High yielding synthesis of dehydroamino acid and dehydropeptide derivatives

Article abstract of DOI:10.1039/a904730a

By using a 4-dimethylaminopyridine (DMAP) catalysed reaction of β-hydroxyamino acid derivatives with tert-butyl pyrocarbonate [(Boc)₂O], dehydroamino acid derivatives are obtained in high yields. The same methodology applied to dipeptides with

Full text of DOI:10.1039/a904730a

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High yielding synthesis of dehydroamino acid and dehydropeptide
derivatives
Paula M. T. Ferreira, Hernâni L. S. Maia, Luís S. Monteiro and Joana Sacramento
Department of Chemistry, University of Minho, Gualtar, P-4700-320 Braga, Portugal
Received (in Cambridge, UK) 14th June 1999, Accepted 19th October 1999
By using a 4-dimethylaminopyridine (DMAP) catalysed reaction of β-hydroxyamino acid derivatives with tert-
butyl pyrocarbonate [(Boc)2O], dehydroamino acid derivatives are obtained in high yields. The same methodology
applied to dipeptides with a β-hydroxyamino acid residue gives the corresponding dipeptides with a dehydroamino
acid residue.
tion, this method is equally suitable for the ready dehydration
of peptide derivatives containing one or more β-hydroxyamino
Introduction
Nisin, a well known preservative in the food industry, epider-
min, a therapeutic agent against acne, and ancovenin, an
immunologically active compound, are examples of the many
different compounds belonging to the important class of
natural bacteriocins known as lantibiotics. Owing to the wide
variety of biological activities and applications found among
the known compounds and also to the economical importance
of many of them, lantibiotics offer a promising field for explor-
ation of new compounds with new biological properties and
new applications. Lantibiotics are polycyclic peptides, which,
in addition to lanthionine, contain α,β-dehydroamino acid
residues. The latter have been found also in the molecules of
several enzymes from plant and bacterial sources and have been
used as linkers in solid phase peptide synthesis and in synthetic
peptides aimed at structure–activity relationship studies, since
they affect both chemical reactivity and conformation.1–4
Another important application for dehydroamino acid deriv-
atives is their use in addition reactions yielding new amino acids
as the corresponding β-substituted products.5,6 A key step to
progress in this field is, thus, the production of α,β-didehydro-
amino acid derivatives suitable for incorporation into peptide
sequences or, otherwise, a method for dehydration of appropri-
ate precursors. An obvious approach to these compounds is the
dehydration of β-hydroxyamino acid derivatives as found in the
biosynthetic route to lantibiotics in which their precursors con-
taining serine and threonine residues are dehydrated to give the
corresponding dehydroalanine (∆Ala) and dehydroaminobut-
yric acid (∆Abu) derivatives. In fact, this has been the most
widely used approach to the chemical synthesis of dehydro-
amino acid derivatives, mainly for the case of dehydroalanine
and dehydroaminobutyric acid.4,7–9 Several other methods have
been developed for the synthesis of these compounds, which
include Hofmann degradation of α,β-diaminopropionyl resi-
dues,10 reduction of azidoacrylates,3 hydrolysis of unsaturated
oxazolinones11 and condensation of α-ketoacids with amides or
nitriles.12,13 As described in the literature, all the above methods
are usually low yielding, multistep processes requiring tedious
purifications to remove side products; this may be related par-
tially to the fact that, together with some of their derivatives,
they undergo polymerisation and hydrolysis fairly easily. In the
case of dehydroaminobutyric acid derivatives the work-up pro-
cedures are complicated by the formation of two stereoisomers.
Thus, there is still a need for developing a simple and efficient
approach to these compounds and in this paper we report such
an approach; it is again based on β-elimination from β-hydroxy-
amino acid derivatives but the reactions are almost quantitative
and no mixtures of isomers have been found, so far. In addi-
acid residues.
Results and discussion
Our approach was initially based on Berkowitz and Peder-
son’s method for simultaneous amine and carboxy protection
of amino acids with benzyl chloroformate in the presence of
DMAP and triethylamine.14 We found that under these condi-
tions serine undergoes elimination, the only product isolated
being the corresponding fully protected dehydroalanine deriv-
ative (Z-∆Ala-OBn) in a yield of 51%.15 Applying the same
procedure to several amino acids protected either at their
N-terminus or at both the N- and the C-terminus, the yields
obtained were within the range 56–76%. With a threonine
derivative, although all the starting material was consumed, we
failed to obtain any pure product and NMR spectroscopy of
the reaction mixture was consistent with the presence of the two
isomers of dehydroaminobutyric acid.
The experimental conditions usually required to cleave most
of the protecting groups we used for N-protection, viz. benzyl-
oxycarbonyl (Z), p-nitrobenzyloxycarbonyl [Z(NO2)] and tosyl
(Tos) are possibly too drastic to be applied to dehydroamino
acid derivatives. However, their use was intended to allow the
investigation of their cleavage from dehydroamino acids by
mild electrolysis according to techniques developed earlier by
our team.16,17 In these studies we showed that electrolysis is
facilitated when two acyl groups are attached to the nitrogen
atom. We also showed that tert-butoxycarbonyl (Boc) not only
fulfils efficiently the role required for the second acyl group but
it is easy to introduce by reaction of the previously protected
material with tert-butyl pyrocarbonate, (Boc)2O, in the presence
of DMAP as catalyst, according to Ragnarsson’s method for
tert-butoxylation of amides.18 This acylation with Boc prior to
electrolysis resembles significantly the method we had taken
advantage of to dehydrate serine. Thus, the use of two equiv-
alents of tert-butyl pyrocarbonate would suit both tasks, i.e.
further acylation for later selective deprotection by electrolysis,
if at all required, and dehydration (2, Scheme 1). Although the
increased bulk thus created at the nitrogen atom was expected
to assist elimination during the dehydration step, the results
obtained exceeded our expectations; in fact, the only product
isolated was the corresponding dehydrated diacyl ester in an
almost quantitative yield.19 Moreover, with both threonine and
β-hydroxyphenylalanine derivatives (threo type) the reaction
was stereoselective, giving only the Z-isomer as shown by NMR
spectroscopy. This selectivity seems to result again from the
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703
This journal is © The Royal Society of Chemistry 1999
3697

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Table 1 Results obtained in the synthesis of dehydroamino acid
Application of our methodology to an N-fluoren-9-ylmethoxy-
derivatives
carbonyl (Fmoc) derivative of serine failed to give Fmoc-
∆Ala(N-Boc)-OMe due to base induced cleavage of Fmoc by
the catalyst (DMAP); when excess (Boc)2O was used, the Fmoc
group was substituted by a further Boc group and, thus, the
only product obtained was Boc-∆Ala(N-Boc)-OMe.
Yield
(%)
Reagent
Product
Boc-Ser-OMe, 1a
Z-Ser-OMe, 1b
Boc-∆Ala(N-Boc)-OMe, 2a
Z-∆Ala(N-Boc)-OMe, 2b
Z(NO2)-∆Ala(N-Boc)-OMe, 2c
Z(NO2)-∆Ala(N-Boc)-OBn, 2d
Tos-∆Ala(N-Boc)-OMe, 2e
Tos-∆Ala(N-Boc)-OBn, 2f
Bz-∆Ala(N-Boc)-OMe, 2g
Bz-∆Ala(N-Boc)-OBn, 2h
Boc-∆ABu(N-Boc)-OMe, 2i
Z-∆Abu(N-Boc)-OMe, 2j
92
85
93
91
99
97
92
92
87
94
DMAP catalysed esterifications with dicarbonate have been
described by Takeda et al.22 With the aim of simplifying our
procedure by avoiding one of the two otherwise required pro-
tection steps, N-Boc protected serine and threonine having a
free carboxy function were reacted with 3 eq. of tert-butyl
pyrocarbonate in the presence of DMAP. As expected, both
dehydration and esterification were obtained in the same step
to give the tert-butyl ester of the N,N-(Boc)2-dehydroamino
acid. However, the reactions were more sluggish and the yields
slightly lower when compared to those of dehydration of the
corresponding methyl or benzyl esters. Having again in mind to
simplify our procedure by avoiding one of the two required
protection steps, we have also investigated the direct dehydra-
tion of the methyl ester of serine. The reaction of this amino
acid derivative with 3 eq. of (Boc)2O allowed the preparation of
2a in an 82% yield, which was lower by 10% than that obtained
when the N-Boc methyl ester of serine was used as starting
material.
Z(NO2)-Ser-OMe, 1c
Z(NO2)-Ser-OBn, 1d
Tos-Ser-OMe, 1e
Tos-Ser-OBn, 1f
Bz-Ser-OMe, 1g
Bz-Ser-OBn, 1h
Boc-Thr-OMe, 1i
Z-Thr-OMe, 1j
Z(NO2)-Thr-OMe, 1k
Tos-Thr-OMe, 1l
Z(NO2)-Phe(β-OH)-OMe,
1m
Boc-Ser-OH, 1n
Boc-Thr-OH, 1o
H-Ser-OMe, 1p
Z(NO2)-∆Abu(N-Boc)-OMe, 2k 92
Tos-∆Abu(N-Boc)-OMe, 2l 87
Z(NO2)-∆Phe(N-Boc)-OMe, 2m 93
Boc-∆Ala(N-Boc)-OBut, 2n
Boc-∆Abu(N-Boc)-OBut, 2o
Boc-∆Ala(N-Boc)-OMe, 2a
73
73
82
As stated previously, the use of most of the protecting groups
was intended to allow the investigation of their cleavage from
dehydroamino acids by mild electrolysis,16,17 which offers a
clean, non-polluting alternative to the classical methods of
reduction. Thus, both Z(NO2) and Tos were selectively removed
by electrolysis at controlled potential from 2c and 2e to give
Boc-∆Ala-OMe in yields of 88 and 73%, respectively, and from
2k and 2l to give Boc-∆Abu-OMe in yields of 88 and 78%,
respectively. However, when electrochemical equipment is
not available selective cleavage can still be achieved by taking
advantage of an alternative strategy we have developed
recently,23 which is based on selective reduction with an
appropriate metal according to the potential required. As an
example, Z(NO2) was cleaved by selective reduction from 2c and
2k with mercury activated aluminium to give Boc-∆Ala-OMe
and Boc-∆Abu-OMe in yields of 87 and 95%, respectively.
Cleavage of Boc from the diacyl derivatives was also easily
performed with trifluoroacetic acid (TFA) and, thus, 2j and 2k
gave Z-∆Abu-OMe and Z(NO2)-∆Abu-OMe in yields of 87 and
85%, respectively. Z-∆Abu-OMe was saponified to yield 77% of
Z-∆Abu-OH, which was then coupled with glycine methyl ester
by dicyclohexylcarbodiimide–hydroxybenzotriazole (DCC–
HOBt) coupling to give the corresponding dehydrodipeptide in
an 85% yield.
Owing to the low reactivity of the α-amine group of
dehydroalanine24 and to the instability of its N-deprotected
derivatives,2 most of the dehydroamino acid derivatives men-
tioned so far are of limited application in peptide synthesis.
This led us to investigate the applicability of our methodology
to the dehydration of peptides containing β-hydroxyamino
acids (3a–g, Table 2) as precursors of dehydropeptides (4a–g).
Thus, dipeptides containing serine or threonine in either the
amine or the carboxy terminus were reacted under the condi-
tions described previously. In these reactions 3 eq. of (Boc)2O
were used, i.e. 2 eq. for acylation of both amide nitrogen atoms
and a third equivalent to generate the carbonate at the β-carbon
atom, which subsequently underwent elimination to form the
double bond. Again, peptides containing threonine gave only
one of the two possible geometric isomers. In the case of a
dipeptide containing both threonine and serine (3e) and of
another containing two residues of threonine (3f), simultaneous
dehydration of both amino acid residues was achieved.
Scheme 1
bulkiness of the groups bound to the nitrogen atom, which
would force and thus facilitate a trans E2-elimination. This is in
agreement with results obtained by Srinivasan et al.20 who have
reported that base induced β-elimination of N-acyl--Thr-
(O-Tos)-OMe (threo type) proceeds via a trans E2-elimination
to give the Z-isomer but only in a 70% yield. As shown in
Table 1, in all cases but one, the starting material had the amine
function previously protected with one of the groups men-
tioned above, or by the benzoyl group, and except for two cases
the carboxy function was protected as the methyl or the benzyl
ester.
Recently, Nugent has patented a method for dehydration of
N-acyl-β-hydroxyamino acid esters by treatment with an excess
of acetic anhydride in the presence of pyridine.21 In this reac-
tion an acetyl group was introduced at the amide function to
give the N-acetyl-N-acyl-dehydroamino acid ester. The second
acyl group bonded to the nitrogen atom would help formation
of the new double bond, but the reported yields are only fair
(≈60%) and the product thus obtained is of limited value: the
acetyl group cannot be easily removed and removal of both acyl
groups would lead to decomposition of the hydroxyamino acid
at the N–C bond.2
The high yields obtained in the dehydration of serine deriv-
atives by our method show the importance of increasing the
bulk of the second acyl group at the nitrogen function. In fact,
by sampling the reaction mixture throughout the preparation
of compound 2c, it was found that the reaction proceeds with
formation of a tert-butyl carbonate, which undergoes β-elimin-
ation to the final product after a tert-butyloxycarbonyl group
has been bound to the amine function. In an attempt to
use an N-trityl serine derivative (Trt-Ser-OMe) as substrate for
β-elimination the only product obtained was Trt-Ser(O-Boc)-
OMe. Steric hindrance related to the trityl group prevented fur-
ther reaction at the nitrogen atom; the absence of dehydration
suggests that, in addition to bulkiness at the nitrogen atom, the
missing acyl group might be required as a driving force for
elimination by stabilisation of the resulting α–β double bond.
In conclusion, the method reported above allows the high
yield preparation of a variety of dehydroamino acid and
dehydroamino acid-containing peptide derivatives by β-elimin-
ation of the respective β-hydroxyamino acid or β-hydroxy-
3698
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703

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Table
2
Results obtained in the synthesis of dehydropeptide
Yield
tert-butyl pyrocarbonate to afford the N-acyl serine methyl
esters: 1b, mp 42.5–44.1 ЊC (from ethyl acetate–n-hexane) (lit.,26
33–35 ЊC); 1e, mp 87–88 ЊC (from ethyl acetate–petroleum
ether) (lit.,27 92–93 ЊC); 1g, mp 84.5–85.5 ЊC (from ethyl
acetate–petroleum ether) (lit.,28 84–86 ЊC).
derivatives
Reagent
Product
(%)
Z(NO2)-Ser-Phe-OEt, Z(NO2)-∆Ala(N-Boc)-Phe(N-Boc)-
93
Synthesis of 1c. The same procedure as described for the
preparation of 1a was followed substituting p-nitrobenzyl
chloroformate for tert-butyl pyrocarbonate to give 1c29 (71%),
mp 74–75 ЊC (from ethyl acetate–petroleum ether) (Found: C,
48.1; H, 4.7; N, 9.6. Calc. for C12H14N2O7: C, 48.3; H, 4.7; N,
9.4%); δH 3.81 (3 H, s, CH3 OMe), 3.91–4.09 (2 H, complex,
βCH2 Ser), 4.46 (1 H, m, αCH Ser), 5.24 [2 H, s, CH2 Z(NO2)],
5.80 (1 H, d, J 7.0, αNH), 7.53, 8.23 [2 H ϩ 2 H, 2d, J 9.0, ArH
Z(NO2)]; δC 52.86, 56.00, 63.15, 65.61, 123.78, 128.12, 134.02,
143.48, 147.63, 156.26, 170.74.
3a
OEt, 4a
Boc-Ala-Ser-OMe, 3b Boc-Ala(N-Boc)-∆Ala(N-Boc)-OMe, 91
4b
Z-Thr-Gly-OMe, 3c
Z-Abu(N-Boc)-Gly(N-Boc)-OMe, 4c
81
Boc-Ala-Thr-OMe, 3d Boc-Ala(N-Boc)-∆Abu(N-Boc)-OMe, 84
4d
Boc-Thr-Ser-OMe, 3e Boc-∆Abu(N-Boc)-∆Ala(N-Boc)-
OMe, 4e
83
74
Boc-Thr-Thr-OMe,
3f
Boc-∆Abu(N-Boc)-∆Abu(N-Boc)-
OMe, 4f
amino acid containing peptides using mild reaction conditions
and simple work-up procedures. In the case of dehydration of
β-alkyl or β-aryl substituted β-hydroxyamino acids, unlike
other methods previously reported, this methodology offers the
further advantage of leading to a single isomer.
Preparation of 1d. The same procedure as described for the
preparation of 1c was followed substituting H-Ser-OBnؒHCl
for H-Ser-OMeؒHCl to give 1d (76%), mp 85–87 ЊC (from ethyl
acetate–petroleum ether) (Found: C, 57.6; H, 4.6; N, 7.2. Calc.
for C18H18N2O7: C, 57.8; H, 4.8; N, 7.5%); δH 4.01 (2 H, m,
βCH2 Ser), 4.50 (1 H, m, αCH2 Ser), 5.22, 5.24 [2 H ϩ 2 H, 2s,
CH2 Z(NO2) and CH2 Bn], 5.78 (1 H, br, αNH), 7.37 (5 H, s,
ArH Bn), 7.51, 8.21 [2 H ϩ 2 H, 2d, J 8.4, ArH Z(NO2)];
δC(75.4 MHz; CDCl3) 56.15, 63.14, 65.56, 67.62, 123.75, 128.06,
128.18, 128.61, 134.98, 143.48, 147.63, 155.70, 170.18.
Experimental
General methods
All melting points were determined on a Gallenkamp melting
point apparatus and are uncorrected. TLC analyses were
carried out on 0.25 mm thick precoated silica plates (Merck
Fertigplatten Kieselgel 60F254) and spots were visualised under
UV light or, preferably, by heating and subsequent dicarb-
oxidine-spray. Preparative chromatography was carried out on
Merck Kieselgel 60 (230–400 mesh). Petroleum ether refers to
the light petroleum fraction of boiling range 40–60 ЊC. 1H
NMR spectra were recorded on a Varian 300 MHz spec-
trometer for ~5% CDCl3 solutions at 25 ЊC, unless otherwise
stated. All shifts are given in δ ppm using δH (Me4Si) = 0 as
reference. J values are given in Hz. Assignments were made by
comparison of chemical shifts, peak multiplicities and J values.
13C NMR spectra were determined for CDCl3 solutions in the
same instrument at 75.4 MHz using the solvent peak as internal
reference. Elemental analyses of crystalline derivatives and
some oils were carried out on a Leco CHNS 932 instrument.
For controlled potential electrolysis experiments a Hi-Tek
potentiostat DT 2101, and a Hi-Tek wave generator PP RI,
connected to a Philips recorder PM 8043 were used. The
electrolysis cell was a conventional two-compartment, three-
electrode, home-built batch cell of the type illustrated else-
where.16
Preparation of 1f. The same procedure as described for the
preparation of 1d was followed substituting tosyl chloride for
p-nitrobenzyl chloroformate to give 1f15 (87%), mp 82–83 ЊC
(from ethyl acetate–petroleum ether) (Found: C, 58.6; H, 5.4;
N, 4.1; S, 9.2. Calc. for C17H19NO5S: C, 58.4; H, 5.5; N, 4.0; S,
9.2%); δH 2.41 (3 H, s, CH3 Tos), 3.91 (2 H, m, βCH2 Ser), 4.04
(1 H, m, αCH Ser), 5.04 (2 H, s, CH2 Bn), 7.27 (5 H, m, ArH
Bn), 7.35, 7.73 (2 H ϩ 2 H, 2d, J 8.1, ArH Tos); δC 21.55, 57.68,
63.78, 65.36, 127.16, 128.12, 128.38, 128.54, 129.75, 134.67,
136.38, 143.84, 169.53.
Preparation of 1h. The same procedure as described for the
preparation of 1d was followed substituting benzoyl chloride
for p-nitrobenzyl chloroformate to give 1h30 (97%), mp 105–
106 ЊC (from ethyl acetate–n-hexane) (Found: C, 68.5; H, 5.8;
N, 4.6. Calc. for C17H17NO4: C, 68.2; H, 5.7; N, 4.7%); δH 4.02–
4.12 (2 H, complex, βCH2 Ser), 4.91 (1 H, m, αCH2 Ser), 5.25
(2 H, s, CH2 Bn), 7.18 (1 H, d, J 6.9, αNH), 7.35–7.84 (10 H,
complex, ArH Bz ϩ ArH Bn); δC 55.29, 63.47, 67.61, 127.15,
128.15, 128.53, 128.64, 131.94, 133.46, 135.07, 167.70, 170.44.
Preparation of 1i. The same procedure as described for prep-
aration of 1a was followed substituting H-Thr-OMeؒHCl for
H-Ser-OMeؒHCl to give 1i31 (76%), oil, δH 1.25 (3 H, d, J 6.3,
γCH3 Thr), 1.46 (9 H, s, CH3 Boc), 3.78 (3 H, s, CH3 OMe), 4.28
(2 H, m, αCH Thr and βCH Thr), 5.33 (1 H, br, αNH);
δC 19.84, 28.25, 52.46, 58.66, 60.08, 80.30, 156.20, 171.97.
Preparation of N-acyl amino acid esters
Synthesis of 1a. H-Ser-OMeؒHCl was dissolved in dichloro-
methane (1 mol dmϪ3) and 2.2 eq. of triethylamine added, then
1.1 eq. of tert-butylpyrocarbonate were slowly added with vig-
orous stirring and cooling in an ice bath. After stirring at 0 ЊC
for 30 min the solution was stirred at room temperature for 3 h.
The reaction mixture was then evaporated and partitioned
between 200 cm3 of ethyl acetate and 100 cm3 of KHSO4 (1 mol
dmϪ3) and washed with KHSO4 (1 mol dmϪ3), NaHCO3 (1 mol
dmϪ3) and brine (3 times, 50 cm3 each). After drying over
MgSO4 the extract was taken to dryness at reduced pressure to
give 1a (85%), oil (lit.,25 oil); δH 1.46 (9 H, s, CH3 Boc), 3.79
(3 H, s, CH3 OMe), 3.88–4.98 (2 H, complex, βCH2 Ser), 4.39
(1H, m, αCH Ser), 5.47 (1 H, br, αNH); δC 28.24, 52.57, 55.69,
63.38, 80.30, 155.72, 171.34.
Preparation of 1j. The same procedure as described for prep-
aration of 1i was followed substituting benzyl chloroformate
for tert-butyl pyrocarbonate to give 1j20 (74%), mp 90–91 ЊC
(from ethyl acetate–diethyl ether) (Found: C, 58.3; H, 6.6; N,
5.3. Calc. for C13H17NO5: C, 58.4; H, 6.4; N, 5.2%); δH 1.26
(3 H, d, J 6.3, γCH3 Thr), 3.78 (3 H, s, CH3 OMe), 4.34 (2 H,
m, αCH Thr and βCH Thr), 5.14 (2 H, s, CH2 Z), 5.57 (1 H, d,
J 8.1, αNH), 7.35 (5 H, m, ArH Z); δC 16.78, 49.49, 56.06,
64.13, 64.88, 124.95, 125.11, 125.44, 133.08, 153.63, 168.56.
Preparation of 1k. The same procedure as described for
preparation of 1i was followed substituting p-nitrobenzyl
chloroformate for tert-butyl pyrocarbonate to give 1k (97%),
mp 63.5–65 ЊC (from diethyl ether–n-hexane) (Found: C, 50.0;
Synthesis of 1b, 1e and 1g. The same procedure as described
for the preparation of 1a was used substituting benzyl chloro-
formate, tosyl chloride and benzoyl chloride respectively for
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703
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H, 5.1; N, 8.9. Calc. for C13H16N2O7: C, 50.0; H, 5.2; N, 9.0%);
δH 1.27 (3 H, d, J 6.6, γCH3 Thr), 3.79 (3 H, s, CH3 OMe), 4.30–
4.38 (2 H, complex, αCH Thr and βCH Thr), 5.24 [2 H, s, CH2
Z(NO2)], 5.69 (1 H, d, J 8.4, αNH), 7.53, 8.22 [2 H ϩ 2 H, 2d,
J 8.7, ArH Z(NO2)]; δC 19.92, 52.65, 59.16, 65.56, 67.77,
123.71, 127.96, 143.63, 147.56, 156.23, 171.46.
ether–n-hexane) (Found: C, 60.4; H, 5.3; N, 6.1. Calc. for
C23H24N2O8: C, 60.5; H, 5.3; N, 6.1%); δH 1.42 (9 H, s, CH3
Boc), 5.23, 5.26 [2 H ϩ 2 H, 2s, CH2 OBn ϩ CH2 Z(NO2)],
5.79, 6.50 (1 H ϩ 1 H, 2s, βCH2), 7.34 (5 H, m, ArH Bn), 7.44,
8.16 [2 H ϩ 2 H, 2d, J 8.4, ArH Z(NO2)]; δC 27.69, 66.80, 67.30,
84.17, 123.73, 126.23, 127.95, 128.18, 128.48, 128.58, 135.21,
135.47, 142.40, 147.66, 149.78, 151.74, 162.88.
Preparation of 1l. The same procedure as described for prep-
aration of 1i was followed substituting tosyl chloride for tert-
butyl pyrocarbonate to give 1l32 (92%), mp 99–100 ЊC (from
ethyl acetate–petroleum ether) (Found: C, 50.2; H, 6.1; N, 4.9;
S, 11.3. Calc. for C12H17NO5S: C, 50.2; H, 6.0; N, 4.9; S, 11.1%);
δH 1.27 (3 H, d, J 6.6, γCH3 Thr), 2.42 (3 H, s, CH3 Tos), 3.53
(3 H, s, CH3 OMe), 3.83 (2 H, dd, J 3.0, J 9.3, βCH Thr), 4.15
(1 H, m, αCH Thr), 5.53 (1 H, br, αNH), 7.30, 7.72 (2 H ϩ 2 H,
d, J 8.1, ArH Tos); δC 19.79, 21.51, 52.59, 60.93, 68.36, 127.21,
129.58, 136.69, 143.69, 170.69.
Synthesis of 2e. The same procedure as described for the
preparation of 2a was followed substituting Tos-Ser-OMe for
Boc-Ser-OMe to give 2e19 (99%).
Synthesis of 2f. The same procedure as described for the
preparation of 2a was followed substituting Tos-Ser-OBn for
Boc-Ser-OMe to give 2f (97%), mp 113–114 ЊC (from diethyl
ether–n-hexane) (Found: C, 61.4; H, 6.1; N, 3.45; S, 7.4. Calc.
for C22H25NO6S: C, 61.2; H, 5.8; N, 3.25; S, 7.4%); δH 1.28 (9 H,
s, CH3 Boc), 2.42 (3 H, s, CH3 Tos), 5.22 (2 H, s, CH2 OBn),
6.09, 6.67 (1 H ϩ 1 H, 2s, βCH2), 7.23 (2 H, d, J 8.1, ArH Tos),
7.34 (5 H, m, ArH Bn), 7.91 (2 H, d, J 8.1, ArH Tos); δC 21.62,
27.70, 67.46, 84.82, 128.21, 128.35, 128.52, 129.02, 129.04,
130.02, 133.25, 135.13, 136.04, 144.49, 149.64, 163.03.
Preparation of 1m. The same procedure as described for
preparation of 1c was followed substituting H-Phe(β-OH)-
OMeؒHCl for H-Ser-OMeؒHCl to give 1m (82%), mp 114–
115 ЊC (from ethyl acetate–petroleum ether) (Found: C, 57.95;
H, 4.3; N, 7.5. Calc. for C18H18N2O7: C, 58.1; H, 4.3; N, 7.5%);
δH 3.80 (3 H, s, CH3 OMe), 4.62 [1 H, dd, J 2.4, J 9.5, βCH2
Phe(β-OH)], 5.10 [2 H, q, J 6.9, CH2 Z(NO2)], 5.34 [1 H, q,
J 2.4, αCH Phe(β-OH)], 5.71 (1 H, d, J 6.6, αNH), 7.35
[5 H ϩ 2 H, m, ArH Phe(β-OH) ϩ ArH Z(NO2)], 8.67 [2 H,
d, J 8.7, ArH Z(NO2)]; δC 52.77, 59.79, 65.33, 73.38, 123.63,
125.75, 126.95, 127.77, 128.18, 128.45, 139.45, 143.69, 155.73,
170.90.
Synthesis of 2g. The same procedure as described for the
preparation of 2a was followed substituting Bz-Ser-OMe for
Boc-Ser-OMe to give 2g (92%), mp 73–73.5 ЊC (from diethyl
ether–n-hexane) (Found: C, 63.0; H, 6.35; N, 4.6. Calc. for
C16H19NO5: C, 62.9; H, 6.3; N, 4.6%); δH 1.24 (9 H, s, CH3 Boc),
3.84 (3 H, s, CH3 OMe), 5.84, 6.53 (1 H ϩ 1 H, 2s, βCH2), 7.42–
7.74 (5 H, complex, ArH Bz); δC 27.38, 52.60, 83.83, 125.90,
128.14, 131.73, 136.00, 136.25, 151.96, 163.68, 172.18.
Synthesis of N-Boc,N-acyl dehydroamino acid esters
Synthesis of 2h. The same procedure as described for the
preparation of 2a was followed substituting Bz-Ser-OBn for
Boc-Ser-OMe to give 2h (92%), mp 76–77 ЊC (from diethyl
ether–n-hexane) (Found: C, 69.3; H, 6.2; N, 3.7. Calc. for
C22H23NO5: C, 69.3; H, 6.1; N, 3.7%); δH 1.19 (9 H, s, CH3 Boc),
5.26 (2 H, s, CH2 OBn), 5.87, 6.58 (1 H ϩ 1 H, 2s, βCH2), 7.35–
7.68 (10 H, complex, ArH Bn ϩ ArH Bz); δC 27.31, 67.49,
83.81, 126.29, 128.06, 128.10, 128.29, 128.33, 128.49, 135.19,
135.91, 136.21, 151.89, 163.04, 172.09.
Synthesis of 2a. To a solution of Boc-Ser-OMe in dry
acetonitrile (1 mol dmϪ3), 0.1 eq. of DMAP was added followed
by 2.2 eq. of tert-butyl pyrocarbonate under rapid stirring at
room temperature. The reaction was stirred for 12 h while
monitored by TLC (diethyl ether–n-hexane, 1:1). Evaporation
under reduced pressure gave a residue that was partitioned
between 200 cm3 of diethyl ether and 100 cm3 of KHSO4 (1 mol
dmϪ3). The organic phase was thoroughly washed with KHSO4
(1 mol dmϪ3), NaHCO3 (1 mol dmϪ3) and saturated brine
(3 × 50 cm3 each), and dried over MgSO4. Removal of the
solvent afforded pure 2a8 (92%).
Synthesis of 2i. The same procedure as described for the
preparation of 2a was followed substituting Boc-Thr-OMe for
Boc-Ser-OMe to give 2i. Since attempts to crystallise the com-
pound were unsuccessful the dehydroamino acid derivative was
chromatographed through silica using diethyl ether–n-hexane
as eluent to give 2i as a pure oil (87%) (Found: C, 56.9; H, 8.05;
N, 4.4. Calc. for C15H25NO6: C, 57.1; H, 8.0; N, 4.4%); δH 1.46
(18 H, s, CH3 Boc), 1.76 (3 H, d, J 7.2, γCH3), 3.77 (3 H, s, CH3
OMe), 6.90 (1 H, q, J 7.2, βCH); δC 13.26, 27.86, 52.03, 82.72,
130.31, 136.00, 150.47, 164.37.
Synthesis of 2b. The same procedure as described for the
preparation of 2a was followed substituting Z-Ser-OMe for
Boc-Ser-OMe to give 2b. Since attempts to crystallise the com-
pound were not successful the dehydroamino acid derivative
was chromatographed on silica using diethyl ether–n-hexane as
eluent to give 2b as a pure oil (85%) (Found: C, 60.8; H, 6.4; N,
4.2. Calc. for C17H21NO6: C, 60.9; H, 6.3; N, 4.2%); δH 1.46 (9
H, s, CH3 Boc), 3.73 (3 H, s, CH3 OMe), 5.22 (2 H, s, CH2
Z), 5.70, 6.41 (1 H ϩ 1 H, 2s, βCH2), 7.34 (5 H, m, ArH Z);
δC 27.76, 52.40, 68.41, 83.72, 125.59, 127.98, 128.27, 128.46,
135.19, 135.50, 150.13, 151.96, 163.64.
Synthesis of 2j. The same procedure as described for the
preparation of 2a was followed substituting Z-Thr-OMe for
Boc-Ser-OMe to give 2j. Since attempts to crystallise the com-
pound were unsuccessful the dehydroamino acid derivative was
chromatographed through silica using diethyl ether–n-hexane
as eluent to give 2j as a pure oil (94%) (Found: C, 61.6; H, 6.8;
N, 4.0. Calc. for C18H23NO6: C, 61.9; H, 6.6; N, 4.0%); δH 1.46
(9H, s, CH3 Boc), 1.70 (3 H, d, J 7.2, γCH3), 3.69 (3 H, s, CH3
OMe), 5.21 (2 H, s, CH2 Z), 6.95 (1 H, q, J 7.2, βCH), 7.33
(5 H, m, ArH Z); δC 13.27, 27.77, 52.08, 68.25, 83.27, 128.19,
127.89, 128.25, 128.43, 129.73, 135.34, 137.69, 150.72, 151.82,
164.06.
Synthesis of 2c. The same procedure as described for the
preparation of 2a was followed substituting Z(NO2)-Ser-OMe
for Boc-Ser-OMe to give 2c (93%), mp 97–98 ЊC (from diethyl
ether–n-hexane) (Found: C, 53.7; H, 5.3; N, 7.0. Calc. for
C17H20N2O8: C, 53.7; H, 5.3; N, 7.4%); δH 1.47 (9 H, s, CH3
Boc), 3.79 (3 H, s, CH3 OMe), 5.32 [2 H, s, CH2 Z(NO2)], 5.77,
6.46 (1 H ϩ 1 H, 2s, βCH2), 7.52, 8.24 [2 H ϩ 2 H, 2d, J 9.0,
ArH Z(NO2)]; δC 27.74, 52.56, 66.86, 84.12, 123.74, 125.89,
128.05, 135.39, 142.47, 147.74, 149.78, 151.83, 163.50.
Synthesis of 2k. The same procedure as described for the
preparation of 2a was followed substituting Z(NO2)-Thr-OMe
for Boc-Ser-OMe to give 2k (92%), mp 93.5–94.5 ЊC (from
diethyl ether–light petroleum, bp 40–60 ЊC) (Found: C, 54.7;
Synthesis of 2d. The same procedure as described for the
preparation of 2a was followed substituting Z(NO2)-Ser-OBn
for Boc-Ser-OMe to give 2d (81%), mp 93–94 ЊC (from diethyl
3700
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703

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H, 5.6; N, 7.1. Calc. for C18H22N2O8: C, 54.8; H, 5.6; N, 7.1%);
δH 1.47 (9 H, s, CH3 Boc), 1.76 (3 H, d, J 7.2, γCH3), 3.75 (3 H,
s, CH3 OMe), 5.32 [2 H, d, J 2.1, CH2 Z(NO2)], 7.01 (1 H, q,
J 7.2, βCH), 7.52, 8.23 [2 H ϩ 2 H, 2d, J 9.0, ArH Z(NO2)];
δC 13.37, 27.77, 52.26, 66.75, 83.73, 123.73, 128.09, 129.63,
137.99, 142.62, 147.74, 149.76, 151.68, 163.89.
to 50 mV lower than the peak potential measured and the
apparatus switched on. When the intensity of the current was
almost zero, the reaction mixture (catholyte) was transferred to
a round-bottomed flask and the solvent evaporated under
reduced pressure. The residue was partitioned between 100 cm3
of diethyl ether and 50 cm3 of KHSO4 (1 mol dmϪ3). The
organic phase was then washed with KHSO4 (1 mol dmϪ3),
NaHCO3 (1 mol dmϪ3) and brine (3 times, 30 cm3 each),
and dried over MgSO4. Concentration by evaporation under
reduced pressure gave Boc-∆Ala-OMe (88%) as a pure oil,
δH 1.49 (9 H, s, CH3 Boc), 3.84 (3 H, s, CH3 OMe), 5.74, 6.17
(1 H ϩ 1 H, 2s, βCH2), 7.01 (1 H, br, αNH); δC 28.20, 52.78,
80.64, 105.14, 131.28, 152.50, 164.40.
Synthesis of 2l. The same procedure as described for the
preparation of 2a was followed substituting Tos-Thr-OMe for
Boc-Ser-OMe to give 2l (87%), mp 85–86.5 ЊC (from diethyl
ether–n-hexane) (Found: C, 55.6; H, 6.3; N, 3.9; S, 8.5. Calc. for
C17H23NO6S: C, 55.3; H, 6.3; N, 3.8; S, 8.7%); δH 1.33 (9 H, s,
CH3 Boc), 2.01 (3 H, d, J 7.5, γCH3), 2.46 (3 H, s, CH3 Tos),
3.77 (3 H, s, CH3 OMe), 7.31 (3 H, complex, βCH ϩ ArH Tos),
8.01 (2 H, d, J 8.4, ArH Tos); δC 14.73, 21.59, 27.71, 52.16,
84.36, 127.70, 128.87, 129.27, 136.22, 143.91, 144.43, 149.62,
163.98.
Controlled potential electrolysis of 2e. The same procedure as
described previously was followed substituting 2e for 2c to give
Boc-∆Ala-OMe (73%).
Synthesis of 2m. The same procedure as described for the
preparation of 2a was followed substituting Z(NO2)-Phe(β-
OH)-OMe for Boc-Ser-OMe 2m. Since attempts to crystallise
the compound were unsuccessful the dehydroamino acid
derivative was chromatographed by column chromatography
through silica using diethyl ether–n-hexane as eluent to give
2m as a pure oil (93%), δH 1.36 (9 H, s, CH3 Boc), 3.82 (3 H,
s, CH3 OMe), 5.22 [2 H, q, J 13.5, CH2 Z(NO2)], 7.35–7.46 (5 H,
complex, ArH ∆Phe), 7.30 [2 H, d, J 8.7, ArH Z(NO2)],
7.63 (1 H, s, βCH ∆Phe), 7.08 [2 H, d, J 8.7, ArH Z(NO2)];
δC 27.75, 52.60, 66.70, 83.97, 123.55, 124.93, 126.21, 128.08,
129.02, 129.66, 130.40, 132.38, 136.99, 142.35, 149.49, 151.34,
164.85.
Controlled potential electrolysis of 2k. The same procedure as
described previously was followed substituting 2k for 2c to give
Boc-∆Abu-OMe (88%) as a pure oil which solidified on stand-
ing, mp 69.5–71 ЊC (Found: C, 56.0; H, 8.0; N, 6.5. Calc. for
C10H17NO4: C, 55.8; H, 8.0; N, 6.5%); δH 1.48 (9 H, s, CH3 Boc),
1.81 (3 H, d, J 7.2, γCH3), 3.78 (3 H, s, CH3 OMe), 5.98 (1 H, br,
αNH), 6.69 (1 H, q, J 7.2, βCH); δC 14.26, 28.17, 52.22, 80.42,
119.20, 132.05, 142.00, 165.30.
Controlled potential electrolysis of 2l. The same procedure as
described previously was followed substituting 2l for 2c to give
Boc-∆Abu-OMe (78%).
Al(Hg)-mediated cleavage of 2c
Synthesis of 2n. The same procedure as described for the
preparation of 2a was followed substituting Boc-Ser-OH for
Boc-Ser-OMe and using 3.3 eq. of (Boc)2O to give 2n. Since
attempts to crystallise the compound were unsuccessful the
dehydroamino acid derivative was chromatographed by column
chromatography through silica using diethyl ether–n-hexane as
eluent to give a pure oil that solidified on standing (73%), mp
56–57 ЊC (Found: C, 59.5; H, 8.3; N, 4.1. Calc. for C17H29NO6:
C, 59.5; H, 8.5; N, 4.1%); δH 1.47 (18 H, s, CH3 Boc), 1.50 (9 H,
s, CH3 OBut), 5.57, 6.27 (1 H ϩ 1 H, 2s, βCH2); δC 27.84, 27.90,
81.56, 82.71, 123.71, 137.39, 150.62, 162.32.
Finely cut aluminium foil (10 mmol) was stirred with a few
drops of mercury for 30 min under a stream of nitrogen. 2c (0.5
mmol) dissolved in ethyl ether with 1% water was then added.
After 2 h, when most of the Al had dissolved and TLC indi-
cated only minor amounts of 2c, more Al(Hg) (5 mmol) was
added and left to react for a further 2 h. The greyish solid
material was then filtered off with suction and rinsed thor-
oughly with ethyl ether. Evaporation of the yellow filtrate gave
an oil that was redissolved in chloroform and filtered to remove
dark insoluble material. Evaporation of the solvent under
reduced pressure gave Boc-∆Ala-OMe (87%) as a pure oil.
Synthesis of 2o. The same procedure as described for the
preparation of 2n was followed substituting Boc-Thr-OH for
Boc-Ser-OH to give 2o. Since attempts to crystallise the com-
pound were unsuccessful the dehydroamino acid derivative was
chromatographed by column chromatography through silica
using diethyl ether–n-hexane as eluent to give 2o as a pure oil
(73%) (Found: C, 60.4; H, 8.9; N, 4.0. Calc. for C18H31NO6: C,
60.5; H, 8.7; N, 3.9%); δH 1.46 (18 H, s, CH3 Boc), 1.49 (9 H, s,
CH3 OBut), 1.73 (3 H, d, J 7.2, γCH3), 6.81 (1 H, q, J 7.2, βCH);
δC 13.04, 27.85, 27.99, 80.98, 82.32, 131.51, 135.25, 150.45,
162.84.
Al(Hg)-mediated cleavage of 2k
The same procedure as described above was followed substitut-
ing 2k for 2c to give Boc-∆Abu-OMe (95%).
Acidolysis of 2j
To 2j (1 mmol) 3 cm3 of TFA were added and the solution left
to stand for 1 h. Excess TFA was removed by evaporation under
reduced pressure to give Z-∆Abu-OMe20 (87%), mp 69.5–
70.5 ЊC (from diethyl ether–n-hexane) (Found: C, 62.6; H, 6.3;
N, 5.6. Calc. for C13H15NO4: C, 62.6; H, 6.1; N, 5.6%); δH 1.83
(3 H, d, J 7.2, γCH3), 3.77 (3 H, s, CH3 OMe), 5.16 (2 H, s, CH2
Z), 6.77 (1 H, q, J 7.2, βCH), 7.36 (5 H, m, ArH Z); δC 14.26,
52.30, 67.33, 128.12, 128.22, 128.52, 133.27, 136.01, 164.98.
Synthesis of 2a from 1p. The same procedure as described for
the preparation of 2n was followed substituting H-Ser-OMe for
Boc-Ser-OH to give 2a8 (82%).
Deprotection of N-Boc,N-acyl dehydroamino acid esters
Acidolysis of 2k
Controlled potential electrolysis of 2c. Both compartments of
a two-compartment controlled-potential electrolysis cell were
filled with acetonitrile containing Et4NCl (0.1 mol dmϪ3) as
supporting electrolyte and Et3NHCl (0.015 mol dmϪ3) as pro-
ton donor. Then 2c (1 mmol) was added to the cathodic com-
partment and a cyclic voltammogram was recorded at a sweep
rate of 100 mV sϪl in order to measure the corresponding peak
potential. The potential was adjusted to a value corresponding
The same procedure as described previously was followed sub-
stituting 2k for 2j to give Z(NO2)-∆Abu-OMe (85%), mp 120–
121 ЊC (from diethyl ether) (Found: C, 53.0; H, 4.9; N, 9.2. Calc.
for C13H14N2O6: C, 53.1; H, 4.8; N, 9.5%); δH 1.83 (3 H, d, J 7.2,
γCH3), 3.79 (3 H, s, CH3 OMe), 5.26 [2 H, s, CH2 Z(NO2)], 6.30
(1 H, br, αNH), 6.82 (1 H, q, J 7.2, βCH), 7.54, 8.24 [2 H ϩ
2 H, 2d, J 8.7, ArH Z(NO2)]; δC 14.26, 52.41, 65.72, 123.74,
127.99, 128.11, 133.93, 143.42, 164.84.
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703
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Saponification of Z-Abu-OMe
phase was thoroughly washed with KHSO4 (1 mol dmϪ3),
NaHCO3 (1 mol dmϪ3) and saturated brine (3 × 50 cm3 each),
and dried over MgSO4. Removal of the solvent afforded 4a.
Since attempts to crystallise the compound were unsuccessful
the dehydroamino acid derivative was chromatographed by
column chromatography through silica using diethyl ether–n-
hexane as eluent to give 4a as a pure oil (93%), δH 1.27 (3 H, t,
J 7.1, CH3 OEt), 1.50, 1.57 (9 H ϩ 9 H, 2s, CH3 Boc ϩ CH3
Boc), 3.51 (2 H, m, βCH2 Phe), 4.30 (2 H, q, J 7.1, CH2 OEt),
5.05 (1 H, dd, J 6.5, J 10.1, αCH Phe), 5.20 [2 H, s, CH2
Z(NO2)], 5.75, 5.95 (1 H ϩ 1 H, 2s, βCH2 ∆Ala), 7.20 (5 H, m,
ArH Phe), 7.55, 8.25 [2 H ϩ 2 H, 2d, J 8.6, ArH Z(NO2)];
δC 13.99, 27.62, 27.78, 33.83, 48.93, 54.23, 62.20, 66.92, 82.90,
85.40, 105.42, 123.67, 126.93, 128.18, 128.60, 128.66, 131.42,
136.18, 142.95, 147.57, 149.44, 152.99, 160.31, 167.75.
To Z-∆Abu-OMe (1 mmol) in dioxane (0.2 mol dmϪ3), 3 cm3 of
NaOH (1 mol dmϪ3) were added. The solution was stirred at
room temperature for 2 h, acidified to pH 2–3 with KHSO4
(1 mol dmϪ3) and then extracted with ethyl acetate. The organic
phase was dried over MgSO4 and evaporated under reduced
pressure to give Z-∆Abu-OH (77%), mp 174–176 ЊC (from ethyl
acetate–diethyl ether) (Found: C, 61.1; H, 5.6; N, 6.0. Calc.
for C12H13NO4: C, 61.3; H, 5.6; N, 5.95%); δH[300 MHz;
(CD3)2SO] 1.66 (3 H, d, J 6.9, γCH3), 5.03 (2 H, s, CH2 Z), 6.47
(1 H, q, J 6.9, βCH), 7.35 (5 H, m, ArH Z), 8.61 (1 H, br, αNH);
δC[75.4 MHz; (CD3)2SO] 13.17, 65.63, 127.62, 128.32, 128.76,
131.36, 136.79, 154.24, 165.62.
Synthesis of Z-ꢀAbu-Gly-OMe
Z-∆Abu-OH was reacted with H-Gly-OMeؒHCl in ethyl acetate
using the standard DCC–HOBt procedure to give Z-∆Abu-Gly-
OMe (85%), mp 90–90.5 ЊC (from diethyl ether–n-hexane)
(Found: C, 58.7; H, 6.1; N, 9.1. Calc. for C15H18N2O5: C, 58.8;
H, 5.9; N, 9.1%); δH 1.77 (3 H, d, J 6.9, γCH3), 3.76 (3 H, s, CH3
OMe), 4.07 (2 H, d, J 4.5, CH2 Gly), 5.16 (2 H, s, CH2 Z), 6.20
(1 H, br, αNH Gly), 6.56 (1 H, q, J 6.9, βCH ∆Abu), 6.61 (1 H,
br, αNH ∆Abu), 7.36 (5 H, s, ArH Z); δC 13.37, 41.13, 52.38,
67.55, 98.57, 128.17, 128.32, 128.55, 129.28, 135.81, 164.80,
170.25.
Synthesis of 4b. The same procedure as described for the
preparation of 4a was followed substituting 3b for 3a to give
4b (91%), mp 102.5–103.5 ЊC (from diethyl ether–n-hexane)
(Found: C, 56.3; H, 7.7; N, 6.1. Calc. for C22H36N2O9: C, 55.9;
H, 7.7; N, 5.9%); δH 1.43 (3 H, d, J 6.9, βCH3 Ala), 1.48 (27 H, s,
CH3 Boc), 3.76 (3 H, s, CH3 OMe), 5.37 (1 H, q, J 6.9, αCH
Ala), 5.77, 6.35 (1 H ϩ 1 H, 2s, βCH2 ∆Ala); δC 14.97, 27.79,
27.96, 52.33, 56.44, 82.94, 83.37, 124.82, 135.89, 150.87, 152.00,
163.97, 173.75.
Synthesis of 4c. The same procedure as described for the
preparation of 4a was followed substituting 3c for 3a to give 4c.
Since attempts to crystallise the compound were unsuccessful
the dehydroamino acid derivative was chromatographed by
column chromatography through silica using diethyl ether–n-
hexane as eluent to give 4c as a pure oil (81%), δH 1.48, 1.50,
1.60 (9 H ϩ 9 H ϩ 9 H, 3s, CH3 Boc ϩ CH3 Boc ϩ CH3 Boc),
2.31 (3 H, d, J 8.1, γCH3 ∆Abu), 3.78 (3 H, s, CH3 OMe), 4.34
(2 H, s, CH2 Gly), 5.1 (2 H, s, CH2 Z), 7.16 (1 H, q, J 8.1, βCH
∆Abu), 7.40–7.33 (5 H, m, ArH Z); δC 13.03, 27.91, 39.08,
52.71, 67.55, 85.05, 125.48, 126.22, 148.35, 149.31, 160.55,
166.96.
Preparation of N-acyl dipeptides esters 3a–3f
In all cases the N-acyl protected amino acid was reacted with
the appropriate amino acid ester in ethyl acetate using the
standard DCC–HOBt procedure to give: 3a (85%), mp 130–
131.5 ЊC (from ethyl acetate–n-hexane) (Found: C, 57.3; H, 5.2;
N, 9.1. Calc. for C22H25N3O8: C, 57.5; H, 5.5; N, 9.15%); δH 1.26
(3 H, t, J 7.2, CH3 OEt), 3.12 (2 H, m, βCH2 Phe), 3.55 (2 H, m,
βCH2 Ser), 4.00 (1 H, m, αCH Ser), 4.19 (2 H, q, J 7.2, CH2
OEt), 4.84 (1 H, q, J 6.6, αCH Phe), 5.18 [2 H, s, CH2 Z(NO2)],
5.92 (1 H, d, J 6.6, αNH Phe), 6.93 (1 H, d, J 7.5, αNH Ser),
7.26 (5 H, m, ArH Phe), 7.49, 8.21 [2 H ϩ 2 H, 2d, J 8.7, ArH
Z(NO2)]; δC 14.04, 37.50, 49.14, 55.52, 61.86, 62.79, 65.56,
123.75, 127.18, 128.01, 128.55, 129.07, 135.60, 143.46, 147.61,
155.78, 156.86, 171.49; 3b (81%), oil (lit.,33 oil), δH 1.41 (3 H,
J 6.9, βCH3 Ala), 1.46 (9 H, s, CH3 Boc), 3.80 (3 H, s, CH3
OMe), 4.04 (2 H, m, βCH2 Ser), 4.16 (1 H, m, αCH Ser), 4.63
(1 H, m, αCH Ala), 5.03 (1 H, d, J 6.0, αNH Ala), 7.11 (1 H,
d, J 6.6, αNH Ser); 3c (71%), mp 105.5–107 ЊC (from ethyl
acetate–petroleum ether) (lit.,34 105–106 ЊC); 3d (92%), mp 94–
95 ЊC (from ethyl acetate–n-hexane) (lit.,35 104–105 ЊC); 3e36
(92%), oil, δH 1.24 (3 H, d, J 6.3, γCH3 Thr), 1.46 (9 H, s, CH3
Boc), 3.80 (3 H, s, CH3 OMe), 3.96 (2 H, m, β CH2 Ser), 4.14
(1 H, m, βCH Thr), 4.36 (1 H, m, αCH Ser), 4.66 (1 H, m, αCH
Thr), 5.61, 7.40 (1 H ϩ 1 H, br, αNH Ser and αNH Thr);
δC 18.49, 28.23, 52.80, 55.72, 58.88, 62.38, 67.29, 80.47, 156.00
170.74, 171.43; 3f (76%), mp 121–123 ЊC (from ethyl acetate–
diethyl ether) (Found: C, 50.3; H, 7.6; N, 8.6. Calc. for
C14H26N2O7: C, 50.3; H, 7.8; N, 8.4%); δH 1.23 (6 H, d, J 6.3,
γCH3 Thr), 1.46 (9 H, s, CH3 Boc), 3.79 (3 H, s, CH3 OMe), 4.15
(1 H, m, αCH Thr), 4.14–4.37 (2 H, complex, βCH Thr ϩ βCH
Thr), 4.60 (1H, m, αCH Thr), 5.61, 7.31 (1 H ϩ 1 H, 2d, J 7.5,
J 9.0, αNH Thr and αNH Thr); δC 18.19, 19.98, 28.29, 52.71,
57.49, 58.07, 67.06, 67.96, 80.55, 156.38, 171.21, 172.08.
Synthesis of 4d. The same procedure as described for the
preparation of 4a was followed substituting 3d for 3a to give 4d
(84%), mp 111.5–112.5 ЊC (from diethyl ether–light petroleum,
bp 40–60 ЊC) (Found: C, 56.7; H, 7.9; N, 5.8. Calc. for
C23H38N2O9: C, 56.8; H, 7.9; N, 5.8%); δH 1.45 (3 H, d, J 6.1,
βCH3 Ala), 1.51 (27 H, s, CH3 Boc), 1.76 (3 H, d, J 7.3, γCH3
∆Abu), 3.75 (3 H, s, CH3 OMe), 5.22 (1 H, q, J 6.1, αCH Ala),
6.94 (1 H, q, J 7.3, βCH ∆Abu); δC 14.43, 18.08, 28.26,
33.88, 52.25, 83.09, 125.87, 134.40, 150.81, 154.38, 164.73,
171.00.
Synthesis of 4e. The same procedure as described for the
preparation of 4a was followed substituting 3e for 3a and using
4.4 eq. of tert-butyl pyrocarbonate to give 4e (83%), mp 118.5–
119.5 ЊC (from diethyl ether–light petroleum, bp 40–60 ЊC)
(Found: C, 57.1; H, 7.7; N, 5.8. Calc. for C23H36N2O9: C, 57.0;
H, 7.5; N, 5.8%); δH 1.46 [18 H, s, CH3 (Boc)2], 1.49 (9 H, s, CH3
Boc), 1.75 (3 H, d, J 7.2, γCH3 ∆Abu), 3.80 (3 H, s, CH3 OMe),
6.17, 6.18 (1 H ϩ 1 H, 2d, J 0.9, βCH2 ∆Ala), 6.80 (1 H, q,
J 7.2, βCH ∆Abu); δC 14.45, 27.80, 27.83, 52.38, 83.36, 123.49,
132.62, 134.78, 138.78, 150.16, 163.78, 166.70.
Synthesis of 4f. The same procedure as described for the
preparation of 4e was followed substituting 3f for 3e to give
4f. Since attempts to crystallise the compound were unsuccess-
ful the dehydroamino acid derivative was chromatographed by
column chromatography through silica using diethyl ether–n-
hexane as eluent to give a pure oil (74%), δH 1.46 [18 H, s, CH3
(Boc)2], 1.49 (9 H, s, CH3 Boc), 1.73, 1.76 (3 H ϩ 3 H, 2d, J 7.0,
γCH3 ∆Abu), 3.75 (3 H, s, CH3 OMe), 6.66, 6.88 (1 H ϩ 1 H,
2q, J 7.0, βCH ∆Abu); δC 13.48, 14.29, 27.72, 27.78, 52.05,
Synthesis of dehydroamino acid containing dipeptides
Synthesis of 4a. To a solution of 3a in dry acetonitrile (1 mol
dmϪ3) 0.3 eq. of DMAP was added followed by 3.3 eq. of tert-
butyl pyrocarbonate under rapid stirring at room temperature.
The reaction was stirred for 12 h while monitored by TLC
(diethyl ether–n-hexane, 2:1). Evaporation under reduced pres-
sure gave a residue that was partitioned between 200 cm3 of
diethyl ether and 100 cm3 of KHSO4 (1 mol dmϪ3). The organic
3702
J. Chem. Soc., Perkin Trans. 1, 1999, 3697–3703

View Article Online
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