Multicomponent Hantzsch cyclocondensation as a route to highly functionalized 2- and 4-dihydropyridylalanines, 2- and 4-pyridylalanines, and their N-oxides: Preparation via a polymer-assisted solution-phase approach

Article abstract of DOI:10.1016/j.tet.2004.01.011

An improved and efficient entry to highly functionalized β-(2-pyridyl)- and β-(4-pyridyl)alanines and the corresponding 1,4-dihydro and N-oxide derivatives has been developed by one-pot thermal Hantzsch-type cyclocondensation of aldehyde-ketoester-enamine

Full text of DOI:10.1016/j.tet.2004.01.011

Tetrahedron 60 (2004) 2311–2326
Multicomponent Hantzsch cyclocondensation as a route to highly
functionalized 2- and 4-dihydropyridylalanines,
2- and 4-pyridylalanines, and their N-oxides: preparation
via a polymer-assisted solution-phase approach
†
,
Alessandro Dondoni, a Alessandro Massi,a Erik Minghinia and Valerio Bertolasib
,
*
aLaboratorio di Chimica Organica, Dipartimento di Chimica, Universita di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
bCentro di Strutturistica Diffrattometrica, Dipartimento di Chimica, Universita di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
Received 29 October 2003; revised 10 December 2003; accepted 8 January 2004
Abstract—An improved and efficient entry to highly functionalized b-(2-pyridyl)- and b-(4-pyridyl)alanines and the corresponding
1,4-dihydro and N-oxide derivatives has been developed by one-pot thermal Hantzsch-type cyclocondensation of aldehyde–ketoester–
enamine systems in which one of the reagents (aldehyde or ketoester) was carrying the unmasked but protected chiral glycinyl moiety. Thus
coupling N-Boc-O-benzyl aspartate b-aldehyde, acetoacetate and aminocrotonate esters afforded tetrasubstituted b-(4-dihydropyridyl)-
alanines (75% yield). One of these products was almost quantitatively transformed into the b-(4-pyridyl)alanine derivative which in turn was
oxidized to the corresponding N-oxide. Each of these enantiomerically pure (Mosher’s amide analysis) heterocyclic a-amino acids was
incorporated into a tripeptide by coupling with (S)-phenylalanine. In a similar way tetrasubstituted b-(2-dihydropyridyl)alanine, b-(2-
pyridyl)alanine and b-(1-oxido-2-pyridyl)alanine were prepared via Hantzsch cyclocondensation reaction using benzaldehyde,
aminocrotonate, and acetoacetate carrying the N-Boc-O-benzyl glycinate moiety. It was shown that the work up of the reaction mixtures
derived from the cyclocondensation and oxidation reactions can be carried out by the use of polymer supported reagents and sequestrants thus
allowing the isolation of the products in high purity without any chromatography.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
occurring amino acids histidine, phenylalanine, and tyro-
sine. Indeed, they have been incorporated as histidine
replacements in angiotensin II8 and as enzyme substrates
have been shown to function as antagonists of phenyl-
alanines9 and inhibitors of histidine decarboxylase.10 More-
over, as free a-amino acids they have been found to possess
anti-inflammatory11 and anti-tumor-antibiotic activities12 as
shown, for example, by the natural products L-azatyrosine
113 and L-mimosine 214 (Fig. 1). Moreover decapeptides
with incorporated homochiral tyrosine analogues such
(S)-b-(4-pyridyl)alanine N-oxide 3 have been shown15 to
act as inhibitors of epidermal growth factor tyrosine kinase
whose aberrant levels of activity can result in unregulated
cell proliferation associated with cancer.16 It has been also
There has been in recent years a growing interest in the
development of new non-proteinogenic a-amino acids1 due
to their biological and toxicological properties2 and their
applications in the fields of peptide and combinatorial
chemistry as conformationally constrained components,
molecular scaffolds, and chiral auxiliaries toward the
identification of new leads in peptidic and non-peptidic
compounds.3 Among the variety of non-proteinogenic
amino acids, the class of heterocyclic a-amino acids,4 i.e.
heterocycles featuring a side carbon-chain with a chiral
glycinyl moiety (–CH(NH2)CO2H), is of special interest
not only for their applications as components of artificial
peptides5 and peptide nucleic acids (PNAs)6 but also
because they display a wide range of their own biological
activities.2 Typically, pyridyl-a-alanines and ring substi-
tuted derivatives4c–e,7 are structural analogues of the natural
Keywords: Multicomponent reaction; Hantzsch reaction; Heterocyclic
a-amino acids; Pyridyl-a-alanines; Unnatural amino acids.
*
Corresponding author. Tel.: þ39-0532-291176; fax: þ39-0532-291167;
e-mail address: adn@dns.unife.it
Address correspondence concerning crystallography to this author. Fax:
†
Figure 1. Typical bioactive pyridyl-, dihydropyridyl-, and 1-oxido-pyridyl-
a-alanines.
þ39-0532-240709.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.01.011

2312
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
found17 that racemic b-(4-pyridyl)alanine N-oxide is an
antagonist to both phenylalanine and tyrosine in Escherichia
coli whereas the 2- and 3-pyridyl isomers are less effective
inhibitors.
acids is crucial for the discovery of new leads. Our
previous18 and present study constitute the first approach
to enantiopure heterocyclic a-amino acid collections
through a MCR. A two-step approach which may also be
applicable to parallel and/or combinatorial syntheses of
diverse families of heterocyclic a-amino acids has been
developed by Baldwin and co-workers via cyclocondensa-
tion of suitable bifunctional nucleophiles with a-amino acid
alkynyl ketones.4a–c Hence, we will report herein the results
of our improved synthesis of the title a-alanine derivatives
of the pyridine family with demonstration of their insertion
into oligopeptides.
Recently, we have been stimulated to contribute novel
chemistry to the interesting field of non-proteinogenic
a-amino acids by developing versatile synthetic routes
towards highly substituted dihydropyrimidinyl-a-glycines
and dihydropyrimidinyl- and pyridyl-a-alanines.18 In both
cases we adopted the technique of one-step heterocyclic
amino acid construction through a multicomponent cyclo-
condensation reaction (MCR), namely the Biginelli19
aldehyde–ketoester–urea reaction and the Hantzsch-
type20 aldehyde–ketoester–enamine reaction in which
one of the reagents (aldehydes 4 and 5 or ketoester 6) was
carrying the masked chiral glycinyl moiety in the form of
the configurationally stable N-Boc-2,2-dimethyl oxazoli-
dine ring21 (Fig. 2). One of the advantages of this approach
relies on exploiting the potential of MCRs in generating
molecular diversity thus opening the route to the preparation
of libraries of highly substituted dihydropyrimidinyl- and
pyridyl-a-amino acids. Moreover, this approach allows us
to overcome the problem of the control of the configuration
at the carbon bearing the amino group since this was already
in place in the masked glycinyl moiety introduced in one of
the reagents taking part in the MCR. However, the oxidative
conditions required to unveiling the glycinyl group from the
N-Boc-2,2-dimethyl oxazolidine ring constituted a limi-
tation in the Hantzsch approach18 for the isolation of
dihydropyridyl a-amino acids because of the concomitant
oxidation of the dihydropyridyl ring to the pyridyl ring.
Hence aiming at overcoming this drawback and improving
the value of the MCR approach by a more rapid entry to the
target product, we have investigated the use of reagents such
as the aldehyde 7 and the ketoester 8 (Fig. 2) carrying the
chiral glycinyl group in a protected instead of a masked
form. Another improvement was possible by the use of
polymer-supported reagents and sequestrants to make
operatively simple and efficient the isolation of the target
products on gram scale. These new conditions should allow
the application of this methodology to a fully automatized
procedure suitable for parallel and/or combinatorial synth-
eses of highly substituted dihydropyridyl (DHP)-, pyridyl-,
and 1-oxido-pyridyl-a-alanines. The access to a variable
substitution pattern in the heterocyclic ring of these amino
2. Results and discussion
2.1. Synthesis of ring substituted b-(4-dihydropyridyl)-,
b-(4-pyridyl)-, and b-(1-oxido-4-pyridyl)alanines
Given the ready access to N-Boc-O-benzyl aspartate
b-aldehyde 7 by different preparative procedures22 and its
sufficient stability as demonstrated by the use in a variety of
stereoselective syntheses,22 we initially considered the
Hantzsch MCR3 by coupling this aldehyde with ethyl
acetoacetate 9a and ethyl aminocrotonate 10a (1:1:1
mixture) in EtOH at 70 8C for 24 h (Scheme 1). From this
reaction the b-(4-dihydropyridyl)alanine derivative 11a was
obtained in 75% isolated yield by column chromatography.
Quite obviously, the level of enantiomeric purity of this
compound depended on the preservation of the stereo-
chemical integrity (S configuration) of the aldehyde 7 during
the cyclocondensation reaction. Gratifyingly this appeared
to be the case since the a-amino ester 11a by treatment
w0ith trifluoroacetic acid (TFA) at 0 8C was converted into its
N -deprotected form 12a (78%) whose enantiomeric purity
1
established by H and 19F NMR analyses of the (R)- and
(S)-Mosher’s amides23 turned out to be greater than 96%.
Owing to the orthogonal protection of the ester gr0oups,
compound 11a was also transformed into the N -Boc
a-amino acid 13a (98% yield) by debenzylation via
Pd(OH)2 catalyzed hydrogenation. Compounds 12a and
13a represent orthogonally protected units suitable0 for
peptide synthesis by chain extension from either the N - or
C-terminus.24
Given the potential practical application of b-(4-dihydro-
pyridyl)alanines in the area of artificial peptides, we
considered the insertion of 13a into a tripeptide as a good
evidence of its reactivity despite the densely substituted
heterocyclic ring. This operation appeared to be also a test
of the stability of the dihydropyridine ring under the
reaction conditions of peptide formation. Hence coupling
of 13a with L-phenylalanine methyl ester hydrochloride
(H-Phe-OMe·HCl), using (benzotriazol-1-yloxy)tripyrro-
lidinophosphonium hexafluorophosphate (PyBop) as
coupling reagent and diisopropylethylamine (DIEA) in
CH2Cl2 (Scheme 2) furnished the dipeptide 20 in 80%
isolated yield. Extension from the N0-terminus was next
carried out. After Boc removal in 20 by treatment with
TFA, the coupling of the resulting free amine with tert-
butoxycarbonyl-L-phenylalanine (Boc-Phe-OH) using
PyBop and DIEA afforded the target tripeptide 21 in 62%
overall yield (two steps).
Figure 2. Masked and protected glycinyl moieties embodied in aldehydes
and ketoesters employed in Ref. 18 and in this work.

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2313
˚
Scheme 1. Reagents and conditions: (a) 4-A MS, ROH, 70 8C, 24 h; (b) TFA–CH2Cl2 (1:4), 0 8C to rt, 1 h; (c) H2, Pd(OH)2, AcOEt–EtOH (2:1), rt, 15 min;
˚
(d) 4-A MS, PCC, CH2Cl2, rt, 2 h; (e) MCPBA, CH2Cl2, rt, 15 h.
The same MCR3 route was followed in the synthesis of a
tetrasubstituted b-(4-pyridyl)alanine and its N-oxide. In this
case, tert-butyl acetoacetate 9b and tert-butyl aminocroto-
nate 10b were employed as components in the cyclo-
condensation with the aldehyde 7 (Scheme 1). In fact it has
been shown18 that the bulky tert-butyl in the carboxylate
groups at C3 and C5 of the pyridine ring of a b-(4-pyridyl)-
alanine inhibits intramolecular condensation reactions to
occur during the manipulation of the amino acid function-
alities. The effectiveness of the cyclocondensation did not
appear to suffer of such a change in the reagents since the
one-pot reaction of the ketoester 9b, enamino ester 10b and
aldehyde 7 (1:1:1 ratio) in tert-BuOH at 70 8C after 24 h
afforded the corresponding ring substituted b-(4-dihydro-
pyridinyl)alanine 11b in 75% isolated yield. This compound
was almost quantitatively transformed into the b-(4-pyridyl)-
alanine 14b by treatment with pyridinium chlorocromate
(PCC) in CH2Cl2 and the latter was oxidized to the N-oxide
17b using m-chloroperoxybenzoic acid (MCPBA) in
CH2Cl2. The N0-Boc removal from these compounds by
Scheme 2. Reagents and conditions: (a) H-Phe-OMe·HCl, PyBOP, DIEA,
CH2Cl2, rt, 2 h; (b) TFA–CH2Cl2 (1:4), 0 8C to rt, 1 h; (c) Boc-Phe-OH,
PyBOP, DIEA, CH2Cl2, rt, 2 h.

2314
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
treatment with TFA furnished the amino free heterocyclic
benzyl alaninates 15b and 18b, respectively, whereas the
debenzylation by Pd(OH)2 catalyzed hydrogenation pro-
vided the corresponding N0-Boc alanines 16b and 19b in
very good yields. Compound 16b was identical by
comparison of the optical rotation value ([a]D¼225.8 in
MeOH) to the same product reported in our earlier
publication18 and which was prepared using the oxazo-
lidinyl-functionalized aldehyde 5 in the cyclocondensation
reaction (Fig. 2). However, the degree of enantiomeric
purity higher than 96% was confirmed also in these cases by
1H and 19F NMR analyses of the (R)- and (S)-Mosher’s
amides derived from compounds 15b and 18b.
aminocrotonate 10a in 1:1:1 ratio was carried out under the
usual conditions (EtOH, 70 8C, 24 h). This reaction afforded
the desired ring substituted b-(2-dihydropyridyl)alaninate
25a (35%) as 1:1 mixture of diastereoisomers having
opposite configuration at C4 together with the dihydro-
pyridine 26 (35%, Scheme 3). Evidently, the latter product
was formed by cyclocondensation of benzaldehyde 24, ethyl
aminocrotonate 10a and ethyl acetoacetate 9a, the latter
reagent having been formed by partial hydrolysis of 10a
despite the use of anhydrous EtOH and the presence of
molecular sieves. The same reaction carried out in the
absence of solvent at 70 8C for 4 h produced the a-amino
ester 25a and the dihydropyridine 26 in 3:1 ratio and 60%
overall yield, thus indicating that the hydrolysis of 10a had
not been totally prevented. Hence, since the main objective
was to achieve a high conversion of the glycinyl
functionalized ketoester 8a into the target a-amino ester
25a, the solvent-free cyclocondensation was carried out at
70 8C for 1 h using 2 equiv. of inexpensive benzaldehyde 24
and aminocrotonate 10a. Under these conditions the target
product 25a was formed in a rewarding 70% yield, as
It has been already demonstrated18 that the two bulky tert-
butyl ester groups at the pyridine ring of the pyridyl-a-
amino acid 16b do not prevent this compound to be
effectively incorporated into a peptide chain as the stepwise
coupling of 16b with two L-phenylalanine units afforded the
tripeptide 22 in 74% overall yield (Fig. 3). The same
appeared to be true for the N-oxide 19b which was
employed to synthesize the tripeptide 23 in 57% overall
yield in analogous fashion to 21 and 22, i.e. by first coupling
19b with H-Phe-OMe·HCl, then N0-Boc removal from the
resulting dipeptide and second coupling of the latter with
Boc-Phe-OH (see Section 3). Hence the N-oxide functional
group appeared to be unaffected by the conditions of this
peptide synthesis.
1
judged by H NMR analysis of the crude reaction mixture,
but still contaminated by 26 (3:1 molar ratio). Unfortu-
nately, the separation of the individual diastereoisomers as
well as the removal of the dihydropyridine 26 was
unsuccessful by the use of column chromatography. There-
fore, we proceeded to the removal of the N0-Boc from
the amino acid functionality by treatment of crude 25a
with TFA at 0 8C. This reaction afforded the amine-free
b-(2-dihydropyridyl)alaninate diastereoisomers (4S)-27a
and (4R)-27a which were individually isolated by
chromatography.
The absolute configuration at C4 of the dihydropyridine ring
Figure 3. Tripeptides prepared using L-phenylalanine and 4-pyridyl-a-
alanine 16b and its N-oxide 19b (for reagents and conditions, see Section 3).
2.2. Synthesis of ring substituted b-(2-dihydropyridyl)-,
b-(2-pyridyl)-, and b-(1-oxido-2-pyridyl)alanines
We next turned our attention to the implementation of the
above cyclocondensation–oxidation procedure by con-
sidering the synthesis of the 2-regioisomers of the above
heterocycle-a-alanines. To this aim the glycinyl substituted
ethyl acetoacetate 8a and tert-butyl acetoacetate 8b (Fig. 2)
were prepared in satisfactory yields (ca. 70%) by BF3·Et2O
promoted coupling of the aspartate b-aldehyde 7 with ethyl
and tert-butyl diazoacetate, respectively, as described18 for
the oxazolidinyl derivative 6 (see Section 3). Then the
cyclocondensation between 8a, benzaldehyde 24 and ethyl
Scheme 3. Reagents and conditions: (a) 70 8C, 1 h; (b) TFA–CH2Cl2 (1:4),
0 8C, 1 h.

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2315
of (4R)-27a was established by the X-ray crystallographic
analysis of its (R)-Mosher’s amide (Fig. 4).25 Furthermore,
1H and 19F NMR analysis of both Mosher’s amides of (4R)-
27a confirmed that the configuration of the glycinyl moiety
was preserved during the cyclocondensation reaction.
In a second instance, 2-pyridylalanines and their N-oxides
were targeted using the glycinyl substituted tert-butyl
acetoacetate 8b and tert-butyl aminocrotonate 10b as
partners of the cyclocondensation with benzaldehyde 24
(Scheme 4). The reasons which led to the use of tert-butyl
esters in this synthesis were the same of those reported
above for the synthesis of 4-pyridylalanines and their
N-oxides.18 We were gratified to observe that the reaction of
8b, 10b, and 24 in 1:1:1 ratio under the standard conditions
(tert-BuOH, 70 8C, 24 h) afforded exclusively the ring
substituted b-(2-dihydropyridyl)alaninate 25b in good yield
(75%) without the formation of a dihydropyridine side
product similar to 26 shown in Scheme 3. Hence, it can be
deduced that the enamino ester 10b unlike the ethyl
derivative 10a is sufficiently stable under the reaction
conditions toward the hydrolytic conversion into a keto-
ester. Then, conversions of 25b to the b-(2-pyridyl)alani-
nate 28b and the latter to the N-oxide 31b were carried out
Figure 4. An ORTEP view of the (R)-Mosher’s amide of (4R)-27a
displaying the thermal ellipsoids at a 30% probability level.
˚
Scheme 4. Reagents and conditions: (a) 4-A MS, tert-BuOH, 70 8C, 24 h; (b) 4-A MS, PCC, CH2Cl2, rt, 2 h; (c) TFA–CH2Cl2 (1:4), 0 8C to rt, 1 h; (d) H2,
˚
Pd(OH)2, AcOEt–EtOH (2:1), rt, 15 min; (e) MCPBA, CH2Cl2, rt, 15 h.

2316
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
by almost quantitative oxidation reactions using PCC and
MCPBA, respectively. The selective deprotection of the
amino (Boc removal) and carboxyl group (Bn removal) of
these compounds furnished the amino alaninates 29b and
32b and the N0-Boc alanines 30b and 33b. The former pair
of these products was employed for the demonstration
through the Mosher’s amides of their enantiomeric purity,
which also in this case resulted to be higher than 96% by
NMR analysis.
appears to be superior to the complementary way to con-struct
compound collections on polymer beads (solid-phase organic
synthesis,28 SPOS), since the reaction monitoring is easier,
reaction optimization is usually faster, and no residue of
attachment to the beads remains in final product. This new
tendency in combinatorial chemistry is confirmed by the
continuousdevelopment ofhighly efficientpolymersupported
sequestrants, reagents, and catalysts to scavenge excess
reactants or side products, and promote reactions in solution.27
Finally, as proof of the potential of this family of hetero-
cyclic amino acids in peptide synthesis, compound 33b was
incorporated into the tripeptide 34 by sequential reaction
with suitably protected L-phenylalanine derivatives as
described above (57% overall yield, three steps).
We have developed a protocol for the isolation of 2- and
4-dihydropyridylalanines 25b and 11b avoiding chromato-
graphic separation by the use of suitable supported
sequestrants for the work up of the crude reaction mixtures
arising from the Hantzsch MCR3. Thus, a mixed resin bed
containing the strongly acidic resin Amberlyst 15 (A-15)
and the strongly basic resin Ambersep 900 OH was
employed to scavenge unreacted enamine and ketoester,
respectively (Scheme 5). Then, nucleophilic aminomethyl-
ated polystyrene (AM-resin) was added to remove unreacted
aldehyde and side products derived from partial conden-
sation reactions such as the enone which is produced by
condensation of aldehyde and ketoester (Knoevenagel
product). By this purification procedure DHP-a-alanines
11b and 25b were obtained in good yield (75%) and
1
high purity (.92%) as judged by H NMR analysis. The
subsequent conversion into the corresponding pyridyl-a-
alanines 14b and 28b was performed in almost quantitative
yield by using PCC immobilized on silica gel. This reagent
which was prepared according to the procedure described by
Eynde and co-workers29 greatly facilitated removal of
reduced chromium by-products affording 14b and 28b in
high purity (.92%). Finally, 1-oxido-pyridyl-a-alanines
17b and 31b were prepared following the previously
described procedure (MCPBA, CH2Cl2, rt, 15 h) but their
purification was performed by sequestering excess MCPBA
and m-chlorobenzoic acid side product with aminomethy-
lated polystyrene resin. At the end of the three-step synthetic
sequence, N-oxides 17b and 31b were obtained in 95 and
92% purity and good overall yield without the need of any
chromatographic separation. However, analytical samples
of 17b and 31b obtained via the polymer-assisted approach
were prepared to compare their optical rotation values with
those of the same compounds obtained by the conventional
solution methodology. The observed identical rotation
values confirmed that the resins employed, especially
the strongly basic Ambersep 900 OH, did not affect the
stereochemical integrity at the a-carbon of the amino acid
moiety.
2.3. Polymer-assisted solution-phase synthesis of 2- and
4-dihydropyridylalanines, 2- and 4-pyridylalanines, and
their N-oxides
Among the modern criteria considered when establishing
the level of efficiency of chemical reactions, such as yield,
selectivity, and atom economy, those regarding the reaction
processing are equally emphasized in present times.26
Hence, we envisaged adapting the above synthetic route to
techniques that would facilitate easy isolation of the target
products. To this aim the use of polymer supported reagents
and sequestrants were considered in the whole sequence
toward the synthesis of dihydropyridyl-, pyridyl-, and
1-oxido-pyridyl-a-alanines. In fact, the immobilization of
reagents on the surface of a polymer bead (or the absorption
onto silica gel) simplifies the purification procedure
dramatically, reducing it to a simple washing and filtration
step. During the last few years polymer-assisted solution-
phase (PASP) synthesis has become the prevalent method
for the generation and rapid purification of chemical
libraries as demonstrated by the increasing number of
publications in this area.27 In many cases, this methodology
˚
Scheme 5. Reagents and conditions: (a) (i) 4-A MS, tert-BuOH, 70 8C, 24 h; (ii) A-15, Ambersep, CH2Cl2, rt, 2 h; (iii) AM-resin, CH2Cl2, rt, 2 h. (b) PCC on
silica gel, CH2Cl2, rt, 24 h. (c) (i) MCPBA, CH2Cl2, rt, 15 h; (ii) AM-resin, CH2Cl2, rt, 2 h. Purities were determined by 1H NMR analysis.

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2317
In conclusion, we described an improved and efficient
approach toward the synthesis of 2- and 4-dihydropyridyl-
alanines, 2-and 4-pyridylalanines, and their N-oxide deriva-
tives. The strategy employed relies on the use of aldehyde 7
and ketoester 8 carrying the chiral glycinyl moiety in a
suitably protected form as components in Hantzsch MCR3.
Highly functionalized heterocyclic amino acid derivatives
have been obtained by this route and their potential appli-
cation as units of artificial peptides demonstrated by their
insertion into tripeptides. Furthermore, the compatibility of
our strategy with a polymer-assisted solution phase
synthesis approach has been established by exploiting an
orchestrated sequence of polymer supported reagents and
sequestrants to produce the title heterocyclic amino acids in
high yield and purity without any chromatographic step.
While we have not used robotic systems to build a large
number of compounds in this study, we believe this route
would be entirely adaptable to these high throughput
methods.
aldehyde 7 (615 mg, 2.00 mmol), ethyl diazoacetate
˚
(0.25 mL, 2.40 mmol), activated 4-A powdered molecular
sieves (300 mg), and anhydrous CH2Cl2 (20 mL) was stirred
at room temperature for 15 min, and then cooled to 0 8C. To
the mixture a solution of BF3·Et2O (127 mL, 1.00 mmol) in
anhydrous CH2Cl2 (1 mL) was added drop by drop,
controlling the N2 evolution at a low steady rate. The
mixture was stirred at 0 8C for an additional 30 min, diluted
with 10% NaHCO3 (10 mL), warmed to room temperature,
filtered through a pad of Celite, and extracted with CH2Cl2
(3£30 mL). The organic layer was dried (Na2SO4) and
concentrated. The residue was eluted from a short column of
silica gel with 3:1 cyclohexane–AcOEt to give 8a (551 mg,
1
70%) as a yellow syrup. [a]D¼þ12.0 (c 1.6, CHCl3). H
NMR: d 7.45–7.30 (m, 5H, Ph), 5.49 (bd, 1H, J2,NH
8.5 Hz, NH), 5.19 (s, 2H, PhCH2), 4.59 (ddd, 1H, J2,3a
¼
¼
4.5 Hz, J2,3b¼4.0 Hz, H-2), 4.27–4.12 (m, 2H, CH2CH3),
3.44 (s, 2H, 2H-5), 3.31 (dd, 1H, J3a,3b¼18.0 Hz, H-3a),
3.13 (dd, 1H, H-3b), 1.44 (s, 9H, t-Bu), 1.28 (t, 3H, J¼
7.0 Hz, CH2CH3). MALDI-TOF MS: 394.7 (MþþH), 416.4
(MþþNa), 432.6 (MþþK). Anal. calcd for C20H27NO7
(393.43): C, 61.06; H, 6.92; N, 3.56. Found: C, 61.05; H,
6.88; N, 3.57.
3. Experimental
3.1. General
3.1.2. (2S)-2-tert-Butoxycarbonylamino-4-oxo-hexane-
dioic acid 1-benzyl ester 6-tert-butyl ester (8b). A mixture
of aldehyde 7 (615 mg, 2.00 mmol), tert-butyl diazoacetate
All moisture-sensitive reactions were performed under a
nitrogen atmosphere using oven-dried glassware. Solvents
were dried over standard drying agent and freshly distilled
˚
(0.33 mL, 2.40 mmol), activated 4-A powdered molecular
˚
sieves (300 mg), and anhydrous CH2Cl2 (20 mL) was stirred
at room temperature for 15 min, and then cooled to 0 8C. To
the mixture a solution of BF3·Et2O (127 mL, 1.00 mmol) in
anhydrous CH2Cl2 (1 mL) was added drop by drop, con-
trolling the N2 evolution at a low steady rate. The mixture
was stirred at 0 8C for an additional 30 min, diluted with
10% NaHCO3 (10 mL), warmed to room temperature,
filtered through a pad of Celite, and extracted with CH2Cl2
(3£30 mL). The organic layer was dried (Na2SO4) and
concentrated. The residue was eluted from a short column of
silica gel with 4:1 cyclohexane–AcOEt to give 8b (590 mg,
prior to use. Commercially available powdered 4 A
molecular sieves (5 mm average particle size) were used
without further activation. Reactions were monitored by
TLC on silica gel 60 F254 with detection by charring with
sulfuric acid, or alcoholic solution of ninhydrin. Flash
column chromatography was performed on silica gel 60
(230–400 mesh). Melting points were determined with a
capillary apparatus. Optical rotations were measured at
20^2 8C in the stated solvent; [a]D values are given in
1021 deg cm2 g21. Infrared spectra were recorded on a
Nicolet 510 P FT-IR instrument. 1H (300 MHz), 19F
(282 MHz), and 13C (75 MHz) NMR spectra were recorded
for CDCl3 solutions at room temperature unless otherwise
specified. Assignments were aided by homo- and hetero-
nuclear two-dimensional experiments. MALDI-TOF mass
spectra were acquired using a-cyano-4-hydroxycinnamic
acid as the matrix. X-ray diffraction data for compound
(4R)-27a (R)-Mosher amide were collected at room
temperature, 295 K, on a Nonius Kappa CCD diffractometer
with graphite monochromated Mo Ka radiation
1
70%) as a yellow syrup. [a]D¼þ9.9 (c 1.7, CHCl3). H
NMR: d 7.40–7.30 (m, 5H, Ph), 5.49 (bd, 1H, J2,NH
8.5 Hz, NH), 5.19 (s, 2H, PhCH2), 4.59 (ddd, 1H, J2,3a
¼
¼
4.5 Hz, J2,3b¼4.0 Hz, H-2), 3.35 (s, 2H, 2H-5), 3.30 (dd, 1H,
J3a,3b¼18.2 Hz, H-3a), 3.12 (dd, 1H, H-3b), 1.48 (s, 9H,
t-Bu), 1.44 (s, 9H, t-Bu). MALDI-TOF MS: 422.8 (MþþH),
444.6 (MþþNa), 460.3 (MþþK). Anal. calcd for
C22H31NO7 (421.48): C, 62.69; H, 7.41; N, 3.32. Found:
C, 62.70; H, 7.44; N, 3.38.
˚
(l¼0.7107 A). The structure was solved by direct methods
3.1.3. (20S)-4-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-2,6-dimethyl-1,4-dihydro-pyridine-3,5-
dicarboxylic acid diethyl ester (11a). A screw-capped vial,
containing a magnetic bar, was charged with aldehyde 7
(615 mg, 2.00 mmol), b-ketoester 9a (255 mL, 2.00 mmol),
(SIR97)30 and refined (SHELXL-97)31 by full matrix least
squares with anisotropic non-H and hydrogen atoms
included on calculated positions riding on their carrier
atoms, except N–H hydrogens which were refined iso-
tropically. ORTEP32 view is shown in Figure 4. Aldehyde
722 was synthesized as described. tert-Butyl 3-aminocroto-
nate 10b33 was prepared by reaction of the corresponding b-
ketoester with ammonium acetate in refluxing tert-butyl
alcohol. Pyridinium chlorochromate immobilized on silica
gel was prepared as described.29 Pyridines 16b and 30b are
known compounds.18
˚
aminocrotonate 10a (258 mg, 2.00 mmol), activated 4-A
powdered molecular sieves (100 mg) and EtOH (3 mL). The
mixture was then vigorously stirred, degassed under
vacuum and saturated with argon (by an Ar-filled balloon)
three times. The mixture was stirred at 70 8C for 24 h then
cooled to room temperature, diluted with AcOEt (5 mL),
filtered through a pad of Celite, and concentrated. The
residue was eluted from a column of silica gel with 2:1
cyclohexane–AcOEt to give 11a (796 mg, 75%) as a yellow
3.1.1. (2S)-2-tert-Butoxycarbonylamino-4-oxo-hexane-
dioic acid 1-benzyl ester 6-ethyl ester (8a). A mixture of

2318
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
foam. [a]D¼þ24.1 (c 1.4, CH3OH); IR (Nujol) nmax: 3500–
0.5H, J1 b,2 ¼10.0 Hz, J1 a,1 b¼13.2 Hz, H-10b), 2.28 (s,
1.5H, CH3), 2.27 (s, 1.5H, CH3), 1.38 (s, 4.5H, t-Bu), 1.36
(s, 4.5H, t-Bu), 1.16 (t, 3H, J¼7.0 Hz, CH2CH3), 1.14 (t, 3H,
J¼7.0 Hz, CH2CH3). MALDI-TOF MS: 593.7 (MþþH),
615.4 (MþþNa), 631.6 (MþþK). Anal. calcd for
C33H40N2O (592.68): C, 66.87; H, 6.80; N, 4.73. Found:
C, 66.85; H, 6.88; N, 4.72.
0
0
0
0
1
3220 (br), 2900, 1750, 1700, 1650. H NMR: d 7.40–7.20
0
(m, 5H, Ph), 5.85 (bs, 1H, NH), 5.41 (bd, 1H, J2 ,NH¼7.5 Hz,
N0H), 5.16 and 5.05 (2d, 2H, J¼12.5 Hz, PhCH2), 4.28 (ddd,
0
0
0
0
0
1H, J1 a,2 ¼3.5 Hz, J1 b,2 ¼8.5 Hz, H-2 ), 4.18 (q, 4H, J¼
0
0
7.0 Hz, 2CH2CH3), 4.05 (dd, 1H, J4,1 a¼6.5 Hz, J4,1 b
¼
5.3 Hz, H-4), 2.31 (s, 3H, CH3), 2.27 (s, 3H, CH3), 1.90
(ddd, 1H, J1 a,1 b¼14.0 Hz, H-10a), 1.69 (ddd, 1H, H-10b),
1.42 (s, 9H, t-Bu), 1.30 (t, 6H, 2CH2CH3). MALDI-TOF
MS: 531.6 (MþþH), 553.7 (MþþNa), 569.7 (MþþK).
Anal. calcd for C28H38N2O8 (530.61): C, 63.38; H, 7.22; N,
5.28. Found: C, 63.40; H, 7.28; N, 5.25.
0
0
3.1.6. (4R,20S) and (4S,20S)-2-(20-Benzyloxycarbonyl-20-
tert-butoxycarbonylamino-ethyl)-6-methyl-4-phenyl-
1,4-dihydro-pyridine-3,5-dicarboxylic acid di-tert-butyl
esters (25b). A screw-capped vial, containing a magnetic
bar, was charged with benzaldehyde 24 (203 mL,
2.00 mmol), b-ketoester 8b (843 mg, 2.00 mmol), amino-
3.1.4. (20S)-4-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-2,6-dimethyl-1,4-dihydro-pyridine-3,5-
dicarboxylic acid di-tert-butyl ester (11b). A screw-
capped vial, containing a magnetic bar, was charged with
aldehyde 7 (1.23 g, 4.00 mmol), b-ketoester 9b (0.66 mL,
4.00 mmol), aminocrotonate 10b (0.63 g, 4.00 mmol),
˚
crotonate 10b (314 mg, 2.00 mmol), powdered 4 A
molecular sieves (100 mg) and tert-BuOH (3 mL). The
mixture was then vigorously stirred, degassed under
vacuum and saturated with argon (by an Ar-filled balloon)
three times. The mixture was stirred at 70 8C for 24 h then
cooled to room temperature, diluted with AcOEt (5 mL),
filtered through a pad of Celite, and concentrated. The
residue was eluted from a column of silica gel with 4:1
cyclohexane–AcOEt to give 25b (973 mg, 75%) as a 1:1
mixture of diastereoisomers. IR (Nujol) nmax: 3500–3210
˚
activated 4-A powdered molecular sieves (200 mg) and
tert-BuOH (5 mL). The mixture was then vigorously stirred,
degassed under vacuum and saturated with argon (by an
Ar-filled balloon) three times. The mixture was stirred at
70 8C for 24 h then cooled to room temperature, diluted with
AcOEt (10 mL), filtered through a pad of Celite, and
concentrated. The residue was eluted from a column of
silica gel with 4:1 cyclohexane–AcOEt to give 11b (1.76 g,
75%) as a yellow foam. [a]D¼þ21.4 (c 1.0, CH3OH); IR
1
(br), 2920, 1740, 1690, 1660, 1650. H NMR (DMSO-d6,
120 8C): d 8.14 (bs, 0.5H, NH), 8.00 (bs, 0.5H, NH), 7.40–
7.00 (m, 10H, 2Ph), 6.48 (bd, 0.5H, J2 ,NH¼8.0 Hz, N0H),
0
6.34 (bd, 0.5H, J2 ,NH¼8.0 Hz, N0H), 5.17 (s, 2H, PhCH2),
0
1
(Nujol) nmax: 3450–3200 (br), 2900, 1760, 1720, 1650. H
4.90 (s, 0.5H, H-4), 4.88 (s, 0.5H, H-4), 4.60–4.46 (m, 1H,
0
H-20), 3.34 (dd, 0.5H, J1 a,2 ¼5.6 Hz, J1 a,1 b¼13.2 Hz,
0
0
0
NMR (DMSO-d6, 120 8C): d 8.21 (bs, 1H, NH), 7.40–7.30
(m, 5H, Ph), 5.92 (bs, 1H, N0H), 5.08 (s, 2H, PhCH2), 4.05
H-10a), 3.16 (dd, 0.5H, J1 a,2 ¼5.4 Hz, J1 a,1 b¼13.4 Hz,
0
0
0
0
0
(dd, 1H, J1 a,2 ¼4.0 Hz, J1 b,2 ¼8.3 Hz, H-2 ), 3.89 (dd, 1H,
H-10a), 3.04 (dd, 0.5H, J1 b,2 ¼10.7 Hz, J1 a,1 b¼13.2 Hz,
0
0
0
0
0
0
0
0
J4,1 a¼6.1 Hz, J4,1 b¼5.6 Hz, H-4), 2.19 (s, 6H, 2CH3), 1.73
H-10b), 2.77 (dd, 0.5H, J1 b,2 ¼10.5 Hz, J1 a,1 b¼13.4 Hz,
H-10b), 2.23 (s, 1.5H, CH3), 2.22 (s, 1.5H, CH3), 1.38 (s, 9H,
t-Bu), 1.37 (s, 9H, t-Bu), 1.36 (s, 4.5H, t-Bu), 1.35 (s, 4.5H,
t-Bu). MALDI-TOF MS: 649.9 (MþþH), 671.6 (MþþNa),
687.5 (MþþK). Anal. calcd for C37H48N2O8 (648.79): C,
68.50; H, 7.46; N, 4.32. Found: C, 68.46; H, 7.44; N, 4.39.
0
0
0
0
0
0
(ddd, 1H, J1 a,1 b¼11.0 Hz, H-10a), 1.54 (ddd, 1H, H-10b),
1.46 (s, 18H, 2t-Bu), 1.38 (s, 9H, t-Bu). MALDI-TOF MS:
587.7 (MþþH), 609.7 (MþþNa), 625.8 (MþþK). Anal.
calcd for C32H46N2O8 (586.72): C, 65.51; H, 7.90; N, 4.77.
Found: C, 65.50; H, 7.48; N, 4.75.
0
0
3.1.5. (4R,20S)- and (4S,20S)-2-(20-Benzyloxycarbonyl-20-
tert-butoxycarbonylamino-ethyl)-6-methyl-4-phenyl-
1,4-dihydro-pyridine-3,5-dicarboxylic acid diethyl esters
(25a). A screw-capped vial, containing a magnetic bar, was
charged with benzaldehyde 24 (407 mL, 4.00 mmol),
b-ketoester 8a (787 mg, 2.00 mmol), aminocrotonate 10a
(517 mg, 4.00 mmol). The mixture was then vigorously
stirred, degassed under vacuum and saturated with argon (by
an Ar-filled balloon) three times. The mixture was stirred at
70 8C for 1 h then cooled to room temperature, diluted with
AcOEt (10 mL), and concentrated. The residue was eluted
from a column of silica gel with 4:1 cyclohexane–AcOEt to
give 25a (830 mg, 70%) as a 1:1 mixture of diastereo-
isomers contaminated by 26 (155 mg, 0.47 mmol) as judged
3.2. General procedure for DHP oxidation leading to
pyridines 14b and 28b
˚
A mixture of DHP 11b or 25b (1.00 mmol), activated 4-A
powdered molecular sieves (200 mg), and anhydrous
CH2Cl2 (15 mL) was stirred at room temperature for
15 min, and then pyridinium chlorochromate (647 mg,
3.00 mmol) was added. The suspension was stirred for
2 h, and then cyclohexane (15 mL) and Et2O (30 mL) were
added. The mixture was filtered through a pad of silica gel
and concentrated. The residue was eluted from a short
column of silica gel with the suitable elution system to give
the analytical sample of the corresponding pyridine.
3.2.1. (20S)-4-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-2,6-dimethyl-pyridine-3,5-dicarboxylic
acid di-tert-butyl ester (14b). Column chromatography
with 4:1 cyclohexane–AcOEt afforded 14b (573 mg, 98%)
1
by H NMR analysis. IR (Nujol) nmax: 3500–3200 (br),
2910, 1710, 1690, 1650, 1640. 1H NMR (DMSO-d6,
120 8C) for 25a: d 8.33 (bs, 0.5H, NH), 8.23 (bs, 0.5H,
0
NH), 7.40–7.00 (m, 10H, 2Ph), 6.46 (bd, 0.5H, J2 ,NH
¼
8.0 Hz, N0H), 6.42 (bd, 0.5H, J2 ,NH¼8.0 Hz, N0H), 5.18
and 5.12 (2d, 2H, J¼13.4 Hz, PhCH2), 4.95 (s, 1H, H-4),
4.60–4.48 (m, 1H, H-20), 4.14–4.00 (m, 4H, 2CH2CH3),
as a white foam. [a]D¼237.0 (c 1.2, CHCl3); IR (Nujol)
0
1
n
max: 3400, 2900, 1740, 1710, 1690. H NMR (DMSO-d6,
120 8C): d 7.40–7.20 (m, 5H, Ph), 6.31 (bs, 1H, N0H), 5.10
3.41 (dd, 0.5H, J1 a,2 ¼5.6 Hz, J1 a,1 b¼13.2 Hz, H-10a), 3.28
(s, 2H, PhCH2), 4.42 (dd, 1H, J1 a,2 ¼7.1 Hz, J1 b,2 ¼9.0 Hz,
0
0
0
0
0
0
0
0
(dd, 0.5H, J1 a,2 ¼5.4 Hz, J1 a,1 b¼13.7 Hz, H-10a), 2.97 (dd,
H-20), 3.22 (dd, 1H, J1 a,1 b¼14.2 Hz, H-10a), 2.89 (dd, 1H,
0
0
0
0
0
0
0.5H, J1 b,2 ¼10.5 Hz, J1 a,1 b¼13.7 Hz, H-10b), 2.76 (dd,
H-10b), 2.44 (s, 6H, 2CH3), 1.59 (s, 18H, 2t-Bu), 1.31 (s, 9H,
0
0
0
0

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2319
t-Bu). MALDI-TOF MS: 585.7 (MþþH), 607.6 (MþþNa),
3.4. General procedure for Boc deprotection of amino
esters 11a, 14b, 17b, 28b and 31b
623.9 (MþþK). Anal. calcd for C32H44N2O8 (584.70): C,
65.73; H, 7.58; N, 4.79. Found: C, 65.71; H, 7.56; N, 4.75.
To a cooled (0 8C), stirred solution of N0-Boc amino ester
(0.50 mmol) in CH2Cl2 (1 mL) was slowly added a solution
of TFA–CH2Cl2 (1–3 mL). Stirring was continued at 0 8C
for an additional 30 min then the solution was warmed to
room temperature. After 30 min at room temperature the
solution was neutralized at 0 8C with saturated aqueous
Na2CO3 and extracted with CH2Cl2 (2£50 mL). The
combined organic phases were dried (Na2SO4), concen-
trated, and eluted from a column of silica gel with the
suitable elution system to give the corresponding
N0-deprotected amino ester.
3.2.2. (20S)-2-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-6-methyl-4-phenyl-pyridine-3,5-dicarb-
oxylic acid di-tert-butyl ester (28b). Column chroma-
tography with 4:1 cyclohexane–AcOEt afforded 28b
(634 mg, 98%) as a white foam. [a]D¼þ5.6 (c 0.7,
CHCl3); IR (Nujol) nmax: 3450, 2910, 1760, 1710, 1690.
1H NMR (DMSO-d6, 120 8C): d 7.45–7.10 (m, 10H, 2Ph),
6.63 (bd, 1H, J2 ,NH¼7.3 Hz, N0H), 5.15 and 5.09 (2d, 2H,
0
0
0
0
0
J¼12.7 Hz, PhCH2), 4.75 (ddd, 1H, J1 a,2 ¼6.6 Hz, J1 b,2
¼
7.1 Hz, H-20), 3.33 (dd, 1H, J1 a,1 b¼15.1 Hz, H-10a), 3.19
(dd, 1H, H-10b), 2.47 (s, 3H, CH3), 1.38 (s, 9H, t-Bu), 1.22
(s, 9H, t-Bu), 1.20 (s, 9H, t-Bu). MALDI-TOF MS: 647.9
(MþþH), 669.6 (MþþNa), 685.3 (MþþK). Anal. calcd for
C37H46N2O8 (646.77): C, 68.71; H, 7.17; N, 4.33. Found: C,
68.66; H, 7.12; N, 4.33.
0
0
3.4.1. (20S)-4-(20-Amino-20-benzyloxycarbonyl-ethyl)-
2,6-dimethyl-1,4-dihydro-pyridine-3,5-dicarboxylic acid
diethyl ester (12a). Column chromatography with 4:1
cyclohexane–AcOEt (containing 1% of Et3N) afforded 12a
(168 mg, 78%) as a white foam. [a]D¼þ40.8 (c 1.3,
CH3OH); IR (Nujol) nmax: 3600–3200 (br), 2900, 1730,
3.3. General procedure for pyridine oxidation leading to
1-oxido-pyridines 17b and 31b
1
1720. H NMR: d 7.40–7.30 (m, 5H, Ph), 5.92 (bs, 1H,
NH), 5.15 and 5.09 (2d, 2H, J¼12.5 Hz, PhCH2), 4.25–4.15
0
0
A mixture of pyridine 14b or 28b (1.00 mmol), 3-chloro-
peroxybenzoic acid (690 mg, 4.00 mmol), and anhydrous
CH2Cl2 (30 mL) was stirred at room temperature for 15 h,
then diluted with 10% NaHCO3 (15 mL), and extracted with
CH2Cl2 (3£30 mL). The combined organic layer was dried
(Na2SO4) and concentrated. The residue was eluted from a
column of silica gel with the suitable elution system to give
the corresponding 1-oxido-pyridine.
(m, 4H, 2CH2CH3), 4.10 (dd, 1H, J4,1 a¼9.0 Hz, J4,1 b¼
4.0 Hz, H-4), 3.48 (bdd, 1H, H-20), 2.32 (s, 3H, CH3), 2.29
(s, 3H, CH3), 2.82 (m, 3H, N0H2, H-10a), 1.44 (ddd, 1H,
0
0
0
0
0
J1 a,1 b¼13.0 Hz, J1 b,2 ¼9.5 Hz, H-1 b), 1.32 (t, 3H, J¼
7.0 Hz, CH2CH3), 1.30 (t, 3H, J¼7.0 Hz, CH2CH3). 13C
NMR: d 175.2, 167.9, 167.1, 146.1, 145.4, 135.9, 128.5,
128.4, 128.3, 128.2, 103.0, 101.8, 66.4, 60.1, 59.8, 51.7,
41.8, 29.9, 19.9, 194, 14.4, 14.0. MALDI-TOF MS: 431.5
(MþþH), 453.7 (MþþNa), 469.5 (MþþK). Anal. calcd for
C23H30N2O6 (430.49): C, 64.17; H, 7.02; N, 6.51. Found: C,
64.20; H, 7.04; N, 6.56.
3.3.1. (20S)-4-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-2,6-dimethyl-1-oxido-pyridine-3,5-
dicarboxylic acid di-tert-butyl ester (17b). Column
chromatography with 1:1 cyclohexane–AcOEt afforded
17b (589 mg, 98%) as a white foam. [a]D¼227.1 (c 2.1,
CH3OH); IR (Nujol) nmax: 3200, 2900, 1750, 1720, 1690,
3.4.2. (20S)-4-(20-Amino-20-benzyloxycarbonyl-ethyl)-
2,6-dimethyl-pyridine-3,5-dicarboxylic acid di-tert-
butyl ester (15b). Column chromatography with 1:1
cyclohexane–AcOEt afforded 15b (182 mg, 75%) as a
1
1250. H NMR (DMSO-d6, 140 8C): d 7.40–7.20 (m, 5H,
Ph), 6.21 (bd, 1H, J2 ,NH¼8.0 Hz, N0H), 5.11 (s, 2H,
0
white foam. [a]D¼þ2.8 (c 1.4, CH3OH); IR (Nujol) nmax
:
0
0
0
0
PhCH2), 4.45 (ddd, 1H, J1 a,2 ¼7.1 Hz, J1 b,2 ¼9.3 Hz,
3600–3300 (br), 2900, 1730, 1700, 1690. 1H NMR: d 7.40–
7.26 (m, 5H, Ph), 5.17 (s, 2H, PhCH2), 3.85 (dd, 1H,
H-20), 3.16 (dd, 1H, J1 a,1 b¼14.4 Hz, H-10a), 2.87 (dd, 1H,
H-10b), 2.40 (s, 6H, 2CH3), 1.60 (s, 18H, 2t-Bu), 1.33 (s,
9H, t-Bu). MALDI-TOF MS: 601.5 (MþþH), 623.7
(MþþNa), 639.8 (MþþK). Anal. calcd for C32H44N2O9
(600.70): C, 63.98; H, 7.38; N, 4.66. Found: C, 63.90; H,
7.33; N, 4.62.
0
0
0
0
0
0
0
0
0
J1 a,2 ¼4.5 Hz, J1 b,2 ¼11.0 Hz, H-2 ), 3.24 (dd, 1H, J1 a,1 b
¼
13.8 Hz, H-10a), 2.84 (dd, 1H, H-10b), 2.53 (s, 6H, 2CH3),
1.57 (s, 18H, 2t-Bu), 1.56 (bs, 2H, N0H2). 13C NMR: d
174.4, 167.6, 154.6, 154.5, 140.6, 135.4, 128.9, 128.5,
128.4, 128.3, 128.2, 83.3, 66.9, 54.8, 35.4, 28.0, 22.9.
MALDI-TOF MS: 485.6 (MþþH), 507.7 (MþþNa), 523.8
(MþþK). Anal. calcd for C27H36N2O6 (484.58): C, 66.92;
H, 7.49; N, 5.78. Found: C, 66.90; H, 7.48; N, 5.75.
3.3.2. (20S)-2-(20-Benzyloxycarbonyl-20-tert-butoxycarbo-
nylamino-ethyl)-6-methyl-1-oxido-4-phenyl-pyridine-
3,5-dicarboxylic acid di-tert-butyl ester (31b). Column
chromatography with 3.5:1 cyclohexane–AcOEt afforded
31b (650 mg, 98%) as a white foam. [a]D¼þ5.8 (c 1.2,
CHCl3); IR (Nujol) nmax: 3250, 2920, 1780, 1730, 1690,
1260. 1H NMR (DMSO-d6, 120 8C): d 7.50–7.10 (m, 10H,
3.4.3. (20S)-4-(20-Amino-20-benzyloxycarbonyl-ethyl)-
2,6-dimethyl-1-oxido-pyridine-3,5-dicarboxylic acid di-
tert-butyl ester (18b). Column chromatography with
AcOEt afforded 18b (175 mg, 70%) as a white foam.
[a]D¼þ1.1 (c 1.1, CH3OH); IR (Nujol) nmax: 3700–3200
(br), 2900, 1730, 1710, 1250. 1H NMR: d 7.40–7.25 (m, 5H,
2Ph), 6.70 (bd, 1H, J2 ,NH¼7.8 Hz, N0H), 5.17 and 5.07 (2d,
0
0
0
2H, J¼12.7 Hz, PhCH2), 4.82 (ddd, 1H, J1 a,2 ¼6.8 Hz,
J1 b,2 ¼8.6 Hz, H-2 ), 3.42 (dd, 1H, J1 a,1 b¼13.4 Hz, H-10a),
3.29 (dd, 1H, H-10b), 2.44 (s, 3H, CH3), 1.35 (s, 9H, t-Bu),
1.22 (s, 9H, t-Bu), 1.19 (s, 9H, t-Bu). MALDI-TOF MS:
663.6 (MþþH), 685.9 (MþþNa), 701.4 (MþþK). Anal.
calcd for C37H46N2O9 (662.77): C, 67.05; H, 7.00; N, 4.23.
Found: C, 67.05; H, 7.08; N, 4.17.
Ph), 5.17 (s, 2H, PhCH2), 3.84 (dd, 1H, J1 a,2 ¼4.7 Hz,
0
0
0
0
0
0
0
0
0
0
0
0
J1 b,2 ¼11.0 Hz, H-2 ), 3.16 (dd, 1H, J1 a,1 b¼14.0 Hz,
H-10a), 2.79 (dd, 1H, H-10b), 2.50 (s, 6H, 2CH3), 1.59 (s,
20H, 2t-Bu, N0H2). 13C NMR: d 174.3, 165.2, 146.4, 135.3,
132.3, 128.6, 128.5, 128.4, 128.3, 127.3, 84.5, 67.1, 54.8,
35.1, 28.0, 15.8. MALDI-TOF MS: 501.8 (MþþH), 522.7

2320
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
(MþþNa), 539.3 (MþþK). Anal. calcd for C27H36N2O7
(500.58): C, 64.78; H, 7.25; N, 5.60. Found: C, 64.80; H,
7.24; N, 5.56.
167.4, 148.0, 144.5, 135.2, 128.6, 128.5, 128.3, 128.2,
128.1, 128.0, 127.9, 127.8, 127.7, 126.0, 104.5, 103.2, 67.3,
59.7, 59.5, 53.4, 39.6, 33.3, 19.3, 14.2, 14.1. MALDI-TOF
MS: 493.3 (MþþH), 515.6 (MþþNa), 531.3 (MþþK).
Anal. calcd for C28H32N2O6 (492.56): C, 68.28; H, 6.55; N,
5.69. Found: C, 68.25; H, 6.57; N, 5.62.
3.4.4. (20S)-2-(20-Amino-20-benzyloxycarbonyl-ethyl)-6-
methyl-4-phenyl-pyridine-3,5-dicarboxylic acid di-tert-
butyl ester (29b). Column chromatography with 1:1
cyclohexane–AcOEt (containing 1% of Et3N) afforded
29b (205 mg, 75%) as a white foam. [a]D¼24.2 (c 0.7,
CHCl3); IR (Nujol) nmax: 3600–3300 (br), 2900, 1730,
1700, 1690. 1H NMR: d 7.40–7.25 (m, 10H, 2Ph), 5.20 (s,
Eluted second was (4R)-27a. [a]D¼þ15.5 (c 1.0, CHCl3);
1
IR (Nujol) nmax: 3600–3200 (br), 2900, 1740, 1730. H
NMR: d 8.54 (s, 1H, NH), 7.40–7.10 (m, 10H, 2Ph), 5.19
and 5.08 (2d, 2H, J¼12.0 Hz, PhCH2), 5.00 (s, 1H, H-4),
0
0
0
0
0
0
2H, PhCH2), 4.14 (bdd, 1H, J1 a,2 ¼4.2 Hz, J1 b,2 ¼7.8 Hz,
4.16–3.98 (m, 4H, 2CH2CH3), 3.85 (dd, 1H, J1 a,2 ¼3.5,
H-20), 3.38 (dd, 1H, J1 a,1 b¼15.0 Hz, H-10a), 3.22 (dd, 1H,
H-10b), 2.53 (s, 3H, CH3), 2.05 (bs, 2H, N0H2), 1.20 (s, 9H,
t-Bu), 1.16 (s, 9H, t-Bu). 13C NMR: d 174.7, 166.7, 166.5,
154.4, 153.7, 145.4, 136.3, 135.7, 128.8, 128.7, 128.6,
128.5, 128.4, 128.3, 128.2, 128.1, 127.9, 127.8, 127.7, 82.6,
82.4, 66.8, 53.7, 39.2, 27.5, 27.4, 22.6. MALDI-TOF MS:
547.9 (MþþH), 569.6 (MþþNa), 585.7 (MþþK). Anal.
calcd for C32H38N2O6 (546.65): C, 70.31; H, 7.01; N, 5.12.
Found: C, 70.35; H, 7.08; N, 5.17.
J1 b,2 ¼8.2 Hz, H-2 ), 3.67 (dd, 1H, J1 a,1 b¼15.0 Hz, H-10a),
2.85 (dd, 1H, H-10b), 2.31 (s, 3H, CH3), 1.77 (bs, 2H, N0H2),
1.24 (t, 3H, J¼7.0 Hz, CH2CH3), 1.19 (t, 3H, J¼7.0 Hz,
CH2CH3). 13C NMR: d 174.1, 167.5, 167.4, 147.9, 144.5,
135.2, 128.6, 128.5, 128.3, 128.2, 128.1, 128.0, 127.9,
127.8, 127.7, 126.1, 104.5, 103.2, 67.3, 59.8, 59.6, 53.5,
39.7, 33.2, 19.5, 14.2, 14.1. MALDI-TOF MS: 493.5
(MþþH), 515.6 (MþþNa), 531.3 (MþþK). Anal. calcd
for C28H32N2O6 (492.56): C, 68.28; H, 6.55; N, 5.69.
Found: C, 68.22; H, 6.53; N, 5.66.
0
0
0
0
0
0
0
3.4.5. (20S)-2-(20-Amino-20-benzyloxycarbonyl-ethyl)-6-
methyl-1-oxido-4-phenyl-pyridine-3,5-dicarboxylic acid
di-tert-butyl ester (32b). Column chromatography with
AcOEt (containing 1% of Et3N) afforded 32b (197 mg,
70%) as a white foam. [a]D¼þ21.9 (c 0.7, CH3OH); IR
3.5. General procedure for debenzylation of amino esters
11a, 14b, 17b, 28b and 31b
A vigorously stirred mixture of 20% palladium hydroxide
on carbon (50 w/w of substrate), AcOEt (2 mL), and EtOH
(2 mL) was degassed under vacuum and saturated with
hydrogen (by a H2-filled balloon) three times. To this
mixture was added a solution of amino benzyl ester
(0.30 mmol) in AcOEt (2 mL) previously degassed and
saturated with hydrogen as described before. After the
solution was stirred under a slightly positive pressure of
hydrogen (balloon) at room temperature for 15 min,
palladium hydroxide on carbon was filtered off through a
plug of cotton and washed thoroughly with MeOH (2 mL).
The combined filtrates were concentrated to give the
corresponding amino acid in almost quantitative yield.
1
(Nujol) nmax: 3650–3200 (br), 2900, 1730, 1710, 1250. H
NMR: d 7.40–7.20 (m, 10H, 2Ph), 5.23 and 5.15 (2d, 2H,
0
0
0
0
J¼12.0 Hz, PhCH2), 4.41 (bdd, 1H, J1 a,2 ¼5.7 Hz, J1 b,2
¼
9.0 Hz, H-20), 3.48 (dd, 1H, J1 a,1 b¼13.0 Hz, H-10a), 3.27
(dd, 1H, H-10b), 2.54 (s, 3H, CH3), 1.76 (bs, 2H, N0H2), 1.20
(s, 9H, t-Bu), 1.16 (s, 9H, t-Bu). 13C NMR: d 174.9, 165.4,
146.3, 145.6, 135.6, 132.6, 132.4, 129.3, 128.9, 128.8,
128.7, 128.5, 128.4, 128.3, 128.2, 128.1, 128.0, 127.9, 83.8,
83.5, 66.9, 51.6, 34.5, 27.4, 27.3, 15.6. MALDI-TOF MS:
563.7 (MþþH), 585.4 (MþþNa), 601.6 (MþþK). Anal.
calcd for C37H46N2O9 (562.65): C, 68.31; H, 6.81; N, 4.98.
Found: C, 68.35; H, 6.88; N, 4.97.
0
0
3.4.6. (4S,20S)- and (4R,20S)-2-(20-Amino-20-benzyloxy-
carbonyl-ethyl)-6-methyl-4-phenyl-1,4-dihydro-pyri-
dine-3,5-dicarboxylic acid diethyl esters ((4S)-27a and
(4R)-27a). To a cooled (0 8C), stirred solution of 25a
(0.50 mmol) contaminated by 26 (0.17 mmol) in CH2Cl2
(1 mL) was slowly added a solution of TFA–CH2Cl2
(1–3 mL). Stirring was continued at 0 8C for 1 h, then the
solution was neutralized at 0 8C with Et3N, concentrated,
and eluted from a column of silica gel with 1:1.5
cyclohexane–AcOEt to give the diastereomeric amino
esters (4S)-27a and (4R)-27a (185 mg, 75%) in 1:1 ratio.
3.5.1. (20S)-4-(20-tert-Butoxycarbonylamino-20-carboxy-
ethyl)-2,6-dimethyl-1,4-dihydro-pyridine-3,5-dicarb-
oxylic acid diethyl ester (13a). White foam. [a]D¼þ28.0
(c 1.1, CH3OH); IR (Nujol) nmax: 3600–3200 (br), 2900,
1
1740, 1730, 1690, 1650. H NMR (DMSO-d6, 120 8C): d
8.41 (bs, 1H, NH), 5.75 (bd, 1H, J2 ,NH¼8.1 Hz, N0H), 4.14
0
(q, 2H, J¼7.0 Hz, CH2CH3), 4.12 (q, 2H, J¼7.0 Hz,
0
0
CH2CH3), 3.94 (dd, 1H, J4,1 a¼6.6 Hz, J4,1 b¼5.4 Hz,
0
0
0
0
0
H-4), 3.92 (dd, 1H, J1 a,2 ¼4.4 Hz, J1 b,2 ¼8.6 Hz, H-2 ),
2.24 (s, 6H, 2CH3), 1.71 (ddd, 1H, J1 a,1 b¼13.9 Hz, H-10a),
1.50 (ddd, 1H, H-10b), 1.41 (s, 9H, t-Bu), 1.27 (t, 3H,
CH2CH3), 1.25 (t, 3H, CH2CH3). MALDI-TOF MS: 441.8
(MþþH), 463.7 (MþþNa), 479.5 (MþþK). Anal. calcd for
C21H32N2O8 (440.49): C, 57.26; H, 7.32; N, 6.36. Found: C,
57.31; H, 7.34; N, 6.35.
0
0
Analytical samples of each diastereoisomer were obtained
by preparative TLC (1:2 cyclohexane–AcOEt containing
1% of Et3N). Eluted first was (4S)-27a. [a]D¼þ9.2 (c 1.0,
CHCl3); IR (Nujol) nmax: 3600–3200 (br), 2900, 1740,
1730. H NMR: d 8.61 (s, 1H, NH), 7.40–7.10 (m, 10H,
2Ph), 5.21 and 5.16 (2d, 2H, J¼12.0 Hz, PhCH2), 4.98 (s,
3.5.2. (20S)-4-(20-tert-Butoxycarbonylamino-20-carboxy-
ethyl)-2,6-dimethyl-1-oxido-pyridine-3,5-dicarboxylic
acid di-tert-butyl ester (19b). White foam. [a]D¼211.3 (c
1.4, CH3OH); IR (Nujol) nmax: 3600–3200 (br), 2900, 1740,
1730, 1690, 1250. 1H NMR (DMSO-d6, 120 8C): d 5.84 (bd,
1
1H, H-4), 4.18–4.00 (m, 4H, 2CH2CH3), 3.88 (dd, 1H,
0
0
0
0
0
0
0
J1 a,2 ¼3.8 Hz, J1 b,2 ¼8.0 Hz, H-2 ), 3.48 (dd, 1H, J1 a,1 b
¼
15.2 Hz, H-10a), 3.03 (dd, 1H, H-10b), 2.28 (s, 3H, CH3),
1.73 (bs, 2H, N0H2), 1.24 (t, 3H, J¼7.0 Hz, CH2CH3), 1.21
(t, 3H, J¼7.0 Hz, CH2CH3). 13C NMR: d 174.1, 167.5,
1H, J2 ,NH¼8.0 Hz, N0H), 4.28 (ddd, 1H, J1 a,2 ¼5.6 Hz,
0
0
0
0
0
0
0
0
J1 b,2 ¼10.5 Hz, H-2 ), 3.22 (dd, 1H, J1 a,1 b¼14.4 Hz,

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2321
H-10a), 2.75 (dd, 1H, H-10b), 2.42 (s, 6H, 2CH3), 1.63 (s,
18H, 2t-Bu), 1.30 (s, 9H, t-Bu). MALDI-TOF MS: 511.4
(MþþH), 533.7 (MþþNa), 549.5 (MþþK). Anal. calcd for
C25H38N2O9 (510.58): C, 58.81; H, 7.50; N, 5.49. Found: C,
58.86; H, 7.46; N, 5.53.
residue was taken into AcOEt, washed with saturated
aqueous NaHCO3 and brine, dried over Na2SO4, and
concentrated. The residue was purified by preparative
TLC affording the corresponding Mosher amide in almost
quantitative yield.
3.5.3. (20S)-2-(20-tert-Butoxycarbonylamino-20-carboxy-
ethyl)-6-methyl-1-oxido-4-phenyl-pyridine-3,5-dicarb-
oxylic acid di-tert-butyl ester (33b). White foam. [a]D¼
210.3 (c 0.8, CH3OH); IR (Nujol) nmax: 3600–3200 (br),
2900, 1740, 1730, 1680, 1250. 1H NMR (DMSO-d6,
120 8C) d: 7.50–7.40 and 7.30–7.20 (2m, 5H, Ph), 6.17
3.6.1. (20S,200S)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-1,4-dihydro-pyridine-3,5-dicarboxylic acid
diethyl ester (12a (S)-Mosher amide). Elution system: 2:1
0
cyclohexane–AcOEt. 1H NMR: d 7.81 (d, 1H, J2 ,NH
¼
7.5 Hz, N0H), 7.70–7.65 and 7.42–7.30 (m, 10H, 2Ph), 5.71
(bd, 1H, J¼7.1 Hz, N0H), 4.66 (ddd, 1H, J1 a,2 ¼4.9, J1 b,2
¼
(bs, 1H, NH), 5.16 and 5.10 (2d, 2H, J¼12.0 Hz,0PhCH2),
0
0
0
0
11.5 Hz, H-20), 3.45 (dd, 1H, J1 a,1 b¼13.2 Hz, H-10a), 3.19
(dd, 1H, H-10b), 2.46 (s, 3H, CH3), 1.32 (s, 9H, t-Bu), 1.22
(s, 18H, 2t-Bu). MALDI-TOF MS: 573.5 (MþþH), 595.9
(MþþNa), 611.6 (MþþK). Anal. calcd for C30H40N2O9
(572.65): C, 62.92; H, 7.04; N, 4.89. Found: C, 62.95; H,
7.08; N, 4.85.
4.56 (ddd, 1H, J1 a,2 ¼4.0 Hz, J1 b,2 ¼8.0 Hz, H-2 ), 4.20–
0
0
0
0
0
0
0
4.05 (m, 4H, 2CH2CH3), 3.95 (dd, 1H, J4,1 a¼6.5 Hz,
0
J4,1 b¼5.0 Hz, H-4), 3.56 (q, 3H, J¼0.7 Hz, OCH3), 2.20 (s,
0
0
3H, CH3), 2.19 (s, 3H, CH3), 1.95 (ddd, 1H, J1 a,1 b¼14.0
Hz, H-10a), 1.84 (ddd, 1H, H-10b), 1.24 (t, 3H, J¼7.0 Hz,
CH2CH3), 1.18 (t, 3H, J¼7.0 Hz, CH2CH3). 19F NMR: d
269.3. Anal. calcd for C33H37F3N2O8 (646.65): C, 61.29;
H, 5.77; N, 4.33. Found: C, 61.30; H, 5.77; N, 4.33.
3.5.4. (4R,20S)- and (4S,20S)-2-(20-tert-Butoxycarbonyl-
amino-20-carboxy-ethyl)-6-methyl-4-phenyl-1,4-dihydro-
pyridine-3,5-dicarboxylic acid diethyl esters (debenzyl-
ated 25a). A vigorously stirred mixture of 20% palladium
hydroxide on carbon (90 mg), AcOEt (2 mL), and EtOH
(2 mL) was degassed under vacuum and saturated with
hydrogen (by a H2-filled balloon) three times. To this
mixture was added a solution of 25a (0.30 mmol)
contaminated by 26 (0.10 mmol) in AcOEt (2 mL) pre-
viously degassed and saturated with hydrogen as described
before. After the solution was stirred under a slightly
positive pressure of hydrogen (balloon) at room temperature
for 15 min, palladium hydroxide on carbon was filtered off
through a plug of cotton and washed thoroughly with MeOH
(2 mL). The combined filtrates were concentrated. The
residue was eluted from a column of silica gel with 1:1
cyclohexane–AcOEt (containing 1% of AcOH) to give
debenzylated 25a (148 mg, 98%) as a white foam. IR
(Nujol) nmax: 3600–3200 (br), 2900, 1740, 1730, 1690,
1650. 1H NMR (DMSO-d6, 120 8C): d 8.36 (bs, 0.5H, NH),
8.21 (bs, 0.5H, NH), 7.25–7.05 (m, 5H, Ph), 6.24 (bd, 0.05H,
3.6.2. (20S,200R)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-1,4-dihydro-pyridine-3,5-dicarboxylic acid
diethyl ester (12a (R)-Mosher amide). Elution system: 2:1
1
0
cyclohexane–AcOEt. H NMR: d 7.89 (bd, 1H, J2 ,NH
¼
7.2 Hz, N0H), 7.65–7.60 and 7.42–7.20 (m, 10H, 2Ph), 5.78
(s, 1H, NH), 5.11 and 5.06 (2d, 2H, J¼12.2 Hz, 0PhCH2),
0
0
0
0
4.69 (ddd, 1H, J1 a,2 ¼4.0 Hz, J1 b,2 ¼8.0 Hz, H-2 ), 4.26–
0
4.09 (m, 4H, 2CH2CH3), 4.07 (dd, 1H, J4,1 a¼6.2 Hz,
0
J4,1 b¼5.2 Hz, H-4), 3.50 (q, 3H, J¼0.7 Hz, OCH3), 2.31 (s,
0
0
3H, CH3), 2.25 (s, 3H, CH3), 2.00 (ddd, 1H, J1 a,1 b¼14.0
Hz, H-10a), 1.85 (ddd, 1H, H-10b), 1.28 (t, 3H, J¼7.0 Hz,
CH2CH3), 1.23 (t, 3H, J¼7.0 Hz, CH2CH3). 19F NMR: d
269.5. Anal. calcd for C33H37F3N2O8 (646.65): C, 61.29;
H, 5.77; N, 4.33. Found: C, 61.27; H, 5.74; N, 4.33.
3.6.3. (20S,200S)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-pyridine-3,5-dicarboxylic acid di-tert-butyl
ester (15b (S)-Mosher amide). Elution system: 3.5:1
0
J2 ,NH¼8.8 Hz, N0H), 6.15 (bd, 0.5H, J2 ,NH¼8.8 Hz, N H),
4.97 (s, 0.5H, H-4), 4.96 (s, 0.5H, H-4), 4.50–4.34 (m, 1H,
H-20), 4.20–4.00 (m, 4H, 2CH2CH3), 3.37 (dd, 0.5H,
0
0
cyclohexane–AcOEt. 1H NMR: d 8.28 (d, 1H, J2 ,NH
¼
8.3 Hz, N0H), 7.40–7.00 (m, 10H, 2Ph), 5.21 (s, 2H,
J1 a,2 ¼5.1 Hz, J1 a,1 b¼13.4 Hz, H-10a), 3.22 (dd, 0.5H,
PhCH2), 5.06 (ddd, 1H, J1 a,2 ¼5.6 Hz, J1 b,2 ¼12.9 Hz,
0
0
0
0
0
0
0
0
J1 a,2 ¼4.9 Hz, J1 a,1 b¼13.7 Hz, H-10a), 2.99 (dd, 0.5H,
H-20), 3.54 (dd, 1H, J1 a,1 b¼13.9 Hz, H-10a), 3.50 (q, 3H,
J¼0.7 Hz, OCH3), 2.70 (dd, 1H, H-10b), 2.40 (s, 6H, 2CH3),
1.59 (s, 18H, 2t-Bu). 19F NMR: d 269.9. Anal. calcd for
C37H43N2F3O8 (700.74): C, 63.42; H, 6.19; N, 4.00. Found:
C, 63.41; H, 6.16; N, 4.07.
0
0
0
0
0
0
J1 b,2 ¼10.7 Hz, J1 a,1 b¼13.4 Hz, H-10b), 2.74 (dd, 0.5H,
0
0
0
0
J1 b,2 ¼10.8 Hz, J1 a,1 b¼13.7 Hz, H-10b), 2.28 (s, 1.5H,
CH3), 2.26 (s, 1.5H, CH3), 1.40 (s, 4.5H, t-Bu), 1.37 (s,
4.5H, t-Bu), 1.21–1.14 (m, 6H, 2CH2CH3). MALDI-TOF
MS: 503.7 (MþþH), 525.8 (MþþNa), 541.3 (MþþK).
Anal. Calcd for C26H34N2O8 (502.56): C, 62.14; H, 6.82; N,
5.57. Found: C, 62.15; H, 6.88; N, 5.52.
0
0
0
0
3.6.4. (20S,200R)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-pyridine-3,5-dicarboxylic acid di-tert-butyl
ester (15b (R)-Mosher amide). Elution system: 4:1
0
3.6. General procedure for Mosher amides formation
cyclohexane–AcOEt. 1H NMR: d 8.23 (d, 1H, J2 ,NH
¼
To a stirred solution of amino ester (0.10 mmol) in
anhydrous CH2Cl2 (1 mL) were added either (R)- or
7.6 Hz, N0H), 7.60–7.25 (m, 10H, 2Ph), 5.22 and 5.16 (2d,
0
0
2H, J¼12.2 Hz, PhCH2), 5.04 (ddd, 1H, J1 a,2 ¼5.6 Hz,
0
0
0
0
0
(S)-a-methoxy-a-(trifluoromethyl)phenylacetic
acid
J1 b,2 ¼12.7 Hz, H-2 ), 3.57 (dd, 1H, J1 a,1 b¼13.9 Hz,
H-10a), 3.04 (q, 3H, J¼0.7 Hz, OCH3), 2.80 (dd, 1H,
H-10b), 2.56 (s, 6H, 2CH3), 1.56 (s, 18H, 2t-Bu). 19F NMR:
d 270.3. Anal. calcd for C37H43N2F3O8 (700.74): C, 63.42;
H, 6.19; N, 4.00. Found: C, 63.40; H, 6.18; N, 4.05.
(29 mg, 0.12 mmol), 1,3-dicyclohexylcarbodiimide
(25 mg, 0.12 mmol), and a catalytic amount of 4-N,N-
(dimethylamino)pyridine. The mixture was stirred for an
additional 12 h at room temperature then concentrated. The

2322
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
3.6.5. (20S,200S)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-1-oxido-pyridine-3,5-dicarboxylic acid di-
tert-butyl ester (18b (S)-Mosher amide). Elution system:
cyclohexane–AcOEt. 1H NMR: d 8.34 (d, 1H, J2 ,NH
8.0 Hz, N0H), 7.60–7.20 (m, 15H, 3Ph), 5.22 and 5.17 (2d,
¼
0
0
0
2H, J¼11.0 Hz, PhCH2), 5.16 (ddd, 1H, J1 a,2 ¼6.8 Hz,
0
0
0
J1 b,2 ¼4.2 Hz, H-2 ), 3.56 (q, 3H, J¼0.7 Hz, OCH3), 3.46
1:1 cyclohexane–AcOEt. H NMR: d 7.99 (d, 1H, J2 ,NH
¼
(dd, 1H, J1 a,1 b¼15.7 Hz, H-10a), 3.34 (dd, 1H, H-10b), 2.38
(s, 3H, CH3), 1.23 (s, 9H, t-Bu), 1.16 (s, 9H, t-Bu). 19F
NMR: d 269.4. Anal. Calcd for C42H45F3N2O8 (762.81):
C, 66.13; H, 5.95; N, 3.67. Found: C, 66.14; H, 5.96; N,
3.65.
1
0
0
0
8.5 Hz, N0H), 7.40–7.00 (m, 10H, 2Ph), 5.22 (s, 2H,
0
0
0
0
PhCH2), 5.07 (ddd, 1H, J1 a,2 ¼5.5 Hz, J1 b,2 ¼12.8 Hz,
H-20), 3.58 (q, 3H, J¼0.7 Hz, OCH3), 3.45 (dd, 1H,
J1 a,1 b¼14.2 Hz, H-10a), 2.57 (dd, 1H, H-10b), 2.34 (s, 6H,
2CH3), 1.60 (s, 18H, 2t-Bu). 19F NMR: d 269.9. Anal. calcd
for C37H43F3N2O9 (716.74): C, 62.00; H, 6.05; N, 3.91.
Found: C, 61.97; H, 6.02; N, 3.93.
0
0
3.7.2. (20S,200R)-2-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-6-
methyl-4-phenyl-pyridine-3,5-dicarboxylic acid di-tert-
butyl ester (29b (R)-Mosher amide). Elution system: 4:1
0
3.6.6. (20S,200R)-4-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
2,6-dimethyl-1-oxido-pyridine-3,5-dicarboxylic acid di-
tert-butyl ester (18b (R)-Mosher amide). Elution system:
cyclohexane–AcOEt. 1H NMR: d 8.77 (d, 1H, J2 ,NH
¼
8.2 Hz, N0H), 7.60–7.10 (m, 15H, 3Ph), 5.16 and 5.12 (2d,
0
0
2H, J¼12.4 Hz, PhCH2), 5.13 (ddd, 1H, J1 a,2 ¼6.6 Hz,
0
2:1 cyclohexane–AcOEt. H NMR: d 8.23 (d, 1H, J2 ,NH
¼
J1 b,2 ¼4.1 Hz, H-2 ), 3.57 (dd, 1H, J1 a,1 b¼16.2 Hz, H-10a),
3.35 (dd, 1H, H-10b), 3.29 (q, 3H, J¼0.7 Hz, OCH3), 2.44 (s,
3H, CH3), 1.19 (s, 9H, t-Bu), 1.15 (s, 9H, t-Bu). 19F NMR: d
270.0. Anal. Calcd for C42H45F3N2O8 (762.81): C, 66.13;
H, 5.95; N, 3.67. Found: C, 66.16; H, 5.93; N, 3.67.
1
0
0
0
0
0
8.0 Hz, N0H), 7.60–7.25 (m, 10H, 2Ph), 5.21 and 5.16 (2d,
0
0
2H, J¼12.0 Hz, PhCH2), 5.05 (ddd, 1H, J1 a,2 ¼5.7 Hz,
0
0
0
0
0
J1 b,2 ¼12.3 Hz, H-2 ), 3.47 (dd, 1H, J1 a,1 b¼14.0 Hz,
H-10a), 3.13 (q, 3H, J¼0.7 Hz, OCH3), 2.77 (dd, 1H,
H-10b), 2.54 (s, 6H, 2CH3), 1.57 (s, 18H, 2t-Bu). 19F NMR:
d 270.4. Anal. calcd for C37H43F3N2O9 (716.74): C, 62.00;
H, 6.05; N, 3.91. Found: C, 61.94; H, 6.06; N, 3.95.
3.7.3. (20S,200S)-2-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-6-
methyl-1-oxido-4-phenyl-pyridine-3,5-dicarboxylic acid
di-tert-butyl ester (32b (S)-Mosher amide). Elution
3.6.7. (4R,20S,200S)-2-[20-Benzyloxycarbonyl-20-(300,300,300-
trifluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
6-methyl-4-phenyl-1,4-dihydro-pyridine-3,5-dicarb-
oxylic acid diethyl ester ((4R)-27a (S)-Mosher amide).
Elution system: 4:1 cyclohexane–AcOEt. 1H NMR: d 8.12
1
system: 4:1 cyclo0hexane–AcOEt. H NMR: d 8.94 (d, 1H,
0
J2 ,NH¼6.8 Hz, N H), 7.50–7.20 (m, 15H, 3Ph), 5.27 and
0
0
5.21 (2d, 2H, J¼12.0 Hz, PhCH2), 5.15 (ddd, 1H, J1 a,2
¼
0
0
0
0
0
12.2 Hz, J1 b,2 ¼4.5 Hz, H-2 ), 3.48 (dd, 1H, J1 a,1 b¼13.2
Hz, H-10a), 3.64 (q, 3H, J¼0.7 Hz, OCH3),3.26 (dd, 1H,
H-10b), 2.08 (s, 3H, CH3), 1.24 (s, 9H, t-Bu), 1.20 (s, 9H,
t-Bu). 19F NMR: d 269.7. Anal. calcd for C42H45F3N2O9
(778.81): C, 64.77; H, 5.82; N, 3.60. Found: C, 64.76; H,
5.83; N, 3.65.
(d, 1H, J2 ,NH¼6.5 Hz, N0H), 7.50–7.10 (m, 15H, 3Ph), 6.43
0
(s, 1H, NH), 5.21 and 5.14 (2d, 2H, J¼12.0 Hz, PhCH2),
0
0
0
0
4.97 (s, 1H, H-4), 4.67 (dd, 1H, J1 a,2 ¼6.8 Hz, J1 b,2
¼
6.7 Hz, H-20), 4.20–3.95 (m, 4H, 2CH2CH3), 3.51 (dd, 1H,
J1 a,1 b¼14.0 Hz, H-10a), 3.26 (q, 3H, J¼0.7 Hz, OCH3),
0
0
3.10 (dd, 1H, H-10b), 2.24 (s, 3H, CH3), 1.24 (t, 3H,
J¼7.0 Hz, CH2CH3), 1.18 (t, 3H, J¼7.0 Hz, CH2CH3). 19
F
NMR: d 269.3. Anal. calcd for C38H39F3N2O8 (708.72): C,
64.40; H, 5.55; N, 3.95. Found: C, 64.48; H, 5.52; N, 3.90.
3.7.4. (20S,200R)-2-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-6-
methyl-1-oxido-4-phenyl-pyridine-3,5-dicarboxylic acid
di-tert-butyl ester (32b (R)-Mosher amide). Elution
3.6.8. (4R,20S,200R)-2-[20-Benzyloxycarbonyl-20-(300,300,300-
trifluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-
6-methyl-4-phenyl-1,4-dihydro-pyridine-3,5-dicarb-
oxylic acid diethyl ester ((4R)-27a (R)-Mosher amide).
Elution system: 4:1 cyclohexane–AcOEt. 1H NMR: d 8.30
system: 4:1 cyclo0hexane–AcOEt. H NMR: d 9.27 (d, 1H,
1
0
J2 ,NH¼6.8 Hz, N H), 7.65–7.25 (m, 15H, 3Ph), 5.21 (2d,
0
0
0
0
2H, PhCH2), 5.11 (ddd, 1H, J1 a,2 ¼11.2, J1 b,2 ¼4.8 Hz,
H-20), 3.80 (dd, 1H, J1 a,1 b¼13.2 Hz, H-10a), 3.44 (dd, 1H,
H-10b), 3.29 (q, 3H, J¼0.7 Hz, OCH3), 2.56 (s, 3H, CH3),
1.21 (s, 9H, t-Bu), 1.16 (s, 9H, t-Bu). 19F NMR: d 270.6.
Anal. calcd for C42H45F3N2O9 (778.81): C, 64.77; H, 5.82;
N, 3.60. Found: C, 64.78; H, 5.84; N, 3.61.
0
0
(d, 1H, J2 ,NH¼6.2 Hz, N0H), 7.60–7.10 (m, 15H, 3Ph), 6.30
0
(s, 1H, NH), 5.20 and 5.10 (2d, 2H, J¼12.1 Hz, PhCH2),
0
0
0
0
4.99 (s, 1H, H-4), 4.68 (dd, 1H, J1 a,2 ¼7.8 Hz, J1 b,2
¼
6.0 Hz, H-20), 4.16–3.98 (m, 4H, 2CH2CH3), 3.38 (dd, 1H,
J1 a,1 b¼14.0 Hz, H-10a), 3.23 (dd, 1H, H-10b), 3.08 (q, 3H,
J¼0.7 Hz, OCH3), 2.28 (s, 3H, CH3), 1.22 (t, 3H, J¼7.0 Hz,
CH2CH3), 1.20 (t, 3H, J¼7.0 Hz, CH2CH3). 19F NMR: d
269.1. Anal. calcd for C38H39F3N2O8 (708.72): C, 64.40;
H, 5.55; N, 3.95. Found: C, 64.45; H, 5.56; N, 3.93.
3.7.5. (20S,200S,100S)-4-[20-(200-tert-Butoxycarbonylamino-
300-phenyl-propionylamino)-20-(100-methoxycarbonyl-200-
phenyl-ethylcarbamoyl)-ethyl]-2,6-dimethyl-1,4-di-
hydro-pyridine-3,5-dicarboxylic acid diethyl ester (21).
To a cooled (0 8C), stirred solution of amino acid 13a
(88 mg, 0.20 mmol), L-phenylalanine methyl ester hydro-
chloride (65 mg, 0.30 mmol), and (benzotriazol-1-yloxy)-
tripyrrolidinophosphonium hexafluorophosphate (125 mg,
0.24 mmol) in anhydrous CH2Cl2 (1 mL) was added
N,N-diisopropylethylamine (105 mL, 0.60 mmol). The solu-
tion was warmed to room temperature, stirred for an
additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with H2O
0
0
3.7. Crystallization from pentane afforded small crystals
of this compound suitable for X-ray diffraction analysis
3.7.1. (20S,200S)-2-[20-Benzyloxycarbonyl-20-(300,300,300-tri-
fluoro-200-methoxy-200-phenyl-propionylamino)-ethyl]-6-
methyl-4-phenyl-pyridine-3,5-dicarboxylic acid di-tert-
butyl ester (29b (S)-Mosher amide). Elution system: 4:1

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2323
(2£10 mL). The organic phase was dried (Na2SO4),
concentrated, and eluted from a column of silica gel
with 1:1 cyclohexane–AcOEt to give (20S,100S)-4-[20-tert-
Butoxycarbonylamino-20-(100-methoxycarbonyl-200-phenyl-
ethylcarbamoyl)-ethyl]-2,6-dimethyl-1,4-dihydro-pyridine-
3,5-dicarboxylic acid diethyl ester 20 (96 mg, 80%) as a
white foam. [a]D¼þ26.0 (c 0.9, acetone). 1H NMR
(DMSO-d6, 120 8C): d 8.37 (s, 1H, NH), 7.41 (bd, 1H,
phenyl-ethylcarbamoyl)-ethyl]-2,6-dimethyl-1-oxido-
pyridine-3,5-dicarboxylic acid di-tert-butyl ester (23). To
a cooled (0 8C), stirred solution of amino acid 19b (102 mg,
0.20 mmol), L-phenylalanine methyl ester hydrochloride
(65 mg, 0.30 mmol), and (benzotriazol-1-yloxy)tripyrro-
lidinophosphonium hexafluorophosphate (125 mg, 0.24
mmol) in anhydrous CH2Cl2 (1 mL) was added N,N-di-
isopropylethylamine (105 mL, 0.60 mmol). The solution
was warmed to room temperature, stirred for an additional
2 h, and then concentrated. The residue was suspended with
AcOEt (80 mL) and washed with H2O (2£10 mL). The
organic phase was dried (Na2SO4), concentrated, and eluted
from a column of silica gel with 1:1 cyclohexane0–AcOEt to
give (20S,100S)-4-[20-tert-butoxycarbonylamino-2 -(100-meth-
oxycarbonyl-200-phenyl-ethylcarbamoyl)-ethyl]-2,6-dimethyl-
1-oxido-pyridine-3,5-dicarboxylic acid di-tert-butyl ester
(121 mg, 90%) as a white foam. [a]D¼214.0 (c 0.9,
J1 ,NH¼7.5 Hz, N00H), 7.30–7.10 (m, 5H, Ph), 5.81 (bd, 1H,
0
J¼7.0 Hz, N0H), 4.57 (ddd, 1H, J1 ,2 a¼6.0 Hz,
00 00
J1 ,2 b¼7.5 Hz, H-100), 4.20–4.05 (m, 4H, 2CH2CH3),
00 00
3.90–3.75 (m, 2H, H-20, H-4), 3.60 (s, 3H, OCH3), 3.04
(dd, 1H, J2 a,2 b¼13.8 Hz, H-200a), 2.94 (dd, 1H, H-200b),
2.25 (s, 3H, CH3), 2.23 (s, 3H, CH3), 1.60–1.30 (m, 11H,
2H-10, t-Bu), 1.27 (t, 3H, J¼7.0 Hz, CH2CH3), 1.23 (t, 3H,
J¼7.0 Hz, CH2CH3). MALDI-TOF MS: 602.5 (MþþH),
624.6 (MþþNa), 640.8 (MþþK). Anal. calcd for
C31H43N3O9 (601.69): C, 61.88; H, 7.20; N, 6.98. Found:
C, 61.82; H, 7.26; N, 6.96.
00
00
1
CHCl3). H NMR: d 7.40–7.10 (m, 5H, Ph), 6.91 (bd, 1H,
J1 ,NH¼7.0 Hz, N00H), 5.97 (bd, 1H, J2 ,NH¼7.0 Hz, N0H),
00
0
4.85 (ddd, 1H, J1 ,2 a¼5.8 Hz, J1 ,2 b¼6.0 Hz, H-100), 4.52
00 00
00 00
0
0
0
0
0
To a cooled (0 8C), stirred solution of the above dipeptide 20
(60 mg, 0.10 mmol) in CH2Cl2 (0.50 mL) was slowly added
a solution of TFA–CH2Cl2 (0.50–1.50 mL). Stirring was
continued at 0 8C for an additional 30 min, then the solution
was warmed to room temperature. After 30 min at room
temperature the solution was neutralized at 0 8C with
saturated aqueous Na2CO3 and extracted with CH2Cl2
(2£50 mL). The combined organic phases were dried
(Na2SO4), and concentrated to give the corresponding
crude free amine (39 mg, ,78%) suitable for the next step.
(bddd, 1H, J1 a,2 ¼4.5 Hz, J1 b,2 ¼13.0 Hz, H-2 ), 3.70 (s,
3H, OCH3), 3.26 (dd, 1H, J1 a,1 b¼14.5 Hz, H-10a), 3.17 (dd,
0
0
1H, J2 a,2 b¼14.0 Hz, H-200a), 3.09 (dd, 1H, H-200b), 2.71
(bdd, 1H, H-10b), 2.55 (s, 6H, 2CH3), 1.63 (s, 18H, 2t-Bu),
1.35 (s, 9H, t-Bu). MALDI-TOF MS: 672.5 (MþþH), 694.8
(MþþNa), 710.5 (MþþK). Anal. Calcd for C35H49N3O10
(671.78): C, 62.58; H, 7.35; N, 6.26. Found: C, 62.55; H,
7.36; N, 6.26.
00
00
To a cooled (0 8C), stirred solution of the above dipeptide
(67 mg, 0.10 mmol) in CH2Cl2 (0.50 mL) was slowly added
a solution of TFA–CH2Cl2 (0.50–1.50 mL). Stirring was
continued at 0 8C for an additional 30 min, then the solution
was warmed to room temperature. After 30 min at room
temperature the solution was neutralized at 0 8C with
saturated aqueous Na2CO3 and extracted with CH2Cl2
(2£50 mL). The combined organic phases were dried
(Na2SO4), and concentrated to give the corresponding
crude free amine (40 mg, ,70%) suitable for the next step.
To a cooled (0 8C), stirred solution of the above free amine
(39 mg, ,0.08 mmol), tert-butoxycarbonyl-L-phenyl-
alanine (32 mg, 0.12 mmol), and (benzotriazol-1-yloxy)tri-
pyrrolidinophosphonium hexafluorophosphate (73 mg,
0.14 mmol) in anhydrous CH2Cl2 (0.5 mL) was added
N,N-diisopropylethylamine (40 mL, 0.23 mmol). The solu-
tion was warmed to room temperature, stirred for an
additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with saturated
aqueous NaHCO3 (10 mL) and brine (10 mL). The organic
phase was dried (Na2SO4), concentrated, and eluted from a
column of silica gel with 1:1 cyclohexane–AcOEt to give
tripeptide 21 (46 mg, 62% from 20) as a white foam.
[a]D¼þ28.0 (c 0.8, CHCl3). 1H NMR (DMSO-d6, 140 8C):
d 8.37 (s, 1H, NH), 7.40–7.00 (m, 12H, 2Ph, N0H, N000H),
To a cooled (0 8C), stirred solution of the above free amine
(40 mg, ,0.07 mmol), tert-butoxycarbonyl-L-phenyl-
alanine (29 mg, 0.11 mmol), and (benzotriazol-1-yloxy)tri-
pyrrolidinophosphonium hexafluorophosphate (68 mg,
0.13 mmol) in anhydrous CH2Cl2 (0.5 mL) was added
N,N-diisopropylethylamine (37 mL, 0.21 mmol). The solu-
tion was warmed to room temperature, stirred for an
additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with saturated
aqueous NaHCO3 (10 mL) and brine (10 mL). The organic
phase was dried (Na2SO4), concentrated, and eluted from a
column of silica gel with 1:1 cyclohexane–AcOEt to give
tripeptide 23 (52 mg, 63% from the corresponding dipep-
6.00 (d, 1H, J2 ,NH¼8.5 Hz, N00H), 4.60 (ddd, 1H, J1 ,2 a
¼
00
000 000
6.5 Hz, J1 ,2 b¼8.0 Hz, H-1000), 4.30–4.05 (m, 6H,
000 000
2CH2CH3, H-20, H-200), 3.90 (dd, 1H, J4,1 a¼5.6 Hz,
0
0
J4,1 b¼6.2 Hz, H-4), 3.60 (s, 3H, OCH3), 3.09 (dd, 1H,
J2 ,3 a¼4.5 Hz, J3 a,3 b¼14.0 Hz, H-300a), 3.05 (dd, 1H,
00 00
00
00
J2 a,2 b¼14.0 Hz, H-2000a), 2.98 (dd, 1H, J2 ,3 b¼6.8 Hz,
000
000
00 00
H-300b), 2.79 (dd, 1H, H-2000b), 2.25 (s, 3H, CH3), 2.23 (s, 3H,
1
0
0
0
0
CH3), 1.73 (ddd, 1H, J1 a,2 ¼5.8 Hz, J1 a,1 b¼14.0 Hz,
tide) as a white foam. [a]D¼236.8 (c 1.0, acetone). H
H-10a), 1.50 (ddd, 1H, J1 b,2 ¼7.8 Hz, H-1 b), 1.34 (s, 9H,
t-Bu), 1.26 (t, 3H, J¼7.0 Hz, CH2CH3), 1.23 (t, 3H,
J¼7.0 Hz, CH2CH3). MALDI-TOF MS: 749.9 (MþþH),
771.3 (MþþNa), 787.9 (MþþK). Anal. calcd for
C40H52N4O10 (748.86): C, 64.15; H, 7.00; N, 7.48. Found:
C, 64.15; H, 7.02; N, 7.46.
NMR (DMSO-d6, 120 8C) d: 7.54 (bd, 1H, J2 ,NH¼8.0 Hz,
0
0
0
0
N0H), 7.30–7.1000 (m, 11H, 2Ph, N000H), 60.017 (bd, 1H,
J2 ,NH¼7.0 Hz, N H), 4.64–4.52 (m, 2H, H-2 , H-1000), 4.18
00
0
0
0
0
0
(ddd, 1H, J1 a,2 ¼5.6 Hz, J1 b,2 ¼4.5 Hz, H-2 ), 3.58 (s, 3H,
OCH3), 3.15 (dd, 1H, J1 a,1 b¼14.0 Hz, H-10a), 3.06 (dd, 1H,
0
0
J1 ,2 a¼6.0 Hz, J2 a,2 b¼14.0 Hz, H-2000a), 2.99 (dd, 1H,
000 000
000
000
J1 ,2 b¼7.0 Hz, H-2000b), 2.92 (dd, 1H, J2 ,3 a¼5.0 Hz,
000 000
00 00
3.7.6. (20S,200S,100S)-4-[20-(200-tert-Butoxycarbonylamino-
J3 a,3 b¼14.0 Hz, H-300a), 2.78 (dd, 1H, J2 ,3 b¼2.2 Hz,
00
00
00 00
300-phenyl-propionylamino)-20-(100-methoxycarbonyl-200-
H-300b), 2.75 (dd, 1H, H-10b), 2.40 (2 s, 6H, 2CH3), 1.61

2324
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
00
000 000
(s, 18H, 2t-Bu), 1.32 (s, 9H, t-Bu). MALDI-TOF MS: 672.5
(MþþH), 694.8 (MþþNa), 710.5 (MþþK). Anal. calcd for
C35H49N3O10 (818.95): C, 64.53; H, 7.14; N, 6.84. Found:
C, 64.56; H, 7.12; N, 6.89.
J2 ,NH¼7.0 Hz, N00H), 4.80 (ddd, 1H, J1 ,2 a¼10.2 Hz,
J1 ,2 b¼4.5 Hz, H-1000), 4.60 (ddd, 1H, J2 ,3 a¼6.0 Hz,
000 000
00 00
J2 ,3 b¼7.2 Hz, H-200), 4.17 (ddd, 1H, J1 a,2 ¼5.0 Hz,
00 00
0
0
0
0
0
J1 b,2 ¼8.5 Hz, H-2 ), 3.62 (s, 3H, OCH3), 3.38 (dd, 1H,
J2 a,2 b¼13.5 Hz, H-2000a), 3.19 (dd, 1H, H-2000b), 3.11 (dd,
000
000
3.7.7. (20S,200S,100S)-2-[20-(200-tert-Butoxycarbonylamino-
300-phenyl-propionylamino)-20-(100-methoxycarbonyl-200-
phenyl-ethylcarbamoyl)-ethyl]-6-methyl-1-oxido-4-phe-
nyl-pyridine-3,5-dicarboxylic acid di-tert-butyl ester
(34). To a cooled (0 8C), stirred solution of amino acid
33b (115 mg, 0.20 mmol), L-phenylalanine methyl ester
hydrochloride (65 mg, 0.30 mmol), and (benzotriazol-
1-yloxy)tripyrrolidinophosphonium hexafluorophosphate
(125 mg, 0.24 mmol) in anhydrous CH2Cl2 (1 mL) was
added N,N-diisopropylethylamine (105 mL, 0.60 mmol).
The solution was warmed to room temperature, stirred for
an additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with H2O
(2£10 mL). The organic phase was dried (Na2SO4),
concentrated, and eluted from a column of s0ilica gel with
2:1 cyclohexane–AcOEt to give (20S,100S)-2-[2 -tert-Butoxy-
carbonylamino-20-(100-methoxycarbonyl-200-phenyl-ethyl-
carbamoyl)-ethyl]-6-methyl-1-oxido-4-phenyl-pyridine-
3,5-dicarboxylic acid di-tert-butyl ester (132 mg, 90%) as a
white foam. [a]D¼þ14.9 (c 0.7, CHCl3). 1H NMR: d 7.50–
7.10 (m, 11H, 2Ph, N00H), 6.59 (bd, 1H, N0H), 4.88 (ddd, 1H,
1H, J3 a,3 b¼14.0 Hz, H-300a), 3.02 (dd, 1H, H-300b), 2.97
00
00
(dd, 1H, J1 a,1 b¼14.0 Hz, H-10a), 2.82 (dd, 1H, H-10b), 2.48
(s, 3H, CH3), 1.34 (s, 9H, t-Bu), 1.22 (s, 9H, t-Bu), 1.21 (s,
9H, t-Bu). MALDI-TOF MS: 882.3 (MþþH), 904.2
(MþþNa), 920.0 (MþþK). Anal. calcd for C40H51N3O10
(881.02): C, 66.80; H, 6.86; N, 6.36. Found: C, 66.83; H,
6.88; N, 6.86.
0
0
3.8. Polymer-assisted solution-phase synthesis of 11b and
25b
A screw-capped vial, containing a magnetic bar, was
charged with aldehyde (0.50 mmol), b-ketoester
(0.50 mmol), aminocrotonate 10b (79 mg, 0.50 mmol),
˚
activated 4-A powdered molecular sieves (50 mg) and
tert-BuOH (2 mL). The mixture was then vigorously stirred,
degassed under vacuum and saturated with argon (by an
Ar-filled balloon) three times. The mixture was stirred at
70 8C for 24 h then cooled to room temperature, diluted with
AcOEt (10 mL), filtered through a pad of Celite, and
concentrated. The residue was dissolved in CH2Cl2 (5 mL)
and Amberlyst 15 (400 mg) and Ambersep 900 OH
(400 mg) were added. The suspension was shaken for 2 h
then the polymers were filtered off and washed thoroughly
with CH2Cl2. The combined filtrates were concentrated. The
residue was dissolved in CH2Cl2 (5 mL) and aminomethy-
lated polystyrene (185 mg, 0.50 mmol of a 2.7 mmol g21
resin) was added. The suspension was stirred for an
additional 2 h then the polymer was filtered off and washed
thoroughly with CH2Cl2. The combined filtrates were
concentrated to yield the target DHP-alanine: 11b
(220 mg, 75%; purity: 95%); 25b (243 mg, 75%; purity:
92%).
J1 ,2 a¼5.8 Hz, J1 ,2 b¼6.5 Hz, J1 ,NH¼7.0 Hz, H-100), 4.70
00 00
00 00
00
(bddd, 1H, H-20), 3.71 (s, 3H, OCH3), 3.45–3.30 (m, 2H,
2H-10), 3.21 (dd, 1H, J2 a,2 b¼13.5 Hz, H-200a), 3.11 (dd, 1H,
H-200b), 2.59 (s, 3H, CH3), 1.38 (s, 9H, t-Bu), 1.20 (s, 18H,
2t-Bu). MALDI-TOF MS: 734.9 (MþþH), 756.4
(MþþNa), 772.5 (MþþK). Anal. calcd for C40H51N3O10
(733.85): C, 65.47; H, 7.00; N, 5.73. Found: C, 65.43; H,
7.08; N, 5.72.
00
00
To a cooled (0 8C), stirred solution of the above dipeptide
(73 mg, 0.10 mmol) in CH2Cl2 (0.50 mL) was slowly added
a solution of TFA–CH2Cl2 (0.50–1.50 mL). Stirring was
continued at 0 8C for an additional 30 min, then the
solution was warmed to room temperature. After 30 min
at room temperature the solution was neutralized at 0 8C
with saturated aqueous Na2CO3 and extracted with
CH2Cl2 (2£50 mL). The combined organic phases were
dried (Na2SO4), and concentrated to give the corre-
sponding crude free amine (44 mg, ,70%) suitable for the
next step.
Purities were determined by 1H NMR analysis of the
reaction mixtures after work up.
3.9. Polymer-assisted solution-phase synthesis of 14b and
28b
A mixture of DHP 11b or 25b (0.50 mmol), pyridinium
chlorochromate immobilized on silica gel28 (1.88 g,
,1.50 mmol of a ,0.8 mmol g21 resin) and anhydrous
CH2Cl2 (7 mL) was stirred at room temperature for 24 h.
Then, the immobilized reagent was filtered off and washed
thoroughly with CH2Cl2. The combined filtrates were con-
centrated to yield the target pyridyl-alanine: 14b (287 mg,
98%; purity: 95%); 28b (317 mg, 98%; purity: 92%).
To a cooled (0 8C), stirred solution of the above free amine
(44 mg, ,0.07 mmol), tert-butoxycarbonyl-L-phenyl-
alanine (29 mg, 0.11 mmol), and (benzotriazol-1-yloxy)tri-
pyrrolidinophosphonium hexafluorophosphate (68 mg,
0.13 mmol) in anhydrous CH2Cl2 (0.5 mL) was added
N,N-diisopropylethylamine (37 mL, 0.21 mmol). The solu-
tion was warmed to room temperature, stirred for an
additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with saturated
aqueous NaHCO3 (10 mL) and brine (10 mL). The organic
phase was dried (Na2SO4), concentrated, and eluted from a
column of silica gel with 1.5:1 cyclohexane–AcOEt to give
tripeptide 34 (56 mg, 63% from the corresponding dipep-
tide) as a white foam. [a]D¼26.2 (c 0.6, CHCl3). 1H NMR
(DMSO-d6, 120 8C) d: 8.28 and 7.62 (2bd, 2H, J¼7.0 Hz,
N0H, N000H), 7.50–7.10 (m, 15H, 3Ph), 6.17 (bd, 1H,
Purities were determined by 1H NMR analysis of the
reaction mixtures after work up.
3.10. Polymer-assisted solution-phase synthesis of 17b
and 31b
A mixture of pyridine 14b or 28b (0.50 mmol), 3-chloro-
peroxybenzoic acid (345 mg, 2.00 mmol), and anhydrous
CH2Cl2 (12 mL) was stirred at room temperature for 15 h,

A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
2325
Didierjean, C.; Aubry, A.; Briand, J. P.; Guichard, G. J. Org.
Chem. 2002, 67, 8440–8449. (j) Dinsmore, A.; Doyle, P. M.;
Young, D. W. J. Chem. Soc., Perkin Trans. 1 2002, 155–164.
(k) Dinsmore, A.; Doyle, P. M.; Steger, M.; Young, D. W.
J. Chem. Soc., Perkin Trans. 1 2002, 613–621. (l) Wasserman,
H. H.; Long, Y. O.; Parr, J. Tetrahedron Lett. 2003, 44,
361–363.
then aminomethylated polystyrene (1.11 g, 3.00 mmol of a
2.7 mmol g21 resin) was added in one portion. The
suspension was stirred for an additional 2 h then the
polymer was filtered off and washed thoroughly with
CH2Cl2. The combined filtrates were concentrated to yield
the target 1-oxido-pyridyl-alanine: 17b (294 mg, 98%;
purity: 95%); 31b (325 mg, 98%; purity: 92%).
5. (a) Martinez, J.; Fehrentz, J. A. Peptides 2000: Proceedings of
the Twenty-Sixth European Peptide Symposium; EDK: Paris,
2001. (b) Van Batemburg, O. D.; Voskuyl-Holtcamp, I.;
Schattenkerk, C.; Hoes, K.; Kerling, K. E. T.; Havinga, E.
Biochem. J. 1977, 163, 385–387.
Purities were determined by 1H NMR analysis of the
reaction mixtures after work up.
3.11. Crystal data for compound ((4R)-27a (R)-Mosher
amide)
6. (a) Howarth, N. M.; Wakelin, L. P. G. J. Org. Chem. 1997, 62,
5441–5450. (b) Kuwahara, M.; Arimitsu, M.; Sisido, M.
Tetrahedron 1999, 55, 10067–10078, and references therein.
7. (a) Bozell, J. J.; Vogt, C. E.; Gozum, J. J. Org. Chem. 1991,
56, 2584–2587, and references cited therein for earlier
preparations. (b) Schow, S. R.; Quinn DeJoy, S.; Wick,
M. M.; Kerwar, S. S. J. Org. Chem. 1994, 59, 6850–6852. (c)
Ye, B.; Burke, Jr. T. R. J. Org. Chem. 1995, 60, 2640–2641.
C38H39F3N2O8; monoclinic, space group P21, a¼9.6979(5),
˚
b¼10.8821(6), c¼17.3140(11) A, b¼91.578(2)8, V¼
3
1826.5(2) A , Z¼2, Dc¼1.289 g cm23. Intensity data col-
˚
lected with u#27.38; 7643 independent reflections
measured; 4103 observed [I(2s(I)]. Final R index¼0.070
(observed reflections). The molecules in the crystal form
intramolecu0lar and intermolecular hydrogen bonds: N10 –
¨
(d) Dobler, C.; Kreuzfeld, H.-J.; Michalik, M.; Krause, H. W.
˚
H…O3 [N1 …O3¼2.780(5) A]; N1–H…O1 (12x, y21/2,
Tetrahedron: Asymmetry 1996, 7, 117–125. (e) Myers, A. G.;
Gleason, J. L. J. Org. Chem. 1996, 61, 813–815. (f) Walker,
M. A.; Kaplita, K. P.; Che, T.; King, H. D. Synlett 1997,
169–170.
˚
12z) [N1…O1¼2.918(6) A].
8. Hsieh, K.; Jorgensen, E. C. J. Med. Chem. 1979, 22,
1199–1206.
Acknowledgements
9. Roberts, D. C.; Vellaccio, F. The peptides. Academic: New
York, 1983.
We thank University of Ferrara for financial support and
Ajinomoto Co., Inc. (Tokyo, Japan) for an unrestricted
grant.
10. (a) Chang, G. W.; Snell, E. E. Biochemistry 1968, 7,
2005–2012. (b) Tanase, S.; Guirard, B. M.; Snell, E. E.
J. Biol. Chem. 1985, 260, 6738–6746.
11. Shimeno, H.; Soeda, S.; Nagamatsu, S. H. Chem. Pharm. Bull.
1977, 25, 2983–2987.
References and notes
12. (a) Inouye, S.; Shomura, T.; Ogawa, Y.; Watanabe, H.;
Yoshida, J.; Niida, T. Chem. Pharm. Bull. 1975, 23,
2669–2677. (b) Izawa, M.; Takayama, S.; Shindo-Okada,
N.; Doi, S.; Kimura, M.; Katsuki, M.; Nishimura, S. Cancer
Res. 1992, 52, 1628–1630.
1. (a) Duthaler, R. O. Tetrahedron 1994, 50, 1539–1650. (b) In
Asymmetric synthesis of novel sterically constrained amino
acids; Symposium-in-print; Tetrahedron, Hruby, V. J.,
Soloshonok, V. A., Eds.; 2001; 57, pp 6329–6650. (c) Ma,
J.-A. Angew. Chem., Int. Ed. 2003, 42, 4290–4299.
13. (a) Isolation: Ref. 12a. (b) Recent synthesis and references to
earlier syntheses: Ref. 4e.
2. Rosenthal, G. A. Plant nonprotein amino and imino acids.
Biological, biochemical, and toxicological properties. Aca-
demic: New York, 1982.
14. Rosenthal, G. A. Plant nonprotein amino and imino acids.
Biological, biochemical, and toxicological properties. Aca-
demic: New York, 1982; p 78.
3. (a) Martinez, J.; Fehrentz, J. A. Peptides 2000: Proceedings of
the Twenty-Sixth European Peptide Symposium; EDK: Paris,
2001. (b) Fenniri, H. Combinatorial chemistry: a practical
approach. Oxford University Press: Oxford, UK, 2000.
4. For very recent selected papers with leading references, see:
(a) Adlington, R. M.; Baldwin, J. E.; Catterick, D.; Pritchard,
G. J. J. Chem. Soc., Perkin Trans. 1 2000, 299–302.
(b) Adlington, R. M.; Baldwin, J. E.; Catterick, D.; Pritchard,
G. J.; Tang, L. T. J. Chem. Soc., Perkin Trans. 1 2000,
303–305. (c) Adlington, R. M.; Baldwin, J. E.; Catterick, D.;
Pritchard, G. J.; Tang, L. T. J. Chem. Soc., Perkin Trans. 1
2000, 2311–2316. (d) Dalla Croce, P.; La Rosa, C.; Pizzatti, E.
Tetrahedron: Asymmetry 2000, 11, 2635–2642.
(e) Adamczyk, M.; Akireddy, S. R.; Reddy, R. E. Org. Lett.
2001, 3, 3157–3159. (f) Kim, Y. B.; Choi, E. H.; Keum, G.;
Kang, S. B.; Lee, D. H.; Koh, H. Y.; Kim, Y. Org. Lett. 2001,
3, 4149–4152. (g) Adamczyk, M.; Akireddy, S. R.; Reddy,
R. E. Tetrahedron: Asymmetry 2001, 12, 2385–2387.
(h) Wang, W.; Cai, M.; Xiong, C.; Zhnag, J.; Trivedi, D.;
Hruby, V. J. Tetrahedron 2002, 58, 7365–7374. (i) Marin, J.;
15. Andrews, D. M.; Gregoriou, M.; Page, T. C. M.; Peach, J. M.;
Pratt, A. J. J. Chem. Soc., Perkin Trans. 1 1995, 1335–1340.
16. (a) Egan, S. E.; Weinberg, R. A. Nature 1993, 365, 781–783.
(b) Ennis, B. W.; Lippman, M. E.; Dickson, R. B. Cancer Inv.
1991, 9, 553–562.
17. Sullivan, P. T.; Kester, M.; Norton, S. J. J. Med. Chem. 1968,
11, 1172–1176.
18. Dondoni, A.; Massi, A.; Minghini, E.; Sabbatini, S.; Bertolasi,
V. J. Org. Chem. 2003, 68, 6172–6183.
19. (a) Biginelli, P. Gazz. Chim. Ital. 1893, 23, 360–413.
(b) Review: Kappe, C. O. Acc. Chem. Res. 2000, 33, 879–888.
20. (a) Hantzsch, A. Justus Liebigs Ann. Chem. 1882, 215, 1–82.
(b) Review: Stout, D. M.; Meyers, A. I. Chem. Rev. 1982, 82,
223–243.
21. For improved one-pot oxidative conditions (Jones reagent)
that transform this heterocycle into N-Boc glycinate without
epimerization, see: (a) Dondoni, A.; Mariotti, G.; Marra, A.
J. Org. Chem. 2002, 67, 4475–4486. (b) Dondoni, A.; Marra,
A.; Massi, A. J. Org. Chem. 1999, 64, 933–944.

2326
A. Dondoni et al. / Tetrahedron 60 (2004) 2311–2326
22. (a) Martin-Martinez, M.; De la Figuera, N.; Latorre, M.;
Herranz, R.; Garcia-Lopez, M. T.; Cenarruzabeitia, E.; Del
Rio, J.; Gonzalez-Muniz, R. J. Med. Chem. 2000, 43,
3770–3777. (b) Matsuura, F.; Hamada, Y.; Shioiri, T.
Tetrahedron 1994, 50, 265–274.
UK [fax: þ44(0)-1223-336033 or e-mail: deposit@ccdc.cam.
ac.uk].
26. (a) Trost, B. M. Science 1991, 254, 1471–1477. (b) Trost,
B. M. Angew. Chem., Int. Ed. 1995, 34, 259–281. (c) Sheldon,
R. A. Pure Appl. Chem. 2000, 72, 1233–1246. (d) Anastas,
P. T.; Warner, J. C. Green chemistry: theory and practice;
Oxford University Press: New York, 1998.
23. (a) Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Org. Chem. 1969,
34, 2543–2549. (b) Eliel, E. L.; Wilen, S. H. Stereo chemistry
of organic compounds; Wiley: New York, 1994; p 221.
24. Foregoing studies aimed at finding suitable protective groups
for the many functionalities displayed by the target class of
dihydropyridyl-a-alanines demonstrated that the presence of
ethyl ester groups at C3 and C5 positions of the DHP ring was
crucial for the elaboration of the amino acid functionality. In
fact, the presence of tert-butyl ester groups at the same ring
positions caused the isolation of a complex reaction mixture
during the N0-Boc removal (1:4 TFA–CH2Cl2, 0 (C, 1 h). The
high reactivity of tert-butyl ester groups at C3 and C5 may be
attributed to the strong conjugation with the adjacent carbon–
carbon double bond of the enaminic system. This speculation
is supported by the unreactivity under the above acidic
conditions of tert-butyl ester groups installed at C3 and C5 of
pyridyl and 1-oxido-pyridyl rings. For a discussion about this
issue, see: (a) Goerlitzer, K.; Baltrusch, H. J. Pharmazie 2000,
10, 747–750. (b) Buckler, R. T.; Hartzler, H. E. J. Med. Chem.
1975, 18, 509–513.
27. For recent reviews, see: (a) Kirschning, A.; Monenschein, H.;
Wittenberg, R. Angew. Chem., Int. Ed. 2001, 40, 650–679.
(b) Ley, S. V.; Baxendale, I. R.; Bream, R. N.; Jackson, P. S.;
Leach, A. G.; Longbottom, D. A.; Nesi, M.; Scott, J. S.; Storer,
R. I.; Taylor, S. J. J. Chem. Soc., Perkin Trans. 1 2000,
3815–4195.
28. For reviews see: (a) Hermkens, P. H. H.; Ottenheijm, H. C. J.;
Rees, D. C. Tetrahedron 1997, 53, 5643–5678. (b) Thompson,
L. A.; Ellman, J. A. Chem. Rev. 1996, 96, 555–600.
(c) Fruchtel, J. S.; Jung, G. Angew. Chem., Int. Ed. Engl.
1996, 35, 17–42.
29. Eynde, J. V.; Mayence, A.; Maquestiau, A. Tetrahedron Lett.
1992, 48, 463–468.
30. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
31. Sheldrick, G. M. SHELXL-97. Program for the Refinement for
¨ ¨
Crystal Structures; University of Gottingen: Gottingen,
25. Complete crystallographic data (excluding structural factors)
for the structure of the (R)-Mosher’s amide of (4R)-27a have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publications number CCDC 222942.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
Germany, 1997.
32. Burnett, M. N.; Johnson, C. K. ORTEP III, Report ORNL-
6895. Oak Ridge National Laboratory: Oak Ridge, TN, 1996.
33. Bagley, M. C.; Dale, J. W.; Bower, J. Synlett 2001,
1149–1151.