Preparation of a protected phosphoramidon precursor via an H-Phosphonate coupling strategy

Article abstract of DOI:10.1016/j.bmcl.2006.08.024

The preparation of a phosphoramidon precursor is described using a phosphorus(III) coupling protocol.

Full text of DOI:10.1016/j.bmcl.2006.08.024

Bioorganic & Medicinal Chemistry Letters 16 (2006) 5602–5604
Preparation of a protected phosphoramidon precursor via an
H-Phosphonate coupling strategy
Matthew G. Donahue and Jeffrey N. Johnston*
Department of Chemistry, Vanderbilt University, 7330 Stevenson Center, Station B 351822, Nashville, TN 37235, USA
Received 28 July 2006; accepted 2 August 2006
Available online 28 August 2006
Abstract—The preparation of a phosphoramidon precursor is described using a phosphorus(III) coupling protocol.
Ó 2006 Elsevier Ltd. All rights reserved.
Phosphoramidon (1) was isolated from Streptomyces
tanashiensis in 1972 by a Japanese group at Keio Uni-
versity.1 It consists of a leucyl-tryptophan dipeptide cou-
pled to a-L-rhamnose via a phosphoramidate linkage
(Fig. 1). Phosphoramidon possesses an array of biolog-
ical activity, including inhibition of the bacterial zinc
protease thermolysin,2 the mammalian plasma mem-
brane zinc peptidase,3 and a metalloprotease endothelin
converting enzyme.4 Its ability to inhibit endothelin con-
verting enzyme has rendered it a valuable tool in the
study of hypertension,5 stroke,6 and diseases of the
kidney.7,8 Phosphoramidon has been particularly scruti-
nized as a potent and specific inhibitor of a zinc metallo-
proteinase identified as an endothelin converting
endopeptidase. As a result, it has been a model for the
development of numerous novel inhibitors developed
over the last decade.9 Phosphoramidon has also been
used in the study of the upstream inhibition of endothe-
lin conversion.
coworkers in 1995 (Fig. 2),11 in which three fragments
served as key intermediates: triacylated L-rhamnose
(2), phenyl dichlorophosphate (3), and the ethyl ester
of leucyl-tryptophan (4). In their report, the three com-
ponents were linked in a one-pot, double condensation
reaction to afford the protected intermediate (11), which
was saponified to afford 1 in 7.5% yield over two steps.
Toward that end, commercially available L-rhamnose 5
was peracylated under Steglich conditions to afford 6 in
99% yield (Scheme 1).12 Chemoselective deacylation of
6 at the anomeric position gave a-anomer 2 as a white
solid in 77% yield.13 The carbobenzyloxy protected
To support studies of this type, phosphoramidon is sold
commercially. In practice however, supplies of phospho-
ramidon are subject to both high cost (ca. $45/mg), and
inconsistent availability.10 This report details our effort
to produce gram quantities of phosphoramidon, which
led ultimately to an improvement of the convergent cou-
pling of the monosaccharide, phosphate ester, and
dipeptide fragments of the natural product.
Figure 1. Phosphoramidon.
Our initial synthetic route to phosphoramidon followed
the first report of its synthesis by De Nanteuil and
Keywords: Phosphoramidon; H-Phosphonate; Synthesis.
*
Figure 2. Key fragments for the synthesis of phosphoramidon (De
Nanteuil).
Corresponding author. Tel.: +1 615 322 7376; fax: +1 615 343
1234; e-mail: jeffrey.n.johnston@vanderbilt.edu
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.08.024

M. G. Donahue, J. N. Johnston / Bioorg. Med. Chem. Lett. 16 (2006) 5602–5604
5603
Scheme 1. Unsuccessful coupling based on literature protocol.
dipeptide 10 was prepared in 98% yield via union of
commercially available N-carbobenzyloxy-L-leucine (7)
with the ethyl ester of L-tryptophan (9) (prepared in
89% yield by esterification of L-tryptophan 8 in refluxing
ethanol with two equivalents of DL-camphorsulfonic
acid) using dicyclohexylcarbodiimide (DCC) and 1-
hydroxybenzotriazole (HOBT).14 Palladium catalyzed
hydrogenolysis of the Cbz group of 10 gave dipeptide
4 in 99% yield.
electrophilic phosphorus(III) H-phosphonates, for
which several bench stable options can be found in
the literature.15 Phosphitylation of the anomeric alco-
hol of 2 with 1416 was followed by a hydrolysis pro-
tocol to afford H-phosphonate 15 in quantitative
yield (Scheme 2).17 This step required chromato-
graphic separation over silica gel to remove the sali-
cylic acid. The subsequent coupling involves four
distinct steps: (1) conversion of the tetracoordinate
P(III) H-phosphonate to the tri-coordinate P(III)
bis(silyl)phosphite, (2) oxidation of phosphorus with
molecular iodine, (3) formation of the presumed
phosphoryl pyridinium ion and (4) trapping with
dipeptide 4 to afford 16. We found that this sequence
furnished the desired aminophosphonate in 44% yield
With ample quantites of 2 and 4 in hand, the litera-
ture coupling conditions involving phenyl dichloro-
phosphate 3 failed to provide any product 11. As
the result of several attempts to effect the coupling,
we successfully isolated glycosyl chloride 12 (anomeric
proton, d 5.87 (d, J = 2 Hz)). Furthermore, we noted
the propensity for dipeptide 4 to transform slowly to
cyclic dipeptide 13, even upon standing in purified
form at low temperature. Through various modifica-
tions to the coupling protocol, we surmised that for-
mation of the glycosyl chloride was both evidence
that phosphorylation of the monosaccharide was
occurring (leading to 12), as well as an indication that
chloride displacement of the phosphate is competitive
with dipeptide coupling. Therefore, we turned to a
more effective phosphorus lynchpin for these
couplings.
after silica gel chromatography. The anomeric center
was assigned as
3
by comparison of JC(1)H-P
a
(7.5 Hz) to known a linked phosphoramidates of
rhamnose.18 The 31P NMR (carbon and proton
decoupled) showed a singlet at 3.83 ppm. As a prac-
tical note, the isolable compounds contain achiral
phosphorus, thereby simplifying the spectral data. A
total of 18.8 g of 16 was produced by this method,
and the final saponification to phosphoramidon has
been reported by De Nanteuil.11
In summary, a two step coupling protocol utilizing
salicylate phosphorus(III) reagent 14 allowed the effi-
cient, multigram scale synthesis of phosphoramidon
precursor 16. This synthesis should result in a more
consistent and inexpensive supply of this highly em-
ployed, pharmacologically important agent as a tool
in chemical biology.
To circumvent the failings of the doubly activated
phosphorus reagent, the coupling route was rede-
signed to a step-wise approach. We hypothesized that
acceleration of the dipeptide-glycosyl phosphate cou-
pling might be achieved through the use of more
Scheme 2. Phosphorus(III) coupling protocol.

5604
M. G. Donahue, J. N. Johnston / Bioorg. Med. Chem. Lett. 16 (2006) 5602–5604
6. Simonson, M. S.; Dunn, M. J. J. Lab. Clin. Med. 1992,
119, 622.
Acknowledgment
7. Kohan, D. E. Am. J. Kidney Dis. 1993, 22, 493.
8. Review: Warner, T. D.; Battistini, D.; Doherty, A. M.;
Corder, R. Biochem. Pharmacol. 1994, 48, 625.
9. (a) Lloyd, J.; Schmidt, J. B.; Hunt, J. T.; Barrish, J. C.;
Little, D. K.; Tymiak, A. A. Bioorg. Med. Chem. Lett.
1996, 6, 1323; (b) Hanessian, S.; Rogel, O. Bioorg. Med.
Chem. Lett. 1999, 9, 2441.
10. (a) http://www.agscientific.com/Item/P1101.html.; (b) This
worked commenced due to an indefinite backorder for
phosphoramidon.
We are grateful to Procter and Gamble for financial sup-
port of this work.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2006.08.024.
´
11. De Nanteuil, G.; Benoist, A.; Remond, G.; Descombes, J.-
J.; Barou, V.; Verbeuren, T. J. Tetrahedron Lett. 1995, 36,
References and notes
1435.
12. Steglich, W.; Hoefle, G. Angew. Chem., Int. Ed. Engl.
1969, 8, 981.
1. Umezawa, S.; Tatsuta, K.; Izawa, O.; Tsuchiya, T.
Tetrahedron Lett. 1972, 1, 97.
`
´
13. Fiandor, J.; Garcıa-Lopez, M. T.; De Las Heras, F. G.;
2. Suda, H.; Aoyagi, T.; Takeuchi, T.; Umezawa, H. J.
Antibiot. 1973, 26, 621.
3. Kenny, A. J. In Proteinases in Mammalian Cells and
Tissues; Barrett, A. J., Ed.; Elsevier: Amsterdam, 1977; p
393.
4. Review of the pharmacology of endothelin converting
enzymes: Turner, A. J.; Murphy, L. J. Biochem. Pharma-
col. 1996, 51, 91.
´
´
Mendez-Castrillon, P. P. Synthesis 1985, 1121.
14. Sheehan, J. C.; Hess, G. P. J. Am. Chem. Soc. 1955, 77,
1067.
15. Stawinski, J.; Kraszewski, A. Acc. Chem. Res. 2002, 35, 952.
16. Young, R. W. J. Am. Chem. Soc. 1952, 74, 1672.
17. Marugg, J. E.; Tromp, M.; Kuyl-Yeheskiely, E.; van der
Marel, G. A.; van Boom, J. H. Tetrahedron Lett. 1986, 27,
2661.
5. Matsumura, Y.; Hisaki, K.; Takaoka, M.; Morimoto, S.
Eur. J. Pharmacol. 1990, 185, 103.
18. Sun, Q.; Xiao, Q.; Ju, Y.; Zhao, Y. F. Chin. Chem. Lett.
2003, 14, 685.