Synthesis of An Advanced Precursor to the Amino Epimer of (+)-Pyloricidins

Article abstract of DOI:10.1007/s00706-003-0094-1

A consice synthesis of an advanced intermediate for the amino epimer of (+)-pyloricidins (the antipode of the natural ones) starting from D-galactose is presented.

Full text of DOI:10.1007/s00706-003-0094-1

Monatshefte f€ur Chemie 134, 1617–1622 (2003)
DOI 10.1007/s00706-003-0094-1
Synthesis of An Advanced Precursor
to the Amino Epimer of (1)-Pyloricidins
ꢀ
Chao-Jun Tang, Xin Shen, and Yikang Wu
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Received June 25, 2003; accepted July 18, 2003
Published online October 20, 2003 # Springer-Verlag 2003
Summary. A consice synthesis of an advanced intermediate for the amino epimer of (þ)-pyloricidins
(the antipode of the natural ones) starting from D-galactose is presented.
Keywords. Amino acids; Azides; Carbohydrates; Drugs; Acylation.
Introduction
Pyloridin A, B, C, and D (1a–1d, isolated from Bacillus sp. HC-70 and HC-72)
were reported [1] for the first time in 2001 by Takeda scientists. These compounds
show significant activity against Helicobacter pylori [2], a Gram-negative bacte-
rium that is a major cause for human gastric and duodental ulcers [3] and probably
[4] also a promoting factor for gastric cancer.
Immediately after the report on the isolation and structure determination, the
Takeda chemists disclosed their total syntheses [5] of pyloricidins using D-galactos-
amine as a chiral template for the (2S,3R,4R,5S)-5-amino-2,3,4,6-tetrahydroxyhexa-
noic acid moiety. They have also conducted investigations [6] on the structure-activity
relationship, but mainly by altering the peptide chain at the amino group. In medicinal
chemistry it is usually of interest to know what consequences in terms of activity might
follow from a change of the configurations of a chiral drug. Herein wewish to report on
the synthesis of an advanced intermediate, which on a few further manipulations
would provide amino epimers of (þ)-pyloricidins (the antipodes of the natural ones).
Results and Discussions
Our synthesis made full use of the stereogenic centers in the inexpensive chiral
pool compound D-galactose. As shown in Scheme 1, D-galactose was first
converted to its allyl acetal [7] by stirring at 60ꢁC in allyl alcohol containing acetyl
ꢀ
Corresponding author. E-mail: yikangwu@mail.sioc.ac.cn

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C.-J. Tang et al.
Fig. 1. Structures of pyloricidins
Scheme 1
Scheme 2
chloride. The crude tetraol thus obtained was then converted [8] to the tetrabenzyl
ether with NaH=BnBr in 53% overall yield. The allyl protecting group was readily
removed in MeOH in the presence of PdCl2 following the work of Liaigre and co-
workers [9]. An oxidation with acetyl anhydride in DMSO as previously reported
by Overkleeft et al. [10] afforded (2R,3S,4S,5R)-3,4,5-tris(benzyloxy)-6-(benzyl-
oxymethyl)tetrahydropyran-2-one (3).
The ꢀ-amino acid was prepared via the route in Scheme 2. (R)-Phenylglycine (4)
was reduced [11] to the corresponding aminol with LiAlH4 and the amino group was
protected [12] with Boc2O (71.3% over two steps). The additional carbon was intro-
duced by converting [13] the alcohol into the mesylate and treatment [14] with KCN.
Methanolysis in HCl gas saturated MeOH with concurrent removal of the Boc pro-
tecting group gave methyl (R)-3-amino-3-phenylpropionate (5) [15] in 84% yield.
Lactone 3 was treated (Scheme 3) with 5 in refluxing toluene in the presence of a
base such as pyridine, DMAP, or NEt3, which led to methyl (2R,3S,4S,5R)-3-phenyl-
3-(2,3,4,6-tetrakis(benzyloxy)-5-hydroxy-hexanoylamino)propionate (6) but only in
rather low (20–30%) yields. Further transformation into the corresponding mesylate
7 proceeded smoothly. However, all efforts to introduce the azido functionality
failed. Direct substitution with either NaN3 or TMSN3 resulted in a complex mixture,
while an attempt to prepare the triflate (instead of the mesylate) of 6 led to formation
of a lactam. Due to these difficulties we decided to adopt a longer route.
Thus, as shown in Scheme 4, the lactone 3 was converted to methyl
(2R,3S,4S,5R)-(þ)-2,3,4,6-tetrakis(benzyloxy)-5-hydroxyhexanoate (9) via base-

An Advanced Precursor to the Amino Epimer of (þ)-Pyloricidins
1619
Scheme 3
Scheme 4
catalyzed hydrolysis followed by treatment with diazomethane. The hydroxyl-
group in 9 was then mesylated with MsCl in the presence of NEt3 in 90% yield.
Interestingly, with a methoxy group instead of an amino acid at the carboxylic
carbonyl carbon, the previously difficult reaction with NaN3 (ca. 3 equiv.) became
rather facile. After 12 h at 110ꢁC in DMF, the desired methyl (2R,3S,4S,5S)-3-(5-
azido-2,3,4,6-tetrakis(benzyloxy)-hexanoate (11) was obtained in 80% yield.
The methyl ester 11 was then readily hydrolyzed with 1 N KOH in 1,4-dioxane.
The resulting acid was activated with EDCI [16] and further treated with 5 in the
presence of BtOH [16] to give methyl (2R,3S,4S,5S)-3-(5-azido-2,3,4,6-tetrakis
(benzyloxy)hexanoylamino)-3-phenylpropionate (12), which is expected to yield
the amino epimer of the antipodes of natural pyloricidins on reduction of the azido
group, attaching a proper peptide chain, and removal of the benzyl protecting
group. To ensure feasibility of cleavage of the terminal methyl ester in the presence
of amido functionality, hydrolysis of 12 was also briefly examined. Indeed, on
treament with 1 N LiOH in THF the methyl ester could be readily hydrolyzed into
the free carboxylic acid 13 in 70% yield after neutralization.
Experimental
The 1H NMR spectra were taken on either a Varian Mercury 300 (300 MHz) or a Bruker Avance 300
(300 MHz). The FT-IR spectra were measured on a Nicolet Avatar 360. EI-MS were recorded on an HP
5989A. ESI-MS spectra were taken on a PE Mariner API-TOF or an Agilent Technologies LC=MSD
SL instrument. ESI-HRMS data were measured on a Bruker APEXIII 7.0 Tesla FT-MS spectrometer.

1620
C.-J. Tang et al.
Optical rotations were measured on a Perkin Elmer Polarimeter 341. Elemental analyses were done
using an Elementar Vario EL III autoanalyzer. Results of elemental analysis were found to agree
favourably with the calculated values.
Methyl (2R,3S,4S,5R)-(þ)-2,3,4,6-Tetrakis(benzyloxy)-5-hydroxy-hexanoate (9, C35H38O7)
Aqueous LiOH (1 N, 2.4 cm3) was added to a solution of 620mg of 3 (1.16 mmol) in 5 cm3 of THF
stirred at 0ꢁC and the stirring was continued at the same temperature until TLC showed disappearance
of 3. The reaction mixture was acidified with 1 N HCl to pH ¼ 2–3, extracted with 3 ꢂ 30cm3 of
CH2Cl2, washed with brine, and dried over anhydrous Na2SO4. After removal of the drying agent and
the solvent, the residue was dissolved in diethyl ether and treated with ethereal CH2N2 at ambient
temperature. When TLC showed complete reaction, the reaction mixture was evaporated to dryness on
a rotary evaportaor and the residue was chromatographed on silica gel (EtOAc:n-hexane ¼ 1:4) to give
460 mg of 9 as a colorless oil (71% yield), along with 200 mg of recovered 8 (0.31 mmol).
20
Data for 9: ½ꢁꢃD ¼ þ14.1ꢁ cm2 gꢄ1 (c ¼ 0.70, CHCl3); 1H NMR (300MHz, CDCl3): ꢂ ¼ 7.40–7.15
(m, 20H), 4.87 (d, J ¼ 11.4Hz, 1H, part of AB system 1), 4.69 (d, J ¼ 11.0 Hz, 1H, part of AB system
1), 4.60 (d, J ¼ 11.9Hz, 1H, part of AB system 2), 4.52 (d, J ¼ 11.9Hz, 1H, part of AB system 2), 4.44
(d, J ¼ 11.8Hz, 1H), 4.42–4.29 (several sharp lines for 2 benzylic CH2, 4H), 4.20 (dd, J ¼ 3.1, 8.1 Hz,
1H), 4.11 (br q, J ¼ 7.0 Hz, 1H), 3.84 (dd, J ¼ 1.3, 8.1 Hz, 1H), 3.62 (s, 3H), 3.54 (dd, J ¼ 6.5, 9.4Hz,
1H), 3.84 (dd, J ¼ 6.2, 9.7 Hz, 1H), 2.53 (d, J ¼ 7.8 Hz, OH) ppm; FT-IR (film): ꢃꢀ¼ 3494 (br), 1754,
1101, 738 cmꢄ1; EI-MS: m=z (%) ¼ 91 (100), 181 (13); ESI-MS: 571.3 ([M þ H]þ).
Methyl (2R,3S,4S,5S)-(þ)-5-azido-2,3,4,6-tetrakis(benzyloxy)hexanoate (11, C35H37N3O6)
MsCl (0.16 cm3, 13.46 mmol) was added to a solution of 980 mg of 9 (1.72 mmol) in 15 cm3 of dry
CH2Cl2 containing 0.4 cm3 of NEt3 and stirred at 0ꢁC. The stirring was continued at the same
temperature until TLC showed disappearance of 9. The reaction mixture was diluted with EtOAc,
washed with brine, and dried over anhydrous Na2SO4. After removal of the drying agent and the
solvent the residue was chromatographed on silica gel (EtOAc:n-hexane¼ 1:3) to afford 1.190 g of 10
as a colorless oil (90% yield): 1H NMR (300MHz, CDCl3): ꢂ ¼ 7.40–7.20 (m, 20 H), 5.14 (dt, J ¼ 6.3,
4.2 Hz, 1H), 4.83 (d, J ¼ 10.9 Hz, 1H), 4.73 (d, J ¼ 10.3Hz, 1H, part of AB system), 4.58 (d,
J ¼ 10.6Hz, 1H, part of AB system), 4.54–4.30 (many sharp lines for 3 benzylic CH2, 6H), 4.17
(dd, J ¼ 7.1, 3.3 Hz, 1H), 3.99 (dd, J ¼ 7.3, 3.8 Hz, 1H), 3.75 (dd, J ¼ 10.7, 6.7Hz, 1H), 3.69 (s, 3H),
3.65 (dd, J ¼ 10.6, 4.0Hz, 1H) ppm.
The above mesylate (390 mg, 0.60 mmol) was dissolved in 8 cm3 of DMF. NaN3 (140mg,
2.15mmol) was added. The mixture was heated to 110ꢁC with stirring over night before cooling to
ambient temperature, it was diluted with water, and extracted with 3 ꢂ 60cm3 of EtOAc. The combined
EtOAc phases were washed with brine and dried over anhydrous Na2SO4. After removal of the drying
agent and the solvent, the residue was chromatographed on silica gel (1:4 EtOAc:n-hexane¼ 1:4) to
20
afford 283 mg of 11 as a colorless oil (80% yield): ½ꢁꢃD ¼ þ8.0ꢁ cm2 gꢄ1 (c ¼ 1.30, CHCl3); 1H NMR
(300 MHz, CDCl3): ꢂ ¼ 7.40–7.15 (m, 20H), 4.80 (d, J ¼ 11.7Hz, 1H), 4.614.19 (many sharp lines for
the 4 benzylic CH2, 8H), 4.11 (m, 1H), 4.07 (dd, J ¼ 7.9, 3.1 Hz, 1H), 3.80 (dd, J ¼ 2.9, 7.9Hz, 1H),
3.66 (s, 3H), 3.66–3.60 (m, 2H) ppm; FT-IR (film): ꢃꢀ¼ 2101, 1751, 1455, 1101, 698cmꢄ1; EI-MS:
m=z (%) ¼ 91 (100), 181 (14); ESI-MS: 618.2 ([M þ Na]þ).
Methyl (3R)-(þ)-3-((2R,3S,4S,5S)-5-azido-2,3,4,6-tetrakis(benzyloxy)hexanoylamino)-
3-phenylpropionate (12, C44H46N4O7)
Aqueous KOH (1 N, 0.43cm3) was added to a solution of 253 mg of 11 (0.43 mmol) in 5 cm3 of 1,4-
dioxane. After stirring for 1.5 h, another portion of KOH (1 N, 0.43cm3) was introduced. The mixture
was then stirred at ambient temperature over night before it was diluted with 20cm3 of H2O, acidified

An Advanced Precursor to the Amino Epimer of (þ)-Pyloricidins
1621
with 1 N HCl to pH¼ 2–3, and extracted with 3 ꢂ 30cm3 of EtOAc. The combined EtOAc phases were
washed with brine and dried over anhydrous Na2SO4. After removal of the drying agent and the
solvent, the residue was dissolved in 5 cm3 of dry CH2Cl2. (R)-ꢀ-Amino-phenylacetic acid (100mg,
0.35mmol) was added with cooling (ice-H2O), followed by 75mg of HOBt (0.55 mmol). Two minutes
later, 110 mg of EDCI (0.57 mmol) were introduced. The bath temperature was allowed to rise
naturally to ambient temperature. About 10 h later, the reaction mixture was diluted with EtOAc,
washed with aq. NH4Cl and brine, and dried over anhydrous Na2SO4. After removal of the drying
agent and the solvent, the residue was chromatographed on silica gel (EtOAc:n-hexane¼ 1:4) to give
190 mg of 12 as a colorless oil (73% yield from 11): 1H NMR (300MHz, CDCl3): ꢂ ¼ 7.73
(d, J ¼ 8.5 Hz, NH), 7.40–7.15 (m, 25H, aromat), 5.32 (br q, J ¼ 6.7 Hz, H-7), 4.75 (d, J ¼ 11.5Hz,
H-1), 4.60 (d, J ¼ 11.8Hz, 1H, part of AB system), 4.57 (d, J ¼ 11.5Hz, 1H, part of AB system),
4.50–4.20 (several sharp lines for the benzylic CH2, 5H), 4.33 (dd, J ¼ 11.4, 4.2 Hz, H-2), 4.16–4.08
(m, H-4 and 1H from the benzylic CH2), 3.86 (dd, J ¼ 8.3, 2.5 Hz, H-3), 3.64 (dd, J ¼ 10.2, 4.0Hz,
H-5), 3.58 (br d, J ¼ 10.9Hz, H-5), 3.48 (s, OCH3), 2.73 (dd, J ¼ 15.9, 5.3 Hz, H-8), 2.44 (dd, J ¼ 15.8,
6.8 Hz, H-8) ppm (an arbitary numbering system as shown in Scheme 4 was adopted for easier
assignment, which was assisted by a COSY spectrum); FT-IR (film): ꢃꢀ¼ 3405 (br), 2100, 1738,
1676, 1092, 737 cmꢄ1. EI-MS: m=z (%) ¼ 91 (100), 121 (10); ESI-MS: 743.3 ([M þ H]þ); ESI-HRMS:
20
calcd. for C44H47O7N4 ([MþH]þ) 743.3433; found 743.3439; ½ꢁꢃD ¼ þ5.6ꢁ cm2 gꢄ1 (c ¼ 1.39,
CHCl3).
(3R)-(þ)-3-((2R,3S,4S,5S)-5-azido-2,3,4,6-tetrakis(benzyloxy)hexanoylamino)-
3-phenylpropionic acid (13, C43H44N4O7)
Aqueous LiOH (1 N, 0.24 cm3) was added to a solution of 183mg of 12 (0.25 mmol) in 2 cm3 of THF.
The mixture was stirred at ambient temperature until TLC showed disappearance of 12. The reaction
mixture was diluted with 20 cm3 of H2O, acidified with 1 N HCl to pH¼ 2–3, and extracted with
3 ꢂ 40cm3 of EtOAc. The combined EtOAc phases were washed with brine and dried over anhydrous
Na2SO4. After removal of the drying agent and the solvent the residue was chromatographed on silica
gel (CH2Cl2=MeOH¼ 20:1) to afford 165 mg of 13 as a colorless oil (71% yield): 1H NMR (300 MHz,
CDCl3): ꢂ ¼ 7.62 (d, J ¼ 8.3Hz, 1H), 7.40–7.15 (m, 25H), 5.30 (m, 1H), 4.75 (d, J ¼ 10.2 Hz, 1H),
4.57–4.22 (several sharp lines for benzylic CH2, 7H), 4.34 (dd, J ¼ 4.3, 10Hz, 1H), 4.10 (dd, J ¼ 1.8,
8.3 Hz, 1H), 4.12–4.09 (several lines for benzylic CH2, 1H), 3.85 (dd, J ¼ 2.0, 8.3 Hz, 1H), 3.63 (dd,
J ¼ 10.7, 4.3 Hz, 1H), 3.56 (br d, J ¼ 10.2Hz, 1H), 2.73 (dd, J ¼ 16.2, 5.4Hz, 1H), 2.43 (dd, J ¼ 7.2,
16.4Hz, 1H) ppm; FT-IR (film): ꢃꢀ¼ 2930 (br), 2100, 1720, 1671, 1011, 737 cmꢄ1; ESI-MS: 729.3
([M þ H]þ); ESI-HRMS: calcd. for C43H44O7N4Na ([M þ Na]þ) 751.3102; found 751.3114;
20
½ꢁꢃD ¼ þ6.0ꢁ cm2 gꢄ1 (c ¼ 2.1, CHCl3).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (20025207, 20272071),
the Major State Basic Research Development Program (G2000077502), and the Chinese Academy of
Sciences (CAS ‘Knowledge Innovation’ Project, KGCX2-SW-209).
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