Reduction of azides to primary amines in substrates bearing labile ester functionality. Synthesis of a PEG-solubilized, "Y"-shaped iminodiacetic acid reagent for preparation of folate-tethered drugs

Article abstract of DOI:10.1021/ol9905248

(equation presented) Anhydride 3 is a useful reagent for the synthesis of triply linked drug conjugates. Examples using paclitaxel are provided. Conversion of the azido moiety to a primary amine in the presence of substrates bearing labile ester functionality requires the use of a tin/mercaptan reducing system which includes methanol exchange equilibration to effect nitrogen-tin bond scission.

Full text of DOI:10.1021/ol9905248

ORGANIC
LETTERS
1999
Vol. 1, No. 2
179-181
Reduction of Azides to Primary Amines
in Substrates Bearing Labile Ester
Functionality. Synthesis of a
PEG-Solubilized, “Y”-Shaped
Iminodiacetic Acid Reagent for
Preparation of Folate-Tethered Drugs1
Jae Wook Lee and Philip L. Fuchs*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
pfuchs@chem.purdue.edu
Received March 16, 1999
ABSTRACT
Anhydride 3 is a useful reagent for the synthesis of triply linked drug conjugates. Examples using paclitaxel are provided. Conversion of the
azido moiety to a primary amine in the presence of substrates bearing labile ester functionality requires the use of a tin/mercaptan reducing
system which includes methanol exchange equilibration to effect nitrogen−tin bond scission.
In conjunction with our program to utilize the folate receptor
as a cell-specific mediator for the delivery of various
oncolytic agents,1 we required a general intermediate capable
of accommodating covalent attachment of folic acid, an
anticancer agent, and a polycarboxylic acid to increase water
solubility (Figure 1, arrows a-c). In addition, we wished to
exploit our synthesis of folates which employs the readily
prepared pteroyl azide as a chemospecific reagent for “late”
construction of the (typically insoluble) folate function via
nitrogen acylation of the glutamate moiety (Figure 1, arrow
d).1b Our initial target was iminodiacetic acid (IDA) anhy-
dride 3 which enables sequential monoacylation of selected
nucleophiles and still bears the azido moiety which serves
as a latent amine. We selected 3 since the termini-differenti-
ated tetraethylene glycol amino azide 4 can be inexpensively
and easily prepared by the excellent method of Schwabacher.2
Reaction of the Schwabacher amino azide 42 with 2 equiv
of allyl bromoacetate in the presence of potassium carbonate
in acetonitrile for 30 h at 25 °C gave dialkylated product 53
in 97% yield which was deprotected4 by phenylsilane in the
(1) Folate-Mediated Multiple Drug Delivery 3. For papers 1 and 2, see:
(a) Wang, S.; Luo, J.; Lantrip, D. A.; Waters, D. J.; Mathias, C. J.; Green,
M. A.; Fuchs, P. L.; Low, P. S. Bioconjugate Chem. 1997, 8, 673. (b) Luo,
J.; Smith, M. D.; Lantrip, D. A.; Wang, S.; Fuchs, P. L. J. Am. Chem. Soc.
1997, 119, 10004.
(2) Schwabacher, A. W.; Lane, J. W.; Schiesher, M. W.; Leigh, K. M.;
Johnson, C. W. J. Org. Chem. 1998, 63, 1727. We find that the use of 5%
HCl in place of phosphoric acid improves the yield of 4 to 90%. For an
earlier preparation of 4, see: Bertozzi, C. R.; Bednarski, M. D. J. Org.
Chem. 1991, 56, 4326-4329.
Figure 1.
10.1021/ol9905248 CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/04/1999

presence of 5% Pd(PPh3)4 to afford iminodiacetic acid 6 in
92% yield. Iminodiacetic acid 6 was initially treated with
1.0 equiv of DCC at room temperature to generate anhydride
3. In a separate sequence, amino azide 4 was acylated with
the readily available bis-protected glutamate 71b in methylene
chloride with 1.1 equiv of DCC and a catalytic amount of
DMAP for 12 h at 25 °C to provide azide 8 in 94% yield
(cf. 47% yield in THF). Reaction of 8 with triphenylphos-
phine2,5 in THF and H2O gave the desired primary amine 9
in 98% yield. Addition of amine 9 to the preformed
anhydride 3 and stirring for an additional 12 h at 25 °C
smoothly provided glutamate 10 (Scheme 1).
Hu¨nig’s base at 25 °C in methylene chloride. Similar reaction
in THF only afforded 12 in 58% yield. Reaction of 2′-alloc
paclitaxel 12 with carboxylic acid 10 in the presence of DCC
and a catalytic amount of DMAP in methylene chloride for
12 h at 25 °C provided Y-shaped intermediate 13 in 93%
yield after column chromatography (Scheme 2).
Scheme 2
Scheme 1
Initially, we tried to reduce azide 13 to amine 15 using
the triphenylphosphine method, but we could not isolate the
pure compound. It was found that the phosphine imine
intermediate formed satisfactorily, but hydrolysis was very
slow and amine 14 was consumed at a rate competitive with
its formation over the course of 48 h. Products arising from
transamination and ester hydrolysis were observed. At this
stage, a variety of azide reduction methods7-10 (NaHTe,
LiMe2NBH3, tin hydrides, and TMS-Cl/NaI) were surveyed
with essentially no success. To preserve our supply of 13,
we returned to azide 8 as a model compound. The TMSCl/
NaI method was ineffective, and HSCH2CH2CH2SH/Et3N/
MeOH underwent reaction with the ester functionality. The
first encouraging reaction utilized SnCl2/PhSH/Et3N,11 which
quickly and completely consumed the starting material and
gave product 9 without affecting the ester group (Scheme
1).
Application of this method also rapidly reduced compound
10 to amine 14 but revealed the problem of effective product
isolation. The recommended protocol11 employs a strongly
basic workup procedure (2 N NaOH) to destroy the tin
complex, but is not applicable to our substrates because of
the labile ester functionality. The workup procedure was
modified, utilizing an exchange method to transfer the tin
from nitrogen to the oxygen of the methanol solvent.9a After
the reaction of compound 10 with SnCl2/PhSH/Et3N in
acetonitrile, a weakly basic workup (saturated NaHCO3
solution) served to neutralize the ammonium salt. The
We needed to protect the C-2′ position of paclitaxel 11 to
selectively functionalize the C-7 site.6 Since we were
employing allyl esters on the glutamate moiety, we elected
to also protect the C-2′ position as an allyloxycarbonate to
provide for eventual simultaneous deblocking of all three
centers. 2′-Alloc paclitaxel 12 was obtained in 98% yield
from reaction of paclitaxel 11 with allyl chloroformate and
(3) All new compounds have been characterized by proton NMR, carbon
NMR, exact mass, and/or elemental analysis.
(4) Dessolin, M.; Guillerez, M.-G.; Thieriet, N.; Guibe, F.; Loffet, A.
Tetrahedron Lett. 1995, 36, 5741.
(7) Alvarez, S. G.; Fisher, G. B.; Singaram, B. Tetrahedron Lett. 1995,
36, 2567.
(8) Suzuki, H.; Kawaguchi, T.; Takaoka, K. Bull. Chem. Soc. Jpn. 1986,
59, 665.
(5) Frisch, B.; Boeckler, C.; Schuber, F. Bioconjugate Chem. 1996, 7,
180. (b) Wentworth, P., Jr.; Vandersteen, A. M.; Janda, K. D. Chem.
Commun. 1997, 759. (c) Urpi, F.; Vilarrasa, J. Tetrahedron Lett. 1986, 27,
4623. (d) Schoetzau, T.; Holletz, T.; Cech, D. Chem. Commun. 1996, 387.
(6) Greenwald, R. B.; Gilbert, C. W.; Pendri, A.; Conover, C. D.; Xia,
J.; Martinez, A. J. Med. Chem. 1996, 39, 424.
(9) Hays, D. S.; Fu, G. C. J. Org. Chem. 1998, 63, 2796. (b) Samano,
M. C.; Robins, M. J. Tetrahedron Lett. 1991, 32, 6293.
(10) Kamal, A.; Rao, N. V.; Laxman, E. Tetrahedron Lett. 1997, 38,
6945.
(11) Bartra, M.; Romea, P.; Urpi, F.; Vilarrasa, J. Tetrahedron 1990,
46, 587.
180
Org. Lett., Vol. 1, No. 2, 1999

methylene chloride extract was evaporated followed by
stirring the residue for 1 h at 25 °C in MeOH. Chromatog-
raphy then provided model amine 14 in 75% yield. Repeating
the protocol with paclitaxel-linked substrate 13 provided
primary amine 15 in 96% yield with the methanol treatment.
Addition of Boc anhydride at this stage of the methanol
treatment afforded urethane 16 in 98% yield (Scheme 2).
Two additional paclitaxel derivatives were prepared from
reaction of the Schwabacher aminoazide 4 with 1.5 equiv
of anhydrides 17 and 18 in CH2Cl2 for 26 h at 25 °C followed
by additon of water and stirring for 10 h to consume the
excess anhydride to afford the azidocarboxylic acids 19 and
20 in >98% yields. Acylation of paclitaxel derivative 12
using DCC and DMAP (catalytic) in CH2Cl2 for 12 h at 25
°C afforded key intermediates 21 and 22 in 92-94% yields.
Application of the azide reduction methodology to these
substrates smoothly provided primary amines 23 and 24 in
>90% yield (Scheme 3).
solution was stirred for several hours to destroy the excess
anhydride. The resulting solution was lyopholized, and the
residue was dissolved in CH2Cl2 and then filtered. The filtrate
was concentrated and recrystallized from CH2Cl2/EtOAc/
ether to provide a 76% yield of the tris-allyl-protected
tetraacid (not shown) which was subjected to treatment with
phenylsilane and palladium [0]4 to afford the desired triply
deprotected amino pentaacid 26 in 96% yield (Scheme 4).
Scheme 4
Scheme 3
The addition of the pteroyl unit to 26 to produce the target
drug 1, as well as pharmacological testing data, will be
presented elsewhere.
Acknowledgment. We acknowledge Arlene Rothwell and
Karl Wood for mass spectral data and Endocyte, Inc., for
financial support of this project. Dr. Douglas Lantrip is
thanked for assistance in manuscript preparation.
Installation of the polyacid functionality was accomplished
by treatment of 15 with 7 equiv of DTPA dianhydride 25 in
DMSO for 3 h at 25 °C. Water was added, and the reaction
OL9905248
Org. Lett., Vol. 1, No. 2, 1999
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