Synthetic studies on tetrazomine: Lipase PS resolution of racemic cis-β-hydroxypipecolic acid

Article abstract of DOI:10.1016/S0040-4039(00)01527-6

An efficient enzymatic resolution of racemic cis-β-hydroxypipecolic acid is described affording both the (2S,3R) and (2R,3S) protected amino acids in good yield and high enantiomeric ratios. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01527-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8413–8416
Synthetic studies on tetrazomine: lipase PS resolution of
racemic cis-b-hydroxypipecolic acid
Jack D. Scott and Robert M. Williams*
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
Received 29 August 2000; accepted 11 September 2000
Abstract
An efficient enzymatic resolution of racemic cis-b-hydroxypipecolic acid is described affording both the
(2S,3R) and (2R,3S) protected amino acids in good yield and high enantiomeric ratios. © 2000 Elsevier
Science Ltd. All rights reserved.
Tetrazomine (1, Fig. 1) is an antitumor antibiotic that was isolated from Saccharothrix
mutabilis and reported by the Yamanouchi Pharmaceutical Co. in Japan.1 Preliminary anti-
tumor/antimicrobial assays of this substance indicate that tetrazomine displays potent antitumor
activity against P388 leukemia in vivo and displays good antimicrobial activity against both
Gram-negative and Gram-positive organisms. Tetrazomine is structurally related to the
quinocarcin and bioxalomycin classes of antitumor antibiotics, but is distinct from these agents
by virtue of containing the unusual amino acid b-hydroxypipecolic acid. The absolute stereo-
chemistry of the b-hydroxypipecolic acid stereoisomer present in tetrazomine has been deter-
mined in these laboratories to be (2S,3R), (−)-2.2
Figure 1.
As part of a program directed at tackling the total synthesis of tetrazomine, we required a
practical synthesis of (−)-2. The previous synthesis3 of (−)-2 was accomplished recently by Corey
* Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01527-6

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et al. This synthesis entailed an aldol condensation using an ammonium salt catalyst, yielding a
1:1 mixture of syn/anti aldol products. Initially, we attempted a Mitsunobu4 inversion of the
secondary alcohol of previously reported 32 (Scheme 1). Unfortunately, under various conditions
the only product that was obtained was acylation with retention of configuration (4). The
mechanism of this acylation was not investigated and an alternative route was explored.
Scheme 1.
The literature revealed that the racemic cis-b-hydroxypipecolic acid [( )-2] was available in
one-step from picolinic acid (7) via reduction using Rh/C.5 Since ( )-2 is thus readily available,
we endeavored to resolve the enantiomeric pair of amino acids. The use of Lipase PS has been
reported by Toyooka et al.6 for the resolution of a structurally related pipecolic acid derivative
and this method was examined in the present context.
As shown in Scheme 2, picolinic acid was reduced as previously described5 and the amine
protected as the corresponding Fmoc derivative ( )-87 in 55% yield for the two steps. Esterifica-
tion of the acid to the allyl ester afforded ( )-98 in 68% yield. Enzymatic resolution of this
protected amino acid racemate9 using Lipase PS (Amano) proceeded smoothly forming the
acetate of the (2S,3R) isomer, providing (+)-1010 in 46% yield. The (2R,3S) amino acid (+)-9 was
recovered in 43% yield and an enantiomeric ratio (er) of 99.5:0.5 was determined by chiral
HPLC analysis.11
Scheme 2.
The acetate (+)-10 was easily cleaved to form (−)-9 in 91% yield (Scheme 3). The er of (−)-9
was found to be 98:2 by chiral HPLC analysis.11 To confirm the absolute stereochemistry, (−)-9
was converted to the free amino acid (−)-2 via treatment with lithium hydroxide. The optical
rotation matched that of the literature value.2 With these amino acids in hand, we next
investigated the conversion of (−)-9 to a suitably protected form that would be compatible with
peptide coupling strategies.

8415
Scheme 3.
The hydroxyl group on the amino acid (−)-9 was protected as the benzyl ether using benzyl
trichloroacetimidate and triflic acid12 to afford (+)-1113 in 69% yield (Scheme 4). Finally, the
allyl ester was cleaved in 93% yield using palladium tetrakistriphenylphosphine and the sodium
salt of hydroxymethanesulfinic acid14 to afford the amino acid (+)-12.15 This substance should be
a useful protected form of (−)-2 suitable for incorporation into peptides as well as synthetic
strategies directed toward tetrazomine. This route to (−)-2 compares favorably to the previous
synthesis due to higher er (98:2 versus 91:9).
Scheme 4.
Acknowledgements
This work was supported by the National Institutes of Health (CA85419). Lipase PS was
generously supplied by Amano (US).
References
1. (a) Sato, T.; Hirayama, F.; Saito, T. J. Antibiot. 1991, 44, 1367–1370; (b) Suzuki, K.; Sato, T.; Morika, M.;
Nagai, K.; Kenji, A.; Yamaguchi, H.; Sato, T. J. Antibiot. 1991, 44, 479–485.
2. Scott, J. D.; Tippie, T. N.; Williams, R. M. Tetrahedron Lett. 1998, 39, 3659–3662.
3. Horikawa, M.; Busch-Peterson, J.; Corey, E. J. Tetrahedron Lett. 1999, 40, 3843–3846. For previous syntheses of
(+)-2 see Ref. 2 and Roemmele, R. C.; Rapoport, H. J. Org. Chem. 1989, 54, 1866–1875.
4. Mitsunobu, O. Synthesis 1981, 1–28.
5. Drummond, J.; Johnson, G.; Nickell, D. G.; Ortwine, D. F.; Bruns, R. F.; Welbaum, B. J. Med. Chem. 1989, 32,
2116–2128.
6. Toyooka, N.; Yoshida, Y.; Yotsui, Y.; Momose, T. J. Org. Chem. 1999, 64, 4914–4919.
1
7. ( )-8: H NMR (300 MHz) (d6-DMSO, 120°C) l 1.35–1.80 (4H, m); 1.93 (1H, s); 3.07 (1H, ddd, J=12.9, 12.9,
3.3 Hz); 3.66 (1H, m); 3.76 (1H, m); 4.26 (1H, d, J=6.3 Hz); 4.37 (2H, d, J=5.7 Hz); 4.68 (1H, d, J=5.7 Hz);
7.32 (2H, t, J=7.5 Hz); 7.40 (2H, t, J=7.5 Hz); 7.63 (2H, t, J=7.5 Hz); 7.84 (2H, d, J=7.5 Hz). HRMS (FAB)
calcd for C21H22NO5 (MH+) 368.1498; found 368.1502.
1
8. ( )-9. H NMR (300 MHz) (d6-DMSO, 120°C) l 1.44 (1H, m); 1.71 (3H, m); 3.20 (1H, ddd, J=12.3, 12.3, 3.3
Hz); 3.78 (2H, m); 4.27 (1H, dd, J=6.0 Hz); 4.42 (2H, d, J=6.6 Hz); 4.54 (1H, s, broad); 4.62 (2H, m); 4.82 (1H,
d, J=7.2 Hz); 5.21 (1H, dd, J=10.2, 1.2 Hz); 5.35 (1H, dd, J=17.1, 1.8 Hz); 5.91 (1H, m); 7.33 (2H, t, J=7.5
Hz); 7.41 (2H, t, J=7.5 Hz); 7.62 (2H, d, J=7.5 Hz); 7.84 (2H, d, J=7.5 Hz). HRMS (FAB) calcd for

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C24H26NO5 (MH+) 408.1811; found 408.1820. (−)-9. [h]2D0=−33.6 (c=1.5 CH2Cl2). (+)-9. [h]2D0=+36.8 (c=1.2
CH2Cl2).
9. Procedure for lipase PS resolution: to a solution of ( )-9 (380 mg, 0.93 mmol) in 20 mL iPr2O at 30°C was added
1.52 g lipase PS. The reaction progress was monitored by HPLC until 50% completion (Waters novapak HR
silica, isocratic 70/30 hexanes/EtOAc, 2 mL/min, UV 254 nm). When complete, the mixture was filtered through
Celite and the solvent was removed in vacuo. The crude product was purified via flash chromatography (gradient
25–50% EtOAc/hex) to afford 191 mg (+)-10 (43%) and 165 mg (+)-9 (46%).
10. (+)-10: 1H NMR (300 MHz) (d6-DMSO, 120°C) l 1.53 (1H, m); 1.78 (3H, m); 2.10 (3H, s); 3.22 (1H, ddd,
J=12.9, 12.9, 3.6 Hz); 3.82 (1H, dd, J=12.9, 5.1 Hz); 4.27 (1H, t, J=6.3 Hz); 4.42 (2H, d, J=6.6 Hz); 4.63 (2H,
m); 4.84 (1H, m); 5.04 (1H, d, J=6.3 Hz); 5.24 (1H, dd, J=11.7, 1.5 Hz); 5.33 (1H, dd, J=17.3, 1.5 Hz); 5.91
(1H, m); 7.32 (2H, t, J=7.5 Hz); 7.41 (2H, t, J=7.5 Hz); 7.61 (2H, d, J=7.5 Hz); 7.84 (2H, d, J=7.5 Hz).
HRMS (FAB) calcd for C26H28NO6 (MH+) 450.1917; found 450.1916. [h]2D0=+12.8 (c=1.1 CH2Cl2).
11. Chiral HPLC conditions: Pirkle Whelk-O 2 (R,R), isocratic 85/15 hex/iPrOH, 1 mL/min, UV 254 nm.
12. Wessel, H.-P.; Iversen, T.; Bundle, D. R. J. Chem. Soc., Perkin Trans. 1 1985, 2247–2250.
1
13. (+)-11: H NMR (300 MHz) (d6-DMSO, 120°C) l 1.39(1H, m); 1.73 (3H, m); 2.89 (1H, s); 3.22 (1H, m); 3.70
(2H, m); 4.03 (2H, m); 4.26 (1H, ddd, J=12.9, 12.9, 5.7 Hz); 4.46 (2H, m); 4.55 (1H, d, J=6.3 Hz); 4.90 (1H,
d, J=6.3 Hz); 5.15 (1H, m); 5.31 (1H, m); 5.84 (1H, m); 7.14–7.49 (10H, m); 7.60 (1H, d, J=7.5 Hz); 7.78 (2H,
m). HRMS (FAB) calcd for C31H32NO5 (MH+) 498.2280; found 428.2280. [h]2D0=+15.0 (c=1.4 CH2Cl2).
14. Honda, M.; Morita, H.; Nagakura, I. J. Org. Chem. 1997, 62, 8932–8936.
1
15. (+)-12: H NMR (300 MHz) (d6-DMSO, 120°C) l 1.41 (1H, m); 1.72 (3H, m); 3.26 (1H, ddd, J=12.6, 12.6, 3.9
Hz); 3.65 (1H, m); 3.76 (1H, m); 4.05 (1H, s); 4.28 (1H, t, J=6.3 Hz); 4.42 (1H, m); 4.55 (1H, 1/2ABq, J=11.7
Hz); 4.67 (1H, 1/2ABq, J=11.7 Hz); 4.87 (1H, d, J=6.3 Hz); 7.18–7.43 (8H, m); 7.50–7.86 (5H, m). HRMS
(FAB) calcd for C28H28NO5 (MH+) 458.1967; found 458.1959. [h]2D0=+12.2 (c=0.9 CH2Cl2).
.