Total synthesis of the multidrug-resistance reversing agent hapalosin

Article abstract of DOI:10.1055/s-1999-2765

Hapalosin, a new reversing MDR agent, has been synthesized by macrolactamisation of a linear precursor 2 derived from the coupling of a β-hydroxy γ-aminoacid 3 and two hydroxy-esters 4 and 5. The aminoacid 3 was stereoselectively obtained by opening of an anti β-aminoepoxide while the 3-hydroxyacid 4 was prepared by condensation of a lithium compound with (R)-2-phenyl-propanal followed by oxidation of the phenyl group.

Full text of DOI:10.1055/s-1999-2765

1118
LETTER
Total Synthesis of the Multidrug-Resistance Reversing Agent Hapalosin
Mansour Haddad*, Candice Botuha, Marc Larchevêque*
Laboratoire de Synthèse Organique associé au CNRS, ENSCP, 11 rue Pierre et Marie Curie, 75231 - Paris Cedex 05, France
Fax +33 1 43 25 79 75; E-mail : larchm@ext.jussieu.fr
Received 8 April 1999
Abstract: Hapalosin, a new reversing MDR agent, has been synthe-
sized by macrolactamisation of a linear precursor 2 derived from the
coupling of a b-hydroxy g-aminoacid 3 and two hydroxy-esters 4
and 5. The aminoacid 3 was stereoselectively obtained by opening
of an anti b-aminoepoxide while the 3-hydroxyacid 4 was prepared
by condensation of a lithium compound with (R)-2-phenyl-propanal
followed by oxidation of the phenyl group.
Key words: multidrug resistance, epoxyamine, styrene oxide,
macrolactamisation
Many cytotoxic drugs are quite effective during initial
stage of cancer chemotherapy. Over time, however, the
tumor cells that survive the treatment often become resis-
tant not only to the particular drugs employed but also to
a broad spectrum of seemingly unrelated drugs. This phe-
nomenon is known as multidrug resistance, or MDR, and
one of the principal mechanisms is the expulsion of a wide
spectrum of drugs from tumor cells by P-glycoprotein (P-
gp).1
Recently, Moore and co-workers have isolated hapalosin
1 a new cyclic depsipeptide from the blue-green alga
Hapalosiphon welwitschii which showed better MDR-re-
Scheme 1
versing activity than verapamil which is currently under
active investigation in the clinic for MDR reversal.2
Hapalosin exists as an inseparable mixture of two con-
formers around the amide function, the major one pos-
sessing the s-cis stereochemistry. Although the respective
activities of both conformers have not been clarified, it
seems that the s-cis conformation is a crucial factor for the
MDR reversing activity.3
benzyl and allyl amine were used until now. The epoxyke-
tone 6 was then prepared in optically pure form by con-
densation at low temperature of a Grignard compound
with ethyl glycidate.7 Reduction in acidic medium of 6
with tetramethylammonium triacetoxyborohydride was
achieved in the presence of methylamine and we were
pleased to isolate the required anti aminoepoxide 7 as a
single diastereomer. To introduce the acidic function, we
submitted this epoxide to the action of different nucleo-
philic agents such as cyanide anion or vinylic cuprates.
However, we were unable to obtain the corresponding
aminoalcohols. In fact, the epoxide 7 was relatively unsta-
ble and it was necessary to protect it as a t-butoxycarbonyl
group before opening. The resulting epoxide 8 was then
reacted with divinylmagnesiocuprate in THF at -30 °C to
afford the unsaturated aminoalcohol 9 in excellent yield.
In order to study the structure-activity relationship, we
wanted to develop a flexible stereocontrolled route to
hapalosin which would be also applicable to the synthesis
of analogs.
The retrosynthetic analysis indicated that hapalosin might
be constructed by cyclisation of the compound 2 resulting
from the coupling of three sub-units: a b-hydroxy g-ami-
noacid 3, a b-hydroxyester 4 and the known (S)-2-hydrox-
yisovaleric acid 5 (Scheme 1).
In all the previous synthesis of hapalosin, the synthon 3
was prepared from phenylalanine.4 In order to develop a
more general access to these hydroxyaminoacids, we de-
cided to use a new route (Scheme 2).5
The latter was protected in two different ways with MOM
chloride and t-butyldimethylsilyl chloride. After cleavage
of the double bond with ruthenium trichloride and excess
of sodium periodate,8 the acids 10 and 11 were obtained in
80 and 67% yield respectively.
We have previously shown that the reductive amination of
epoxyketones leads almost exclusively to the formation of
the corresponding anti aminoepoxides.6 However, only
Synlett 1999, No. 07, 1118–1120 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Total Synthesis of the Multidrug-Resistance Reversing Agent Hapalosin
1119
With this two synthons in hand, we were able to achieve
the synthesis of hapalosin. A first route was explored us-
ing the MOM protected acid 10 (Scheme 4). This com-
pound was esterified by reaction with the hydroxyester 4
using 2,4,6-trichlorobenzoyl chloride in the presence of
triethylamine13 to afford the benzyl ester 17 in 75% yield.
After removal of the benzyl protection by hydrogenolysis
with Pearlman’s catalyst, the resulting acid was condensed
with benzyl (S)-2-hydroxy-3-methylbutanoate obtained in
two straightforward steps from L-valine14 using again the
Yamaguchi’s conditions.
Scheme 2 (a) Me4NBH(OAc)3 (3 eq), MeNH2 (4eq), CH3COOH (4
eq), 4 Å molecular sieves, CH2Cl2, 0 °C, 15 h, 70%. (b) Boc2O (1.2
eq), 3.75 M NaOH (5 eq), THF, 20 °C, 2h, 75%. (c) (Vinyl)2CuMgBr
(3 eq), THF, -30 °C, 6h, 88%. (d) MOMCl or t-BuMe2SiCl, iPr2NEt,
CH2Cl2, 80% for 10 and 78% for 11. (e) NaIO4 (5 eq), RuCl3 (2%),
CCl4-CH3CN- H2O (2/2/3), 20 °C, 4h, 80% for 10 and 67% for 11.
The synthesis of the second fragment, hydroxyester 4, is
illustrated in Scheme 3. A phenyl group was used as pre-
cursor of the acid function. Some years ago, it was report-
ed that the condensation of organolithium compounds
with 2-phenylpropionaldehyde in the presence of crown
ether afforded the syn diastereomers almost exclusively
according to a Cram selectivity.9 Following the same pro-
cedure, n-heptyllithium was reacted with optically pure
aldehyde 12 prepared by regioselective opening of styrene
oxide by methyl copper compound followed by oxidation
of the resulting primary alcohol.10 The corresponding al-
cohol 13 was obtained in a nearly pure syn stereochemis-
try (syn/anti 95/5) and in high enantiomerical purity.11
After purification by chromatography on silica gel, this al-
cohol was protected under its acetate form and oxidized
into acid 15 with sodium periodate in the presence of a
catalytic amount of ruthenium trichloride. Treatment of
15 by hydrobromic acid in ethanol allowed to effect the
removal of the acetate group and the esterification of the
acid function at the same time. The resulting hydroxyester
16 was then transformed into the more usable benzylester
4 by transesterification in the presence of titanium isopro-
pylate.12
Scheme 4 (a) 4 (1.1 eq), 2,4,6-trichlorobenzoyl chloride (1.1 eq),
Et3N (1.2 eq), cat. DMAP, 20 °C, 2h, 75% (for 16), 72% (for 17). (b)
H2, cat Pd(OH)2, Abs. EtOH, 20 °C, 2h, 98%. (c) (S)-(+)-2-hydroxy-
3-methylbutanoate benzyl ester (1.2 eq), 2,4,6-trichlorobenzoyl chlo-
ride (1.2 eq), Et3N (1.2 eq), cat. DMAP, 20 °C, 6h, 82%. (e) TFA,
CH2Cl2, 20 °C, 2h, 87%.
The resulting compound 19 was successively reacted with
trifluoroacetic acid for removing the Boc protection and
hydrogenated over palladium hydroxyde in order to pre-
pare the intermediate 20. Although the mass spectrum ob-
tained for this compound was correct, its NMR spectra
(1H and 13C) were fairly different from those given for the
same intermediate.4b Moreover, attempts to remove the
MOM protection before cyclisation by using a variety of
reagents have so far been unsuccessful.
We decided then to use a more labile protective group for
the protection of the aminoacid 3 in order to achieve the
cyclisation on the fully deprotected secoacid 2. For this
purpose, the ester 18 was prepared according a similar
method by reaction of the acid 11 with the hydroxyester 4.
After catalytic removal of the benzyl protection, the re-
sulting acid was reacted with allyl (S)-2-hydroxy-3-
methylbutanoate3b using the Yamaguchi’s conditions to
give the allyl ester 21 in 78% yield (Scheme 5).15
This intermediate 21 was in full accordance with that de-
scribed by Yamamura and al.3b The next steps were con-
ducted following the described procedure leading to the
free seco acid 2. This latter was cyclized under high dilu-
tion using diphenylphosphoryl azide to give after purifica-
tion hapalosine 1 in 28% isolated yield as a 3/1 mixture of
Scheme 3 (a) C7H15Li (2 eq), 15-crown-5 (2 eq), ether, -78 °C, 2h,
60%. (b) Ac2O, Et3N, cat. DMAP, CH2Cl2, 20 °C, 2h, 90%. (c) NaIO4
(15 eq), RuCl3 (2%), CCl4-CH3CN- H2O (2/2/3), 20 °C, 24h. (d) 30%
HBr in acetic ac.(4 eq), EtOH, reflux, 24h, 67% from 14. (e) BnOH
(5 ml), Ti(Oi-Pr)4, (0.4 eq), 120 °C, 12h, 63%.
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M. Haddad et al.
LETTER
(7) Pégorier, L.; Petit, Y.; Mambu, A.; Larchevêque, M. Synthesis
1994, 1403.
(8) Carlsen, P.J.; Katsuki, T.; Martin, V.; Sharpless, B.K. J. Org.
Chem. 1981, 46, 3936.
(9) Yamamoto, Y.; Maruyama, K. J. Am. Chem. Soc. 1985, 107,
6411.
(10) Botuha, C.; Haddad, M.; Larchevêque, M. Tetrahedron :
Asymmetry 1998, 9, 1929.
(11) The enantiomeric purity of the alcohol 13 was checked by
HPLC using a ChiralPak AD (Daicel) with hexane-iPrOH (99/
1) and was found better than 98%.
(12) Seebach, D.; Hungerbühler, E.; Naef, R.; Schnurrenberger, P.;
Weidmann, B.; Züger, M. Synthesis 1982, 138.
(13) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989.
Scheme 5 (a) H2, Pd(OH)2, abs. EtOH, 2h, 93%. (b) (S)-2-hydroxy-
3-methylbutanoate allyl ester (1.2 eq), 2,4,6-trichlorobenzoyl chloride
(1.2 eq), Et3N (1.2 eq), cat. DMAP, 20 °C, 6h, 78%. (c) HF-pyridine,
20 °C, 3h, 75%. (d) cat. (Ph3P)4Pd, morpholine/THF, 20 °C, 2h, 92%.
(e) TFA, CH2Cl2, 20 °C, 2h, 87%. (f) DPPA(1.5 eq), i-Pr2NEt (5 eq),
DMF (2mmol/l), 0 °C (2h) then 20 °C (48h), 28%.
(14) (S)-2-hydroxy-3-methylbutanoic acid was prepared by diazo-
tation of L valine (Mori, K. Tetrahedron 1976, 32, 1101)
followed by reaction with benzylbromide and NaHCO3 in
DMF according a procedure described in ref 3b for the allyl
ester (Yield:80%).
conformers in agreement with the results previously re-
ported.16
(15) [a]20D -25.2 (c = 0.98, CHCl3); 1H NMR (250 MHz, CDCl3)
dppm 0.09-0.18 (m, 6H), 0.86-1.75 (m, 24H), 2.12-2.88 (m,
8H), 3.10-3.24 (m, 1H), 4.02-4.20 (m, 1H), 4.28-4.48 (m, 1H),
4.62 (d, J :5.6 Hz, 1H), 4.81 (d, J :4.4 Hz, 1H), 5.09-5.20 (m,
3H), 5.31 (dd, J :1.6 and 16.9 Hz, 1H), 5.78-5.98 (m, 1H),
7.11-7.33 (m, 5H).
In summary, using two synthons prepared by opening of
chiral epoxides, we have achieved an efficient and flexi-
ble synthesis of hapalosin easily applicable to other ana-
logs.
13C NMR (CDCl3) dppm -3.9, 12.8, 13.1, 14.0, 17.1, 18.7,
22.5, 25.3, 25.9, 28.1, 29.0, 29.3, 29.9, 31.7, 34.5, 39.9, 42.8,
43.2, 65.5, 70.1, 70.3, 74.5, 76.7, 79.0, 79.6, 118.6, 125.9,
126.0, 128.0, 128.3, 128.9, 131.5, 139.1, 155.3, 155.7, 168.8,
170.8, 173.3. MS (EI) m/z 570 (M-191, 43), 352 M-409, 44),
207 M-554, 100).
References and Notes:
(1) For reviews, see : (a) Simon, S.M.; Schindler, M. Proc. Natl.
Acad. Sci. USA 1994, 91, 3497. (b) Kavallaris, M. Anti-
Cancer Drugs 1997, 8, 17. (c) Robert, J. Drugs of the Future
1997, 22, 149.
(2) Stratmann, K.; Burgoyne, D.L.; Moore, R.E.; Patterson,
G.M.L.; Smith, C.D. J. Org. Chem. 1994, 59, 7219.
(3) (a) Dinh, T.Q.; Du X.; Armstrong, R.W. J. Org. Chem. 1996,
61, 6606. (b) Okuno, T.; Ohmori, K.; Nishiyama, S.;
Yamamura, S.; Nakamura, K.; Houk, K.N.; Okamoto, K.
Tetrahedron 1996, 52, 14723.
(4) (a) Dinh, Q.; Armstrong, R.W. J. Org. Chem. 1995, 60, 8118.
(b) Ghosh, A.K.; Liu, W.; Xu, Y.; Chen, Z. Angew. Chem. Int.
Ed. Engl. 1996, 35, 74. (c) Ohmori, K.; Okuno, T.; Nishiyama,
S.; Yamamura, S. Tetrahedron Lett. 1996, 37, 3467. (d)
Wagner, B.; Beugelmans, R.; Zhu, J. Tetrahedron Lett. 1996,
37, 6557.
(5) For the synthesis of analogs, see: Dinh, T.Q.; Smith, C.D.;
Armstrong, R.W. J. Org. Chem. 1997, 62, 790; Dinh, T.Q.;
Du, X.; Smith, C.D.; Armstrong, R.W. J. Org. Chem. 1997,
62, 6713; Dinh, T.Q.; Smith, C.D.; Du, X.; Armstrong, R.W.
J. Med. Chem. 1998, 41, 981.
(16) Hapalosin : [a]20D-42.5 (c = 0.65, CH2Cl2); 1H NMR (400
MHz, CDCl3) dppm (major conformer) 0.23 (d, J :6.6 Hz,
1H), 0.55 (d, J :6.9 Hz, 1H), 0.89 (m, 3H), 1.10-1.40 (m, 13H),
1.45-2.05 (m, 3H), 2.61 (m, 1H), 2.65 (m, 1H), 2.75-2.88, (m,
3H), 2.92 (dd, J :5.0 and 17.8 Hz, 1H), 3.22 (m, 3H), 3.40 (dd,
J :2.6 and 13.9 Hz, 1H), 3.69 (dt, J :2.7 and 11.0 Hz, 1H), 3.85
(m, 1H), 4.31 (d, J :8.4 Hz, 1H), 5.09-5.19 (m, 1H), 7.17-7.34
(m, 5H).
13C NMR (CDCl3) dppm 12.1, 14.1, 17.6, 18.3, 22.6, 26.0,
28.1, 28.8, 29.1, 29.2, 31.7, 36.4, 37.1, 40.7, 61.4, 70.2, 73.8,
76.5, 128.9, 129.7, 137.4, 168.5, 168.7, 172.7; MS (EI) m/z
490 (M+H+, 10), 398 (100), 298 (9), 267 (27), 217 (19), 167
91).
Article Identifier:
1437-2096,E;1999,0,07,1118,1120,ftx,en;L05099ST.pdf
(6) Pégorier, L.; Petit, Y.; Larchevêque, M. J. Chem. Soc., Chem.
Commun. 1994, 633.
Synlett 1999, No. 07, 1118–1120 ISSN 0936-5214 © Thieme Stuttgart · New York