NELISSEN ET AL.
Representative Experimental Procedure for the Synthesis
of 2-[(2-{[(benzyloxy)carbonyl]amino}-3-phenylpropyl)(tert-
butoxycarbonyl)amino]Ethyl tert-butyl Carbonate (Cbz-
Phaol-Boc2) 12
H
N
R
N
OH
H
The product was synthesized using the general procedure for
protection with a Boc-group: Cbz-Phaol 5 is used as a starting
material (1 g, 3 mmol) Yield: 85%; m.p.: 98–101 ◦C; IR: 3332, 2979,
2939, 1742, 1710, 1685, 1604, 1589, 1531.
Phaol
Figure 1. Phaol [2-{[(2S)-2-amino-3-phenylpropyl]amino}ethanol].
The molecule exists as a mixture of two conformers:
1H NMR (400 MHz, DMSO, ppm): δ = 7·37 − 7·13 (m, 11H, Ar +
NH), 4·94 − 4·92 (m, 2H, CH2Cbz), 4·08 − 4·05 (m, 2H, CH2O), 4·96
(br s, 1H, αH), 3·50−3·02 (m, 7H, CH2NCH2CH2 + H2O), 2·71−2·60
(m, 2H, βH), 1·41 (s, 9H, tBu) 1·38 + 1·35 (2 × s, 9H, tBu).
13C NMR (75 MHz, DMSO, ppm): δ = 155·7 (CO Cbz), 154·7
(CO O-Boc), 152·8 (CO N-Boc), 138·7 (C Cbz), 137·2 (C Phe)
129·0+128·2+128·1+127·6+127·3+126·0 (CH Cbz + CH Phe),
81·4+78·8 (C Boc), 64·8 (CH2Cbz), 64·2+63·8 (CH2O), 51·8+50·9
(αC), 51·3 (CH2NCH2CH2), 46·3 (CH2NCH2CH2), 38·7 + 38·2 (βC),
28·0 + 27·3 (tBu).
1H NMR (300 MHz, CDCl3, ppm, with β = αCHCH2Ph):
δ = 7·38 − 7·18 (m, 10H, Ph), 6·16 (br s, 1H, CONH), 5·43 (s,
1H, OCONH), 5·05 (t (AB), J = 14 Hz, 2H, CH2Cbz), 4·37 (q, J = 7 Hz,
1H, αH), 3·59 − 3·46 (m, 2H, CH2OH), 3·31 − 3·25 (m, 2H, CH2 NH),
3·15 − 2·98 (d, J = 7 Hz, 2H, βH).
13C NMR (75 MHz, CDCl3, ppm): δ = 172·2 (CONH), 156·5
(OCONH), 136·8 + 136·4 (C Phe + Cbz), 129·7 + 129·1 + 129·0 +
128·7+128·4+127·5 (C Phe + Cbz), 67·5 (CH2Cbz), 62·0 (CH2OH),
56·9 (αC), 42·6 (CH2 NH), 39·2 (βC).
ESI: 365·9 [M+Na]+, 707·0 [2M+Na]+.
ESI: 551·9 [M+Na]+, 1079·5 [2M+Na]+.
General Procedure for Reduction with Red-Al (Sodium bis(2-
methoxyethoxy)Aluminum Hydride)
Results and Discussion
N-protected N-(2-hydroxyethyl)phenylalaninamide is dissolved
in a mixture of dry THF and dry toluene (ratio 1 : 2). The
mixture is cooled to −20 ◦C, after which Red-Al (sodium bis(2-
methoxyethoxy)aluminum hydride) is added very slowly. The
reaction mixture is stirred at room temperature. When the reaction
is completed, it is quenched with a 2 N NaOH solution at −20 ◦C.
More toluene is added and the organic layer is washed twice with
a 2 N NaOH solution, after which the organic layer is dried over
Na2SO4 and evaporated in vacuo. The residue is crystallized from
chloroform/n-hexane or purified via chromatography.
Two syntheses of Phaol, on a small scale, have been reported by
Almquist et al. [3] and Kumazawa et al. [1], both using a coupling-
reduction scheme. In this scheme first Cbz-Phe-OH is elongated,
after which the formed amide is reduced to an amine with borane
(Scheme 1). In the first approach (Almquist) glycine is coupled
to Cbz-Phe-OH, followed by the reduction of the amide and the
carboxylic acid functions with borane, yielding 46% of product.
Ph
H
N
O
O
DCC-HOBt
ethanolamine
73 %
OH
O
Representative Experimental Procedure for the Synthesis
of Benzyl {1-benzyl-2-[(2-hydroxyethyl)amino]ethyl}
Carbamate (Cbz-Phaol) 5
Ph
O
H
N
O
The product was synthesized using the general procedure for the
reduction using Red-Al starting from compound 8 (10 g): Reaction
time: 10 h; Yield: 45–60%; m.p.: 70–73 ◦C; IR: 3340, 3290, 3030,
2936, 2908, 2728.
N
O
OH
H
Ph
H
O
O
1H NMR (300 MHz, CDCl3, ppm): δ = 7·37 − 7·15 (m, 10H, 2 ×
Ar), 5·07 (t (AB), J = 13 Hz, 2H, CH2Cbz), 4·89 (br s, 1H, NH or OH),
3·98 (br s, 1H, αH), 3·60 − 3·57 (m, 2H, CH2OH), 2·90 − 2·59 (m, 6H,
CH2NHCH2 + βH).
N
N
O
OH
H2/Pd
91 %
H
O
13C NMR (75 MHz, CDCl3, ppm): 156·7 (CO), 137·9 + 136·9 (C
Phe + C Cbz), 129·7 + 128·9 + 128·5 + 128·4 + 127·0 (CH Cbz+
CH Phe), 67·1 (CH2Cbz), 61·1 (CH2OH), 52·3 + 52·2 + 51·4 (αC
+ CH2 NHCH2), 39·5 (βC).
O
H2N
BH3
N
H
OH
46 %
Ph
ESI: 329·5 [M+H]+, 351·4 [M+Na]+, 679·4 [2M+Na]+.
O
H
N
BH3
65 %
N
H
O
OH
General Procedure for Protection with a Boc-group
H2N
N
The substrate is dissolved in either Et2O or dichloromethane,
depending on the solubility of the specific substrate. Di-tert-
butyl dicarbonate (Boc2O, 3 eq.) and TEA (1 eq.) are added. After
refluxing the mixture for 12 h, it is washed with 5% KHSO4 solution,
5% NaHCO3 solution and water. The organic fraction is dried over
Na2SO4 and the solvent is evaporated in vacuo, after which the
resulting crude product is purified via column chromatography.
OH
H
Almquist et al.1
Kumazawa et al.2
Scheme 1. Synthesis according to Almquist et al. [3] and Kumazawa et al.
[1].
c
wileyonlinelibrary.com/journal/jpepsci Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. J. Pept. Sci. 2011; 17: 527–532