An improved procedure for the synthesis of dehydroamino acids and dehydropeptides from the carbonate derivatives of serine and threonine using tetrabutylammonium fluoride

Article abstract of DOI:10.1002/psc.1210

Dehydroamino acids are important precursors for the synthesis of a number of unnatural amino acids and are structural components in many biologically active peptide derivatives. However, efficient synthetic procedures for their production in large amounts

Full text of DOI:10.1002/psc.1210

Rapid Communication
Received: 12 October 2009
Revised: 28 November 2009
Accepted: 1 December 2009
Published online in Wiley Interscience: 28 January 2010
(www.interscience.com) DOI 10.1002/psc.1210
An improved procedure for the synthesis of
dehydroamino acids and dehydropeptides
from the carbonate derivatives of serine and
threonine using tetrabutylammonium fluoride
Ramesh Ramapanicker, Roli Mishra and Srinivasan Chandrasekaran∗
Dehydroamino acids are important precursors for the synthesis of a number of unnatural amino acids and are structural
components in many biologically active peptide derivatives. However, efficient synthetic procedures for their production
in large amounts and without side reactions are limited. We report here an improved procedure for the synthesis of
dehydroalanine and dehydroamino butyric acid from the carbonate derivatives of serine and threonine using TBAF. The
antiselective E2 elimination of the carbonate derivatives of serine and threonine using TBAF is milder and more efficient
than other available procedures. The elimination reaction is completed in less than 10 min with various carbonate derivatives
studied and the methodology is very efficient for the synthesis of dehydroamino acids and dehydropeptides. The procedure
thus provides an easy access to key synthetic precursors and can be used to introduce interesting structural elements to
c
designed peptides. Copyright ꢀ 2010 European Peptide Society and John Wiley & Sons, Ltd.
Keywords: amino acids; peptides; elimination; carbonates
Results and Discussion
corresponding dehydroamino acid derivatives using Boc2O and
DMAP, we developed a very useful and high yielding procedure for
the β-elimination of carbonate derivatives of serine and threonine
to dehydroalanine and dehydroamino butyric acid derivatives,
respectively, using K2CO3 (2 equiv, 60 ◦C, 1 h, DMF) [6]. The
method is very useful for the selective preparation of Z-isomers
of dehydroamino acids in very high yields and does not lead
to any side reactions. Preparation of dehydropeptides using the
procedure is simple and the reaction conditions do not result in
racemization of other stereogenic centers present in the peptide
[6]. However, the procedure could still be improved by using a low
boiling solvent instead of DMF and also by finding a suitable base
that can affect the elimination reaction at a lower temperature.
Our initial experiments had revealed that the use of organic bases,
such as triethylamine, DMAP, NMM, DIPEA etc. did not result in
the expected elimination reaction of the carbonates of serine and
threonine and the use of more basic reagents, such as NaOH,
NaOMe etc. resulted in side reactions including the hydrolysis of
ester functionalities present in the molecules. The reactions were
best performed in DMF and the use of other low boiling solvents,
such as THF, CH3CN, CH2Cl2 etc. resulted in low yields of the
products as the solubility of K2CO3 in these solvents is very poor.
A recent report on the use of TBAF as a base for the dehy-
drobromination of vinyl bromides to terminal acetylenes [7] and
preceding reports on the use of TBAF for the dehydrohalogena-
α,β-Dehydroamino acids are important constituents of many
peptide antibiotics and are very useful synthetic precursors for the
synthesisofanumberofunnaturalaminoacidsandpeptides[1–3].
The dehydration of β-hydroxyamino acids is the most common
strategy employed for the synthesis of α,β-dehydroamino acids
and α,β-dehydropeptides [1,2]. There are a number of procedures
available in the literature for the preparation of dehydroalanine
(ꢀAla) and dehydroamino butyric acid (ꢀAbu) derivatives from
serine and threonine, respectively, either in one step by direct
elimination of water using various reagents or in two steps by
converting the hydroxyl group to a good leaving group followed
by a β-elimination [1,2,4]. In general, most of the procedures
used for the elimination reactions of β-hydroxyamino acids
give low to moderate yields and the intermediates are quite
reactiveandundergosidereactions.Thesynthesisofβ-substituted
dehydroamino acids using many of these methods generally
results in the formation of a mixture of both the stereoisomers
of the olefin. The generation of dehydroalanine or dehydroamino
butyric acid residue within a peptide by dehydrating a serine
or a threonine residue faces additional problems. The hydroxyl
groups of serine or threonine need to be protected prior to
peptide synthesis and later these protecting groups need to
be removed before the peptides are subjected to elimination
reactions; all of which add to the total number of steps involved
in the synthesis. Thus, although there are a number of procedures
available for the synthesis of dehydroamino acids, none of them
have completely satisfied the increased requirement of these
compounds as commodity chemicals.
∗
Correspondence to: Srinivasan Chandrasekaran, Department of Organic
Chemistry, Indian Institute of Science, Bangalore 560012, India.
E-mail: scn@orgchem.iisc.ernet.in
Inspired by a mild procedure employed by Ferreira et al. [5]
for the selective elimination of β-hydroxyamino acids to the
Department of Organic Chemistry, Indian Institute of Science, Bangalore
560012, India
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J. Pept. Sci. 2010; 16: 123–125
Copyright ꢀ 2010 European Peptide Society and John Wiley & Sons, Ltd.

RAMAPANICKER, MISHRA AND CHANDRASEKARAN
BnO
O
Table 1. Preparation of dehydroamino acid derivatives from the
carbonate derivatives of serine and threonine using TBAF
O
O
TBAF (1 equiv)
10 min,THF
Carbonate
derivatives of
serine and threonine
Dehydroamino acid
derivatives and
peptides
OCH3
OCH3
BocHN
BocHN
Yield
(%)a
Entry
1
O
1a
2a (88%)
80
80
80
89
O
O
Scheme 1. Preparation of the dehydroalanine derivative (2a) from the
benzyl carbonate derivative (1a) of serine using TBAF.
BocHN COOBn
2b
O
BocHN COOBn
1b
tion of alkyl halides to alkenes [8–11] prompted us to attempt the
elimination of the benzyl carbonate derivative (1a) of DL-serine
using TBAF (The basic nature of the unsolvated fluoride ion, as
in TBAF/THF has long been exploited in carrying out a number
of organic transformations) [12]. When 1a was treated with 1
equiv of TBAF (1 M solution in THF), the elimination occurred
readily (10 min, 28 ◦C) and the dehydroalanine derivative 2a was
obtained in excellent yield (88%, Scheme 1).
2
3
4
O
O
CbzHN COOMe
2c
O
CbzHN COOMe
1c
O
O
BocHN COOMe
O
2d
BocHN COOMe
1d
The efficient elimination of the benzyl carbonate group from
1a in the presence of TBAF was surprising as none of the organic
bases studied (DMAP, Et3N, DIPEA etc.; There was considerable
elimination reaction observed on using DBU as the base. 1a
was quantitatively converted to 2a in l h with 1 equiv of DBU)
[13] effected these reactions under the same conditions (THF,
28 ◦C) even after 6 h. These observations offered the scope for an
improved methodology for the synthesis of dehydroamino acids
and dehydropeptides from the carbonate derivatives of serine and
threonine. We examined the usefulness of this reaction by treating
various carbonate derivatives of serine and threonine (1b–h) with
1 equiv of TBAF (Table 1). The reactions were complete within
10 min and the corresponding dehydroamino acid derivatives
(2b–h) could be isolated in excellent yields (Table 1). The reactions
proceeded via a trans E2-elimination resulting in the formation of
only the Z-isomer of the dehydroamino butyric acid derivatives
(2f–h) from the carbonate derivatives (1f–h) of threonine (A
comparison of 1H NMR data of 2f-k with those of compounds
containing ꢀAbu residues available in the literature confirmed the
Z-configuration of the olefin). The procedure was highly efficient
for the synthesis of dehydropeptides (2i–l) (entries 8–11, Table 1)
and proceeded without racemization of the other chiral centers
present in the peptide (A comparison of the optical rotation of
the dehydropeptide 2l ([α]2D5 = −64 c 1, MeOH) prepared using
TBAF and the one prepared using K2CO3 ([α]2D5 = −64 c 1, MeOH)
showed similar values). The use of TBAF effected the elimination
reaction at a faster rate and at rt (28 ◦C) which is a definite
advantage over the method previously reported using K2CO3
[6].
Cl3C
O
O
BocHN COOMe
O
2e
BocHN COOMe
1e
5
6
88
88
Ph
O
O
O
BocHN COOMe
2f
BocHN COOMe
1f
Ph
O
O
O
BocHN COOBn
2g
BocHN COOBn
1g
7
8
81
84
O
O
CbzHN COOMe
O
2h
CbzHN COOMe
1h
O
O
H
N
COOMe
BocHN
O
O
O
O
H
N
COOMe
2i
BocHN
BocHN
1i
9
84
H
N
H
COOMe
COOMe
N
COOMe
BocHN
BocHN
O
2j
We examined the possibility of using catalytic amount of TBAF
for carrying out these elimination reactions. Accordingly, 1a was
treated with 0.1 equiv of TBAF and the dehydroalanine derivative
2a could be isolated in 87% yield after 4 h. When 0.5 equiv of
TBAF was used, the elimination reaction was complete in 30 min
to give 2a in 85% yield. Although it is possible to carry out this
elimination reaction using catalytic amount of TBAF, the rate of
the reaction is considerably reduced. Quite surprisingly, a similar
elimination of an acetyl derivative of serine, Boc-Ser(Ac)-OMe (3)
was not successful using 1 equiv of TBAF; the elimination product
2a could be isolated only in 42% after 24 h (Scheme 2).
O
1j
O
O
O
O CCl3
10
11
86
83
Ph
Ph
H
H
N
N
COOMe
BocHN
BocHN
O
2k
O
O
1k
O CCl3
Ph
Ph
H
N
H
N
COOMe
O
COOMe
BocHN
O
O
2l
In conclusion, TBAF is found to be a better base for the
elimination of carbonate derivatives of serine and threonine
to the corresponding dehydroamino acid derivatives. We have
thus developed an improved procedure for the synthesis of
1l
O
Ph
a Yields correspond to those of products isolated using column
chromatography.
c
www.interscience.com/journal/psc Copyright ꢀ 2010 European Peptide Society and John Wiley & Sons, Ltd. J. Pept. Sci. 2010; 16: 123–125

SYNTHESIS OF DEHYDROAMINO ACIDS AND DEHYDROPEPTIDES
Boc-Phe-ꢀAbu-OMe (2k)
O
Colorless oil.
O
[α]2D5 − 44 (c 1, ethanol)
TBAF (1 equiv)
IR (Neat): 3372, 1741 cm−1
24 h, 28 °C
BocHN
COOMe
BocHN
COOMe
1H NMR (400 MHz, CDCl3): δ = 7.37 (s, 1H), 7.32–7.24 (m, 6H), 6.78
(q, J = 1.2 Hz, 1H), 5.03 (br, s, 1H), 4.48 (br, s, 1H), 3.73 (s, 3H), 3.16
(dd, J1 = 8 Hz, J2 = 8 Hz, 1H), 3.35 (dd, J1 = 8 Hz, J2 = 8 Hz, 1H),
2.17 (s, 1H), 2.68 (d, J = 3.2 Hz, 4H), 1.41 (s, 9H).
13C NMR (100 MHz, CDCl3): δ = 169.9, 164.5, 136.4, 134.4, 129.3,
128.6, 125.7, 52.2, 28.3, 28.2, 14.5.
3
2a (42%)
Scheme 2. Elimination of an acetyl derivative of serine using TBAF.
dehydroamino acids and dehydropeptides from the carbonate
derivatives of hydroxy amino acids, exploiting the basicity of the
unsolvated fluoride ion in TBAF.
HRMS: m/z calculated for C19H26N2O5 + H = 363.1920; Observed
= 363.1942.
Acknowledgements
Experimental Part
RR thanks CSIR, New Delhi for a senior research fellowship and
IISc for financial assistance, and RM thanks DBT for a postdoctoral
fellowship.
All reagents were purchased from commercial sources and
were used without further treatment. The amino acid and
peptide derivatives (1a–l) were synthesized using procedures
reported [5]. 1H (400 MHz) and 13C NMR (100 MHz) were
recorded on a Bruker 400 spectrometer. IR spectra were
recorded on a JASCO FTIR spectrophotometer. High resolu-
tion mass spectra were recorded on a Micromass QTOF ESIMS
instrument.
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Procedure for the Preparation of Dehydroalanine and De-
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c
J. Pept. Sci. 2010; 16: 123–125 Copyright ꢀ 2010 European Peptide Society and John Wiley & Sons, Ltd. www.interscience.com/journal/psc