z = 362/364 (100) [M+]. Anal. Calcd for C14H21BrNO3P: C, 46.42; H, 5.84; N, 3.87. Found: C, 46.38; H, 5.87; N, 3.81.
10
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