Oppolzer sultam directed aldol as a key step for the stereoselective syntheses of antitumor antibiotic belactosin C and its synthetic congeners

Article abstract of DOI:10.1021/jo0516887

An efficient protocol has been developed using D-(2R)-Oppolzer sultam as a chiral auxiliary for generating anti/syn diastereomers with high enantiopurity and utilized in the efficient synthesis of natural product belactosin C and their synthetic congeners

Full text of DOI:10.1021/jo0516887

Oppolzer Sultam Directed Aldol as a Key Step
for the Stereoselective Syntheses of Antitumor
Antibiotic Belactosin C and Its Synthetic
Congeners†,‡
Gullapalli Kumaraswamy,*,§ Mogilisetti Padmaja,§
Bekkam Markondaiah,§ Nivedita Jena,§
Balasubramanian Sridhar,| and Marelli Udaya Kiran
Organic DiVision-III, Laboratory of X-ray crystallography, and
NMR DiVision, Indian Institute of Chemical Technology,
Hyderabad 500 007, India
FIGURE 1. Belactosin C (1) and its congeners 2 and 3.
SCHEME 1. Retrosynthesis of Belactosin C and Congeners
gkswamy@iictnet.org
ReceiVed August 10, 2005
An efficient protocol has been developed using D-(2R)-
Oppolzer sultam as a chiral auxiliary for generating anti/
syn diastereomers with high enantiopurity and utilized in the
efficient synthesis of natural product belactosin C and their
synthetic congeners. It has been observed that a variation in
the stoichiometry of the Lewis acid led to a difference in
anti/syn selectivity.
Small molecules that target the 20S proteosome inhibition
appear to be an apt choice for cancer chemotherapy.1 Lacta-
cystin, a Steptomyces metabolite isolated by Omura et al., is an
irreversible, covalent inhibitor of the chymotrypsin-like and
trypsin-like activity and a weak, reversible inhibitor of the PGPH
activity of the 20S proteasome.2 A related compound of class
PS-519 is currently in preclinical development for the treatment
of ischemia-reperfusion injury in stroke and myocardial
infarction.3 Recently, Asai et al. identified belactosin A and C
molecules that inhibits the 20S proteasome in vitro (IC ) 0.4
µM, chymotrypsin-like activity)4 in a yeast-based assay of
Steptomyces metabolite (Figure 1). Initial studies of the belac-
tosins revealed that this could be a good lead compound for
cancer by regulating the ubiquitin-proteosome pathways.5
Interestingly, both belactosins A and C (1) exhibit inhibitory
activity comparable to that of lactacystin. In addition, the
degradation studies suggested the â-lactone moiety to be
responsible for antiproliferative activity. Two impressive total
syntheses of belactosins A and C (1) and their homoanalogues
have since been reported.6
To broaden the therapeutical value and also to understand
the mode of action, we initiated a synthetic program to prepare
new variants of this unique natural product, belactosin C (1).
Since the â-lactone moiety is pivotal for bioactivity, we set out
to investigate the influence of the relative stereochemistry of
â-lactone ring and stereochemistry adjacent to the â-lactone on
the anti cancer activity.
* To whom correspondence should be addressed. Tel: +91-40-27193275.
Fax: +91-40-27160387.
† IICT communication no. 050720.
‡ Dedicated to Dr. Ganesh Pandey on occasion of his 51st birthday.
§ Organic Division-III.
| Laboratory of X-ray crystallography.
NMR Division.
In this paper, we describe the synthesis of belactosin C (1)
and their variants 2 and 3 (Scheme 1) based on Oppolzer sultam-
(1) Garcia-Echeverria, C Mini ReV. Med. Chem. 2002, 247.
(2) (a) Omura, S.; Fujimoto, T.; Otoguru, K.; Matsuzaki, K.; Moriguchi,
R.; Tanaka, H. J. Antibiot. 1991, 44, 113. (b) Omura, S.; Matsuzaki, K.;
Fujimoto, T.; Kosuge, K.; Furuya, T.; Fujita, S.; Nakagawa, A. J. Antibiot.
1991, 44, 117.
(3) (a) Campbell, B.; Adams, J.; Shin, Y. K.; Lefer, A. M. J. Mol. Cell
Cardiol. 1999, 31, 467. (b) Elliot, P. J.; Pein, C. S.; McCormack, T. A.;
Chapman, I. D.; Adams, J. J. Allergy Clin. Immunol. 1999, 104, 294.
(4) Asai, A.; Hasegawa, A.; Ochiai, K.; Yamashita, Y.; Mizukami, T. J.
Antibiot. 2000, 53, 81.
(5) (a) Mizukami, T.; Asai, A.; Yamashita, Y.; Katahira, R.; Hasegawa,
A.; Ochiai, K.; Akinaga, S. Eur. Patent 768317, 1997; Chem. Abstr. 1997,
126 338840. (b) Asai, A.; Tsujita, T.; Sharma, S. V.; Yamashita, Y.;
Akinaga, S.; Funakoshi, M.; Kobayashi, H.; Mizukami, T. Biochem.
Pharmacol. 2004, 67, 227.
(6) (a) Armstrong, A.; Scutt, J. N.; Chem. Commun. 2004, 510-511.
(b) Larionov, O. V.; de Meijere, A. Org. Lett. 2004, 6, 2153-2156.
10.1021/jo0516887 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/17/2005
J. Org. Chem. 2006, 71, 337-340
337

FIGURE 2. Proposed transition state for aldolization.
SCHEME 2. Synthesis of Aldol Products 12a, 13, and 14
FIGURE 3. X-ray crystal structure of 12a, 12b, and 14.
TABLE 1. Dependence of Diasteroselectivity on the Stoichiometry
of Lewis Acid
TiCl4
aldehyde
(equiv)
de
entry substrate T (°C) (equiv)
(anti/syn) % yield
1
2
3
4
5
9
9
10
10
10
-10
-10
-10
-78
-78
1
2
2
2
1
1.5
1.5
1.5
3a
70:30
95:5
60:40
90:10
4:96
55
67
15
33
70
directed asymmetric aldol reaction as a key step to obtain three
diastereo pure â-lactones 16, 18, and 21. The study was initiated
by the treatment of a cold (-78 °C) solution of acylsultam 97
with 1 equiv of TiCl4 and 1.2 equiv of diisopropylethylamine
followed by addition of aldehyde 11.
1.5
a Use of 3 equiv of aldehyde increased the product yield.
Surprisingly, after workup, only starting material was recov-
ered. Also, the same reaction conducted at an elevated temper-
ature (-45 °C) did not yield any trace of aldol product, while
the reaction carried out between -10 to 0 °C for 3 h afforded
a product in moderate yield and 70:30 diastereoselectivity in
favor of 12a (Scheme 2). The major and minor aldol products
were readily separated by flash column chromatography on silica
gel. Using 2 equiv of TiCl4 under otherwise identical conditions,
the anti isomer 12a was obtained exclusively (anti/syn
>95:<5, 67% yield). The stereochemical assignments to major
and minor products were unambiguously confirmed by X-ray
crystallography8 (Figure 3). Interestingly, contrary to the
expected syn selectivity,9 the major isomer with absolute
configuration of 1′R,2S was found to result from an anti-selective
aldol reaction.
equiv of diisopropylethylamine between -10 and 0 °C for 3 h
followed by addition of aldehyde 11 to yield product as a
60:40 mixture of anti-13/syn-14 isomers, respectively. The di-
asteroeoselectivity markedly improved when the same reaction
was conducted at -78 °C, where the anti/syn products were
formed in 90:10 ratio.10 Treatment of the compound 10 with 1
equiv of TiCl4, 1.2 equiv of Hu¨nig’s base at -78 °C followed
by the aldehyde yielded 14 as sole product which was isolated
by column chromatography as a crystalline solid (Table 1). The
absolute stereochemistry of 14 was confirmed by X-ray crystal-
lography (Figure 3) and was found to be in agreement with the
expected syn selectivity (syn/anti >96:<4, 70% yield).9b
The observed anti selectivity in the reaction of 9 or 10 with
aldehyde 11 can be explained by invoking open transition state
A# as proposed by Oppolzer et. al (Figure 2).11-13
To check the influence of the resident stereogenic center in
9 (i.e., Me) on the observed sense of stereoselectivity, we treated
10 (lacking the methyl group) with 2 equiv of TiCl4 and 1.2
Proceeding ahead toward the synthesis of belactosin C (1),
attempted hydrolysis of 12a under basic hydroperoxide condi-
tions led to the recovery of starting material. Consequently, a
(7) The acyl-sultam prepared in two steps in overall yield 80% (a)
Michael, R.; Ofer, S.; Volker, S.; Meir, B.; Boris, Y. Tetrahedron:
Asymmetry 1999, 10, 841-853. (b) Ashot, K.; Armenak, Kh. M.; Walter,
F. S. Tetrahedron 2003, 59, 5475-5480.
(8) The major diastereomer 12a was crystallized as silyl ether. The minor
distereomer 12b initially isolated as liquid but slowly it was crystallized
out after a period of one month. The absolute stereochemistry 1′R,2R was
assigned.
(9) (a) Oppolzer, W.; Blagg, J.; Rodriguez.; Walther, E. J. Am. Chem.
Soc. 1990, 112, 2167-2172. (b) Evans, D. A.; Rieger, D. L.; Bilodeau,
Bilodeau, M. T.; Urpi, F. J. Am. Chem. Soc. 1991, 113, 1047-1049. (c)
Evans, D. A.; Tedrow, J. S.; Shaw, J. T.; Downey, C. W. J. Am. Chem.
Soc. 2002, 124, 392-393. (d) Crimmins, M. T.; King, B. W.; Tabet, E. A.
J. Am. Chem. Soc. 1997, 119, 7883. (e) Ghosh, A. K.; Liu, C. J. Am. Chem.
Soc. 2003, 125, 2374-2375.
(10) The anti/syn ratio was estimated by 1H NMR spectrum of crude
product. Our attempt to obtain a crystal of 13 was not successful. The
absolute stereochemistry was assigned by analogy of 12a.
(11) Oppolzer, W.; Lienard, P. Tetrahedron Lett. 1993, 34, 4321-4324.
(12) When compared to the chiral amide 9, the achiral amide 10
undergoes aldolization at lower temperature (-78 °C), probably due to its
greater reactivity. The N, O-Ketone acetal derived from silylation of the
enolate of 9 with TBSCl failed to yield the aldol product upon reaction
with a mixture of aldehyde 11 and TiCl4.13
(13) (a) Walker, M. A.; Heathcock. C. H. J. Org. Chem. 1991, 56, 5747.
(b) Oppolzer, W.; Christian, S.; Ines, R.; Bernardinelli, G. Tetrahedron Lett.
1991, 32, 61-64. (c). Ghosh. A. K.; Onishi, M. J. Am. Chem. Soc. 1996,
118, 2527-2528.
338 J. Org. Chem., Vol. 71, No. 1, 2006

derivatives 24, 2, and 3 in moderate isolated yields (Schemes 3
and 4).16 The analytical data of compound 24 were in full
agreement with the reported data.6b Finally, the compound 24
was subjected to hydrogenolysis followed by filtration and
removal of water with lyophilizer to give the product 1 in 20%
overall yield from â-lactone 16. The analytical data of 1 were
in full agreement with reported data of natural product.4
In conclusion, an efficient method was developed using
D-(2R)-sultam as a chiral auxiliary for generating anti and syn
diastereomers with high enantiopurity, which facilitated the
synthesis of natural product belactosin C (1) and synthetic
congeners 2 and 3. This process is significant in that the Lewis
acid here induces different chirogenicity with variation of
stoichiometry. Further work is in progress to prepare various
analogues and evaluate for the anti cancer activity.
Experimental Section
FIGURE 4. NOE studies of 18 and 21.
N-[(2S,3S-Methyl)-2-[(2′-benzyloxy-1′R-hydroxy)ethyl]pen-
tanoyl]borane-10,2-sultam (12a). TiCl4 (24.3 g or 14.0 mL, 128
mmol) was added dropwise to a 0.2 M solution of 9 (20.0 g, 63.9
mmol) in CH2Cl2 at -10 °C under nitrogen atmosphere resulting
in a yellow slurry. To this was added diisopropylethylamine (13.4
mL, 76.7 mmol) dropwise, and the resulting deep red solution was
stirred maintaining the temperature between -10 and 0 °C. After
45 min, aldehyde 11 (13.4 mL, 95.8 mmol) was added dropwise,
and stirring was continued for a further 3 h at 0 °C. The reaction
was terminated by addition of 1:1 (v/v) saturated aq NH4Cl solution,
and the reaction mixture was warmed to ambient temperature. The
contents were diluted with water and extracted with organic solvent.
The combined organic layers were evaporated and dried. The
residue was purified by column chromatography to give the aldol
reduction/oxidation strategy was adopted. Reduction of 12a with
LAH to alcohol 15 followed by a two-step oxidation afforded
8 in 40% yield. On the other hand, compounds 13 and 14 were
smoothly hydrolyzed to the corresponding â-hydroxy acids 17
and 20. Despite a plethora of methods available for lactoniza-
tion,14 we met success only with BOPCl/Et3N. Thus, lacton-
ization of 8, 17, and 20 afforded 16, 18 and 21 respectively.
The presence of strong NOE between Hf and Hf′ and Hc and
Hc′ for compound 21 confirmed syn stereochemistry, whereas
the absence of the NOE between the same protons in the
compound 18 and between protons Hd and He (Figure 4)
supports anti stereochemistry (see the Suporting Information).
The lactones 16, 18, and 21 were subjected to hydrogenolysis
followed by a one-step oxidation to afford â-lactone carboxylic
acids 7, 19, and 22, respectively. Attempted coupling of 7, 19,
and 22 with dipeptide 2315 using the reported conditions6a
suffered from low yields. After considerable experimentation,
we developed a procedure that furnished protected belactosin
product 12a as white solid. Further, the product was crystallized
1
from EtOH (19.8 g, 67%,): [R]25 -61.5 (c 1, CHCl3); H NMR
D
(200 MHz) δ 0.88 (3H, t, J ) 7.4 Hz), 0.90 (3H, s), 1.00 (3H, d,
J ) 6.7 Hz), 1.08 (3H, s), 1.20-1.41 (4H, m), 1.72-2.11(6H, m),
2.90-3.02 (1H, m), 3.40-3.50 (3H, m), 3.57 (1H, m), 3.85 (1H,
t, J ) 7.4 Hz), 4.05 (1H, m), 4.49 (1H, d, J ) 11.8), 4.58 (1H, d,
J ) 11.8), 7.20-7.41 (5H, m); 13C NMR (75 MHz) δ 12.4, 16.0,
SCHEME 3. Synthesis of Belactosin C
SCHEME 4. Synthesis of Belactosin Congeners 2 and 3
J. Org. Chem, Vol. 71, No. 1, 2006 339

19.9, 19.9, 20.7, 26.4, 27.4, 33.0, 35.1, 38.6, 44.6, 47.9, 52.8, 53.3,
65.6, 70.7, 73.3, 127.6, 138.0, 173.9; IR (KBr) 3521, 3091, 2910,
2961, 1673, 1453, 1394, 1317, 1220, 1131, 1065, 736, 542, 467
cm-1; LSIMS (FAB) m/z 464 (M + H+), 306 (71), 249 (100), 216
(40), 154 (92), 136 (90), 107 (52); HRMS (M + Na+) calcd for
C25H37NO5NaS 486.2290, found 486.2286. (For isomers 13 and
14, see the Supporting Information.)
zyl Ester (2). This compound was prepared using the above typical
procedure 5: yield (900 mg, 50%); [R]25D -3.88 (c 1, CHCl3); 1H
NMR (200 MHz) δ 0.92 (3H, t, J ) 7.2 Hz), 1.35 (3H, d, J ) 7.2
Hz), 1.42-1.7 (4H, m), 1.7-1.9 (4H, m), 3.1-3.35 (2H, m), 3.55-
3.65 (1H, m), 4.25-4.35 (1H, m), 4.45 (1H, d, J ) 4.0 Hz), 4.52-
4.65 (1H, m), 5.09 (2H, s), 5.2 (2H, d, J ) 4.8 Hz), 5.51(1H, d, J
) 7.2 Hz), 6.75(1H, brs), 6.95 (1H, d, J ) 7.2 Hz), 7.26-7.4 (10H,
m); 13C NMR (75 MHz) δ 13.5, 18.4, 19.9, 25.2, 29.3, 30.0, 38.4,
50.5, 51.8, 57.5, 67.0, 67.4, 72.9, 128.0, 128.2, 128.4, 128.5, 128.6,
128.7, 135.1, 136.2, 156.1, 167.9, 169.5, 171.6, 172.4; IR (KBr)
3317, 2965, 1837, 1736, 1654, 1531, 1453, 1248, 1188, 11189,
746, 698 cm-1; HRMS [M + H+] cacld for C30H38N3O8 568.2658,
found 568.2660.
(2S)-[(2S)-Benzyloxycarbonylaminopropionylamino]-5-{[(3R)-
propyl-4-oxo-oxetane-(2R)-carbonyl]amino}pentanoic Acid Ben-
zyl Ester (3). This compound was prepared using the above typical
procedure 5: yield (1.0 g, 56%); [R]25D +21 (c 1,CHCl3); 1H NMR
(200 MHz) δ 0.9 (3H, t, J ) 6.95 Hz), 1.35 (3H, d, J ) 6.95 Hz),
1.42-1.7 (6H, m), 1.70-1.91 (2H, m), 3.11-3.40 (2H, m), 3.85-
3.88 (1H, m), 4.28-4.29 (1H, m), 4.60 (1H, m), 4.80 (1H, d, J )
6.95 Hz), 5.1 (2H, s), 5.15 (2H, d, J ) 4.6 Hz), 5.51 (1H, d, J )
6.95 Hz), 6.80 (1H, brs), 6.90 (1H, d, J ) 8.5 Hz), 7.26-7.41 (10H,
m); 13C NMR (50 MHz) δ 13.5, 18.4, 19.9, 25.2, 27.1, 29.3, 38.4,
50.5, 51.8, 57.5, 67.0, 67.4, 72.9, 128.0, 128.2, 128.3, 128.5, 128.6,
128.7, 135.1, 136.2, 156.1, 167.9, 169.5, 171.6, 172.0; IR (KBr)
3311, 2965, 1837, 1736, 1654, 1531, 1453, 1248, 1188, 11189 746,
698 cm-1; LSIMS (FAB) m/z 568 (M + H+), 154 (30), 91(95), 69
(69), 55 (100); HRMS [M + H+] calcd for C30H38N3O8 568.2658,
found 568.2682.
Typical Procedure for the Peptide Coupling 5: (2S)-[(2S)-
Benzyloxycarbonylaminopropionylamino]-5-{[(3S)-((1S)-meth-
ylpropyl)-4-oxo-oxetane-(2R)-carbonyl]amino}pentanoic Acid
Benzyl Ester (24). To a solution of 23 (1.16 g, 2.50 mmol) in
H2O/EtOAc (1:1; 7.3 mL) were added sequentially DCC (1.24 g,
6.01 mmol), HOBT (810 mg, 6.00 mmol), and acid 7 (500 mg,
2.90 mmol) under nitrogen atmosphere at rt. After 2 h, the reaction
mixture was filtered and washed with EtOAc (5 mL). The aqueous
layer was separated, and the organic layer was washed with 1 M
aq NaOH solution (2 × 20 mL) followed by saturated aq NaHCO3
solution (2 × 20 mL). The organic layer was concentrated under
reduced pressure. The residue was purified by column chromatog-
raphy to give 24 as a solid (840 mg, 50%): [R]25 +3.0 (c 1,
D
1
CHCl3); H NMR (300 MHz) δ 0.94 (3H, t, J ) 7.5 Hz), 1.06
(3H, d, J ) 7.5 Hz), 1.18-1.35 (1H, m), 1.37 (3H, d, J ) 7.5 Hz),
1.4-1.5 (2H, m), 1.5-1.7 (2H, m), 1.8-2.0 (2H, m), 3.07-3.35
(2H, m), 3.56 (1H, dd, J ) 4.5, 7.5 Hz), 4.21-4.39 (1H, m), 4.53
(1H, d, J ) 4.5 Hz), 4.54-4.64 (1H, m), 5.08 (2H, s), 5.15 (2H, d,
J ) 8.3 Hz), 5.60 (1H, d, J ) 7.5 Hz), 6.73 (1H, brs), 6.95 (1H, d,
J ) 7.5 Hz), 7.26-7.38 (10H, m); 13C NMR (75 MHz) δ 10.9,
16.3, 18.4, 25.2, 26.6, 29.3, 33.8, 38.4, 50.5, 51.8, 62.9, 67.1, 67.4,
70.8, 128.0, 128.2, 128.4, 128.6, 128.5, 128.6, 135.1, 136.2, 156.2,
168.2, 169.1, 171.6, 172.4; IR (KBr) 3317, 3066, 2963, 2931, 1835,
1727, 1666, 1536, 1455, 1250, 1101, 908, 746, 698 cm-1; LSIMS
(FAB) m/z 582 (M + H+), 181(10), 154 (15), 136 (18), 109 (21),
91 (100), 81(40), 69 (61), 55 (80); HRMS [M + H+] calcd for
C31H40N3O8 582.2815, found 582.2818.
Acknowledgment. We are grateful to Dr. J. S. Yadav,
Director, IICT, for his constant encouragement. Financial
assistance from DST, New Delhi (Grant No.SR/SI/OC-39/2002),
is gratefully acknowledged. We thank Dr. B. Jagdeesh, NMR
division, for the useful discussions. Thanks are also due to Dr.
T. K. Chakraborthy for his support. B.M., M.P., N.J., and
M.U.K. are thankful to CSIR and UGC (New Delhi) for
awarding them fellowships.
(2S)-[(2S)-Benzyloxycarbonylaminopropionylamino]-5-{[(3S)-
propyl)-4-oxo-oxetane-(2R)-carbonyl]amino}pentanoic Acid Ben-
(14) (a) Corey, E. J.; Li, W.; Reichard. G. A. J. Am. Chem. Soc. 1998,
120, 2330-2336. (b) Among surveyed reagents used for this transformation,
only PhSO2Cl/pyridine gave the product but in poor yields.
(15) For preparation of dipeptide 23, see the Supporting Information.
(16) Synthetic analogues 3, 4, and 24, which were intended to be applied
for testing against cancer cell lines, were not deprotected; belactosin C itself
has greater activity in protected form.5
Supporting Information Available: Experimental details and
analytical data of all compounds as well as crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO0516887
340 J. Org. Chem., Vol. 71, No. 1, 2006