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methanol, the solvent evaporated, and the residue chromatographed on silica gel (eluent: toluene/
ethyl acetate 2/1!1/1), yielding 50 mg (79%) syrupy 33.
1H NMR (CDCl3, ꢀ, 500 MHz): 1.28 (t, 3H, CH2CH3), 2.87 (d, 2H, CH2C6H5), 3.11 (dd, 1H,
CHCH=), 3.43 (1/2AB, 1H, NCHC6H4), 3.62 (dd, 1H, CHO), 3.62 (1/2AB, 1H, NCHC6H4), 3.79 (s,
3H, OCH3), 3.89 (s, 2H, COCH2), 4.20 (q, 2H, CH2CH3), 4.26 (dq, 1H, NHCHCH2), 5.91 (d, 1H,
=CH), 6.77 (dd, 1H, CH=), 7.02 (d, 1H, CONH), 7.16 (d, 2H, aromatic H), 7.19±7.35 (m, 7H,
aromatic H) ppm; MS (FAB): m/z 475 [MH ].
4(R)-Benzyl-5(S)-O-t-butyldimethylsilyl-6(R)-((O-t-butylidimethylsilyl)-methyl)-
1,3-bis-(4-methoxybenzyl)-tetrahydropyrimidin-2-one (34; C40H60N2O5Si2)
227 mg (9.48 mmol) NaH were dispersed in 20 cm3 DMSO at 0ꢁC. Then 2.77 g (4.74 mmol) 26 were
added, and the reaction mixture was stirred for 15 min at 0ꢁC. A solution of 3.056 g (9.48 mmol)
tetrabutylammonium bromide, 975 mg (9.48 mmol) NaBr, and 1.29 cm3 (9.48 mmol) 4-methox-
ybenzyl chloride dissolved in 20 cm3 DMSO was added. After stirring for 12 h at 4ꢁC the reaction
mixture was acidi®ed to pH 4±5 with acetic acid, diluted with CH2Cl2, washed with H2O, the organic
solvent was evaporated, and the residue was chromatographed on silica gel (eluent: toluene/ethyl
acetate 4/1) yielding 2.82 g (70%) syrupy 34.
1H NMR (CDCl3, ꢀ, 500 MHz): ꢂ0.31 (s, 3H, SiCH3), ꢂ0.18 (s, 3H, SiCH3), 0.10 (s, 3H,
SiCH3), 0.20 (s, 3H, SiCH3), 0.81 (s, 9H, SiC(CH3)3), 0.95 (s, 9H, SiC(CH3)3), 2.41 (1/2ABX, 1H,
CHC6H5), 2.57 (1/2AB, 1H, NCHC6H4), 2.88 (1/2ABX, 1H, CHC6H5), 3.12±3.18 (m, 1H,
NCHCH2), 3.51 (1/2ABX, 1H, CHOSi), 3.76 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 3.81±3.84 (m, 1H,
CHOSi), 3.91 (1/2ABX, 1H, CHOSi), 5.10 (1/2AB, 1H, NCHC6H4), 5.60 (1/2AB, 1H, NCHC6H4),
7.16 (d, 2H aromatic H), 7.26±7.38 (m, 7H, aromatic H) ppm; MS (FAB): m/z [MH ].
4(R)-Benzyl-5(S)-O-t-butyldimethylsilyl-6(R)-hydroxymethyl)-1,3-bis-(4-methoxybenzyl)-
tetrahydropyrimidin-2-one (35; C34H46N2O5Si)
2.34 g (3.32 mmol) 34 were reacted with 1 cm3 40% HF for 4 d according to the general procedure
yielding 1.12 g (56%) syrupy 35 after chromatography on silica gel (eluent: toluene/ethyl acetate
6/4!8/1).
1H NMR (CDCl3, ꢀ, 500 MHz): ꢂ0.31 (s, 3H, SiCH3), ꢂ0.18 (s, 3H, SiCH3), 0.81 (s, 9H,
SiC(CH3)3), 1.63 (1/2ABX, 1H, CHC6H5), 2.32 (1/2ABX, 1H, CHC6H5), 2.63 (1/2AB, 1H,
NCHC6H4), 2.91 (1/2ABX, 1H, CHOH), 3.04 (d, 1H, CHCH2), 3.10±3.19 (m, 1H, CHOSi), 3.49 (1/
2ABX, 1H, CHOH), 3.75 (s, 3H, OCH3), 3.79 (s, 3H, OCH3), 3.92 (dt, 1H, NCHCH2), 4.22 (1/2AB,
1H, NCHC6H4), 5.11 (1/2AB, 1H, NCHC6H4), 5.18 (1/2AB, 1H, NCHC6H4), 6.75±6.68 (m, 4H,
aromatic H), 6.90 (d, 2H, aromatic H), 7.08±7.15 (m, 2H, aromatic H), 7.22±7.43 (m, 5H, aromatic
H) ppm; MS (FAB): m/z 591 [MH ].
3(R)-(6(R)-Benzyl-5(S)-O-t-butyldimethylsilyl-1,3-bis-(4-methoxybenzyl)-2-oxo-hexahydropyrimi-
din-4-yl)-acrylic acid ethylester (36; C38H50N2O6Si)
200 mg (0.34 mmol) 35 were dissolved in 3 cm3 CH2Cl2. 173 mg (0.408 mmol) Dess-Martin reagent
and 53 mm3 (0.68 mmol) pyridine were added, and after 1 h 284 mg (0.82 mmol) carboxyethyl
triphenylphosphorane were added together with 4 cm3 toluene. The reaction mixture was heated at
60ꢁC for 1 h; then the solvent was evaporated and the residue chromatographed on silica gel (eluent:
toluene/methanol 95/5) to yield 172.5 mg (77%) syrupy 36.
1H NMR (CDCl3, ꢀ, 500 MHz): ꢂ0.31 (s, 3H, SiCH3), ꢂ0.18 (s, 3H, SiCH3), 0.79 (s, 9H,
SiC(CH3)3), 1.32 (t, 3H, CH2CH3), 2.20 (1/2ABX, 1H, CHC6H5), 2.58 (1/2AB, 1H, NCHC6H4), 2.87
(1/2ABX, 1H, CHC6H5), 3.12±3.20 (m, 1H, NCHCH2), 3.41 (dd, 1H, CHOSi), 3.61 (t, 1H, CHCH=),