An efficient RCM-based synthesis of orthogonally protected meso-DAP and FK565

Article abstract of DOI:10.1021/jo0485738

A condensation-ring-close-ring-open sequence was employed for the synthesis of orthogonally protected meso-2,6-diaminopimelic acid, starting from easily accessible chiral synthons. Condensation of suitably protected L-allyl-glycine and D-vinylglycinol der

Full text of DOI:10.1021/jo0485738

thesis of DAP, the goal of orthogonal protection is
generally considered to be equally important. Recently,
a number of reports have appeared describing the ste-
reospecific synthesis of differentially protected DAP
derivatives.4,5 Most of these syntheses involve setting one
of the two chiral centers by stereoselective susbtrate-
directed reactions or the use of chiral reagents.
An Efficient RCM-Based Synthesis of
Orthogonally Protected meso-DAP and
FK565†
Juan R. Del Valle* and Murray Goodman‡
Department of Chemistry and Biochemistry, University of
California, San Diego, La Jolla, California 93027
The structure of DAP suggests that ring-closing
metathesis (RCM) can be an effective tool for construction
of the seven-membered carbon chain.6 Toward this end,
we envisioned the condensation of two olefin building
blocks, providing a suitable nine-carbon RCM substrate.
Following ring closure, hydrolysis of the cyclic intermedi-
ate at the original point of attachment would then
provide a linear carbon-bridged product. Here, we present
an efficient preparation of orthogonally protected meso-
DAP, and of the potent immunostimulatory DAP-
containing peptide FK565, based on this condensation-
ring-close-ring-open sequence.
In planning our synthesis, D-vinylglycine and L-allyl-
glycine (1) were initially selected as appropriate chiral
synthons, but the known tendency for vinylglycine to
isomerize5e prompted us to turn to D-vinylglycinol (2) as
an alternative four-carbon building block. Vinylglycinol
is also advantageous in providing an additional site for
condensation, via the hydroxyl group. Thus, two viable
retrosynthetic routes are depicted in Scheme 1. The cyclic
olefins represent key lactone and lactam intermediates
resulting from the RCM of linear ester and amide
precursors, respectively.
Our synthesis of the desired ester RCM substrate, from
Boc-D-vinylglycinol7 and Cbz-L-allylglycine (3), is depicted
in Scheme 2. Condensation of these building blocks
proceeded in good yield under Mitsunobu conditions to
give compound 4.
With the diene ester in hand, we turned our attention
to the corresponding amide intermediates for the lactam-
based strategy (Scheme 3). Although the lactam and
lactone routes are conceptually similar, the introduction
of a Boc group onto the amide nitrogen would be neces-
sary to allow for eventual hydrolysis.8 To avoid concomi-
tant protection of the carbamate nitrogen during this
jdvalle@ucsd.edu
Received August 16, 2004
Abstract: A condensation-ring-close-ring-open sequence
was employed for the synthesis of orthogonally protected
meso-2,6-diaminopimelic acid, starting from easily accessible
chiral synthons. Condensation of suitably protected L-allyl-
glycine and D-vinylglycinol derivatives was followed by
Grubbs’ ring-closing metathesis to generate the key lactam
intermediate. This strategy has been applied to a concise
total synthesis of the potent immunostimulatory peptide
FK565.
Peptidoglycan is a key structural component in the cell
walls of most pathogenic bacteria.1 The structural integ-
rity of the rigid peptidoglycan layer is highly dependent
on meso-2,6-diaminopimelic acid (meso-DAP), which acts
as a cross-linking agent between glycan strands.2 Pre-
sumably, therapeutics that inhibit the diaminopimelate
pathway will serve to disrupt the biopolymerization
process necessary for peptidoglycan formation. Because
mammals do not produce DAP, such inhibitors are not
likely to exhibit toxicity in humans, making them at-
tractive drug targets in the search for novel antibacterial
agents.2 In addition, a number of peptidoglycan frag-
ments featuring the DAP residue exhibit antitumor,
immunostimulant, and sleep-inducing biological activity.3
DAP is thus a versatile building block with a number of
potential medicinal applications.
To obtain DAP derivatives of practical synthetic value,
a preparative strategy should allow for functional group
differentiation and be amenable to analogue synthesis.
Although stereospecificity is a key objective in the syn-
† Dedicated to the memory of Professor Murray Goodman, for a
lifetime of achievements in bioorganic chemistry.
(4) For examples of DAP derivative syntheses based on asymmetric
alkylations, see: (a) Williams, R. M.; Yuan, C. G. J. Org. Chem. 1992,
57, 6519-6527. (b) Williams, R. M.; Yuan, C. G. J. Org. Chem. 1994,
59, 6190-6193. (c) Jurgens, A. R. Tetrahedron Lett. 1992, 33, 4727-
4730. (d) Paradisi, F.; Porzi, G.; Sandri, S. Tetrahedron: Asymmetry
2001, 12, 3319-3324.
(5) For examples of DAP derivative syntheses based on chiral
catalysis, see: (a) Wang, W.; Xiong, C. Y.; Yang, J. Q.; Hruby, V. J.
Synthesis (Stuttgart) 2002, 94-98. (b) Collier, P. N.; Campbell, A. D.;
Patel, I.; Taylor, R. J. K. Tetrahedron 2002, 58, 6117-6125. (c) Collier,
P. N.; Patel, I.; Taylor, R. J. K. Tetrahedron Lett. 2001, 42, 5953-
5954. (d) Sutherland, A.; Vederas, J. C. Chem. Commun. 2002, 224-
225. (e) Gao, Y.; Lane-Bell, P.; Vederas, J. C. J. Org. Chem. 1998, 63,
2133-2143. (f) Davis, F. A.; Srirajan, V. J. Org. Chem. 2000, 65, 3248-
3251. (g) Hernandez, N.; Martin, V. S. J. Org. Chem. 2001, 66, 4934-
4938. (h) Roberts, J. L.; Chan, C. Tetrahedron Lett. 2002, 43, 7679-
7682.
‡ Deceased June 1, 2004.
(1) (a) In Bacterial Cell Wall; Ghuysen, J.-M., Hakenbeck, R., Eds.;
Elsevier Science BV: Amsterdam, 1994. (b) Lazar, K.; Walker, S. Curr.
Opin. Chem. Biol. 2002, 6, 786-793. (c) Katz, A. H.; Caufield, C. E.
Curr. Pharm. Des. 2003, 9, 857-866. (d) van Heijenoort, J. Nat. Prod.
Rep. 2001, 18, 503-519. (e) van Heijenoort, J. Glycobiology 2001, 11,
25R-36R.
(2) Patte, J.-C. In Amino Acids: Biosynthesis and Genetic Regula-
tion; Herrmann, K. M., Somerville, R. L., Eds.; Addison-Wesley:
Reading, MA, 1983.
(3) (a) Johannsen, L.; Wecke, J.; Obal, F.; Krueger, J. M. Am. J.
Phys. 1991, 260, R126-R133. (b) Luker, K. E.; Tyler, A. N.; Marshall,
G. R.; Goldman, W. E. Mol. Microbiol. 1995, 16, 733-743. (c) Takada,
H.; Kawabata, Y.; Kawata, S.; Kusumoto, S. Infect. Immun. 1996, 64,
657-659. (d) Gotoh, T.; Nakahara, K.; Iwami, M.; Aoki, H.; Imanaka,
H. J. Antibiot. 1982, 35, 1280-1285. (e) Hemmi, K.; Takeno, H.; Okada,
S.; Nakaguchi, O.; Kitaura, Y.; Hashimoto, M. J. Am. Chem. Soc. 1981,
103, 7026-7028. (f) Kitaura, Y.; Nakaguchi, O.; Takeno, H.; Okada,
S.; Yonishi, S.; Hemmi, K.; Mori, J.; Senoh, H.; Mine, Y.; Hashimoto,
M. J. Med. Chem. 1982, 25, 335-337. (g) Gotoh, T.; Nakahara, K.;
Nishiura, T.; Hashimoto, M.; Kino, T.; Kuroda, Y.; Okuhara, M.;
Kohsaka, M.; Aoki, H.; Imanaka, H. J. Antibiot. 1982, 35, 1286-1292.
(6) (a) The groups of Vederas and Williams have investigated the
use of RCM in the synthesis of differentially protected 2,7-diamino-
suberic acid derivatives. See ref 5e and (b) Williams, R. M.; Liu, J. W.
J. Org. Chem. 1998, 63, 2130-2132.
(7) Campbell, A. D.; Raynham, T. M.; Taylor, R. J. K. Synthesis
(Stuttgart) 1998, 1707-1709.
10.1021/jo0485738 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/09/2004
8946
J. Org. Chem. 2004, 69, 8946-8948

TABLE 1. RCM of Diene Substrates 4, 8, and 10a
SCHEME 1. Retrosynthesis of Orthogonally
Protected meso-DAP
diene
catalystb
mol %
time (h)
productc
yieldd (%)
4
A
A
B
A
A
A
A
B
B
5
30
5
20
28
18
18
48
48
20
2
5
5
NR
NR
8
4
4
5
8
5
9
9
11
11
11
11
NR
NRe
19
8
50
5
30
2
2
10
10
10
10
40
99
48
64f
a All reactions, except those noted, were carried out at 0.01 M
in DCM at reflux. b A ) Grubbs’ first-generation catalyst, B )
Grubbs’ second-generation catalyst. c Desired cyclic product. d Iso-
lated yield after silica gel column. NR ) no reaction. e Reaction
concentration was 0.05 M. f Reaction was carried out at room
temperature.
acetylation to give 8 in 83% yield. Because Boc protection
of the resulting amide could theoretically be carried out
before or after RCM, diene 8 was deemed a suitable
substrate for ring closing. Alternatively, compound 10
was prepared by treatment of 8 with excess Boc2O and
catalytic DMAP.
SCHEME 2. Synthesis of Ester Diene 4a
Dienes 4, 8, and 10 were subjected to a variety of olefin
metathesis conditions along with Grubbs’ first- and
second-generation catalysts (Table 1).9 As expected, we
encountered difficulty in forming the eight-membered
lactone 5. Successful examples of eight-membered ring
formation by Grubbs’ RCM are relatively scarce and
generally rely on a conformational bias to position the
olefins within a reactive proximity.6b,10 While no reaction
was observed employing Grubbs’ first-generation catalyst
(A), utilization of the second-generation catalyst (B)
resulted in a complex, largely inseparable mixture of
products.
a Reagents and conditions: (a) (N-Boc)-D-vinylglycinol, PPh3,
DIAD, THF; (b) RCM (see Table 1).
SCHEME 3. Synthesis of Amide Dienes 8 and 10a
Amide 8 surprisingly failed to give any of the desired
lactam product (9) when treated with catalyst A, even
after extended reaction times at reflux. We were encour-
aged to see that N-Boc amide 10, when subjected to the
same conditions, underwent ring closure to give lactam
11 in 19% yield. Although the presence of a Boc group in
10 most likely increases the population of the reactive
cis amide rotamer, high catalyst loading and extended
reaction times failed to push this reaction to completion.
By employing 2 mol % of catalyst B, however, we found
that quantitative formation of 11 was complete within 2
h at reflux. We have prepared lactam 11 on up to a 20
mmol scale without any compromise in yield. It is
interesting to note that the same reaction carried out at
room temperature was still incomplete after 2 days.
The synthesis of orthogonally protected meso-DAP from
intermediate 10 was completed as shown in Scheme 4.
Beginning with the linear amide, we have developed an
efficient five-step sequence which requires only minimal
purificiation en route to the final product. After olefin
metathesis, as described above, the crude lactam was
treated with LiOH in THF/water to effect simultaneous
hydrolysis of the amide and acetate ester bonds. The
crude hydroxy acid was then esterified by alkylation, and
a Reagents and conditions: (a) SOCl2, MeOH; (b) PhCHO, TEA,
MeOH; (c) NaBH4, MeOH; (d) CbzCl, 10% aq Na2CO3, EtOAc; (e)
2 M aq NaOH, MeOH; (f) D-vinylglycinol (2), PyBOP, TEA, DCM;
(g) Ac2O, pyridine, DCM; (h) RCM (see Table 1); (i) Boc2O, DMAP,
THF.
process, N,N-diprotected derivative 7 was synthesized for
condensation with D-vinylglycinol (2). Amide bond forma-
tion mediated by PyBOP was followed by hydroxyl
(9) (a) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2039-2041. (b) Nguyen, S. T.; Grubbs,
R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 9858-9859.
(10) Creighton, C. J.; Reitz, A. B. Org. Lett. 2001, 3, 893-895.
(8) Burk, M. J.; Allen, J. G. J. Org. Chem. 1997, 62, 7054-7057.
J. Org. Chem, Vol. 69, No. 25, 2004 8947

SCHEME 4. Synthesis of Orthogonally Protected
DAP Derivative 13 from Diene 10a
SCHEME 5. Synthesis of FK565 (16) from Lactam
11a
a Reagents and conditions: (a) 2 mol % Grubbs’ second-genera-
tion catalyst, DCM, reflux, 2 h; (b) 2 M aq LiOH, THF; (c) MeI,
K2CO3, DMF; (d) 2 mol % [Ir(COD)PyPCy3]PF6, H2, DCM; (e)
NaClO, NaClO2, TEMPO, MeCN/H2O.
the cis olefin was selectively hydrogenated using 2 mol
% of the Crabtree catalyst. Alcohol 12 was isolated in
58% overall yield from 10 after rapid filtration through
a pad of silica gel. Finally, oxidation with sodium chlorite,
bleach, and TEMPO11 gave orthogonally protected DAP
derivative 13 in good yield.
Although the 1H NMR spectrum of 13 taken in CDCl3
was complicated by the presence of multiple rotamers,
a Reagents and conditions: (a) 2 M aq LiOH, THF; (b) D-Ala-
OtBu, DEPBT, TEA, DCM; (c) 20% Pd(OH)2/C, H2, MeOH; (d)
R-tert-butyl-(N-heptanoyl)-D-glutamate, PyBOP, TEA, DCM; (e)
NaClO, NaClO2, TEMPO, MeCN/H2O; (f) 95% TFA/DCM.
1
the 13C NMR spectrum of 13 and a H NMR spectrum
taken in DMSO-d6 both exhibit only one set of reso-
nances. After treatment of an analytical sample of 13
with 50% TFA/DCM (for Boc group removal), a RP-HPLC
spectrum also showed a single compound, supporting the
purity of the product.12
In an effort to extend our strategy to the preparation
of biologically active DAP-containing structures, we un-
dertook the stereoselective synthesis of the potent im-
munostimulatory peptide FK565 (16).3d,g A close analogue
of the natural product FK156, FK565 was discovered at
Fujisawa Pharmaceutical Co. and was found to exhibit
various biological activities including tumoricidal and
antimicrobial effectiveness in animal models.13 Alone, and
in combination with zidovudene (AZT), FK565 also
inhibits retroviral infection by the Friend leukemia virus
in mice, indicating its potential for use in the treatment
of AIDS.14
Conceptually, acyl tripeptide 16 should be easily ac-
cessible from an appropriately protected meso-DAP de-
rivative. Although the synthesis of FK565 has been
described previously, both routes rely on enzymatic
hydrolysis to obtain the key DAP building block.13d,15 By
employing lactam intermediate 11, we envisioned a more
direct route to the target structure.
As shown in Scheme 5, hydrolysis of lactam 11 afforded
the crude hydroxy acid, which was used directly in
peptide coupling. Condensation with H-Ala-OtBu, medi-
ated by DEPBT, provided the dipeptide alcohol (14) in
53% overall yield. None of the eight-membered lactone
byproduct was observed despite the possibility for in-
tramolecular ring closure. Compound 14 was hydroge-
nated in the presence of 20% Pd(OH)2/C to effect con-
comitant removal of the Cbz and benzyl protecting groups
and reduction of the olefin. The crude hydroxyamine was
then coupled with R-tert-butyl-(N-heptanoyl)-D-glutamate16
to give tripeptide alcohol 15.
Mild oxidation of 15 and treatment of the product with
95% TFA/DCM afforded 16 in 82% overall yield. The
RP-HPLC spectrum of 16 indicated >95% purity without
the need for further purification. High-resolution mass
spectrometry confirmed the chemical composition of 16,
and the 1H NMR spectrum we obtained was in good
agreement with that reported in the literature for
FK565.15
In summary, a novel route toward orthogonally pro-
tected meso-DAP and FK565 has been described. The
starting materials for our synthesis are easily accessible
chiral synthons, obviating the need for stereoselective
reactions. This strategy features a Grubbs’ ring-closing
metathesis reaction as the key carbon-carbon bond-
forming step and is complementary to recent reports
utilizing asymmetric reduction or alkylation. These
results should find application in the concise synthesis
of various biologically relevant DAP-containing com-
pounds as well as close structural analogues of DAP.
(11) Zhao, M. Z.; Li, J.; Mano, E.; Song, Z. G.; Tschaen, D. M.;
Grabowski, E. J. J.; Reider, P. J. J. Org. Chem. 1999, 64, 2564-2566.
(12) See the Supporting Information.
Acknowledgment. We thank Joseph Taulane for
helpful discussions and the National Institutes of
Health for support of this research (NIH DA 05539).
(13) (a) Sone, S.; Mutsuura, S.; Ishii, K.; Shirahama, T.; Tsubura,
E. Gann 1984, 75, 920-928. (b) Inamura, N.; Nakahara, K.; Kino, T.;
Gotoh, T.; Kawamura, I.; Aoki, H.; Imanaka, H.; Sone, S. J. Biol.
Response Modif. 1985, 4, 408-417. (c) Hemmi, K.; Aratani, M.; Takeno,
H.; Okada, S.; Miyazaki, Y.; Nakaguchi, O.; Kitaura, Y.; Hashimoto,
M. J. Antibiot. 1982, 35, 1300-1311. (d) Yokota, Y.; Mine, Y.; Wakai,
Y.; Watanabe, Y.; Nishida, M.; Goto, S.; Kuwahara, S. J. Antibiot. 1983,
36, 1051-1058.
(14) Yokota, Y.; Wakai, Y.; Watanabe, Y.; Mine, Y. J. Antibiot. 1988,
41, 1479-1487.
(15) Kolodziejczyk, A. M.; Kolodziejczyk, A. S.; Stoev, S. Int. J. Pept.
Protein Res. 1992, 39, 382-387.
Supporting Information Available: Experimental pro-
cedures and spectral data for selected compounds. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
JO0485738
(16) See the Supporting Information for the preparative procedure.
8948 J. Org. Chem., Vol. 69, No. 25, 2004