New stereoselective synthesis of the peptidic aminopeptidase inhibitors bestatin, phebestin and probestin

Article abstract of DOI:10.1016/S0040-4039(03)01799-4

Peptidic aminopeptidase inhibitors, bestatin, phebestin and probestin have been prepared by stereo- and regiocontrolled reactions from a common α,β-epoxy ester precursor.

Full text of DOI:10.1016/S0040-4039(03)01799-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6999–7002
New stereoselective synthesis of the peptidic aminopeptidase
inhibitors bestatin, phebestin and probestin
Giuliana Righi,a,* Claudia D’Achille,a Giovanna Pescatorea and Carlo Boninib
aCNR Istituto di Chimica Biomolecolare—Sezione di Roma, c/o Dip. di Chimica, Universita` ‘La Sapienza’, P. le A. Moro 5,
Box 34, 00185 Roma 62, Italy
bDipartimento di Chimica, Universita` della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
Received 20 May 2003; accepted 23 July 2003
Abstract—Peptidic aminopeptidase inhibitors, bestatin, phebestin and probestin have been prepared by stereo- and regiocontrolled
reactions from a common a,b-epoxy ester precursor.
© 2003 Elsevier Ltd. All rights reserved.
A recent screening for new amino peptidase inhibitors
has allowed the isolation of some b-amino a-hydroxy
amides from Streptomyces cultures having this biologi-
cal activity.1 Structure-modification studies on these
compounds indicated that the presence of syn amino
alcohol fragments and the 2S configuration of the
a-hydroxy group are the most important factors for a
tight interaction with the enzyme.2 The current interest
in the synthesis of these bioactive compounds and the
need of having stereocontrolled structures,3 prompted
us to employ our well-known methodologies to prepare
these types of fragments.
alcohol moiety;4 however, the use of this asymmetric
procedure still suffers from some drawbacks, especially
as to the regioselectivity of the reaction. Also, the Salen
asymmetric epoxidation, followed by opening of the
epoxide by nitrogen nucleophiles, appears suitable for
this purpose.5 However, in this methodology the limita-
tions in the choice of the olefinic substrate (cis conju-
gated olefins) and the unpredictable regioselectivity in
the opening of the epoxide with nitrogen nucleophiles
still limit this approach to a small number of substrates.
By way of contrast, the older Sharpless asymmetric
epoxydation6 allows a more flexible access to a large
variety of regio- and diastereoisomers than the direct
AA and Salen-ae, despite the number of steps required
for the transformation of the starting chiral epoxy
alcohols into the final amino alcohols.
Among the methods already reported for obtaining syn
amino alcohols, the recently developed Sharpless asym-
metric amino hydroxylation of olefins is the most
straightforward route to build up the chiral b-amino
Scheme 1.
Keywords: asymmetric synthesis; a,b-epoxy esters; aminopeptidase inhibitors.
* Corresponding author.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01799-4

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G. Righi et al. / Tetrahedron Letters 44 (2003) 6999–7002
Following this last approach, our method, starting
from a suitable chiral a,b-epoxy ester, affords syn 3-
amino-2-hydroxy esters7 or syn 2-amino-3-hydroxy
esters8 with virtually all kinds of R substituents.
While the MgBr2-mediated opening of 2 (Scheme 3)
afforded, as expected, the bromohydrin 3 in nearly
quantitative yield, several attempts were necessary until
conditions were eventually found for the displacement
of bromine by azide.13 Finally, by employing NaN3 in
DMSO at 40°C,14 4 was readily obtained with complete
inversion of configuration; its catalytic hydrogenation
in the presence of di-tert-butyl carbonate15 followed by
hydrolysis afforded 1 (Scheme 3).16
Indeed, during our studies we succeeded in finding
proper conditions for effecting the stereo- and regiose-
lective ring opening of the a,b-epoxy esters by metal
halides.
As shown in Scheme 1, the substitution of the halogen
by azide (the most used precursor of an amino group)
gives vicinal azido alcohols with a syn relationship
between the stereocenters, which is not an easy goal to
reach with other approaches.
As shown in Scheme 4, from 1 it is possible to synthe-
size the three peptidic aminopeptidase inhibitors using
simple and well-known procedures. The coupling of the
benzyl ester of
deprotection by hydrogenation afforded the dipeptide
9. Finally, its coupling with the benzyl ester of -phenyl-
L
-valine with acid 1 and the subsequent
L
Some of the recently isolated b-amino a-hydroxy amide
aminopeptidase inhibitors, viz. bestatin,9 phebestin10
and probestin,11 contain a (2S,3R)-3-amino-2-hydroxy-
4-phenylbutanoic acid fragment in their structure
(Scheme 2). Thus, we deemed very convenient to
employ our method for the synthesis of these com-
pounds starting from a common optically active precur-
sor, i.e. the (2S,3R)-3-[(N-t-butoxycarbonyl)amino]-2-
hydroxy-4-phenylbutanoic acid, 1, easily obtained from
(2S,3R)-methyl-2,3-epoxy-4-phenylbutanoic acid, 2.12
alanine and the sequential removal of the protecting
groups by hydrogenation and TFA treatment afforded
the tripeptide phebestine.
In order to prepare bestatin, 1 was coupled with the
benzyl ester of L-leucine, hydrogenated to the dipeptide
10 and then deprotected with TFA. On the other hand,
the coupling of 10 with the benzyl ester of prolylpro-
line, followed by the removal of the protecting groups
as usual, furnished the tetrapeptide probestin.
Scheme 2.
Scheme 3. Reagents and conditions: (a) MgBr2, Et2O, rt, 2 h, 92%; (b) NaN3, DMSO, 40°C, 6 h, 73%; (c) H2, Pd/C, EtOAc,
(BocO)2O, rt, 5 h, 95%; (d) Na2CO3, H2O/MeOH, rt, 12 h, 79%.

G. Righi et al. / Tetrahedron Letters 44 (2003) 6999–7002
7001
Scheme 4. Reagents and conditions: (a) EDAC, HOBt, DIPEA,
MeOH, rt, 3h, 96%; (c) EDAC, HOBt, DIPEA, -Phe-OBn-TsOH, DMF, CH2Cl2, rt, 12 h, 85%; (d) H2, Pd/C, MeOH, rt, 3 h,
95%; (e) TFA, CH2Cl2, rt, 12 h, 85%; (f) EDAC, HOBt, DIPEA, -Leu-OBn-TsOH, DMF, CH2Cl2, rt, 12 h, 87%; (g) H2, Pd/C,
L-Val-OBn-TsOH, DMF, CH2Cl2, rt, 12 h, 87%; (b) H2, Pd/C,
L
L
MeOH, rt, 3 h, 95%; (h) TFA, CH2Cl2, rt, 12 h, 85%; (i) EDAC, HOBt, DIPEA, L-Pro-Pro-OBn-HCl, DMF, CH2Cl2, rt, 12 h,
86%; (l) H2, Pd/C, MeOH, rt, 3 h, 94%; (m) TFA, CH2Cl2, rt, 12 h, 85%.
The physico-chemical properties of the three peptidic
inhibitors were consistent with those reported in the
literature for the same compounds.
Catalysis; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H.,
Eds.; Springer: New York, 1999; Vol. II, p. 621.
7. (a) Righi, G.; Rumboldt, G.; Bonini, C. J. Org. Chem.
1996, 61, 3557; (b) Righi, G.; Chionne, A.; D’Achille, R.;
Bonini, C. Tetrahedron: Asymmetry 1997, 8, 903.
8. Righi, G.; Rumboldt, G.; Bonini, C. Tetrahedron 1995,
51, 13401.
9. (a) Herranz, R.; Vinuesa, S.; Castro-Pichel, J.; Pe´rez, C.
J. Chem. Soc., Perkin Trans. 1 1992, 1825; (b) Umezaca,
H.; Aoyagi, T.; Suda, H.; Hamada, M.; Takeuchi, T. J.
Antibiot. 1976, 29, 97.
In conclusion, the already successfully used highly
regio- and stereoselective sequence from trans a,b-
epoxy esters to syn b-amino-a-hydroxy esters proved to
be also a straightforward approach for the synthesis of
this type of molecules. Moreover, the generality of the
method allows the preparation of a large number of
variously substituted analogues.
10. Nagai, M.; Kojima, F.; Naganawa, H.; Aoyagi, T.;
Hamada, M.; Takeuchi, T. J. Antibiot. 1997, 50, 82.
11. Aoyagi, T.; Yoshida, S.; Nakamura, Y.; Shigihara, Y.;
Hamada, M.; Takeuchi, T. J. Antibiot. 1990, 43, 149.
12. Compound 5 was easily prepared by standard procedure
(Ref. 7a) in four high yielding steps starting from the
commercially available phenylacetaldehyde.
Acknowledgements
This work was partially supported by FIRB-Pro-
gramma Strategico Post Genoma, Obiettivo3,
Cod.RBNE017F8N.
13. Several reaction conditions failed: (a) NaN3 in DMF; (b)
NaN3/R4PBr in H2O; (c) TMSN3/Bu4NF in DMF. In the
case (a) the reaction was not stereoselective and in the
cases (b) and (c) the starting material 3 remained
unchanged when the reaction temperature was :60°C
and decomposed when it was heated to 80–90°C.
14. Evans, D. A.; Britton, T. C.; Ellman, J. A.; Dorow, R. L.
J. Am. Chem. Soc. 1990, 112, 4011.
References
1. (a) Umezawa, K.; Ikeda, Y.; Uchihata, Y.; Naganawa,
H.; Kondo, S. J. Org. Chem. 2000, 65, 459; (b) Com-
pound 5: Umezawa, K.; Nakazawa, K.i; Ikeda, Y.;
Naganawa, H.; Kondo, S. J. Org. Chem. 1999, 64, 3034–
3038.
2. Harbeson, S. L.; Rich, D. H. Biochemistry 1988, 27, 7301.
3. Review: Bergmeier, S. C. Tetrahedron 2000, 56, 2561.
4. (a) Kolb, H. C.; Sharpless, K. B. In Transition Metals for
Organic Synthesis; Beller, M.; Bolm, C., Eds.; Wiley-
VCH: New York, 1998; p. 243; (b) O’Brien, P. Angew.
Chem., Int. Ed. Engl. 1999, 38, 326.
15. Saito, S.; Nakajima, H.; Inaba, M.; Moriwate, T. Tetra-
hedron Lett. 1989, 30, 1969.
16. NMR data for most significant new compounds. Com-
pound 3: 1H NMR: l 7.40–7.22 (m, 5H), 4.54–4.43 (m,
2H), 3.78 (s, 3H), 3.56 (d, J=6.7 Hz, 1H), 3.38 (dd,
J=6.7, 13.9 Hz, 1H), 3.27 (dd, J=8, 13.9 Hz, 1H). 13C
NMR: l 171.4, 137.3, 129.1, 128.5, 126.9, 73.3, 56.0, 52.7,
5. See for example: Deng, L.; Jacobsen, E. N. J. Org. Chem.
1
40.3. Compound 4: H NMR: l 7.41–7.24 (m, 5H), 4.15
1992, 57, 4320.
(dd, J=2.2, 5.8 Hz, 1H), 3.80 (s, 3H), 3.72 (dt, J=2.2, 8
Hz, 1H), 3.14 (d, J=7.3 Hz, 2H), 3.04 (d, J=5.8 Hz,
1H). 13C NMR: l 173.1, 136.5, 129.3, 128.8, 127.1, 71.3,
6. For most recent reviews covering the different aspects of
the AE, see: (a) Katsuki, T.; Martin, V. S. Org. React.
1996, 1; (b) Katsuki., T. In Comprehensive Asymmetric

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G. Righi et al. / Tetrahedron Letters 44 (2003) 6999–7002
1
1
38.1, 28.1. Compound 1: H NMR: l 7.32 (m, 5H), 6.5
(bs, 1H), 5.05 (bd, J=8.8 Hz, 1H), 4.35–4.05 (m, 2H),
4.15 (d, J=6.6 Hz, 1H), 2.95 (d, J=7.3 Hz, 2H), 1.37 (s,
9H). 13C NMR: l 157.1, 156.1, 137.8, 129.4, 128.5, 126.5,
81.5, 70.4, 56.1, 54.4, 38.2, 28.1.
64.1, 53.0, 36.1. Compound 5: H NMR: l 7.36 (m, 5H),
4.81 (bd, J=9.5 Hz, 1H), 4.32–4.18 (m, 1H), 4.05 (d,
J=4.4 Hz, 1H), 3.75 (s, 3H), 3.15 (d, J=4.4 Hz, 1H),
2.95–2.78 (m, 2H), 1.35 (s, 9H). 13C NMR: l 172.4,
155.1, 143.8, 129.5, 128.5, 126.5, 79.6, 70.2, 54.2, 52.7,