An efficient method for the preparation of peptide alcohols

Article abstract of DOI:10.1039/b905730g

N-Protected ll-dipeptide alcohols 3a-p, diastereomeric mixture (3d + 3d′) and tripeptide alcohols 6a-c were synthesized by treatment of various amino alcohols with N-protected(α-aminoacyl)benzotriazoles 1a-c, 1f-m, (1a + 1a′) and N-protected(α-dipeptidoyl)benzotriazoles 5a, 5b respectively in good yields with complete retention of chirality.

Full text of DOI:10.1039/b905730g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
An efficient method for the preparation of peptide alcohols†
Alan Roy Katritzky,*a Nader Elmaghwry Abo-Dya,a,b Srinivasa Rao Tala,a Kapil Gyandaa and
Zakaria Kamel Abdel-Samiib
Received 23rd March 2009, Accepted 13th July 2009
First published as an Advance Article on the web 26th August 2009
DOI: 10.1039/b905730g
N-Protected LL-dipeptide alcohols 3a–p, diastereomeric mixture (3d + 3d¢) and tripeptide alcohols 6a–c
were synthesized by treatment of various amino alcohols with N-protected(a-aminoacyl)benzotriazoles
1a–c, 1f–m, (1a + 1a¢) and N-protected(a-dipeptidoyl)benzotriazoles 5a, 5b respectively in good yields
with complete retention of chirality.
Introduction
Results and discussion
Peptide alcohols are synthetic intermediates for peptide alde-
hydes which are inhibitors of m-calpain,1a Subtilisin 72,1b
a-Chymotrypsin,1b HIV-protease,1c,d multicatalytic proteinase
complex (MPC)1e and other biologically and pharmaceutically
active molecules.1f ,g Peptide alcohols serve as reversible activators
of catalytic activity of the 20S proteasome.2 Peptide alcohols
are also used as ligands or precursors for ligands in asymmetric
catalysis,3a–g and in the preparation of a prototype calix[4]arene-
based receptor for carbohydrate recognition.4
Reported preparations of peptide alcohols have utilized:
(i) coupling reactions with DCC/HOBT,1c,3d EDC/HOBT,3g
HATU, DIPEA,1a NMM, BuiOCOCl,3b DEPBT,5 (ii) aminoacyl
chlorides,6 (iii) activated esters,3b,e (iv) reduction of esters,4,7
ketones,6 (v) enzymatic synthesis,8 and (vi) solid phase synthesis.9
Utilization of these methods involves: (i) cumbersome reaction
conditions,3b,3e,6 (ii) producing mixtures of diastereomeric alcohols
on reduction,6 (iii) long reaction times (up to 53 hours),8 (iv) use
of expensive reagents and (v) low yields.6 Thus a mild and efficient
approach to peptide alcohols would be of utility.
N-Acylbenzotriazoles are advantageous for N-, O-, C-,
S-acylation,10a–j especially where the corresponding acid chlorides
are unstable or difficult to prepare.10k,l N-Fmoc-(a-aminoacyl)-
benzotriazoles and their Boc- and Cbz- analogs enabled the
preparation of chiral di-, tri- and tetra-peptides in good yields
from natural amino acids in the solution phase.10b,11a–b Recently,
we have also prepared tri-, tetra-, penta-, hexa-, and hepta-
peptides in 71% average crude yields by microwave-assisted
solid phase peptide synthesis utilizing N-Fmoc-(a-aminoacyl)-
benzotriazoles and N-Fmoc-protected(a-dipeptidoyl)benzo-
triazoles.12a,b We now report the preparation of di- and
tri-peptide alcohols in solution phase by reaction of N-
1. Preparation of dipeptide alcohols 3a–p and diastereomeric
mixture (3d + 3d¢)
The starting N-protected(a-aminoacyl)benzotriazoles 1a–e,
1g–m and (1a + 1a¢) (N-Cbz-L-Ala-Bt 1a, N-Cbz-DL-Ala-Bt
(1a + 1a¢), N-Fmoc-L-Val-Bt 1b, N-Fmoc-L-Met-Bt 1c, N-
Fmoc-L-Phe-Bt 1d, N-Fmoc-Gly-Bt 1e, N-Cbz-L-Phe-Bt 1g, N-
Fmoc-L-Glu(OtBu)-Bt 1h, N-Fmoc-L-Asp(OtBu)-Bt 1i, N-Fmoc-
L-Lys(Boc)-Bt 1j, N-Fmoc-L-Trp-Bt 1k, N-Fmoc-L-His(Trt)-Bt
1l and N-Fmoc-L-Cys(Trt)-Bt 1m) were prepared in 75–90%
yield from corresponding N-protected amino acids following our
previously published one-step procedure.10b N-Boc-L-Val-Bt 1f
was prepared as described previously.10e
Reaction of N-protected(a-aminoacyl)benzotriazoles 1a–c,
1f–m and racemic mixture (1a + 1a¢) with various amino alcohols
2 in THF at room temperature for 6 h gave N-protected dipeptide
alcohols 3a–p and diastereomeric mixture (3d + 3d¢) in 46–89%
yield, all isolated without column chromatography (Scheme 1,
Table 1). The dipeptide alcohols 3a–p were characterized by
1H-NMR, 13C-NMR, HRMS and elemental analysis. Diastere-
omeric mixtures were prepared to confirm that the original
chirality of the amino acid used was maintained during the
formation of peptide alcohol by means of chiral HPLC analysis.
Chiral HPLC (detection at 254 nm, flow rate 1 mL/min, hexanes–
isopropanol (9:1) as solvent using Whelk-O1 chiral column)
showed a single peak for 3d at 21.12 min. By contrast, two peaks
were observed for the corresponding racemic mixtures (3d + 3d¢)
at 19.25 and 21.12 min, confirming the enantiopurity of 3d.
protected(a-aminoacyl)benzotriazoles
and
N-protected(a-
dipeptidoyl)benzotriazoles with amino alcohols.
Scheme 1 Preparation of dipeptide alcohols.
aCenter for Heterocyclic Compounds, Department of Chemistry, Univer-
sity of Florida, Gainesville, FL 32611-7200, USA. E-mail: katritzky@
chem.ufl.edu; Fax: +1-352-392-9199; Tel: +1-352-392- 0554
2. Preparation of N-Fmoc-a-dipeptides 4a, 4b from
N-Fmoc-(a-aminoacyl)benzotriazoles 1d, 1e
bDepartment of Pharmaceutical Organic Chemistry, Faculty of Pharmacy,
Zagazig University, Zagazig-44519, Egypt
N-Fmoc-protected dipeptides 4a and 4b were prepared by
reaction of N-Fmoc-protected(a-aminoacyl)benzotriazoles 1d
and 1e, respectively, with the corresponding amino acids in
† Electronic supplementary information (ESI) available: General experi-
mental procedures with spectral data for compounds 3a–p, 4a, 4b, 5a, 5b
and 6a–c. See DOI: 10.1039/b905730g
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Table 1 Preparation of dipeptide alcohols 3a–p and (3d + 3d¢)
Entry
1
Amino alcohol 2
Product 3, yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1f
L-Phenylalaninol
L-Leucinol
L-t-Leucinol
N-Boc-L-Val-L-Phenylalaninol 3a, 86
N-Boc-L-Val-L-Leucinol 3b, 77
1f
1f
1a
N-Boc-L-Val-L-t-Leucinol 3c, 81
L-Phenylalaninol
L-Phenylalaninol
L-Leucinol
L-Phenylalaninol
L-Phenylglycinol
L-Phenylglycinol
L-Phenylglycinol
L-Leucinol
L-Phenylalaninol
L-Phenylglycinol
L-Leucinol
L-Phenylalaninol
L-t-Leucinol
L-Leucinol
N-Cbz-L-Ala-L-Phenylalaninol 3d, 88
N-Cbz-DL-Ala-L-Phenylalaninol (3d + 3d¢), 89
N-Cbz-L-Ala-L-Leucinol 3eb, 82
(1a + 1a¢)
1a
1c
1b
1a
1g
1g
1h
1i
N-Fmoc-L-Met-L-Phenylalaninol 3f, 60
N-Fmoc-L-Val-L-Phenylglycinol 3g, 54
N-Cbz-L-Ala-L-Phenylglycinol 3h, 85
N-Cbz-L-Phe-L-Phenylglycinol 3i, 60
N-Cbz-L-Phe-L-Luecinol 3j, 46
N-Fmoc-L-Glu(OtBu)-L-Phenylalaninol 3k, 80
N-Fmoc-L-Asp(OtBu)-L-Phenylglycinol 3l, 77
N-Fmoc-L-Lys(Boc)-L-Leucinol 3m, 85
N-Fmoc-L-Trp-L-Phenylalaninol 3n, 83
N-Fmoc-L-His(Trt)-L-t-Leucinol 3o, 60
N-Fmoc-L-Cys(Trt)-L-Leucinol 3p, 70
1j
1k
1l
1m
a Isolated yield. b Lit.,13 mp 90 ◦C.
Table 2 Preparation of N-Fmoc-a-dipeptides 4a, 4b from N-Fmoc-
protected(a-aminoacyl)benzotriazoles 1d, 1e
Table 3 Preparation of N-Fmoc-(a-dipeptidoyl)benzotriazoles 5a, 5b
Entry
4
Product 5
5, yield (%)a 5, mp (◦C)
Entry
1
Amino acid Product 4, yield (%)a
4, mp (◦C)
1
2
4a N-Fmoc-L-Phe-L-Ala-Bt, 5a
4b N-Fmoc-Gly-L-Leu-Bt, 5b
5a, 82
5b, 78
155–156b
165–167
1
2
1d L-Ala-OH
1e L-Leu-OH
N-Fmoc-L-Phe-L-Ala-OH 4a, 87 208–210b
N-Fmoc-Gly-L-Leu-OH 4b, 85
133–134c
a Isolated yield. b Lit.,12b mp 155.2–156.9 ◦C.
a Isolated yield. b Lit.,12b mp 208.7–210.6 ◦C. c Lit.,14 mp 133–135 ◦C.
4. Preparation of tripeptide alcohols 6a–c
acetonitrile–water (3:1) in the presence of triethylamine at room
temperature in 85–87% yield as described previously (Scheme 2,
Table 2).10b
N-Fmoc-protected tripeptide alcohols 6a–c were prepared in solu-
tion phase by treatment of N-Fmoc-(a-dipeptidoyl)benzotriazoles
5a and 5b with amino alcohols 2 in THF for 4 h at 0 ◦C followed by
3 h at room temperature in good yields, all isolated without column
chromatography (Scheme 4, Table 4). These compounds were
characterized by 1H-NMR, 13C-NMR, and elemental analysis. No
detectable racemization of the tripeptide alcohols was observed in
chiral HPLC analysis.
Scheme 2 Preparation of N-Fmoc-a-dipeptides 4a, 4b from N-Fmoc-
protected(a-aminoacyl)benzotriazoles 1d, 1e.
3. Preparation of N-Fmoc-(a-dipeptidoyl)benzotriazoles 5a, 5b
N-Fmoc-(a-dipeptidoyl)benzotriazoles 5a and 5b were prepared
by treatment of N-Fmoc-a-dipeptides 4a and 4b, respectively, with
benzotriazole and thionyl chloride in CH2Cl2 at -15 ◦C for 3 h in
78–82% yield as described previously (Scheme 3, Table 3).12b Com-
1
pounds 5a, 5b were characterized by H-NMR, 13C-NMR and
elemental analysis. The N-Fmoc-(a-dipeptidoyl)benzotriazoles
5a, 5b were utilized for the synthesis of N-Fmoc-protected
tripeptide alcohols 6a–c.
Scheme 4 Preparation of tripeptide alcohols 6a–c from N-Fmoc-
(a-dipeptidoyl)benzotriazoles 5a, 5b.
In comparison our methodology offers: (1) mild and simple
reaction and work-up conditions: we isolated all products under
milder reaction conditions and by simply washing or recrystal-
lization; (2) high yields: most of our products were obtained in
Scheme 3 Preparation of N-Fmoc-(a-dipeptidoyl)benzotriazoles 5a, 5b
from N-Fmoc-a-dipeptides 4a, 4b.
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Table 4 Preparation of tripeptide alcohols 6a–c from N-Fmoc-(a-dipeptidoyl)benzotriazoles 5a, 5b
Entry
5
Amino alcohol 2
Product 6, yield (%)a
1
2
3
5a
5a
5b
L-Phenylalaninol
L-Phenylglycinol
L-Phenylalaninol
N-Fmoc-L-Phe-L-Ala-L-Phenylalaninolb 6a, 69
N-Fmoc-L-Phe-L-Ala-L-Phenylglycinolb 6b, 65
N-Fmoc-Gly-L-Leu-L-Phenylalaninolb 6c, 69
a Isolated yield. b Retention times for 6a, 28.7; 6b, 13.65; and 6c, 26.67 min.
high yield (46–89%); (3) no racemization occurred; (4) less time to
complete; and (5) uses inexpensive reagents.
dC (75 MHz; CDCl3; Me4Si): 172.0, 156.2, 137.8, 129.3, 128.6,
126.6, 80.2, 63.4, 60.6, 52.9, 37.0, 30.6, 28.4, 19.3 and 17.9.
Preparation of tripeptide alcohols 6a–c; general procedure
Conclusions
To
a solution of N-Fmoc-(a-dipeptidoyl)benzotriazole 5
In conclusion, a convenient and efficient method for the
preparation of di-, and tri-peptide alcohols under mild and
simple reaction conditions has been developed by react-
ing N-protected(a-aminoacyl)benzotriazoles and N-protected(a-
dipeptidoyl)benzotriazoles with amino alcohols at room temper-
ature. All the peptide alcohols were obtained in moderate to good
yields with no detectable racemization for chiral compounds.
(0.29 mmol◦) in THF (5 mL), amino alcohol (0.29 mmol) was
◦
added at 0 C. The reaction mixture was stirred at 0 C for 4 h
followed by 3 h at room temperature. Hexanes (5 mL) were added
and the solid separated was washed with diethyl ether (2 ¥ 10 mL)
and dried to give the corresponding tripeptide alcohol 6.
Fmoc-L-Phe-L-Ala-L-Phenylalaninol◦6a. (Yield 0.11 g, 69%.)
White microcrystals; mp 185.0–186.0 C (from tetrahydrofuran–
hexanes). [a]2D4 -67.0 (c 1.0 in MeOH). (Found: C, 71.29; H, 6.25;
N, 6.83. Calc. for C36H37N3O5H2O: C, 70.92; H, 6.12; N, 6.89%.)
dH (300 MHz; DMSO-d6; Me4Si): 8.12 (1 H, d, J 7.2, NH), 7.87
(2 H, d, J 7.5, ArH), 7.73 (1 H, d, J 8.1, NH), 7.61–7.66 (2 H, m,
ArH), 7.41(2 H, t, J 8.1, ArH), 7.15–7.34 (13 H, m, ArH, NH),
4.81 (1 H, br s, CH), 4.25–4.29 (2 H, m, CH2), 4.13–4.17 (4 H, m,
2 ¥ CH2), 3.82–3.89 (1 H, m, CH), 2.62–3.01 (5 H, m, CH2, 2 ¥
CH, OH) and 1.18 (3 H, d, J 6.9, CH3); dC (75 MHz: DMSO-
d6; Me4Si): 171.8, 171.2, 155.9, 143.8, 140.7, 138.3, 129.3, 129.2,
128.2, 128.1, 127.7, 127.2, 126.3, 126.0, 125.3, 120.2, 65.7, 62.1,
56.1, 52.4, 48.41, 46.6, 37.5, 36.4 and 18.6.
Experimental
Starting materials and solvents were purchased from commercial
sources and used without further purification. Melting points were
determined on a Fisher melting apparatus and are uncorrected.
Column chromatography was carried out using silica gel 200–
425 mesh. HPLC analyses were performed on Shimadzu SPD-20-
A using a Whelk-O1 chiral column with detection at 254 nm, a flow
rate of 1.0 mL/min and hexanes–isopropanol (9:1) as the eluting
1
solvent. H NMR (300 MHz) and 13C NMR (75 MHz) spectra
were recorded on a 300 MHz NMR spectrometer with CDCl3 or
DMSO-d6 as solvents. J values are given in Hz. [a]D values are
given in 10-1 deg cm2 g-1. Elemental analyses were performed on a
Carlo Erba-1106 instrument.
Acknowledgements
We thank Dr C. D. Hall for helpful discussions and Dr Q.-Y. Chen
for some preliminary experiments.
Preparation of dipeptide alcohols 3a–p and (3d + 3d¢); general
procedure
To a mixture of N-protected(a-aminoacyl)benzotriazoles 1
(1 mmol) in dry THF (10 mL) was added the a-amino alcohol 2
(1 mmol) and the mixture was stirred for 6 h at room temperature.
The solvent was evaporated under vacuum and the residue was
dissolved in ethyl acetate (15 mL), washed with Na2CO3 (3 ¥
5 mL), brine (1 ¥ 5 mL), water (1 ¥ 5 mL) and dried over Na2SO4.
The solvent was evaporated and the solid residue was crystallized
from ethyl acetate–hexanes to afford the corresponding dipeptide
alcohols 3.
Notes and references
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