Synthesis of L-Carbafuranomycin, an unnatural analogue of the antibiotic amino acid furanomycin

Article abstract of DOI:10.1021/ol0504493

(Chemical Equation Presented) L-(+)-Carbafuranomycin is a novel analogue of L-(+)-furanomycin, an unusual antibiotic α-amino acid that attracted great interest due to its activity as an isoleucine antagonist. We present here a concise and efficient asymme

Full text of DOI:10.1021/ol0504493

ORGANIC
LETTERS
2005
Vol. 7, No. 12
2317-2320
Synthesis of -Carbafuranomycin, an
L
Unnatural Analogue of the Antibiotic
Amino Acid Furanomycin†,‡
|
Ja Young Lee,§ Guido Schiffer, and Volker Ja
1
ger*,§
Institut fu¨r Organische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany, and Bayer HealthCare AG, Pharma Research EU,
D-42096 Wuppertal, Germany
jager.ioc@oc.uni-stuttgart.de
Received March 1, 2005
ABSTRACT
L
-(+)-Carbafuranomycin is a novel analogue of
L
-(
+)-furanomycin, an unusual antibiotic
r
-amino acid that attracted great interest due to its
activity as an isoleucine antagonist. We present here a concise and efficient asymmetric synthesis of this carba-analogue starting with the
1,3-dipolar cycloaddition of a chiral nitrile oxide with cyclopentadiene. Notably, the methyl group was introduced by an SN2
′ cuprate substitution
with high stereo- and regioselectivity.
Furanomycin 1 is a naturally occurring antibiotic R-amino
acid that was isolated from metabolites of Streptomyces
threomyceticus in 1967 by Katagiri et al.1 This unusual
R-amino acid suppresses the growth of several bacterial
species such as E. coli, Bacillus subtilis, or Shigella and
Salmonella strains.1
great interest for the preparation of peptides or proteins
containing unusual, i.e., nonproteinogenic amino acids.3,4
Until now, several syntheses of 1, both as a racemic
mixture and as pure enantiomer, have been published. The
starting points for most of the enantioselective syntheses were
either substituted furans5 or carbohydrates.6 The latest reports
include a Ag(I)-catalyzed cyclization of an allenic alcohol4
and a 1,3-dipolar cycloaddition of a chiral nitrile oxide with
methylfuran, respectively.7 Apart from this, several strategies
for routes to stereoisomers of 1 like 5-epi-furanomycin have
likewise employed substituted furans6a or carbohydrate
precursors.8,9
Furanomycin 1 competitively inhibits the isoleucyl ami-
noacyl tRNA synthetase. In vitro experiments have shown
that furanomycin is incorporated in proteins instead of
isoleucine.2 It is further remarkable that furanomycin is a
substrate for isoleucyl tRNA synthetase, considering that the
structures of isoleucine and furanomycin differ considerably.
NMR studies showed, however, that the conformation of
enzyme-bound furanomycin is very similar to that of
isoleucine.2 These types of translatable amino acids are of
(3) (a) Richmond, M. H. Bacteriol. ReV. 1962, 26, 398-420. (b)
Jakubowski, H.; Goldman, E. Microbiol. ReV. 1992, 56, 412-29.
(4) (a) VanBrunt, M. P.; Standaert, R. F. Org. Lett. 2000, 2, 705-708.
(b) Jayathilaka, L. P.; Deb, M.; Standaert, R. F. Org. Lett. 2004, 6, 3659-
3662.
(5) Masamune, T.; Ono, M. Chem. Lett. 1975, 625-626.
(6) (a) Semple, J. E.; Wang, P.-C.; Lysenko, Z.; Joullie´, M. M. J. Am.
Chem. Soc. 1980, 102, 7505-7510. (b) Joullie´, M. M.; Wang, P.-C.; Semple,
J. E. J. Am. Chem. Soc. 1980, 102, 887-889. (c) Kang, S. H.; Lee S. B.
Chem. Commun. 1998, 761-762. (d) Zhang, J.; Clive, D. L. J. Org. Chem.
1999, 64, 1754-1757.
(7) (a) Zimmermann, J. P.; Blanarikova, I.; Ja¨ger, V. Angew. Chem. 2000,
112, 936-938; Angew. Chem., Int. Ed. 2000, 39, 907-910. (b) Zimmer-
mann, J. P.; Lee, J.-Y.; Hlobilova, I. (ne´e Blanarikova); Endermann, R.;
Ha¨bich, D.; Ja¨ger, V. Eur. J. Org. Chem. 2005, in print.
† Syntheses via Isoxazolines, Part 27. For Part 26, see ref 7b.
‡ Dedicated to Professor Gu¨nther Helmchen on the occasion of his 65th
birthday.
§ Universita¨t Stuttgart.
| Bayer HealthCare AG.
(1) Katagiri, K.; Tori, K.; Kimura, Y.; Yoshida, T.; Nagasaki, T.; Minato,
H. J. Med. Chem. 1967, 10, 1149-1154.
(2) Kohno, T.; Kohda, D.; Haruki, M.; Yokoyama, S.; Miyazawa, T. J.
Biol. Chem. 1990, 265, 6931-6935.
10.1021/ol0504493 CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/13/2005

Concerning furanomycin analogues, syntheses of norfur-
anomycin,10 cyclopentylglycine,11 cyclopentenyl-glycine,12
and cyclohexenylglycine12b,13 and their biological activities
have been disclosed. Norfuranomycin showed antibiotic
activity against E. coli and several species of Pseudomonas
strains. The other compounds also inhibited the growth of
E. coli, but details were not reported in these papers. Re-
cently, syntheses of a series of racemic furanomycin ana-
logues such as norfuranomycin, the 3-, 4-, and R-methyl
regioisomers of furanomycin, cyclopentylglycine, and cy-
clopentenylglycine were published, based on aldol additions
and ring-closing metathesis or ester-enolate Claisen rear-
rangements.14 According to the biological evaluation, no
activities were found apart from the case of norfuranomycin,
however.14 In contrast to this, 1-cyclobutenylglycine was
shown to substitute valine and isoleucine, but not leucine,
in the translation of GFP in vitro.4b
Scheme 1. Retrosynthesis of Carbafuranomycin 2
The required nitrile oxide precursor, the hydroximoyl
chloride 6, was obtained according to earlier work15,16 from
the mannitol bis(acetonide) 7 by periodate cleavage, oxima-
tion, and subsequent chlorination with N-chloro-succinimide
(NCS)16b in 75% overall yield (Scheme 2).
In view of these structure-activity studies, which showed
a very narrow activity profile of the natural structure, we
reasoned that carbafuranomycin 2 would present only a
minute and tolerable change (Figure 1).
Scheme 2. Synthesis of the Hydroximoyl Chloride 6
Figure 1. L-Furanomycin and carba-analogues.
The nitrile oxide was released in situ in low concentration
by slow addition of triethylamine to 6, in order to minimize
furoxan formation (dimerization). In the presence of excess,
freshly distilled cyclopentadiene, the dipole underwent
cycloaddition to provide a diastereomeric mixture of 8 and
9 in 80% yield (dr 56:44).
This paper describes an enantioselective synthesis of 2
where the dihydrofuran oxygen atom has been replaced by
a CH2 unit, along with preliminary tests of its biological
activity.
Separation of 8 and 9 proved to be difficult, so diol pro-
tection was switched from the acetonide to benzyl in 10 and
11, which were separable by MPLC. Thus, the mixture of 8
and 9 was treated with dilute trifluoroacetic acid in methanol
and water (4:1) to give the diols, which were benzylated with
benzyl bromide and sodium hydride in DMF in 78% yield
for the two steps. At this stage, the configuration of 10 and
11 was assigned as (3aS,6aS,1′S)- (“anti”) and (3aR,6aR,1′S)-
(“syn”), respectively, on the basis of the specific rotations
amounting to [R]2D0 ) -36.9 and +135.3.17
Next, the stereoselective reduction of the isoxazoline to
the â-amino alcohol was envisaged, a reliable transformation
well known from earlier work.7,18 The cyclopentaisoxazoline
10 was reduced to the amino alcohol by action of lithium
aluminum hydride in diethyl ether. Due to the bicyclic, bowl-
shaped structure, hydride attack occurred preferentially from
the sterically less hindered exo-side, as expected.7,18 The free
A main feature of this synthesis (Scheme 1) is the 1,3-
dipolar cycloaddition of cyclopentadiene and the chiral nitrile
oxide 3. On one hand, the cyclopentene moiety of carbafura-
nomycin 2 is supplied by cyclopentadiene; on the other hand,
the chiral cyclopentaisoxazoline 4 contains two stereogenic
centers in the carbocycle, from which the two others, in the
ring and in the side-chain of the target structure, were to be
elaborated. Thus, the stereoselective reduction of 4 would
lead to the cyclopentenol derivative 5, the precursor for the
crucial step, the introduction of the methyl group by an SN2′
reaction.
(8) Robins, M. J.; Parker, J. M. R. Can. J. Chem. 1983, 61, 317-322.
(9) Chen, S.-Y.; Joullie´, M. M. J. Org. Chem. 1984, 49, 1769-1772.
(10) Divanfard, H. R.; Lysenko, Z.; Semple, J. E.; Wang, P.-C.; Joullie´,
M. M. Heterocycles 1981, 16, 1975-1985.
(11) (a) Harding, W. M.; Shive, W. J. Biol. Chem. 1954, 206, 401-
410. (b) Hill, J. T.; Dunn, F. W. J. Org. Chem. 1965, 30, 1321-1322.
(12) (a) Dennis, R. L.; Plant, W. J.; Skinner, C. G.; Sutherland, G. L.;
Shive, W. J. Am. Chem. Soc. 1955, 77, 2362-2354. (b) Santoso, S.;
Kemmer, T.; Trowitzsch, W. Liebigs Ann. Chem. 1981, 658-667.
(13) Edelson, J.; Fissekis, J. D.; Skinner, C. G.; Shive, W. J. Am. Chem.
Soc. 1958, 80, 2698-2700.
(15) (a) Chittenden, G. J. F. Carbohydr. Res. 1980, 84, 350-352. (b)
Ha¨fele, B.; Ja¨ger, V. Liebigs Ann. Chem. 1987, 85-87.
(16) (a) Mu¨ller, R.; Leibold, T.; Pa¨tzel, M.; Ja¨ger, V. Angew. Chem.
1994, 106, 1305-1308; Angew. Chem., Int. Ed. Engl. 1994, 33, 1295-
1298. (b) Liu, K.-C.; Shelton, B. R.; Howe, R. K. J. Org. Chem. 1980, 45,
3916-3918.
(14) Kazmaier, U.; Pa¨hler, S.; Endermann, R.; Ha¨bich, D.; Kroll, H.-P.;
Riedl, B. Bioorg. Med. Chem. 2002, 10, 3905-3913.
2318
Org. Lett., Vol. 7, No. 12, 2005

amino group was then protected with Boc2O in dioxane and
water (3:1) to give the cyclopentenol 12, plus a minor product
(probably a diastereoisomer, ratio ) 90:10), which was
separated by MPLC (87% yield of 12 from 10). To act as a
leaving group, the hydroxy group was acetylated with acetic
anhydride in pyridine with a catalytic amount of DMAP
to provide the cyclopentenyl acetate 13 in 97% yield
(Scheme 3).
The acetate 13 was added dropwise to the methyl cuprate,21
prepared from 5 equiv of CuCN and 10 equiv of MeMgBr
in diethyl ether, and provided compound 14 in 88% yield
with a ratio of 88:12 (Scheme 4). The minor product,
Scheme 4. Synthesis of Carbafuranomycin 2
Scheme 3. Synthesis of the Cyclopentenyl Acetate 13 Starting
with 1,3-Dipolar Nitrile Oxide Cycloaddition
probably the regioisomer from the competing SN2 reaction,
was not identified. The isomer ratio was improved from 88:
12 to >95:5 by recrystallizing the diol 15 (vide infra). For
this substitution, 5 equiv of the “higher order” cyanocuprate
were required, probably because of the additional functional
groups in 13 with complexing ability. Experiments with less
cuprate proceeded too slowly or with incomplete reaction.
For the next step, twofold O-debenzylation, catalytic
hydrogenation was presumed to be incompatible with the
For the next step, the key introduction of the methyl group,
use of an organocuprate seemed promising. Organocuprates19
are known to react with suitable cyclic systems in an SN2′
fashion, i.e., nucleophilic attack at the position γ to the
leaving goup, with relocation of the double bond.20 Further-
more, anti attack is stereochemically favored.20 On this basis,
we assumed that the required methylcyclopentenyl moiety
might be obtained with the correct regio- and stereochemical
preference.
(19) For reviews on organocuprates, see: (a) Krause, N. Modern
Organocopper Chemistry; Wiley-VCH: Weinheim, 2002. (b) Normant, J.
F. Synthesis 1972, 63-80. (c) Posner, G. An Introduction to Synthesis Using
Organocopper Reagents; Wiley: New York, 1980. (d) Yamamoto, Y.
Angew. Chem. 1986, 98, 945-957; Angew. Chem., Int. Ed. Engl. 1986,
25, 947-959. (e) Nakamura, E. Synlett 1991, 539-547. (f) Lipshutz, B.;
Sengupta, S. Org. React. 1992, 41, 135-631. (g) Taylor, R. Organocopper
Reagents; Oxford University Press: Oxford, 1994. (h) Lipshutz, B. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Ed.; Perga-
mon: Oxford, 1995; Vol. 12, pp 59-130. (i) Krause, N.; Gerold, A. Angew.
Chem. 1997, 109, 194-213; Angew. Chem., Int. Ed. Engl. 1997, 36, 186-
204. (j) Ibuka, Y.; Yamamoto, Y. Synlett 1992, 769-777.
(20) (a) Goering, H. L.; Singleton, V. D. J. Am. Chem. Soc. 1976, 98,
7854-7855. (b) Claesson, A.; Olsson, L. I. Chem. Commun. 1978, 621-
623. (c) Gendreau, Y.; Normant, J. F. Tetrahedron 1979, 35, 1517-1521.
(d) Roberts, S. M.; Woolley, G. T.; Newton, R. F. J. Chem. Soc., Perkin
Trans. 1 1981, 1729-1733. (e) Goering, H. L.; Singleton, V. D. J. Org.
Chem. 1983, 48, 1531-1533. (f) Corey, E. J.; Boaz, N. W. Tetrahedron
Lett. 1984, 25, 3063-3066. (g) Curran, D. P.; Chen, M.-H.; Leszczweski,
D.; Elliott, R. L.; Rakiewicz, D. M. J. Org. Chem. 1985, 51, 1612-1614.
(h) Tseng, C. C.; Yen, S.-Y.; Goering, H. L. J. Org. Chem. 1986, 51, 2892-
2895. (i) Tseng, C. C.; Paisley, S. D.; Goering, H. L. J. Org. Chem. 1986,
51, 2884-2891. (j) Ito, M.; Matsuumi, M.; Murugesh, M. G.; Kobayashi,
Y. J. Org. Chem. 2001, 66, 5881-5889. (k) Kobayashi, Y.; Ito, M.; Igarashi,
J. Tetrahedron Lett. 2002, 43, 4829-4832.
(17) (a) Such large, consistent differences in optical rotation had been
noted earlier with each one of some 60 5- and 4,5-cis-substituted isoxazolines
whose absolute configuration had been established by chemical correlation
and/or crystal structure analyses; see refs 7, 16a, 18b, and 18c. For example,
in the furanomycin series, the respective furoisoxazolines (methylfuran
adducts of the nitrile oxide generated from 6 in a 60:40 ratio) showed
[R]2D0) -171 for the (3aS,6aS,1′S)-isomer (“anti”), corresponding to 10,
and [R]2D0 ) +199 for the (3aR,6aR,1′S)-diastereomer (“syn”), corre-
sponding to 11; see ref 7. (b) Mu¨ller, R. Dissertation, Universita¨t Wu¨rzburg,
Wu¨rzburg, Germany, 1992. (c) Leibold, T. Dissertation, Universita¨t Stuttgart,
Stuttgart, Germany, 1995.
(18) (a) Mu¨ller, I.; Ja¨ger, V. Tetrahedron Lett. 1982, 4777-4780. (b)
Ja¨ger, V.; Mu¨ller, I.; Paulus, E. F. Tetrahedron Lett. 1985, 2997-3000. (c)
Ja¨ger, V.; Mu¨ller, I. Tetrahedron 1985, 41, 3519-3528. (d) Schaller, C.;
Vogel, P.; Ja¨ger, V. Carbohydr. Res. 1998, 314, 25-35. (e) For a recent
review, see: Ja¨ger, V.; Colinas, P. In Synthetic Applications of 1,3-Dipolar
Cycloaddition Chemistry Towards Heterocycles and Natural Products;
Padwa, A., Pearson, W. H., Eds.; Wiley: New York, 2002; Vol. 59, Chapter
6: Nitrile Oxide, pp 361-472.
(21) To a solution of 5 equiv of CuCN in diethyl ether at -20 °C was
added 10 equiv of CH3MgBr in diethyl ether (3.0 M). After 20 min at -20
°C, the acetate 13 in diethyl ether was added slowly and kept for 1.5 h at
-20 °C.
Org. Lett., Vol. 7, No. 12, 2005
2319

The optical rotation of 2 was found to be [R]2D0 ) +164 (c
) 0.08, H2O), mp 215-217 °C. Antibacterial tests with 2
were performed with several bacterial species such as E. coli,
S. aureus, and B. subtilis; no significant activity was detected,
however.26,27 The minimal inhibitory concentration (MIC)
of 2 was >100 µM, with the exception of the efflux pump-
deficient E. coli HN818 (MIC ) 25 µM (12.5 µM in the
presence of membrane permeabilizer polymycin B nonapep-
tide)).27 This again confirms the narrow structure-activity
relationship of furanomycin.27 In summary, an efficient
synthesis of the new carba-analogue of L-furanomycin is
presented, featuring 1,3-dipolar nitrile oxide cycloaddition
and SN2′ organocuprate substitution as key steps. This
strategy also would give access to the enantiomer of 2 using
the minor cycloadduct 11 and to the respective cyclopente-
nylglycines.
C-C double bond present; therefore, sodium metal in liquid
ammonia was used. Thus, at -78 °C and short exposure,22
the benzyl ether was transformed to the diol 15 in 92% yield.
The diol moiety served as a latent carboxyl group as had
been exploited earlier.23 The diol 15 was cleaved by lead
tetraacetate in CH2Cl2 to afford the intermediate aldehyde,
which was directly oxidized to the carboxylic acid by means
of sodium chlorite (NaClO2) in a phosphate buffer.24 Despite
these very weakly acidic conditions, some epimerization was
observed (probably at the R-amino aldehyde stage), which
required immediate conversion to the diastereomerically pure
ester 16 using diazomethane (77% yield over three steps).
Finally, the Boc group was removed with 90% trifluoroacetic
acid, and the ester was hydrolyzed under mildly basic
conditions (pH 10.5). After ion-exchange column purification
(Dowex 50WX8, H+ form), L-carbafuranomycin 2 was
obtained in analytically and spectroscopically pure form in
61% yield from the N-Boc methyl ester 16.
Acknowledgment. We thank Bayer AG (Wuppertal) and
Fonds der Chemischen Industrie for financial support of this
work. We are further grateful to Dr. D. Ha¨bich, Bayer
HealthCare AG, for discussions.
The structure of 2, with the methyl substituent at C5′, and
its configuration were confirmed notably by 1H NMR
coupling constants and chemical shifts. The chemical shift
difference for 4′-Ha and 4′-Hb in 2 was a mere 0.26 ppm,
characteristic for trans-3,5-disubstituted cyclopentenes.20k,25
Supporting Information Available: Detailed experi-
(22) 14 in THF was added to liquid ammonia at -78 °C, and then 10
equiv of Na was added in small pieces. The reaction was quenched after a
permanent blue color appeared (ca. 5 s later). This kind of careful handling
was necessary to avoid coreduction of Boc-group, observed on a small scale
(<0.1 mmol).
(23) (a) Zimmermann, P. Z. Dissertation, Universita¨t Stuttgart, Stuttgart,
Germany, 2000. For aryl/furyl as a latent carboxyl group in amino acid
syntheses via isoxazolines, see reviews: (b) Ja¨ger, V.; Mu¨ller, I.; Schohe,
R.; Frey, M.; Ehrler, R.; Ha¨fele, B.; Schro¨ter, D. Lect. Heterocycl. Chem.
1985, 8, 79-98. (c) Ja¨ger, V.; Franz, R.; Schwab, W.; Ha¨fele, B.; Schro¨ter,
D.; Scha¨fer, D.; Hu¨mmer, W.; Guntrum, E.; Seidel, B. In Chemistry of
Heterocyclic Compounds; Kova´c, J., Za´lupsky, P., Eds.; Elsevier: Amster-
dam, 1988; Vol. 35, p 58.
mental procedures, spectral data (1H and 13C NMR, IR,
1
elemental analysis, and MS), H NMR spectra of 10, 12-
16, and 2, and 13C NMR of 10, 13-15, and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0504493
(25) Trost, B. M.; Verhoeven, T. R. J. Am. Chem. Soc. 1980, 102, 4730-
4743.
(26) Biological evaluation was done at Bayer HealthCare AG, Wuppertal;
cf. ref 14.
(27) The lack of toxicity of the carbafuranomycin does not mean
nontranslatability; this may be elicited through further testing of aminoacyl
tRNA synthetase assays. We are grateful to one of the referees for this
comment/suggestion.
(24) (a) Lindgren, B. O.; Nilsson, T. Acta Chem. Scand. 1973, 27, 888-
890. (b) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981,
37, 2091-2096. (c) Kraus, G. A.; Roth, B. J. Org. Chem. 1980, 45, 4825-
483. (d) Kraus, G. A.; Taschner, M. J. J. Org. Chem. 1980, 45, 1175-
1176.
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