Side-Chain Protection of Cysteine and Asparagine/Glutamine
and stirring was continued for a further 2 h. The mixture was co-
oled to 0 °C and a 1 aqueous HCl solution was added to reach
pH 2–3. The volatiles were then removed under vacuum to half
volume. The resulting mixture was washed with AcOEt
(3ϫ15 mL), the organic layers were combined, and the final or-
ganic solution was washed with brine (20 mL) and dried with anhy-
drous MgSO4. Evaporation of the solvent under vacuum afforded
an oily foaming solid. This solid was chromatographed on SiO2
(CH3Cl/MeOH, 95:5) to yield 4a (0.347 g, 70%) as a white solid.
M.p. 115–118 °C. HPLC (3:7 to 10:0 over 30 min): tR = 25.5 min.
C2), 32.7 (CH2 CH2-β), 34.2 (CH2 CH2S), 42.9 (CH2 C3), 47.1
(CH Fmoc), 52.7 (CH CH-α), 67.2 (CH2 Fmoc), 85.5 (C2), 116
(C7), 119.9 (CH Fmoc), 122.7 (C4Ј), 125 (CH Fmoc), 127 (CH
Fmoc), 127.7 (CH Fmoc), 131.1 (C4), 136.5 (C6), 156.2 (C7Ј),
141.2 (C Fmoc), 143.6 (C Fmoc), 155.3 (CO), 175.1 (COOH) ppm.
MS (ESI+): m/z (%) = 546.6 (8) [M + 1]+, 203.9 (100) [Pmbf + 1]
+. [α]D = +17.8 (c = 1.13, CHCl3).
N-(9-Fluorenylmethoxycarbonyl)-S-(4,5,6-trimethoxy-2,2-dimethyl-
2,3-dihydrobenzofuran-7-yl)-L-cysteine [Fmoc-Cys(Tmbm)-OH, 7]
4,5,6-Trimethoxy-2,2-dimethyl-2,3-dihydrobenzofuran: This product
was prepared following the procedure described for 1b. 3,4,5-Trime-
thoxyphenol (15 g, 81.4 mmol) afforded the dihydrobenzofuran de-
rivative (7.8 g, 40%) as a white solid, which was used in the next
step without further purification. M.p. 37–39 °C. HPLC (3:7 to
10:0 over 30 min): tR = 13.7 min. TLC (SiO2): Rf = 0.63 (petroleum
TLC (SiO ): R = 0.44 (CHCl MeOH 9:1). IR (KBr): ν = 2929,
˜
2
f
3
1725, 1509, 1449, 1320, 1223, 1167, 1106 cm–1 1H NMR
.
(300 MHz, CDCl3): δ = 1.27 (s, 6 H, 2 CH3), 1.76 (t, J = 9.2 Hz,
2 H, CH2), 2.08 (s, 3 H, CH3), 2.19 (s, 3 H, CH3), 2.24 (s, 3 H,
CH3), 2.58 (t, J = 9.2 Hz, 2 H, CH2), 3.10 (br. s, 2 H, CH2-β), 3.82
(s, 2 H, CH2S), 4.23 (t, J = 7.8 Hz, 1 H, Fmoc), 4.44 (d, J = 7.7 Hz,
2 H, Fmoc), 4.68 (m, 1 H, CH-α), 5.67 (d, J = 8.0 Hz, 1 H, NH),
7.31 (m, 2 H, Fmoc), 7.38 (m, 2 H, Fmoc), 7.58 (m, 2 H, Fmoc),
7.74 (m, 2 H, Fmoc) ppm. 13C NMR (75 MHz, CDCl3): δ = 11.9
(CH3 aryl), 14.8 (CH3 aryl), 15.7 (CH3 aryl), 21.2 (CH2 C4), 26.7
(2 CH3 C2), 32.9 (CH2 CH2-β), 33.1 (CH2 C3), 34.9 (CH2 CH2S),
47.1 (CH Fmoc), 53.6 (CH CH-α), 67.3 (CH2 Fmoc), 72.9 (C2),
116.9 (C8), 119.9 (CH Fmoc), 122.5 (C5Ј), 123.6 (C6), 125 (CH
Fmoc), 127 (CH Fmoc), 127.7 (CH Fmoc), 132.8 (C5), 134.1 (C7),
141.2 (C Fmoc), 143.7 (C Fmoc), 151.1 (C8Ј), 155.9 (CO), 175.1
(COOH) ppm. MS (ESI+): m/z (%) = 560.7 (23) [M + 1]+, 217.9
(100) [Pmcm + 1]+. [α]D = +19.1 (c = 1.1, CHCl3).
ether/AcOEt, 8:2). IR (KBr): ν = 2971, 2936, 1615, 1474, 1416,
˜
1
1300, 1198, 1121 cm–1. H NMR (300 MHz, CDCl3): δ = 1.46 (s,
6 H, 2 CH3), 3.02 (s, 2 H, CH2), 3.78 (s, 3 H, OCH3), 3.80 (s, 3 H,
OCH3), 3.92 (s, 3 H, OCH3), 6.13 (s, 1 H, aryl) ppm. 13C NMR
(75 MHz, CDCl3): δ = 28.1 (2 CH3 C2), 41.2 (CH2 C3), 56.0
(OCH3 aryl), 59.8 (OCH3 aryl), 61.2 (OCH3 aryl), 87.2 (C2), 90.0
(CH C7), 108.8 (C4Ј), 134.7 (C5), 150.1 (C6), 153.7 (C4), 155.3
(C7Ј) ppm. MS (QI, NH3): m/z (%) = 239 (100) [M + 1]+.
4,5,6-Trimethoxy-2,2-dimethyl-2,3-dihydrobenzofuran-7-carbalde-
hyde: A solution of 4,5,6-trimethoxy-2,2-dimethyl-2,3-dihydroben-
zofuran (0.5 g, 2.1 mmol) in anhydrous DCM (5 mL) was purged
with N2 and TiCl4 (1.15 mL, 10.5 mmol) was added over 20 min.
The mixture was left for 1 h at room temperature and dichlorome-
thoxymethane (0.8 mL, 8.84 mmol) was added over 15 min. The
reaction mixture was left for 1 h and saturated aqueous NH4Cl
solution (20 mL) was then carefully added. The mixture was shaken
for 2 h, the resulting two phases were separated, and the aqueous
layer was washed with DCM (3ϫ15 mL). The combined organic
layers were washed with 1 HCl solution (2ϫ25 mL), saturated
aqueous NaHCO3 solution (3ϫ25 mL), and brine (25 mL). The
resulting organic solution was dried with anhydrous MgSO4, and
the solvent was removed under vacuum to afford an oil, which was
chromatographed on SiO2 (hexanes/Et2O, 2:8) to yield the desired
compound (0.41 g, 73%) as a white solid. M.p. 64–65 °C. HPLC
(3:7 to 10:0 over 30 min): tR = 11.8 min. TLC (SiO2): Rf = 0.47
N-(9-Fluorenylmethoxycarbonyl)-S-(2,2,4,6,7-pentamethyl-2,3-dihy-
drobenzofuran-5-yl)-L-cysteine [Fmoc-Cys(Pbfm)-OH, 4b]: This
product was obtained from 3b and ()-cysteine hydrochloride fol-
lowing the procedure described for 4a.
S-(2,2,4,6,7-Pentamethyl-2,3-dihydrobenzofuran-5-yl)-L-cysteine,
Trifluoroacetate Salt: ()-Cysteine (1.43 g, 9.09 mmol) and 3b (2 g,
9.09 mmol) yielded the trifluoroacetate salt (3.6 g, 88%) of the pro-
tected amino acid as a white solid. M.p. 144–147 °C. HPLC (3:7 to
10:0 over 30 min): tR = 8.9 min. TLC (SiO2): Rf = 0.84 (MeOH/
AcOH/H O, 8:1:1). IR (KBr): ν = 2973, 1701, 1578, 1414, 1288,
˜
2
1207, 1086 cm–1. 1H NMR (300 MHz, CD3OD/basic D2O): δ =
1.40 (s, 6 H, 2 CH3), 2.03 (s, 3 H, CH3), 2.23 (s, 3 H, CH3), 2.26
(s, 3 H, CH3), 2.74 (dd, J = 8.3, 13.3 Hz, 1 H, CH2-β), 2.91 (s, 2
H, CH2), 3.05 (dd, J = 4.2, 13.3 Hz, 1 H, CH2-β), 3.43 (dd, J = 8.3,
4.2 Hz, 1 H, CH-α), 3.83 (s, 2 H, CH2S) ppm. 13C NMR (75 MHz,
CD3OD/basic D2O): δ = 12.4 (CH3 aryl), 15.7 (CH3 aryl), 16.3
(CH3 aryl), 28.7 (2 CH3 C2), 32.8 (CH2 CH2-β), 40.6 (CH2 CH2S),
43.7 (CH2 C3), 56.5 (CH CH-α), 86.6 (C2), 116.5 (C7), 124.3 (C4Ј),
126.6 (C5), 131.8 (C4), 136.6 (C6), 157.2 (C7Ј), 180.8 (COOH)
ppm. MS (ESI+): m/z (%) = 324.9 (100) [M – CF3COO]+, 204.2
(28) [Pmbf + 1]+.
(hexanes/Et O, 2:8). IR (KBr): ν = 2975, 2939, 1684, 1594, 1457,
˜
2
1416, 1358, 1200, 1047 cm–1. 1H NMR (300 MHz, CDCl3): δ =
1.52 (s, 6 H, 2 CH3), 3.02 (s, 2 H, CH2), 3.80 (s, 3 H, OCH3), 3.94
(s, 3 H, OCH3), 4.03 (s, 3 H, OCH3), 10.2 (s, 1 H, CHO) ppm. 13
C
NMR (75 MHz, CDCl3): δ = 28.2 (2 CH3 C2), 40.3 (CH2 C3), 59.7
(OCH3 aryl), 61.3 (OCH3 aryl), 62.1 (OCH3 aryl), 89.4 (C2), 109.9
(C7), 112.8 (C4Ј), 138.1 (C5), 154.1 (C6), 155.8 (C4), 157.0 (C7Ј)
ppm. MS (QI, NH3): m/z (%) = 267 (100) [M + 1]+.
N-(9-Fluorenylmethoxycarbonyl)-S-(2,2,4,6,7-pentamethyl-2,3-dihy-
drobenzofuran-5-yl)-L-cysteine (4b): The trifluoroacetate salt (1 g,
2.38 mmol) of the protected amino acid and FmocOSu (0.98 g,
2.9 mmol) afforded 4b (0.97 g, 75%) as a white solid. M.p. 100–
104 °C. HPLC (3:7 to 10:0 over 30 min): tR = 24.6 min. TLC
(4,5,6-Trimethoxy-2,2-dimethyl-2,3-dihydrobenzofuran-7-yl)meth-
anol: This product was obtained from 4,5,6-trimethoxy-2,2-di-
methyl-2,3-dihydrobenzofuran-7-carbaldehyde by following
the procedure described for 3b. The aldehyde derivative (1 g,
3.76 mmol) yielded the alcohol derivative (1 g, 91%) as an oily so-
lid, which was used in the next step without further purification.
HPLC (3:7 to 10:0 over 30 min): tR = 9.6 min. TLC (SiO2): Rf =
(SiO ): R = 0.44 (CHCl MeOH 9:1). IR (KBr): ν = 2971, 1698,
˜
2
f
3
1530, 1451, 1368, 1254, 1084 cm–1. 1H NMR (300 MHz, CDCl3):
δ = 1.43 (s, 6 H, 2 CH3), 2.06 (s, 3 H, CH3), 2.17 (s, 3 H, CH3),
2.22 (s, 3 H, CH3), 2.88 (s, 2 H, CH2), 3.10 (br. s, 2 H, CH2-β),
3.79 (s, 2 H, CH2S), 4.22 (t, J = 7.1 Hz, 1 H, Fmoc), 4.43 (d, J =
7.1 Hz, 2 H, Fmoc), 4.67 (m, 1 H, CH-α), 5.70 (d, J = 7.8 Hz, 1
H, NH), 7.27 (m, 2 H, Fmoc), 7.35 (m, 2 H, Fmoc), 7.58 (m, 2 H,
Fmoc), 7.76 (m, 2 H, Fmoc) ppm. 13C NMR (75 MHz, CDCl3): δ
0.44 (hexanes/AcOEt, 1:1). IR (KBr): ν = 3504, 2962, 1613, 1457,
˜
1420, 1368, 1268, 1104, 1055 cm–1. 1H NMR (300 MHz, CDCl3):
δ = 1.41 (s, 6 H, 2 CH3), 3.02 (s, 2 H, CH2), 3.80 (s, 3 H, OCH3),
3.90 (s, 3 H, OCH3), 3.91 (s, 3 H, OCH3), 4.63 (s, 2 H, CH2OH)
ppm. 13C NMR (75 MHz, CDCl3): δ = 28.2 (2 CH3 C2), 41.1 (CH2
C3), 55.8 (CH2 CH2OH), 59.9 (OCH3 aryl), 61.1 (OCH3 aryl), 61.6
= 12.3 (CH3 aryl), 15.3 (CH3 aryl), 16.0 (CH3 aryl), 28.5 (2 CH3 (OCH3 aryl), 87.5 (C2), 111.4 (C7), 112.7 (C4Ј), 138.1 (C5), 149.8
Eur. J. Org. Chem. 2010, 3631–3640
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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