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A solution of 40 (32
mg, 0.035 mmol, 1.0 equiv), 38 (39 mg, 0.042
mmol, 1.2 equiv) and 2,6-di-tert-butyl-4-methyl-pyridine
(21 mg, 0.11 mmol, 3.0 equiv) in CH2Cl2 (5
mL) in the presence of 4A molecular sieves was stirred for 30 min at
room temperature and cooled to -40 ºC, then Tf2O
(8.8 µL, 0.055 mmol, 1.5 equiv) was added. The reaction
mixture was stirred at -40 ºC for further 1 h
and allowed to warm over 2 h to 0 °C. The reaction mixture
was quenched with saturated aqueous NaHCO3 and the organic
phase was washed with brine, dried (MgSO4), concentrated
in vacuo. The residue was purified by silica gel flash column chromatography
(33% ethyl acetate in hexane) to afford compound 41 (44 mg, 75%): colorless oil,
Rf = 0.37 (33% ethyl acetate in hexane); [α]D
20 = +46.3
(c 1.5, CHCl3); 1H NMR
(250 MHz, CDCl3) δ 1.14 (s, 9 H), 3.38-3.47
(m, 2 H), 3.56-3.63 (m, 2 H), 3.69 (dd, J = 3.0
Hz, 5.0 Hz, 1 H), 3.83-3.91 (m, 5 H), 4.00-4.11
(m, 5 H), 4.14 (dd, J = 2.5
Hz, 5.0 Hz, 1 H), 4.22-4.35 (m, 6 H), 4.39-4.47
(m, 2 H), 4.52-4.65 (m, 5 H), 4.70-4.82 (m, 5
H), 4.92 (d, J = 6.0 Hz, 1 H),
5.00-5.12 (m, 3 H), 5.29 (m, 1 H), 5,33 (s, 2 H), 5.53
(s, 1 H), 5.59 (s, 1 H), 5.61 (s, 1 H), 7.06-7.55 (m, 46
H), 7.68 (dd, J = 3.7 Hz, 3.7
Hz, 1 H), 7.94 (d, J = 3.7 Hz,
1 H), 8.00 (d, J = 3.7 Hz, 1
H); 13C NMR (63 MHz, CDCl3) δ 27.3,
39.0, 64.9, 66.7, 67.8, 68.1, 68.4, 68.6, 69.1, 69.6, 70.0, 70.8,
71.4, 72.7, 73.9, 74.4, 74.7, 76.7, 78.6, 80.1, 97.3, 98.0, 98.8,
100.9, 101.4, 101.5, 101.6, 125.8, 125.9, 126.0, 126.1, 127.3, 127.6,
127.9, 128.0, 128.1, 128.2, 128.3, 128.4, 128.6, 129.0, 134.0, 137.7,
146.5, 166.7, 177.1, 194.6; Anal. Calcd for C100H102O24:
C, 71.16; H, 6.09. Found: C, 71.15; H, 6.11; MALDI-TOF MS Calcd
for 1725.7761 (M + K). Found 1725.7712.