Nonproteinogenic amino acids: An efficient asymmetric synthesis of (S)-(-)-acromelobic acid and (S)-(-)-acromelobinic acid

Article abstract of DOI:10.1016/S0040-4020(02)00670-1

An efficient synthesis of (S)-(-)-acromelobic acid (1) and (S)-(-)-acromelobinic acid (2) is described via asymmetric hydrogenation protocol. Asymmetric hydrogenation of dehydroamino acid derivative 23 using (R,R)-[Rh(DIPAMP)(COD)]BF₄ catalyst followed by removal of the protective groups afforded (S)-(-)-acromelobic acid (1) in >98% ee. The key intermediate 23 was prepared from citrazinic acid (8). The dehydroamino acid derivative 33 required for the synthesis of (S)-(-)-2 was prepared from 2,5-lutidine (27), which upon hydrogenation using (S,S)-[Rh(Et-DuPHOS)(COD)]BF₄ catalyst afforded (S)-(+)-34 in 93% yield and >96% ee. Removal of protective groups in (S)-(+)-34 afforded (S)-(-)-acromelobinic acid (2) in good overall yield.

Full text of DOI:10.1016/S0040-4020(02)00670-1

Tetrahedron 58 (2002) 6951–6963
Nonproteinogenic amino acids: an efficient asymmetric synthesis
of (S)-(2)-acromelobic acid and (S)-(2)-acromelobinic acid
*
Maciej Adamczyk, Srinivasa Rao Akireddy and Rajarathnam E. Reddy
Department of Chemistry (09MD, Bldg AP20), Diagnostics Division, Abbott Laboratories, 100 Abbott Park Road, Abbott Park,
IL 60064-6016, USA
Received 24 April 2002; accepted 18 June 2002
Abstract—An efficient synthesis of (S)-(2)-acromelobic acid (1) and (S )-(2)-acromelobinic acid (2) is described via asymmetric
hydrogenation protocol. Asymmetric hydrogenation of dehydroamino acid derivative 23 using (R,R )-[Rh(DIPAMP)(COD)]BF4 catalyst
followed by removal of the protective groups afforded (S)-(2)-acromelobic acid (1) in .98% ee. The key intermediate 23 was prepared from
citrazinic acid (8). The dehydroamino acid derivative 33 required for the synthesis of (S )-(2)-2 was prepared from 2,5-lutidine (27), which
upon hydrogenation using (S,S )-[Rh(Et-DuPHOS)(COD)]BF4 catalyst afforded (S )-(þ)-34 in 93% yield and .96% ee. Removal of
protective groups in (S )-(þ)-34 afforded (S )-(2)-acromelobinic acid (2) in good overall yield. q 2002 Elsevier Science Ltd. All rights
reserved.
1. Introduction
acids.6 These amino acids exhibit weak depolarizing
activity in the preparation of newborn rat spinal cord.6
The first synthesis of (S )-(2)-1 and (S)-(2)-2 was achieved
by chemical conversion6b of L-stizolobic acid (3) and
L-stizolobinic acid (4) (Fig. 1), respectively, a related
nonproteinogenic amino acids. The amino acids 3 and 4
were also isolated from C. acromelalga,6b and from other
sources such as, Stizolobium hassjoo8a,b and Amanita
pantherina.8c Subsequently, Baldwin et al.9 reported the
first chemical synthesis of 1 in a racemic form starting from
catechol in thirteen steps. Our interest in nonproteinogenic
amino acids led to the synthesis of a number of heterocyclic
Interest in nonproteinogenic a-amino acids1 continues to
arise due to their importance in medicinal and biotechno-
logical fields.2 The nonproteinogenic a-amino acids, which
are also the constituents of a number of peptides and
proteins, are shown to have exceptional utility as chiral
building blocks.3 Synthesis of nonproteinogenic amino
acids is critically important not only for their production
but also for the preparation structural analogs, since in many
cases, only small quantities of were isolated from natural
sources. Since the chirality of a-amino acids is critically
important for molecular recognition and biological activity,
the synthesis must be designed to produce them in high
enantiomeric purity.4 The poisonous mushroom Clitocybe
acromelalga found exclusively in Japan has been the source
for a variety of potent neuroexcitatory nonproteinogenic
amino acids related to the kainoid family.5 Shirahama et al.6
isolated (S )-(2)-acromelobic acid [3-(6-carboxy-2-oxo-4-
pyridyl)-L-alanine, 1] and (S )-(2)-acromelobinic acid
[3-(6-carboxy-2-oxo-3-pyridyl)-L-alanine, 2],7 (Fig. 1)
from the fruit bodies of this mushroom (C. acromelalga )
by a combination of ion-exchange column chromatography,
and paper electrophoresis. The amino acids (S)-(2)-1 and
(S)-(2)-2 are isomers and differ in the attachment of
a-amino acid side chain on the pyridone ring. It was
proposed that these two nonproteinogenic amino acids
(S)-(2)-1 and (S)-(2)-2 are biosynthetically derived from
L-DOPA and are the precursors for various acromelic
Keywords: nonproteinogenic amino acids; total synthesis; acromelobic
acid; acromelobinic acid.
*
Figure 1. Structure of nonproteinogenic amino acids (S)-(2)-1 and
(S)-(2)-2.
Corresponding author. Tel.: þ1-847-937-0225; fax: þ1-847-938-8927;
e-mail: maciej.adamczyk@abbott.com
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00670-1

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the strategy (Fig. 2) for the synthesis of (S)-(2)-1 and (S )-
(2)-2 first involves the preparation of an appropriately
functionalized bromo compound 6, which is suitable for
alkylation with the lithiated (R )-(2)-7. The bromo com-
pound 6 contains a methoxy group at the 2-position of
pyridine ring, which can be cleaved at the final stages of the
synthesis to form the pyridone ring, and a carboxylic acid
group in the form of an ester at the 6-position. Thus, the
stereoselective alkylation of Schollkopf’s reagent (R )-(2)-7
with bromo compound 6 followed by hydrolysis of the
resulting dihydropyrazine ring and cleavage of protective
groups should yield (S )-(2)-1 and (S)-(2)-2. Initially, we
directed our efforts to implement this strategy for the
synthesis of (S )-(2)-acromelobic acid (1).
Figure 2. Retrosynthetic analysis of (S )-(2)-1 and (S )-(2)-2 via
asymmetric alkylation approach.
3. Studies on the synthesis of (S)-(2)-1 via asymmetric
alkylation approach
a-amino acids10,11 for applications in the area of
osteoporosis and neuroscience research. In this context,
we describe an efficient enantioselective synthesis of (S )-
(2)-1 and (S)-(2)-2 starting from commercially available
citrazinic acid (8) and 2,5-lutidine (27), respectively.12
The bromo compound 13 required for the synthesis of (S )-
(2)-1 was envisioned from a commercially available
citrazinic acid (8). Thus, 8 was treated (Scheme 1) with
phosphorous oxychloride and tetramethylammonium
chloride to obtain 2,6-dichloroisonicotinic acid (9) in 71%
yield.15 Treatment of 9 with 4.0 equiv. of NaOMe, which
was added in two portions, in refluxing MeOH afforded
2-chloro-6-methoxyisonicotinic acid (10) in excellent yield
(96%). Interestingly, addition of NaOMe 2,6-dichloroiso-
nicotinic acid (9) in one portion resulted in the formation of
the corresponding bis-methoxy compound (2,6-dimethoxy-
isonicotinic acid). The carboxylic acid group in 10 was
reduced using BH3–THF to afford the alcohol 11 in 74%
yield after purification by silica gel column chromato-
graphy. The hydroxy compound 11 was then subjected to
the carbonylation reaction using palladium acetate, 1,3-
bis(diphenylphosphino)propane (DPPP), anhydrous potas-
sium carbonate and 1-propanol in DMF and carbon
monoxide at 908C to afford the ester 12 in 45% yield. The
2. Results and discussion
The two nonproteinogenic amino acids (S )-(2)-1 and (S )-
(2)-2 have common structural feature, e.g. 2-pyridone-6-
carboxylic acid, and therefore, we envisioned a general
strategy (Fig. 2) for their synthesis. The exceptional utility
of (2R )-(2)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyra-
zine (Schollkopf’s reagent, 7)13 in asymmetric synthesis
of a-amino acids, coupled with our recent success in
utilization this remarkable reagent for synthesis of a bone
collagen cross-link,14 prompted us to make use (R )-(2)-7
for the synthesis of (S)-(2)-1 and (S )-(2)-2. Accordingly,
Scheme 1. Studies on the synthesis of (S)-(2)-1 via asymmetric alkylation approach.

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6953
Scheme 2. Studies on the synthesis of (S)-(2)-1 via asymmetric alkylation approach.
ester 12 was then converted to the bromide 13 by treatment
with carbon tetrabromide, triphenylphosphine and imida-
zole in THF in 60% yield. The crucial step in the synthesis
of (S)-(2)-1, alkylation of (R)-(2)-Schollkopf’s reagent (7,
2.0 equiv.) with bromo compound 13, was carried out by
using n-BuLi in THF at 2788C. After 2.5 h, the reaction was
quenched at 2788C and the crude product was purified by
silica gel column chromatography to afford a compound,
this transformation, the methyl ester of a-amino acid was
trans-esterified into n-propyl ester. The compound 18 has all
functional groups that are present in (S )-(2)-1 and thus the
cleavage of the protective groups should complete its
synthesis. In order to determine if any racemization has
occurred during the carbonylation and to ensure the integrity
of the stereocenter in 18, we determined the optical purity of
18 by converting it to the Mosher’s amide. Thus, Boc group
in 18 was first hydrolyzed with trifluoroacetic acid and the
resulting amine was treated with (R )-MTP-Cl to afford the
Mosher’s amide 19. Analysis of 1H and 19F NMR of
Mosher’s amide 19 showed a diastereomeric mixture in a
1:1 ratio, indicating that complete racemization has
occurred presumably during the transformation of (S)-
(þ)-17 to 18. From these results, it became clear that having
an ester group at the 2-position of pyridine ring during the
alkylation of 13 with (R)-(2)-Schollkopf’s reagent (7) or
introduction of ester group via carbonylation in (S )-17 after
the installation of a-amino acid chain, is problematic.
Therefore, we devised a different strategy for the synthesis
of (S )-(2)-1.
1
which did not correspond to the desired product. In the H
NMR spectrum, the resonance corresponding to the
n-propyl ester was missing and four doublets at 1.07, 0.96,
0.79 and 0.64 with integration of three protons each, were
present. The ESI-MS of this undesired compound showed
(MþH)þ at 516 corresponding to the compound 14, which
was apparently formed by nucleophilic substitution at both
bromo and ester groups. When the alkylation reaction using
bromo compound 13 was carried out with one equivalent of
(R)-(2)-7, a mixture of 14 and unreacted starting material
13 were isolated and thus indicating that there was no
selectivity in the reactivity between the bromide and
n-propyl ester groups with lithiated (R)-(2)-7 reagent.
Alternately, we thought that the ester functionality at
2-position of pyridine ring could be installed after the
introduction of amino acid side chain at 4-position in order
to avoid the reaction of (R )-(2)-7 with ester group. Thus,
alcohol 11 was first converted (Scheme 2) to the bromo
compound 15 in 60% yield. Alkylation of bromo compound
15 with the lithium anion of (R )-(2)-7 in THF at 2788C
afforded (S,R)-(2)-16 in 78% yield as a single isomer (de:
. 98%). The next step in the synthesis of (S)-(2)-1 was to
transform the dihydropyrazine ring in (S,R )-(þ)-16 into the
amino acid moiety. Thus, (S,R )-(þ)-16 was first hydrolyzed
with HCl in acetonitrile and the resulting crude amine
hydrochloride was treated with (Boc)2O and triethylamine
in THF to afford the amino acid derivative (S)-(þ)-17 in
79% yield. The palladium catalyzed carbonylation of (S)-
(þ)-17 in n-propanol and DMF, under the conditions
optimized for preparation of 12, gave 18 in 61% yield. In
4. Synthesis of (S)-(2)-1 via asymmetric hydrogenation
protocol
Synthesis of a-amino acids via catalytic asymmetric
hydrogenation protocol16 has become increasingly valuable
in recent years mainly due to this protocol’s ability to
produce them in excellent enantiomeric purities in good
yields and also its compatibility with various functional
groups to the reaction conditions. Although, application of
hydrogenation protocol for the synthesis of (S)-(2)-1 (Fig.
3) looked promising in light of the recent success in the
preparation of a variety of a-amino acids by various
research groups,16 a careful literature survey revealed that
there were only few reports17 related to the catalytic
asymmetric hydrogenation of pyridyldehydroamino acid
derivatives. The catalytic asymmetric hydrogenation of

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M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
Accordingly, the hydroxy compound 12 was first oxidized
with MnO2 to the corresponding aldehyde 22, and then
reacted with N-(benzyloxycarbonyl)phosphonoglycine tri-
methyl ester (23) in the presence of N,N,N,N-tetramethyl-
guanidine (TMG) to afford dehydroamino acid derivative 20
as a mixture of Z/E isomers (ratio 93:7) (Scheme 3). The
minor E-isomer was separated during silica gel column
chromatography (25% EtOAc in hexanes) to afford
Z-isomer 20 in 76% yield. Asymmetric hydrogenation of
dehydroamino acid derivative 23 was carried out by using a
catalytic amount of (R,R )-[Rh(DIPAMP)(COD)]BF4 in
MeOH18 and hydrogen (65 psi) at 488C for 16 h to afford
the amino acid derivative (S )-(þ)-21 in 89% yield. The
enantiomeric purity of (S)-(þ)-21 was found to be .98%
by the analysis of its Mosher’s amide 24, which was
obtained in 81% yield by hydrogenation of (S)-(þ)-21
followed by reaction of the resulting amine with (R )-(2)-
MTP-Cl. We were gratified with the successful outcome of
the hydrogenation approach to install the a-amino acid
chain in an excellent optical purity (.98%) and proceeded
to complete the synthesis of (S )-(2)-1. Accordingly, the
next step in the synthesis of (S)-(2)-1 was to remove all the
protecting groups in (S)-(þ)-21, which we thought of
achieving in one-pot by using iodotrimethylsilane (TMS-I).
However, treatment of (S )-(þ)-21 with 10 equiv. of TMS-I
in chloroform under reflux conditions cleaved only the
urethane and methyl ether groups leaving the n-propyl and
methyl esters intact. Alternately, (S )-(þ)-21 was first
treated with lithium hydroxide in a mixture of THF–water
to give the corresponding di-acid, which without purifi-
cation was subjected to a reaction with TMS-I in CHCl3.
Purification of the crude product by Dowex CCR-3 ion
exchange resin followed by Biorad AG 11 A8 resin
chromatography and lyophilization afforded (S )-(2)-acro-
melobic acid (1) in 68% yield as a white fluffy solid.
Figure 3. Asymmetric hydrogenation approach to the synthesis of (S )-(2)-
1: retrosynthetic analysis.
heteroaromatic dehydroamino acid was reported17 to be
difficult due to the participation of hetero-atom, e.g.
pyridine ring nitrogen, in blocking the formation of an
active metal–substrate complex. Asymmetric hydrogen-
ation of 3- and 4-pyridyldehydroamino acid derivatives was
carried out at high temperature/pressure17a,b or in the
presence of noncomplexing acids such as HBF4.17c,d The
required pyridyldehydroamino acid derivative 23 for
implementation of the asymmetric hydrogenation
protocol to synthesize (S )-(2)-1 was envisioned from
n-propyl 4-(hydroxymethyl)-6-methoxy-2-pyridinecar-
boxylate (12), which was prepared in four steps from
commercially available citrazinic acid (8) as described in
Scheme 1.
Scheme 3. Synthesis of (S)-(2)-1 via asymmetric hydrogenation protocol.

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6955
5. Synthesis of (S )-(2)-acromelobinic acid (2)
TMG under the conditions developed for 20, afforded the
dehydroamino acid derivative 31 in 76% yield as a mixture
of Z/E isomers in 9:1 ratio. Our attempts to separate the Z/E
mixture of 31 by silica gel column chromatography were not
successful. The asymmetric hydrogenation of dehydro-
amino acid 31 was initially carried out with (R,R )-
[Rh(DIPAMP)(COD)]BF4 catalyst in MeOH and hydrogen
gas (65 psi) at 488C to afford the amino acid derivative
(S)-(þ)-32 in 97% yield. Disappointingly, the enantiomeric
purity of (S )-(þ)-32 was found to be 92% as determined by
the analysis of 19F NMR and HPLC of the Mosher’s amide,
which was prepared from (S )-(þ)-32 by hydrogenation
followed by reaction of the corresponding amine with
(R )-MTP-Cl. Alternatively, the hydrogenation of
dehydroamino acid derivative 31 was carried out using
(S,S)-[Rh(Et-DUPHOS)(COD)]BF4 catalyst22 in a mixture
of MeOH–EtOAc and hydrogen at 35 psi afforded (S )-(þ)-
32 in 93% yield. To our delight, the enantiomeric purity of
(S)-(þ)-32 was found to be .96% as determined by the
analysis of 19F NMR and HPLC of its Mosher’s amide.
Finally, alkaline hydrolysis of methyl esters in (S )-(þ)-32
was carried out using lithium hydroxide and the resulting
crude acid was treated with TMS-I to cleave the methyl
ether and Cbz protecting groups. Purification of the crude
product by Dowex CCR-3 ion exchange resin followed by
Biorad AG 11 A8 resin chromatography and lyophilization
afforded (S)-(2)-acromelobinic acid (2) in 88% yield as
pale yellow powder.
Having successfully achieved the enantioselective synthesis
of (S)-(2)-acromelobic acid (1) via asymmetric hydrogen-
ation protocol, we proceeded to the synthesis of second
nonproteinogenic amino acid, (2)-acromelobinic acid (2).
A similar asymmetric hydrogenation strategy was
envisioned for the synthesis of (S)-(2)-2, which required
3-pyridyldehydroamino acid derivative 31 for implemen-
tation of this protocol (Scheme 4). The key dehydroamino
acid derivative 31 was envisioned from a commercially
available and inexpensive 2,5-lutidine (25). Thus, 25 was
first reacted with selenium dioxide followed by treatment
with sulfuric acid in methanol to afford 5-methylpicolic acid
methyl ester,19 which was subsequently converted to the
corresponding N-oxide 26 in 79% yield. The N-oxide 26
was then treated with acetic anhydride under reflux
conditions followed by treatment with NaOMe to afford
compound 27 in 60% yield. We decided to protect the newly
generated 2-phenolic group as a methyl ether in order to
avoid its interference in the synthesis of (S )-(2)-2. Thus, 27
was treated with silver carbonate and methyl iodide and the
crude compound was purified by silica gel column
chromatography to afford 28 in 93% yield. The next step
in the synthesis was the oxidation of 3-methyl group into
aldehyde functionality. Direct oxidation of methyl group in
28 using selenium dioxide to form the aldehyde 30 was
unsuccessful. Therefore, a two-step protocol was devised
for the conversion of 28 to the aldehyde 30. Thus,
bromination20 of the 5-methyl group in 28 using NBS and
AIBN in CCl4 afforded the bromomethyl compound 29,
which was then treated with hexamethylenetetramine in
aqueous acetic acid (Sommelet reaction)21 to afford the
aldehyde 30. Treatment of 30 with N-(benzyloxycarbonyl)
phosphonoglycine trimethyl ester (23) in the presence of
In summary, an efficient asymmetric synthesis of two
nonproteinogenic amino acids, (S )-(2)-acromelobic
acid (1) and (S )-(2)-acromelobinic acid (2) was
developed via catalytic asymmetric hydrogenation protocol
starting from commercially available and inexpensive
materials.
Scheme 4. Synthesis of (S)-(2)-acromelobinic acid (2).

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M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6. Experimental
separated and the aqueous layer was extracted with EtOAc
(200 mL). The combined organic extracts were dried
(Na2SO4) and concentrated to afford 11.3 g of 10 in 96%
yield as a pale brown solid. Mp: 213–2158C; Analytical RP
HPLC: MeCN–0.05% aqueous acetic acid/30:70,
2.0 mL/min at 225 nm, tR: 11.91 min, 96%; 1H NMR
(CDCl3): d 7.36 (d, J¼1.0 Hz, 1H,), 7.14 (d, J¼1.0 Hz, 1H),
3.88 (s, 3H); 13C NMR (CDCl3): d 164.6, 164.0, 148.1,
144.2, 115.5, 109.3, 54.5; ESI-MS (m/z): 186 (M2H)2, 373
(2£M2H)2.
6.1. General methods and materials
1H and 13C NMR spectra were recorded on a Varian Gemini
spectrometer (300 MHz) and the chemical shifts (d) were
reported in ppm relative to TMS and coupling constants (J )
were reported in Hz. Electrospray ionization mass spec-
trometry (ESI-MS) were carried out on a Perkin–Elmer
(Norwalk, CT) Sciex API 100 Benchtop system employing
Turbo Ionspray ion source and HRMS were obtained on a
Nermang 3010 MS-50, JEOL SX102-A mass spectrometer.
Thin layer chromatography was performed on a pre-coated
6.1.3. (2-Chloro-6-methoxy-4-pyridinyl)methanol (11). A
solution of BH3–THF (1.0 M solution in THF, 120 mL,
120.0 mmol, 3.0 equiv.) was added to compound 10 (7.5 g,
40.1 mmol) in THF (150 mL) at 08C under nitrogen over a
period of 15 min. After the addition was complete, cooling
bath was removed, the mixture was allowed to warm to
room temperature and stirred for 5 h. The mixture was
cooled to 08C and quenched with 1.0N aqueous NaOH
(65 mL). The mixture was then diluted with saturated
aqueous NH4Cl (150 mL) solution and ether (200 mL).
Organic layer was separated and the aqueous layer was
extracted with ether (200 mL). The combined organic
extracts were dried (Na2SO4) and concentrated on a rotary
evaporator. Purification of the crude compound by silica gel
column chromatography (30–35% EtOAc in hexanes)
afforded 5.1 g of 11 as a colorless solid. Rf: 0.53 (40%
EtOAc in hexanes); Mp: 100–1018C; Analytical RP HPLC:
MeCN–0.05% aqueous acetic acid/30:70, 2.0 mL/min at
˚
Whatman MK6F silica gel 60 A plates (layer thickness:
250 mm) and visualized with UV light and/or using KMnO4
solution (prepared from KMnO4 (2.0 g) and NaHCO3
(8.0 g) in water (200 mL)) or phosphomolybdic acid reagent
(20 wt% solution in ethanol). Column chromatography was
performed on silica gel, Merck grade 60 (230–400 mesh).
THF was freshly distilled from a purple solution of sodium
and benzophenone under nitrogen. CH2Cl2 was freshly
distilled from CaH2 under nitrogen. Analytical reversed
phase (RP) HPLC was performed using a Waters Symmetry,
˚
RCM C18, particle size: 7.0 mm, pore size: 100 A,
8£100 mm column. Optical rotations were measured on
Autopol III polarimeter from Rudolph Research, Flanders,
NJ. Melting points were recorded in open capillary tubes on
an Electrothermal Melting Point Apparatus and were
uncorrected. IUPAC names of all new compounds were
obtained using the ACD/Ilab Web service version 3.5 at
http://www.acdlabs.com/ilab.
1
225 nm, tR: 5.22 min, 99%; H NMR (CDCl3): d 6.90 (s,
1H), 6.65 (d, J¼0.8 Hz, 1H), 4.67 (d, J¼5.8 Hz, 2H), 3.94
(s, 3H), 1.86 (t, J¼6.0 Hz, 1H); 13C NMR (CDCl3): d 164.2,
155.4, 148.6, 113.9, 105.9, 63.0, 54.1; ESI-MS (m/z): 174
(MþH)þ, 191 (MþNH4)þ; HRMS (FAB, m/z ): Calcd for
C7H9ClNO2, 174.0322 (MþH)þ, observed, 174.0328.
6.1.1. 2,6-Dichloroisonicotinic acid (9). Citrazinic acid (8,
51.0 g, 0.329 mol) and tetramethylammonium chloride
(37.7 g, 0.342 mol, 1.04 equiv.) were suspended in POCl3
(92 mL, 0.987 mol, 3.0 equiv.) and the mixture was heated
to 1408C (bath temperature). The reaction mixture became
homogenous in 30 min, which was stirred 28 h and then
heated at 1508C for an additional 1 h. The mixture was
cooled to room temperature, poured over ice (700 g) and the
contents were stirred for 2 h. The resulting solid was
filtered, washed with water (50 mL) and dried under
vacuum. The solid was suspended in EtOAc (500 mL),
stirred for 15 min, and the insoluble material was filtered.
The solid was collected and dried under vacuum (0.1 mm
Hg) to afford 45.0 g of 9 in 71% yield as a pale brown
solid.15 Analytical RP HPLC: MeCN–0.05% aq acetic
6.1.4. n-Propyl 4-(hydroxymethyl)-6-methoxy-2-
pyridinecarboxylate (12). A mixture of chloro compound
11 (100 mg, 0.578 mmol), Pd(OAc)2 (6.5 mg, 0.029 mmol,
0.05 equiv.), 1,3-bis(diphenylphosphino)propane (DPPP,
13.0 mg, 0.032 mmol, 0.055 equiv.), anhydrous K2CO3
(120 mg, 0.867 mmol, 1.5 equiv.) in a mixture of n-propa-
nol (2.2 mL) and DMF (1.1 mL) was purged with nitrogen
and then with carbon monoxide. The mixture was then
stirred at 908C (bath temperature) under carbonmonoxide
atmosphere (15 psi). The reaction mixture was cooled to
room temperature, filtered through celite powder and
washed with THF (30 mL). The combined filtrates were
concentrated on a rotary evaporator and the residue was
partitioned between methyl-tert-butylether (MTBE, 50 mL)
and water (50 mL). Organic layer was separated and the
aqueous layer was extracted with MTBE (20 mL). The
combined organic extracts were dried (Na2SO4), concen-
trated on a rotary evaporator and the crude product was
purified by silica gel column chromatography (40–50%
EtOAc in hexanes) to afford 0.059 g of 12 in 45% yield as
viscous oil. Rf: 0.29 (30% EtOAc in hexanes); Analytical
RP HPLC: MeCN–0.05% aqueous acetic acid/30:70,
2.0 mL/min at 225 nm, tR: 7.71 min, 99%; 1H NMR
(CDCl3): d 7.55 (s, 1H), 6.85 (d, J¼1.1 Hz, 1H), 4.66 (s,
2H), 4.24 (t, J¼6.8 Hz, 2H), 3.95 (s, 3H), 3.08 (br s, 1H,
OH), 1.81–1.69 (m, 2H), 0.98 (t, J¼7.7 Hz, 3H); 13C NMR
(CDCl3): d 165.4, 164.3, 153.9, 145.4, 116.4, 111.8, 67.1,
1
acid/30:70, 2.0 mL/min at 225 nm, tR: 10.55 min, 98%; H
NMR (DMSO-d6): d 7.80 (s, 2H); 13C NMR (DMSO-d6): d
163.7, 150.2, 144.7, 122.9; ESI-MS (m/z): 190 (M2H)2.
6.1.2. 2-Chloro-6-methoxyisonicotinic acid (10). NaOMe
(25 wt% solution in MeOH, 47.0 mL, 219.89 mmol,
3.5 equiv.) was added to a solution of 2,6-dichloroisonico-
tinic acid (9, 12.0 g, 62.8 mmol) in MeOH (200 mL) and the
mixture was heated to reflux for 24 h. An additional amount
of NaOMe solution (7.0 mL, 32.0 mmol, 0.5 equiv.) was
added to the reaction mixture and was refluxed for an
additional 24 h. The mixture was cooled to room tempera-
ture, acidified to pH 6.5 using 6.0N aqueous HCl (45 mL,
270.0 mmol, 4.2 equiv.) and the mixture was concentrated
on a rotary evaporator. The residue was partitioned between
water (100 mL) and EtOAc (600 mL). Organic layer was

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6957
63.0, 53.7, 21.9, 10.3; ESI-MS (m/z): 226 (MþH)þ, 243
(MþNH4)þ, 451 (2£MþH)þ; HRMS (FAB, m/z): Calcd for
C11H16NO4, 226.1079 (MþH)þ, observed, 226.1071.
6.1.7. 4-(Bromomethyl)-2-chloro-6-methoxypyridine
(15). Triphenylphosphine (2.72 g, 10.40 mmol, 1.5 equiv.),
imidazole (0.71 g, 10.40 mmol, 1.5 equiv.) and carbon
tetrabromide (3.45 g, 10.40 mmol, 1.5 equiv.) were added
sequentially to a solution of hydroxy compound 11 (1.2 g,
6.94 mmol) in THF (60 mL) at room temperature under
nitrogen. After stirring the mixture for 4 h, the solvent was
removed on a rotary evaporator and the crude product was
purified by silica gel column chromatography (10% EtOAc
in hexanes) to afford 0.25 g of 15 in 60% as a pale yellow
solid. Rf: 0.88 (25% EtOAc in hexanes); Mp: 55–568C;
Analytical RP HPLC: MeCN–0.05% aqueous acetic
6.1.5. n-Propyl 4-(bromomethyl)-6-methoxy-2-pyridine-
carboxylate (13). Triphenylphosphine (0.463 g, 1.766
mmol, 1.5 equiv.), imidazole (0.12 g, 1.766 mmol,
1.5 equiv.) and carbon tetrabromide (0.586 g, 1.766 mmol,
1.5 equiv.) were added sequentially to a solution of hydroxy
compound 12 (0.265 g, 1.177 mmol) in THF (15 mL) at
room temperature under nitrogen. After stirring the mixture
for 3.5 h, the solvent was removed on a rotary evaporator
and the crude product was purified by silica gel column
chromatography (20% EtOAc in hexanes) to afford 0.25 g
of 13 in 74% as viscous oil. Rf: 0.52 (15% EtOAc in
hexanes); Analytical RP HPLC: MeCN–0.05% aqueous
acetic acid/60:40, 2.0 mL/min at 225 nm, tR: 6.34 min,
99%; 1H NMR (CDCl3): d 7.70 (d, J¼1.3 Hz, 1H), 6.92 (d,
J¼1.3 Hz, 1H), 4.38 (s, 2H), 4.33 (t, J¼6.8 Hz, 2H), 4.02 (s,
3H), 1.88–1.76 (m, 2H), 1.04 (t, J¼7.4 Hz, 3H); 13C NMR
(CDCl3): d 164.8, 164.4, 149.4, 146.4, 118.7, 114.5, 67.2,
53.9, 29.8, 22.0, 10.4; ESI-MS (m/z): 288 (MþH)þ, 310
1
acid/60:40, 2.0 mL/min at 225 nm, tR: 6.60 min, 99%; H
NMR (CDCl3): d 6.93 (d, J¼1.1 Hz, 1H), 6.65 (d,
J¼1.1 Hz, 1H), 4.29 (s, 2H), 3.94 (s, 3H); 13C NMR
(CDCl3): d 164.1, 151.0, 148.8, 116.4, 109.0, 54.2, 29.4;
ESI-MS (m/z): 236 (MþH)þ.
6.1.8. (1)-(2S,5R)-2-[(2-Chloro-6-methoxy-4-pyridinyl)-
methyl]-5-isopropyl-3,6-dimethoxy-2,5-dihydropyrazine
(16). n-BuLi (2.5 M solution in hexanes, 1.46 mL,
3.66 mmol, 2.0 equiv.) was added dropwise to a solution
of (R)-(2)-Schollkopf’s reagent (7, 0.674 g, 3.66 mmol,
2.0 equiv.) dissolved in THF (25 mL) at 2788C under
nitrogen. The resulting pale yellow solution was stirred for
20 min. A solution of bromo compound 15 (0.43 g,
1.83 mmol) in THF (25 mL) was added dropwise via
double ended needle over a 5 min period. The reaction
mixture was then stirred for 2.5 h and quenched with
saturated aqueous NH4Cl solution (10 mL) at 2788C. The
mixture was allowed to warm to room temperature and
diluted with EtOAc (50 mL) and H2O (20 mL). The aqueous
layer was separated and extracted with EtOAc (25 mL). The
combined organic layers were dried (Na2SO4) and concen-
trated on a rotary evaporator. The crude compound was
purified by silica gel column chromatography (10% EtOAc
in hexanes) to afford 0.485 g of (S,R)-(þ)-16 in 78% yield
(.98% de) as colorless viscous oil. Rf: 0.37 (15% EtOAc in
hexanes); Analytical RP HPLC: MeCN–0.05% aqueous
trifluoroacetic acid/60:40, 2.0 mL/min at 225 nm, tR:
(MþNa)þ; HRMS (FAB, m/z): Calcd for C11H BrNO3,
79
15
288.0235 (MþH)þ, observed, 288.0235; HRMS (FAB,
81
15
m/z): Calcd for C11H BrNO3, 290.0215 (MþH)þ,
observed, 290.0227.
6.1.6. [(2S,5R )-5-Isopropyl-3,6-dimethoxy-2,5-dihydro-
2-pyrazinyl](4-{[(2S,5R)-5-isopropyl-3,6-dimethoxy-2,5-
dihydro-2-pyrazinyl]methyl}-6-methoxy-2-pyridinyl)
methanone (14). n-BuLi (2.5 M solution in hexanes,
0.654 mL, 1.637 mmol, 2.0 equiv.) was added dropwise to
a solution of (R)-(2)-Schollkopf’s reagent (7, 0.293 g,
1.637 mmol, 2.0 equiv.) in THF (15 mL) at 2788C under
nitrogen. The resulting pale yellow solution was stirred for
20 min. A solution of bromo compound 13 (0.235 g,
0.82 mmol) in THF (15 mL) was added dropwise at
2788C via double ended needle over a 5 min period. The
reaction mixture was then stirred for 4 h and quenched with
saturated aqueous NH4Cl solution (1 mL) at 2788C. The
mixture was allowed to warm to room temperature and
diluted with EtOAc (30 mL) and H2O (20 mL). Organic
layer was separated and the aqueous layer was extracted
with EtOAc (30 mL). The combined organic layers were
dried (Na2SO4) and concentrated on a rotary evaporator.
The crude compound was purified by silica gel column
chromatography (15% EtOAc in hexanes) to afford 0.265 g
of bis-dihydropyrazine compound 14 in 64% yield as
colorless viscous oil. Rf: 0.21 (15% EtOAc in hexanes);
Analytical RP HPLC: MeCN–0.05% aqueous trifluoroace-
tic acid/80:20, 2.0 mL/min at 225 nm, tR: 9.57 min, 96%; 1H
NMR (CDCl3): d 7.54 (d, J¼1.1 Hz, 1H), 6.73 (d,
J¼1.1 Hz, 1H), 5.94 (d, J¼3.54 Hz, 1H), 4.32–4.27 (m,
1H), 4.13–4.09 (m, 1H), 3.88 (s, 3H), 3.71 (s, 3H), 3.68 (s,
3H), 3.65 (s, 3H), 3.63–3.60 (m, 4H), 3.16 (dd, J¼13.2,
4.4 Hz, 1H), 3.04 (dd, J¼13.2, 6.3 Hz, 1H), 2.36–2.25 (m,
1H), 2.23–2.12 (m, 1H), 1.07 (d, J¼6.9 Hz, 3H), 0.96 (d,
J¼6.9 Hz, 3H), 0.79 (d, J¼6.9 Hz, 3H), 0.64 (d, J¼6.9 Hz,
3H); 13C NMR (CDCl3): d 195.0, 165.3, 164.2, 163.2,
161.9, 160.1, 150.5, 148.8, 119.0, 116.2, 61.3, 60.7, 60.5,
55.6, 53.6, 52.9, 52.7, 52.5, 52.4, 39.3, 32.2, 31.7, 19.1,
18.9, 16.9, 16.5; ESI-MS (m/z): 516 (MþH)þ, 1031
(2£MþH)þ.
1
17.51 min, 99%; [a]D23¼þ28.8 (c 0.75, CHCl3); H NMR
(CDCl3): d 6.73 (d, J¼0.8 Hz, 1H), 6.44 (d, J¼0.8 Hz, 1H),
4.29–4.24 (m, 1H), 3.89 (s, 3H), 3.72 (s, 3H), 3.70–3.67
(m, 4H), 3.05 (dd, J¼12.9, 4.39 Hz, 1H), 2.95 (dd, J¼13.2,
6.0 Hz, 1H), 2.24–2.14 (m, 1H), 0.99 (d, J¼6.8 Hz, 3H),
0.65 (d, J¼6.8 Hz, 3H); 13C NMR (CDCl3): d 164.2, 163.6,
161.7, 152.3, 147.6, 118.1, 110.1, 60.7, 55.5, 53.9, 52.4,
52.3, 39.0, 31.7, 18.9, 16.5; ESI-MS (m/z): 340 (MþH)þ;
HRMS (FAB, m/z): Calcd for C16H23N3O3Cl, 340.1428
(MþH)þ, observed, 340.1433.
6.1.9. (1)-Methyl N-(tert-butoxycarbonyl)-3-(2-chloro-6-
methoxy-4-pyridinyl)-L-alaninate (17). 1.0N aqueous HCl
(6.0 mL) was added to a solution of (S,R )-(þ)-16 (0.45 g,
1.22 mmol) in acetonitrile (20 mL) at room temperature.
After stirring the mixture for 1 h 10 min, it was concentrated
on a rotary evaporator. The resulting crude amine–TFA salt
was dissolved in acetonitrile (20 mL) and Et3N (0.85 mL,
6.10 mmol, 5.0 equiv.) followed by (Boc)2O (0.80 g,
3.66 mmol, 3.0 equiv.) were added to the reaction mixture.
After stirring the mixture for 16 h at room temperature, it
was concentrated on a rotary evaporator and purified by
silica gel column chromatography (15% EtOAc in hexanes)

6958
M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
to give 0.36 g of (S )-(þ)-17 in 79% yield as a colorless
solid. Rf: 0.28 (15% EtOAc in hexanes); Mp: 90–918C;
Analytical RP HPLC: MeCN–0.05% aqueous acetic
acid/60:40, 2.0 mL/min at 225 nm, tR: 5.67 min, 99%;
21.5, 10.3, 10.0; ESI-MS (m/z): 325 (MþH)þ, 649
(2£MþH)þ.
The above prepared crude amine–TFA salt (0.021 g,
0.049 mmol) was dissolved in CH2Cl2 (5.0 mL) and cooled
to 08C. Et3N (0.034 mL, 0.245 mmol, 5.0 equiv.) followed
by (R )-(2)-a-methoxy-a-trifluoromethylphenylacetic acid
chloride (MTP-Cl, 0.014 mL, 0.0735 mmol, 1.5 equiv.)
were added and the mixture was stirred for 45 min. The
reaction was quenched with water (10 mL), mixture was
stirred for an additional 10 min and extracted with CH2Cl2
(2£25 mL). The combined organic layers were washed with
saturated aqueous NaHCO3 (20 mL), dried (Na2SO4), and
concentrated on a rotary evaporator to afford 0.024 g of
Mosher amide 19 in 92% yield as a 1:1 diastereomeric
mixture. Analytical RP HPLC: MeCN–0.05% aqueous
acetic acid/62:38, 2.0 mL/min at 225 nm, tR: 12.37 min,
96%; 1H NMR (CDCl3): d 7.52 (m, 5H), 7.19 (d,
J¼8.24 Hz, 1H), 6.72 (d, J¼1.1 Hz, 1H), 6.52 (d,
J¼1.1 Hz, 1H), 5.02–4.89 (m, 1H), 4.34–4.27 (m, 2H),
4.15–4.09 (m, 2H), 4.01 and 3.98 (two s, 3H), 3.44–3.42
and 3.30–3.27 (two m, 3H), 3.24–3.01 (m, 2H), 1.88–1.74
(m, 2H), 1.73–1.59 (m, 2H), 1.06–1.00 (m, 3H), 0.96–0.90
(m, 3H); 19F NMR (CDCl3þ2.0 mL of a,a,a-trifluoroto-
luene): d 26.04 and 26.00; ESI-MS (m/z): 541 (MþH)þ,
1081 (2£MþH)þ.
1
[a ]2D3¼þ47.0 (c 0.6, CHCl3); H NMR (CDCl3): d 6.71
(s, 1H), 6.44 (s, 1H), 5.07 (d, J¼7.4 Hz, 1H), 4.63–4.55
(m, 1H), 3.91 (s, 3H), 3.75 (s, 3H), 3.09 (dd, J¼13.7, 5.5 Hz,
1H), 3.94 (dd, J¼13.7, 6.6 Hz, 1H), 1.43 (s, 9H); 13C
NMR (CDCl3): d 171.5, 163.9, 154.9, 150.5, 148.5,
117.2, 109.6, 80.3, 54.0, 53.5, 52.5, 37.3, 28.2; ESI-MS
(m/z): 345 (MþH)þ, 689 (2£MþH)þ; HRMS (FAB, m/z):
Calcd for C15H22N2O5, (MþH)þ 345.1217, observed,
345.1214.
6.1.10. n-Propyl N-(tert-butoxycarbonyl)-3-(2-n-propyl-
oxycarbonyl-6-methoxy-4-pyridinyl) alaninate (18). A
mixture of chloro compound (S )-(þ)-17 (300 mg,
0.0436 mmol,
0.87 mmol),
Pd(OAc)2
(9.8 mg,
0.05 equiv.), 1,3-bis(diphenylphosphino)propane (DPPP,
20.0 mg, 0.048 mmol, 0.055 equiv.), anhydrous K2CO3
(180 mg, 1.31 mmol, 1.5 equiv.) in a mixture of n-propanol
(6.6 mL) and DMF (3.3 mL) was purged with nitrogen and
then with carbon monoxide. The mixture was then stirred at
908C under carbon monoxide atmosphere (15 psi). The
reaction mixture was cooled to room temperature, filtered
through celite powder and washed with THF (50 mL). The
combined filtrates were concentrated on a rotary evaporator
and the residue was partitioned between methyl-tert-
butylether (MTBE, 100 mL) and water (75 mL). Organic
layer was separated and the aqueous layer was extracted
with MTBE (50 mL). The combined organic extracts were
dried (Na2SO4), concentrated on a rotary evaporator and the
crude product was purified by silica gel column chroma-
tography (20% EtOAc in hexanes) to afford 0.225 g of 18 in
61% yield as viscous oil. Rf: 0.57 (20% EtOAc in hexanes);
Analytical RP HPLC: MeCN–0.05% aqueous acetic
6.1.12. n-Propyl 4-formyl-6-methoxy-2-pyridinecar-
boxylate (22). MnO2 (6.5 g, 63.55 mmol, 13.0 equiv.) was
added to a solution of 12 (1.1 g, 4.88 mmol) in CHCl3
(45 mL) at room temperature and the mixture was stirred for
24 h. An additional amount of MnO2 (2.0 g, 2.0 equiv.) was
added to the reaction mixture and the stirring was continued
for an additional 24 h at room temperature. The mixture was
filtered through celite powder and washed with CHCl3
(50 mL). The combined filtrates were concentrated on a
rotary evaporator and the residue was purified by silica gel
column chromatography (10–20% EtOAc in hexanes) to
afford 0.32 g of aldehyde 22 in 30% yield as pale yellow
solid. Rf: 0.37 (15% EtOAc in hexanes); Mp: 86–878C;
Analytical RP HPLC: MeCN–0.05% aqueous acetic
1
acid/60:40, 2.0 mL/min at 225 nm, tR: 9.71 min, 99%; H
NMR (CDCl3): d 7.49 (d, J¼1.1 Hz, 1H), 6.71 (d,
J¼0.8 Hz, 1H), 5.07 (d, J¼7.7 Hz, 1H), 4.64–4.56 (m,
1H), 4.31 (d, J¼6.8 Hz, 2H,), 4.12–4.07 (m, 2H), 4.00 (s,
3H), 3.17 (dd, J¼13.73, 6.0 Hz, 1H), 3.05 (dd, J¼13.73,
6.2 Hz, 1H), 1.87–1.75 (m, 1H), 1.71–1.59 (m, 1H), 1.42
(s, 9H), 1.03 (t, J¼7.4 Hz, 3H), 0.92 (t, J¼7.4 Hz, 3H); 13C
NMR (CDCl3): d 171.1, 165.1, 164.2, 154.9, 148.7, 145.7,
119.7, 115.7, 80.2, 67.3, 67.1, 53.6, 37.5, 28.2, 22.0, 21.8,
10.4, 10.3; ESI-MS (m/z): 425 (MþH)þ, 447 (MþNa)þ;
HRMS (FAB, m/z): Calcd for C21H33N2O7, 425.2288
(MþH)þ, observed, 425.2293.
1
acid/60:40, 2.0 mL/min at 225 nm, tR: 3.76 min, 99%; H
NMR (CDCl3): d 10.07 (s, 1H), 8.08 (d, J¼1.1 Hz, 1H),
7.32 (d, J¼1.1 Hz, 1H), 4.36 (t, J¼6.8 Hz, 2H), 4.09 (s, 3H),
1.89–1.78 (m, 2H), 1.05 (t, J¼7.4 Hz, 3H); 13C NMR
(CDCl3): d 190.3, 165.0, 164.3, 147.4, 145.3, 116.0, 115.4,
67.5, 54.4, 22.0, 10.4; ESI-MS (m/z ): 224 (MþH)þ, 241
(MþNH4)þ; HRMS (FAB, m/z ): Calcd for C11H14NO4,
224.0923 (MþH)þ, observed, 224.0920.
6.1.11. Mosher’s amide 19. Trifluoroacetic acid (1.2 mL)
was added to a solution of 18 (0.026 g, 0.061 mmol) in
methylenechloride (1.0 mL) at room temperature. After
stirring the mixture for 1 h, it was concentrated on a rotary
evaporator and dried under vacuum (0.1 mm/Hg) to afford
the corresponding amine as its TFA salt of (0.022 g, 82%).
Analytical RP HPLC: MeCN–0.05% aqueous acetic
6.1.13. n-Propyl 4-((1Z )-2-{[(benzyloxy)carbonyl]-
amino}-3-methoxy-3-oxo-1-propenyl)-6-methoxy-2-
pyridinecarboxylate (20). N,N,N0,N0-Tetramethylguani-
dine (TMG, 0.127 mL, 1.01 mmol, 1.1 equiv.) followed by
a solution of aldehyde 22 (0.205 g, 0.92 mmol) in THF
(4.0 mL) were added to a solution of and N-(benzylox-
ycarbonyl)phosphonoglycine trimethyl ester (23, 0.335 g,
1.01 mmol, 1.1 equiv.) in THF (6.0 mL) at 08C under
nitrogen. The cooling bath was removed, the mixture was
allowed to warm to room temperature and stirred for 6 h.
The mixture was then quenched with water (50 mL) and
extracted with EtOAc (2£50 mL). The combined organic
extracts were dried (Na2SO4), concentrated on a rotary
1
acid/30:70, 2.0 mL/min at 225 nm, tR: 3.69 min, 99%; H
NMR (CD3OD): d 7.55 (br s, 1H), 6.90 (br s, 1H), 4.42–
4.33 (m, 1H), 4.26 (t, J¼6.6 Hz, 2H), 4.12 (t, J¼6.6 Hz,
2H), 3.92 (s, 3H), 3.31 (br d, J¼5.2 Hz, 2H), 1.84–1.72 (m,
2H), 1.66–1.54 (m, 2H), 1.01 (t, J¼7.4 Hz, 3H), 0.85 (t,
J¼7.4 Hz, 3H); 13C NMR (CD3OD): d 168.5, 165.2, 164.5,
147.1, 146.0, 119.3, 115.2, 68.6, 67.5, 54.0, 53.3, 35.3, 21.9,

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6959
evaporator and the residue was purified by silica gel column
chromatography (25% EtOAc in hexanes) to afford 0.301 g
of dehydro amino acid derivative 20 in 76% yield (Z-isomer,
.98%). Rf: 0.15 (25% EtOAc in hexanes); Analytical RP
HPLC: MeCN–0.05% aqueous acetic acid/60:40,
2.0 mL/min at 225 nm, tR: 5.95 min, 96%; 1H NMR
(CDCl3): d 7.74 (d, J¼0.8 Hz, 1H), 7.36–7.25 (m, 5H),
7.16 (s, 1H), 6.91 (s, J¼0.6 Hz, 1H). 6.74 (br s, 1H), 5.07 (s,
2H), 4.29 (t, J¼6.8 Hz, 2H), 4.01 (s, 3H), 3.85 (s, 3H),
1.84–1.72 (m, 2H), 1.01 (t, J¼7.4 Hz, 3H); 13C NMR
(CDCl3): d 164.9, 164.8, 164.4, 152.7, 145.9, 145.0, 135.4,
128.5, 128.4, 128.2, 127.5, 125.4, 118.2, 114.2, 67.8, 67.1,
53.8, 53.1, 22.0, 10.4; ESI-MS (m/z): 429 (MþH)þ, 451
(MþNa)þ; HRMS (FAB, m/z): Calcd for C22H25N2O7,
429.1662 (MþH)þ; observed 429.1667.
corresponding amine as its hydrochloride salt in 88%
yield. Analytical RP HPLC: MeCN–0.05% aqueous acetic
acid/20:80, 2.0 mL/min at 225 nm, tR: 6.49 min, 99%; H
NMR (CD3OD): d 7.52 (d, J¼0.8 Hz, 1H), 6.85 (s, 1H),
4.21 (t, J¼6.6 Hz, 2H), 3.87 (s, 3H), 3.72 (s, 3H), 3.23–3.33
(m, 2H), 1.77–1.65 (m, 2H), 0.94 (t, J¼7.1 Hz, 3H); 13C
NMR (CD3OD): d 170.1, 166.5, 166.1, 148.8, 147.4, 120.4,
116.7, 68.4, 54.4, 54.2, 53.8, 36.4, 23.1, 10.7; ESI-MS
(m/z): 297 (MþH)þ, 593 (2£MþH)þ.
1
The above prepared crude amine hydrochloride salt
(0.011 g, 0.033 mmol) was dissolved in CH2Cl2 (5.0 mL)
and cooled to 08C. Et3N (0.019 mL, 0.132 mmol, 4.0 equiv.)
followed by (R )-(2)-a-methoxy-a-trifluoromethylphenyl-
acetic acid chloride (MTP-Cl, 0.0093 mL, 0.0497 mmol,
1.5 equiv.) were added to the above solution and the mixture
was stirred for 2.5 h. The reaction was quenched with water
(10 mL), stirred for an additional 10 min and extracted with
CH2Cl2 (10 mL). Organic layer was separated, washed with
saturated NaHCO3 (10 mL), dried (Na2SO4), and concen-
trated on a rotary evaporator. The residue was purified by
silica gel column chromatography (30% EtOAc in hexanes)
to afford 0.013 g of Mosher amide 24 in 78% yield. Rf: 0.46
(30% EtOAc in hexanes); Analytical RP HPLC: MeCN–
0.05% aqueous acetic acid/55:45, 2.0 mL/min at 225 nm, tR:
Also, 0.025 g of the E-isomer of 20 in 6% yield was
isolated. Rf: 0.21 (25% EtOAc in hexanes); Analytical RP
HPLC: MeCN–0.05% aqueous acetic acid/60:40,
2.0 mL/min at 225 nm, tR: 6.79 min, 91%; 1H NMR
(CDCl3): d 7.59 (br s, 1H), 7.44 (m, 1H), 7.33–7.27 (m,
5H), 7.17 (s, 1H), 7.12 (br s, 1H), 6.65 (m, 1H), 5.09 (s, 2H),
4.22 (t, J¼6.8 Hz, 2H), 3.93 (s, 3H), 3.54 (s, 3H), 1.78–1.66
(m, 2H), 0.94 (t, J¼7.4 Hz, 3H); ESI-MS (m/z): 429
(MþH)þ.
1
11.92 min, 99%; H NMR (CDCl3): d 7.41 (d, J¼1.3 Hz,
6.1.14. (1)-Methyl N-(benzyloxycarbonyl)-3-(n-propyl-
oxycarbonyl-6-methoxy-4-pyridinyl)-L-alaninate (21).
Dehydroamino acid derivative 20 (0.117 g, 0.27 mmol)
was dissolved in MeOH (10 mL) and nitrogen was bubbled.
(R,R)-[Rh(DIPAMP)(COD)]BF4 (13.0 mg, 0.018 mmol,
0.065 equiv.) was added under nitrogen atmosphere and
the mixture was degassed using vacuum. The mixture was
then hydrogenated (65 psi) at 488C for 16 h. The reaction
mixture was cooled to room temperature, concentrated on a
rotary evaporator and the residue was purified by silica gel
column chromatography (25% EtOAc in hexanes) to afford
0.105 g of (S)-(þ)-21 in 89% yield as viscous oil. Rf: 0.15
(25% EtOAc in hexanes); Analytical RP HPLC: MeCN–
0.05% aqueous acetic acid/60:40, 2.0 mL/min at 225 nm, tR:
1H), 7.39–7.33 (m, 5H), 7.18 (d, J¼8.5 Hz, 1H), 6.50 (d,
J¼1.0 Hz, 1H), 5.02–4.95 (m, 1H), 4.32–4.27 (m, 2H),
3.98 (s, 3H), 3.79 (s, 3H), 3.45–3.22 (m, 3H), 3.20 (dd,
J¼14.0, 5.2 Hz, 1H), 3.03 (dd, J¼14.0, 7.4 Hz, 1H), 1.86–
1.74 (m, 2H), 1.03 (t, J¼7.4 Hz, 3H); 19F NMR (CDCl3þ2.0
mL of a,a,a-trifluorotoluene): d 26.03; ESI-MS (m/z): 513
(MþH)þ, 1025 (2£MþH)þ.
6.1.16. (S)-(2)-Acromelobic acid (1). A solution of LiOH
(monohydrate, 0.024 g, 0.558 mmol, 4.0 equiv. in 2.0 mL of
water) was added to a solution of (S )-(þ)-21 (0.06 g,
0.139 mmol) in THF (5.0 mL) at room temperature. After
stirring the mixture for 2 h, it was concentrated on a rotary
evaporator to about 2.0 mL volume and diluted with water
(2.0 mL) and EtOAc (15 mL). 2.0N aqueous HCl (0.28 mL,
0.558 mmol, 4.0 equiv.) was added to the above mixture
with stirring. Organic layer was separated and the aqueous
layer was extracted with EtOAc (2£10 mL). The combined
organic extracts were washed with water (5.0 mL), dried
(Na2SO4) and concentrated on a rotary evaporator to afford
the corresponding di-acid (0.05 g) in 96% yield.
1
5.44 min, 99%; [a ]2D3¼þ50.5 (c 0.525, CHCl3); H NMR
(CDCl3): d 7.47 (d, J¼1.1 Hz, 1H), 7.38–7.30 (m, 5H), 6.68
(d, J¼0.8 Hz, 1H), 5.33 (d, J¼7.9 Hz, 1H), 5.10 (s, 2H),
4.69 (dd, J¼13.4, 6.0 Hz, 1H), 4.30 (t, J¼6.8 Hz, 2H), 3.99
(s, 3H), 3.74 (s, 3H), 3.17 (dd, J¼13.7, 5.8 Hz, 1H), 3.07
(dd, J¼13.7, 6.3 Hz, 1H), 1.86–1.74 (m, 2H), 1.02 (t,
J¼7.4 Hz, 3H); 13C NMR (CDCl3): d 171.2, 165.0, 164.3,
155.5, 148.3, 145.9, 135.9, 128.5, 128.2, 128.0, 119.5,
115.3, 67.1, 53.9, 53.6, 52.6, 37.4, 22.0, 10.4; ESI-MS
(m/z): 431 (MþH)þ, 453 (MþNa)þ; HRMS (FAB, m/z):
Calcd for C22H27N2O7, 431.1818 (MþH)þ, observed,
431.1824.
The above isolated crude di-acid (0.05 g, 0.133 mmol) was
suspended in CHCl3 (24 mL), iodotrimethylsilane
(0.19 mL, 1.336 mmol, 10.0 equiv.) was added and the
mixture was refluxed under nitrogen for 7 h. The reaction
mixture became homogeneous in 10 min of heating. MeOH
(16 mL) was added and the reflux was continued for an
additional 12 h. The mixture was concentrated to dryness on
a rotary evaporator and the residue was triturated with
CH3CN (2£10 mL) and the CH3CN layer was separated
using a pipette. The resulting solid was suspended in water
(5.0 mL) and basified to pH 11 using 1.0N aqueous NaOH.
The resulting clear solution was then passed through
Dowex, CCR-3 ion exchange (Aldrich Chemical Co.)
resin column (eluent: water). The fractions containing the
product were combined and lyophilized. Analysis of the
6.1.15. Preparation of Mosher’s amide (24). 1.0N aqueous
HCl solution (0.046 mL, 0.046 mmol, 1.1 equiv.) was added
dropwise to a stirred solution of (S)-(þ)-21 (0.018 g,
0.0418 mmol) in MeOH (5 mL). After the addition was
complete, the mixture was concentrated on a rotary
evaporator to dryness. The residue was dissolved in
MeOH (5 mL) and hydrogenated in presence of 10% Pd/C
(50% wet with water, 4.0 mg, 10% wt/wt) at 30 psi for 2 h.
The reaction mixture was filtered through celite powder and
the filtrate was concentrated to afford 0.011 g of the

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M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
product by RP HPLC (MeCN–0.05% aqueous acetic
acid/2:98, 1.0 mL, 225 nm, tR: 2.69 min (NaI) and tR:
4.18 min, 80%;), indicated that approximately 20% NaI was
present. The lyophilized product was further purified with
AG11A8 (Biorad Laboratories) resin (eluent: water) to
afford 0.019 g of (S)-(2)-acromelobic acid (1) in 63%
yield. [a]2D3¼2139.1 (c 0.0575, H2O), Lit.6b [a ]2D3¼2131.0
(c 0.05, H2O); Analytical RP HPLC: MeCN–0.05%
aqueous acetic acid/2:98, 1.0 mL/min at 225 nm, tR:
140.7, 138.8, 138.7, 126.7, 126.0, 53.1, 18.3; ESI-MS (m/z):
168 (MþH)þ, 190 (MþNa)þ.
6.1.18. Methyl 6-hydroxy-5-methyl-2-pyridinecarboxyl-
ate (27). 5-Methylpicolic acid-N-oxide methyl ester (26,
0.225 g, 1.347 mmol) was dissolved in acetic anhydride
(5.0 mL) and the mixture was heated to reflux for 3 h. The
reaction was cooled to room temperature and acetic
anhydride was removed on a rotary evaporator. The residue
was dissolved in CH2Cl2 (50 mL) and washed with
saturated aqueous NaHCO3 (10 mL). Organic layer was
dried (Na2SO4) and concentrated on a rotary evaporator.
The residue was dissolved in methanol (10 mL) and cooled
to 08C. NaOMe (25 wt% solution in methanol, 0.288 mL,
1.347 mmol) was added to the above solution and the
mixture was stirred 10 min. The reaction was quenched with
solid NH4Cl (1.0 g) and the mixture was concentrated on a
rotary evaporator. The residue was partitioned between
CH2Cl2 (50 mL) and water (30 mL). Organic layer was
separated and aqueous layer was extracted with CH2Cl2
(20 mL). The combined organic extracts were dried
(Na2SO4), concentrated on a rotary evaporator and purified
by silica gel column chromatography (70–80% EtOAc in
hexanes) to obtain 0.115 g of 27 in 51% yield as a pale
yellow solid. Rf: 0.71 (80% EtOAc in hexanes); Analytical
RP HPLC: MeCN–0.05% aqueous acetic acid/10:90,
2.0 mL/min at 225 nm, tR: 11.12 min, 99%; 1H NMR
(CDCl3): d 9.86 (brs, 1H), 7.33–7.30 (m, 1H), 6.92 (d,
J¼6.9 Hz, 1H), 3.95 (s, 3H), 2.22 (s, 3H); 13C NMR
(CDCl3): d 162.4, 161.5, 137.3, 136.6, 131.0, 109.7, 53.1,
17.1; ESI-MS (m/z ): 168 (MþH)þ, 190 (MþNa)þ.
1
4.57 min, 98%; H NMR (D2O): d 6.86 (s, 1H), 6.48 (s,
1H), 3.54 (dd, J¼7.9, 5.5 Hz, 1H,), 2.91 (dd, J¼13.7,
5.5 Hz, 1H,), 2.71 (dd, J¼13.7, 8.2 Hz, 1H); 13C NMR
(D2Oþ0.05 mL of CD3OD): d 180.3, 167.2, 165.1, 155.9,
140.7, 122.0, 112.3; ESI-MS (m/z ): 225 (M2H)2, 451
(2£M2H)2.
6.1.17. Methyl 5-methyl-2-pyridinecarboxylate N-oxide
(26). A mixture of 2,5-lutidine (25, 20.0 g, 0.187 mol) and
SeO2 (31.1 g, 0.28 mol, 1.5 equiv.) in pyridine (100 mL)
was heated to reflux for 26 h. The mixture was then cooled
to room temperature, filtered and the solid was washed with
pyridine (20 mL) and water (2£20 mL). The combined
filtrate was concentrated on a rotary evaporator and the
resulting residue was suspended in water (200 mL).
Activated carbon (5 g) was added to the suspension and
the mixture was heated to reflux for 15 h. The warm mixture
was then filtered and filtrate was concentrated to dryness on
a rotary evaporator to afford 5-methylpicolic acid, which
was used in the next step without purification.
The above prepared crude 5-methylpicolic acid was
dissolved in methanol (130 mL) and conc H2SO4 (15 mL)
was added. After refluxing the mixture for 8 h, it was cooled
to room temperature and basified carefully with NaHCO3
(20 g). The mixture was concentrated on a rotary evaporator
and the residue was partitioned between water (200 mL) and
ether (200 mL). The organic layer was separated and
aqueous layer was extracted with ether (2£200 mL). The
combined organic extracts were dried (Na2SO4), concen-
trated on a rotary evaporator and the residue was purified by
silica gel column chromatography to afford 8.2 g of the
5-methylpicolic acid methyl ester in 34% yield. Analytical
RP HPLC: MeCN–0.05% aqueous acetic acid/25:75,
2.0 mL/min at 225 nm, tR: 3.14 min, 95%; 1H NMR
(CDCl3): d 8.57 (m, 1H), 8.04 (d, J¼7.9 Hz, 1H,), 7.66–
7.62 (m, 1H), 4.00 (s, 3H), 2.43 (s, 3H); 13C NMR (CDCl3):
d 165.7, 150.2, 145.3, 137.4, 137.2, 124.7, 52.6, 18.5; ESI-
MS (m/z): 152 (MþH)þ.
6.1.19. Methyl 6-methoxy-5-methyl-2-pyridinecarboxyl-
ate (28). Ag2CO3 (11.52 g, 41.91 mmol, 2.0 equiv.) fol-
lowed by MeI (1.3 mL, 20.95 mmol, 1.0 equiv.) were added
to a solution of 27 in CHCl3 (140 mL) at room temperature
under nitrogen. After stirring the mixture for 3 days an
additional amount of MeI (2.6 mL, 41.9 mmol, 2.0 equiv.)
was added in portions over two days period. After stirring
the reaction mixture for a total of five days, it was filtered
and the filtrate was concentrated concentrated on a rotary
evaporator. Purification of the crude product by silica gel
column chromatography (20% EtOAc in hexanes) afforded
3.5 g of methyl ether 28 in 93% yield as a pale yellow solid.
Rf: 0.80 (40% EtOAc in hexanes); Mp: 124–1258C;
Analytical RP HPLC: MeCN–0.05% aqueous acetic
1
acid/45:55, 2.0 mL/min at 225 nm, tR: 5.82 min, 98%; H
NMR (CDCl3): d 7.63 (d, J¼7.4 Hz, 1H,), 7.49–7.46 (m,
1H), 4.04 (s, 3H), 3.95 (s, 3H), 2.25 (s, 3H); 13C NMR
(CDCl3): d 165.9, 162.1, 142.7, 138.5, 126.1, 118.8, 53.6,
52.5, 16.2; ESI-MS (m/z ): 182 (MþH)þ.
m-CPBA (70%, 16.6 g, 67.54 mmol, 1.5 equiv.) was added
to a solution of above prepared 5-methylpicolic acid methyl
ester (6.8 g, 45.0 mmol) in CHCl3 (200 mL) and the mixture
was stirred at room temperature for 15 h. The reaction was
quenched with saturated aqueous Na2SO3 (75 mL) and the
mixture was stirred for 10 min. The organic layer was
separated, washed with saturated aqueous NaHCO3
(100 mL), dried (Na2SO4) and concentrated on a rotary
evaporator to afford 5.84 g of 5-methylpicolic acid-N-oxide
methyl ester (26) in 78% yield. Analytical RP HPLC:
MeCN–0.05% aqueous acetic acid/20:80, 1.0 mL/min at
6.1.20. Methyl 5-(bromomethyl)-6-methoxy-2-pyridine-
carboxylate (29). AIBN (0.041 g, 0.248 mmol,
0.045 equiv.) was added to a refluxing mixture of compound
28 (1.0 g, 5.53 mmol) and NBS (1.08 g, 6.077 mmol,
1.1 equiv.) in CCl4 (30 mL) at room temperature. After
stirring the mixture for 15 min, an additional amount of
AIBN (0.123 g in three portions over a period of 1.5 h and
the stirring was continued for 0.5 h. The reaction mixture
was cooled to room temperature, filtered and the solid was
washed with CCl4 (50 mL). The combined filtrates
were washed with water (100 mL), dried (Na2SO4) and
1
225 nm, tR: 4.08 min, 98%; H NMR (CDCl3): d 8.12 (d,
J¼0.8 Hz, 1H,), 7.55 (d, J¼8.2 Hz, 1H)), 7.12–7.08 (m,
1H), 3.99 (s, 3H), 2.34 (s, 3H); 13C NMR (CDCl3): d 162.0,

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6961
concentrated on a rotary evaporator. The crude compound
was purified by silica gel column chromatography (15%
EtOAc in hexanes) to afford 1.1 g of 29 in 77% yield as a
pale yellow solid. Rf: 0.36 (10% EtOAc in hexanes); Mp:
94–958C; Analytical RP HPLC: MeCN–0.05% aqueous
acetic acid/45:55, 2.0 mL/min at 225 nm, tR: 7.48 min,
98%; 1H NMR (CDCl3): d 7.73 (d, J¼7.4 Hz, 1H,), 7.69 (d,
J¼7.4 Hz, 1H,),4.49 (s, 2H), 4.11 (s, 3H), 3.96 (s, 3H); 13C
NMR (CDCl3): d 165.3, 161.1, 145.3, 139.4, 124.9, 118.8,
54.1, 52.7, 26.8; ESI-MS (m/z ): 260 (MþH)þ.
and bubbled with nitrogen. (R,R)-[Rh(DIPAMP)-
(COD)]BF4 (17.4 mg, 0.023 mmol, 0.05 equiv.) was added
under nitrogen atmosphere and the mixture was degassed
using vacuum. The mixture was then hydrogenated (65 psi)
at 458C for 17 h. The reaction mixture was cooled to room
temperature, concentrated on a rotary evaporator and
residue was purified by silica gel column chromatography
(40% EtOAc in hexanes) to afford 0.18 g of (S )-(þ)-32 in
97% yield as viscous oil. Rf: 0.29 (30% EtOAc in hexanes);
Analytical RP HPLC: MeCN–0.05% aqueous acetic
acid/45:55, 2.0 mL/min at 225 nm, tR: 8.84 min, 99%;
[a ]2D3¼þ24.2 (c 0.76, CHCl3); 1H NMR (CDCl3): d 7.61 (d,
J¼7.1 Hz, 1H), 7.43 (d, J¼7.4 Hz, 1H), 7.36–7.26 (m, 5H),
5.51 (d, J¼7.9 Hz, 1H), 5.08–5.00 (m, 2H), 4.71–4.63 (m,
1H), 4.01 (s, 3H), 3.94 (s, 3H), 3.70 (s, 3H), 3.18 (dd,
J¼13.7, 6.0 Hz, 1H), 3.04 (dd, J¼13.7, 7.7 Hz, 1H); 13C
NMR (CDCl3): d 171.8, 165.4, 161.8, 155.5, 144.0, 139.6,
136.0, 128.4, 128.0, 127.9, 123.8, 118.7, 66.8, 53.7, 53.2,
52.4, 52.3, 32.7; ESI-MS (m/z): 403 (MþH)þ, 805
(2£MþH)þ, 822 (2£MþNH4)þ.
6.1.21. Methyl 5-formyl-6-methoxy-2-pyridinecarboxyl-
ate (30). A mixture of bromo compound 29 (1.1 g,
4.23 mmol) and hexamethylenetetramine (1.3 g, 9.3 mmol,
2.2 equiv.) in 50% aqueous acetic acid (25 mL) was heated
to reflux. After 1 h, the mixture was cooled to room
temperature, neutralized carefully with solid NaHCO3
(~35 g) diluted with water (200 mL) and extracted with
CH2Cl2 (2£50 mL). The combined organic extracts were
dried (Na2SO4), concentrated on a rotary evaporator and
purified by silica gel column chromatography (15% EtOAc
in hexanes) to afford 0.285 g of 30 in 34% yield as a pale
yellow solid. Rf: 0.50 (20% EtOAc in hexanes); Mp: 82–
838C; Analytical RP HPLC: MeCN–0.05% aqueous acetic
6.1.24. (1)-Methyl N-(benzyloxycarbonyl)-3-(methoxy-
carbonyl-6-methoxy-5-pyridinyl)-L-alaninate (32) using
DuPHOS catalyst. Dehydroamino acid derivative 31
(0.16 g, 0.40 mmol) was dissolved in MeOH–EtOAc
(10 mL, 7:3) and bubbled with nitrogen. (S,S)-[Rh(Et-
DuPHOS)(COD)]BF4 (3.0 mg, 0.0045 mmol, 0.011 equiv.)
was added under nitrogen atmosphere and the mixture was
degassed using vacuum. The mixture was then hydrogen-
ated (35 psi) at room temperature for 24 h. The reaction
mixture was concentrated on a rotary evaporator and residue
was purified by silica gel column chromatography (40%
EtOAc in hexanes) to obtain 0.15 g of (S )-(þ)-32 in 93%
yield as a viscous oil. Rf: 0.29 (30% EtOAc in hexanes);
Analytical RP HPLC: MeCN–0.05% aqueous acetic
acid/45:55, 2.0 mL/min at 225 nm, tR: 8.81 min, 99%;
[a ]2D3¼þ25.1 (c 0.74, CHCl3); 1H NMR (CDCl3): d 7.61 (d,
J¼7.1 Hz, 1H), 7.43 (d, J¼7.4 Hz, 1H), 7.36–7.26 (m, 5H),
5.51 (d, J¼7.9 Hz, 1H), 5.08–5.00 (m, 2H), 4.71–4.63 (m,
1H), 4.01 (s, 3H), 3.94 (s, 3H), 3.70 (s, 3H), 3.18 (dd,
J¼13.7, 6.0 Hz, 1H), 3.04 (dd, J¼13.7, 7.7 Hz, 1H); 13C
NMR (CDCl3): d 171.8, 165.4, 161.8, 155.5, 144.0, 139.6,
136.0, 128.4, 128.0, 127.9, 123.8, 118.7, 66.8, 53.7, 53.2,
52.4, 52.3, 32.7; ESI-MS (m/z): 403 (MþH)þ, 805
(2£MþH)þ, 822 (2£MþNH4)þ; HRMS (FAB, m/z):
Calcd for C20H23N2O7, 403.1505 (MþH)þ; observed,
403.1514.
1
acid/45:55, 2.0 mL/min at 225 nm, tR: 3.42 min, 99%; H
NMR (CDCl3): d 10.41 (d, J¼0.8 Hz, 1H), 8.22 (d,
J¼7.4 Hz, 1H), 7.79 (d, J¼7.7, 0.8 Hz, 1H), 4.17 (s, 3H),
4.00 (s, 3H); 13C NMR (CDCl3): d 188.5, 164.6, 163.9,
149.7, 138.5, 121.1, 118.4, 54.2, 52.9; ESI-MS (m/z): 196
(MþH)þ, 391 (2£MþH)þ, 408 (2£MþNH4)þ; HRMS
(FAB, m/z ): Calcd for C9H10NO4, 196.0610 (MþH)þ,
observed, 196.0615.
6.1.22. Methyl 5-((1Z)-2-{[(benzyloxy)carbonyl]amino}-
3-methoxy-3-oxo-1-propenyl)-6-methoxy-2-pyridinecar-
boxylate (31). N,N,N0,N0-tetramethylguanidine (TMG,
0.194 mL, 1.55 mmol, 1.1 equiv.) followed by a solution
of aldehyde 20 (0.275 g, 1.41 mmol) in THF (6.0 mL) were
added to a solution of and N-(benzyloxycarbonyl)phos-
phonoglycine trimethyl ester (23, 0.514 g, 1.55 mmol,
1.1 equiv.) in THF (9.0 mL) at 08C under nitrogen. The
cooling bath was removed and the mixture was allowed
warm to room temperature and stirred for 6 h. The mixture
was then quenched with water (50 mL) and extracted with
EtOAc (2£40 mL). The combined organic extracts were
dried (Na2SO4), concentrated on a rotary evaporator and the
residue was purified by silica gel column chromatography
(40% EtOAc in hexanes) to afford 0.375 g of dehydroamino
acid derivative 31 in 66% yield as a mixture of Z/E isomers
(ratio 9:1). Rf: 0.27 (30% EtOAc in hexanes); Analytical RP
HPLC: MeCN–0.05% aq acetic acid/45:55, 2.0 mL/min at
225 nm, tR: 9.93 min (90%) and 11.47 min (10%); 1H NMR
(CDCl3) of Z-isomer (major): d 7.77 (d, J¼7.7 Hz, 1H,),
7.62 (d, J¼7.7 Hz, 1H), 7.41–7.26 (m, 6H), 6.79 (br s, 1H),
5.06 (s, 2H), 4.06 (s, 3H), 3.97 (s, 3H), 3.85 (s, 3H); ESI-MS
(m/z): 401 (MþH)þ, 801 (2£MþH)þ; HRMS (FAB, m/z):
Calcd for C20H21N2O7, 401.1349 (MþH)þ, observed,
401.1337.
6.1.25. Preparation of the Mosher amide of (S )-(1)-32.
1.0N aqueous HCl solution (0.14 mL, 0.14 mmol,
2.1 equiv.) followed by 10% Pd/C (50% wet with water,
11.0 mg, 20% wt/wt) were added to a solution of (S )-(þ)-32
(0.027 g, 0.0671 mmol) in MeOH (5 mL). The mixture was
hydrogenated at 30 psi for 1 h. The reaction mixture was
filtered through celite powder and the filtrate was concen-
trated to afford the corresponding amine hydrochloride
(19.0 mg, 84%). Analytical RP HPLC: MeCN–0.05%
aqueous acetic acid/15:85, 2.0 mL/min at 225 nm, tR:
4.69 min, 95%; ESI-MS (m/z): (MþH)þ, (2£MþH)þ.
6.1.23. (1)-Methyl N-(benzyloxycarbonyl)-3-(methoxy-
carbonyl-6-methoxy-5-pyridinyl)-L-alaninate (32) using
DIPAMP catalyst. Dehydroamino acid derivative 31
(0.184 g, 0.46 mmol) was dissolved in MeOH (15 mL)
The above prepared crude amine hydrochloride (15.0 mg,
0.044 mmol) was suspended in CH2Cl2 (7.0 mL) and cooled
to 08C. Et3N (0.025 mL, 0.176 mmol, 4.0 equiv.) followed

6962
M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
by (R )-(2)-a-methoxy-a-trifluoromethylphenylacetic acid
chloride (MTP-Cl, 0.0123 mL, 0.066 mmol, 1.5 equiv.)
were added to the above solution and the mixture was
stirred for 1 h. The reaction was quenched with water
(10 mL). After stirring reaction for an additional 10 min, the
mixture was extracted with CH2Cl2 (20 mL). The aqueous
layer was separated and the organic layer was washed with
saturated aqueous NaHCO3 (10 mL), dried (Na2SO4), and
concentrated on a rotary evaporator. The residue was
purified by silica gel column chromatography (40% EtOAc
in hexanes) to afford 0.021 g of Mosher amide of 32 in 98%
yield. Rf: 0.24 (30% EtOAc in hexanes); Analytical RP
HPLC: MeCN–0.05% aqueous acetic acid/50:50,
2.0 mL/min at 225 nm, tR: 10.30 min, 98%; 1H NMR
(CDCl3): d 7.53 (d, J¼7.4 Hz, 1H), 7.36–7.27 (m, 7H),
4.98–4.90 (m, 1H), 3.97 (s, 3H), 3.94 (s, 3H), 3.75 (s, 3H),
3.50–3.47 (m, 3H), 3.16 (dd, J¼14.01, 5.5 Hz, 1H), 3.07
(dd, J¼14.01, 8.5 Hz, 1H); 19F NMR (CDCl3þ2.0 mL of
a,a,a-trifluorotoluene): d 26.04; ESI-MS (m/z ): 485
(MþH)þ, 502 (MþNH4)þ, 986 (2£MþNH4)þ.
References
1. (a) In Chemistry and Biochemistry of the Amino Acids;
Barrett, G. C., Ed.; Chapman and Hall: London, 1985.
(b) Amino Acids, Peptides and Proteins; The Chemical
Society: Cambridge, 1968–1991; Vols. 1–22. (c) Greenstein,
J. P.; Wintz, M. Chemistry of the Amino Acids; Robert, E., Ed.;
Krieger: Florida, 1984; Vols. 1–3.
2. (a) Begley, J. Pharm. Pharmacol. 1996, 48, 136. (b) Cromelin,
D. J. A.; Storm, G. Eur. J. Pharm. Sci. 1994, 2, 17–18.
(c) McMartin, C. Handb. Exp. Pharmaco. 1994, 110, 371.
(d) Kompella, U. B.; Lee, V. H. L. Adv. Drug Delivery Rev.
1992, 8, 115.
3. (a) Coppala, G. M.; Schuster, H. F. Asymmetric Synthesis:
Construction of Chiral Molecules using Amino Acids; Wiley:
New York, 1987. (b) Hanessian, S. Total Synthesis of Natural
Products: The Chiron Approach; Pergamon: Oxford, 1983.
4. (a) Duthaler, R. O. Tetrahedron 1994, 50, 1539. (b) Ohfune, Y.
Acc. Chem. Res. 1992, 25, 360. (c) Williams, R. M. Organic
Chemistry Series Volume 7: Synthesis of Optically Active
a-Amino Acids; Pergamon: Oxford, 1989.
6.1.26. (S )-(2)-Acromelobinic acid (2). LiOH (mono-
hydrate, 0.046 g, 0.1.09 mmol, 4.0 equiv. in 3.0 mL of
water) was added to a solution of (S)-(þ)-32 (0.11 g,
0.27 mmol) in THF (7.0 mL) at room temperature. After
stirring the mixture for 1 h, it was concentrated on a rotary
evaporator to 3.0 mL volume and diluted with water
(5.0 mL) and EtOAc (30 mL). 1.0N aqueous HCl (1.1 mL,
1.1 mmol, 4.0 equiv.) was added to the above mixture with
vigorous stirring. Organic layer was separated and the
aqueous layer was extracted with EtOAc (15 mL). The
combined organic extracts were washed with water
(10 mL), dried (Na2SO4) and concentrated on a rotary
evaporator to obtain the corresponding di-acid (0.10 g) in
98% yield.
5. (a) For a recent review on the kainoid amino acid chemistry
see: Parsons, A. F. Tetrahedron 1996, 52, 4149. (b) Also see
Konno, K.; Hashimoto, K.; Ohfune, Y.; Shiramaha, H.;
Matsumoto, T. J. Am. Chem. Soc. 1988, 110, 4807.
6. (a) Yamano, K.; Hashimoto, K.; Shirahama, H. Heterocycles
1992, 34, 445. (b) Yamano, K.; Shirahama, H. Tetrahedron
1993, 49, 2427.
7. The name acromelobinic acid was assigned based on its
derivatization from (S )-(2)-acromelobic acid (1) and analogy
between L-stizolobic acid (3) and L-stizolobinic acid (4).
8. (a) Hattori, S.; Komamine, A. Nature 1959, 183, 1116.
(b) Senoh, S.; Imamoto, S.; Maeno, Y. Tetrahedron Lett. 1964,
3441. (c) Chilton, W. S.; Hsu, C. P.; Zdybak, W. T.
Phytochemistry 1974, 13, 1179.
9. Baldwin, J. E.; Spyvee, M. R.; Whitehead, R. C. Tetrahedron
Lett. 1994, 35, 6575.
The above prepared crude di-acid (0.10 g, 0.267 mmol) was
suspended in CHCl3 (40 mL), iodotrimethylsilane
(0.38 mL, 2.67 mmol, 10.0 equiv.) was added and the
mixture was heated to reflux under nitrogen for 7 h. The
reaction mixture became homogeneous in 10 min of
heating. MeOH (26 mL) was added and the reflux was
continued for an additional 12 h. The mixture was
concentrated to dryness on a rotary evaporator and the
residue was triturated with CH3CN (10 mL). CH3CN layer
was separated using a pipette. The resulting solid was
suspended in water (5.0 mL) and basified to pH 10 with
1.0N aqueous NaOH. The resulting clear solution was then
passed through Dowex, CCR-3 ion exchange (Aldrich
Chemical Co.) resin column (eluent: water). The fractions
containing the product were combined and lyophilized. The
lyophilized product was further purified with AG11A8
(Biorad Laboratories) resin (eluent: water) to afford 0.053 g
of (S )-(2)-acromelobinic acid (2) in 88% yield.
[a ]2D3¼256 (c 0.15, H2O), Lit.6b [a]2D3¼25.8 (c 0.13,
H2O); Analytical RP HPLC: MeCN–0.05% aqueous acetic
10. (a) Adamczyk, M.; Johnson, D. D.; Reddy, R. E. Tetrahedron
1999, 55, 63. (b) Adamczyk, M.; Akireddy, S. R.; Reddy, R. E.
Tetrahedron: Asymmetry 1999, 10, 3107. (c) Adamczyk, M.;
Johnson, D. D.; Reddy, R. E. Tetrahedron Lett. 1999, 40,
8993. (d) Adamczyk, M.; Johnson, D. D.; Reddy, R. E.
Tetrahedron: Asymmetry 2000, 11, 2289.
11. (a) Adamczyk, M.; Johnson, D. D.; Reddy, R. E. Angew.
Chem., Int. Ed. Engl. 1999, 38, 3537. (b) Adamczyk, M.;
Johnson, D. D.; Reddy, R. E. Bioorg. Med. Chem. Lett. 2000,
10, 269. (c) Adamczyk, M.; Johnson, D. D.; Reddy, R. E.
Tetrahedron: Asymmetry 2000, 11, 3063.
12. For preliminary results on the synthesis of (S )-(2)-1 and (S )-
(2)-2, see: (a) Adamczyk, M.; Akireddy, S. R.; Reddy, R. E.
Org. Lett. 2000, 2, 3421. (b) Adamczyk, M.; Akireddy, S. R.;
Reddy, R. E. Tetrahedron: Asymmetry 2001, 12, 2385.
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Engl. 1981, 20, 798.
14. Adamczyk, M.; Akireddy, S. R.; Reddy, R. E. Tetrahedron
2000, 56, 2379.
1
acid/2:98, 1.0 mL/min at 225 nm, tR: 6.13 min, 98%; H
15. Henegar, K. E.; Ashford, S. W.; Baughman, T. A.; Sih, J. C.;
Gu, R.-L. J. Org. Chem. 1997, 62, 6588.
NMR (D2O): d 7.50 (d, J¼7.1 Hz, 1H), 6.86 (d, J¼7.1 Hz,
1H), 3.76 (dd, J¼7.9, 4.6 Hz, 1H), 3.01 (dd, J¼14.5, 4.6 Hz,
1H), 2.77 (dd, J¼14.5, 8.2 Hz, 1H); 13C NMR
(D2Oþ0.05 mL of CD3OD): d 176.7, 167.4, 164.8, 143.6,
140.0, 131.1, 110.5, 55.6, 34.1; ESI-MS (m/z ): 225
(M2H)2, 451 (2£M2H)2.
16. (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis;
Wiley: New York, 1994; Vol. 2. pp 16–94. (b) Ohkuma, T.;
Kitamura, M.; Noyori, R. Catalytic Asymmetric Hydrogen-
ation; Ojima, I., Ed.; Wiley–VCH: New York, 2000; Vol. 1,
pp 1–110.

M. Adamczyk et al. / Tetrahedron 58 (2002) 6951–6963
6963
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