A practical, enantioselective synthetic route to a key precursor to the tetracycline antibiotics

Article abstract of DOI:10.1021/ol071377d

A practical, enantioselective synthetic route to a key precursor to the tetracycline antibiotics is reported. The route proceeds in nine steps (21% yield) from the commercial substance methyl 3-hydroxy-5-isoxazolecarboxylate. Key steps in the route involve enantioselective addition of divinylzinc to 3-benzyloxy-5-isoxazolecarboxaldehyde and an endo-selective intramolecular furan Diels-Alder cycloaddition reaction. The route described has provided more than 40 g of chromatographically pure 1 with 93% ee.

Full text of DOI:10.1021/ol071377d

ORGANIC
LETTERS
2007
Vol. 9, No. 18
3523-3525
A Practical, Enantioselective Synthetic
Route to a Key Precursor to the
Tetracycline Antibiotics
Jason D. Brubaker and Andrew G. Myers*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
myers@chemistry.harVard.edu
Received June 11, 2007
ABSTRACT
A practical, enantioselective synthetic route to a key precursor to the tetracycline antibiotics is reported. The route proceeds in nine steps
(21% yield) from the commercial substance methyl 3-hydroxy-5-isoxazolecarboxylate. Key steps in the route involve enantioselective addition
of divinylzinc to 3-benzyloxy-5-isoxazolecarboxaldehyde and an endo-selective intramolecular furan Diels−Alder cycloaddition reaction. The
route described has provided more than 40 g of chromatographically pure 1 with 93% ee.
We recently described a convergent synthetic route to the
tetracyclines, broad-spectrum antibiotics used widely in
Scheme 1
human and veterinary medicine.1 The key and final steps
involve a Michael-Dieckmann condensation2 of the AB
precursor 1 with any of a number of structurally diverse
D-ring precursors, followed typically by two chemical
“deprotection” reactions3 (illustrated in Scheme 1 for the
specific construction of (-)-6-deoxytetracycline). Using this
general route, a number of novel broad-spectrum antibiotic
substances have been identified, including molecules effec-
tive against tetracycline-resistant bacterial strains.1,4 Here,
we describe a completely different route to the intermediate
1 than that we first reported and show that this new route is
amenable to the production of multigram quantities of 1 in
optically active form (40 g in the largest single run to date).
We believe that this innovation will fuel the discovery of
large numbers of new candidate antibiotics and may provide
the basis for novel tetracyclines to be synthesized on a scale
sufficient for their clinical development, where hundreds of
grams and, ultimately, kilograms of material will be re-
quired.5
(1) Charest, M. G.; Lerner, C. D.; Brubaker, J. D.; Siegel, D. R.; Myers,
A. G. Science 2005, 308, 395.
(2) (a) Leeper, F. J.; Staunton, J. J. Chem. Soc., Chem Commun. 1978,
406. (b) Hauser, F. M.; Rhee, R. P. J. Org. Chem. 1978, 43, 178. (c) Dodd,
J. H.; Weinreb, S. M. Tetrahedron Lett. 1979, 20, 3593.
(3) Stork, G.; Hagedorn, A. A., III. J. Am. Chem. Soc. 1978, 100, 3609.
(4) (a) Myers, A. G.; Charest, M. G.; Lerner, C. D.; Brubaker, J. D.;
Siegel, D. R. Synthesis of Tetracyclines and Analogues Thereof. PCT/US05/
17831, May 20, 2005. (b) Myers, A. G. PCT/US07/66253, April 6, 2007.
The new, convergent route to 1 begins with 3-benzyloxy-
5-isoxazole carboxaldehyde (2, Scheme 2), prepared in multi-
hundred gram amounts in two steps (O-benzylation, then
10.1021/ol071377d CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/11/2007

Scheme 2
Figure 1. Model for the reversal of enantioselectivity observed
with the heterocyclic aldehyde 2 (other zinc ligands are omitted
for clarity).
1). The reversal in enantioselectivity we observe with a
potentially bis-chelate substrate has not been documented
in the Oppolzer system so far as we are aware. Dinuclear
zinc catalysts apparently do not show such behavior.10 The
lithium cation may prove a key organizational element, as
suggested in Figure 1 and originally proposed by Oppolzer
and Radinov. Because of the ready availability of the
ligand,8c,d thus far we have not pursued efforts to reduce its
levels in the addition, though this may be possible. A second
route to (R)-3 proceeds by kinetic resolution. Acylation of
(()-3 (prepared in 96% yield from 2 and vinylmagnesium
bromide) with vinyl acetate in hexanes using Amano Lipase
AK as a catalyst afforded the (R)-allylic acetate (49% yield)
and the (S)-allylic alcohol (50% yield, 93% ee), isolated
separately after purification by flash column chromatogra-
phy.11 Cleavage of the (R)-allylic acetate with potassium
carbonate in methanol then provided (R)-3 of 93% ee (99%
yield). Produced by either method, the unpurified (R)-allylic
alcohol 3 was transformed into the corresponding mesylate
derivative at -12 °C,12 and the latter intermediate was treated
in situ with a solution of dimethylamine in THF (-30 f 15
°C) to afford, after extractive isolation and purification
through a short column of silica gel, the (S)-allylic amine 5
reduction with diisobutylaluminum hydride)6 from methyl
3-hydroxy-5-isoxazolecarboxylate (a commercial product),
which is available in large quantity from the reaction of
N-hydroxyurea with dimethyl acetylenedicarboxylate.7 We
have developed two procedures to transform 2 into the
optically enriched (R)-allylic alcohol 3. The procedure
currently used for large-scale preparations is derived from
multiple precedents8 and involves addition at -75 °C of 1
equiv of 2 in toluene to 2 equiv of a 1:1 complex formed
from the norephedrine-derived lithium alkoxide 4 and
divinylzinc. After an aqueous workup, wherein the ligand is
recovered in >95% yield, the (R)-allylic alcohol 3 is obtained
with an optical purity of 93% ee (>90% yield). The
configuration of the product (3) is opposite to that observed
in related additions to non-heteroaromatic aldehydes,8a,9
which suggests that 2 may bind to the metal complex in
bidentate fashion during the addition (thus presenting the
enantiotopic π-face of the aldehyde to the nucleophile, Figure
1
(5) The Muxfeldt route to the tetracyclines (involving late-stage C- and
D-ring constructions) was used to synthesize a number of analogs, and was
scaled to provide (()-6-thiatetracycline for early clinical trials (discontinued
due to an apparent CNS-based toxicity). (a) Cunha, B. A. Clinical Uses of
the Tetracyclines. In Handbook of Experimental Pharmacology; Hlavka, J.
J., Boothe, J. H., Eds.; Springer-Verlag: New York, 1985; Vol. 78, p 393.
(b) Tetracyclines in Biology, Chemistry, and Medicine; Nelson, M., Hillen,
W., Greenwald, R. A., Eds.; Birkha¨user Verlag: Boston, MA, 2001; p 237.
(6) Riess, R.; Scho¨n, M.; Laschat, S.; Ja¨ger, V. Eur. J. Org. Chem. 1998,
473.
as a pale yellow oil (76 g, 80% yield over two steps). H
NMR analysis of the latter product in the presence of the
(R)-Mosher acid13 (1.5 equiv, C6D6) established that the
product was of 93% ee and thus that the transformation of
3 to 5 had proceeded with complete stereospecificity, within
experimental error.
A convergent coupling was achieved by metalation of the
isoxazole ring of the (S)-allylic amine 5 with n-butyllithium
in tetrahydrofuran at -100 f -65 °C followed by trapping
of the resulting anion at -65 °C with 3-methoxyfurfural (6),14
(7) Frey, M.; Ja¨ger, V. Synthesis 1985, 1100.
(8) (a) Use of lithium N-methylephedrate for the enantioselective addition
of 1-alkenylzinc bromides to aldehydes: Oppolzer, W.; Radinov, R. N.
Tetrahedron Lett. 1991, 32, 5777. (b) Synthesis of dialkylzinc reagents by
addition of Grignard, reagents to zinc chloride followed by precipitation
with 1,4-dioxane: von dem Bussche-Hu¨nnefeld, J. L.; Seebach, D.
Tetrahedron 1992, 48, 5719. (c) Use of (1S,2R)-2-morpholin-4-yl-1-
phenylpropanol for the enantioselective addition of diisopropylzinc to
aldehydes: Soai, K.; Hayase, T.; Takai, K.; Sugiyama, T. J. Org. Chem.
1994, 59, 7908. (d) Synthesis of (1S,2R)-2-pyrrolidiny-1-phenylpropanol
by alkylation of norephedrine with 1,4-dibromobutane: Pierce, M. E. et al.
J. Org. Chem. 1998, 63, 8536.
(10) Furan 2-carboxaldehydes: (a) Noyori, R.; Suga, S.; Kawai, K.;
Okada, S.; Kitamura, M.; Oguni, N.; Hayashi, M.; Kaneko, T.; Matsuda,
Y. J. Organomet. Chem. 1990, 382, 19. (b) Sato, I.; Saito, T.; Omiya, D.;
Takizawa, Y.; Soai, K. Heterocycles 1999, 51, 2753.
(11) Marshall, J. A.; Chobanian, H. Org. Synth. 2005, 82, 43.
(12) Crossland, R. K; Servis, K. L. J. Org. Chem. 1970, 35, 3195.
(13) Benson, S. C.; Cai, P.; Colon, M.; Haiza, M. A.; Tokles, M.; Snyder,
J. K. J. Org. Chem. 1988, 53, 5335.
(9) Layton, M. E.; Morales, C. A.; Shair, M. D. J. Am. Chem. Soc. 2002,
124, 2002.
3524
Org. Lett., Vol. 9, No. 18, 2007

of the cycloaddition reaction is notable, given that the
majority of furan Diels-Alder reactions are exo-selective,
reversible processes.17 Retro-cycloaddition apparently does
not occur in the present case, as evidenced by the fact that
each of the (purified) individual diastereomers remains
unchanged after heating at 110 °C for 24 h. The lack of
reversibility is likely due to an enthalpic effect of the
3-methoxyl substituent of the furan ring, as discussed in detail
by Pieniazek and Houk.18
Scheme 3
Swern oxidation19 of the Diels-Alder product mixture
gave two ketone products of different polarities; these were
readily separated by flash column chromatography. The more
polar isomer proved to be the desired product 8, and was
obtained in 73% yield (55 g, Scheme 2). Treatment of 8 with
boron trichloride (3 equiv) in dichloromethane at -40 °C
led to opening of the oxabicyclic ring system and demethy-
lation of the methyl enol ether. Protection of the tertiary
hydroxyl group of the product with tert-butyldimethylsilyl
triflate then gave the target 1 (58% over two steps, 23% from
aldehyde 2 over seven steps, 21% from the commercial
precursor to 2 over nine steps, 40 g after chromatographic
purification). The purified product was also recrystallized
(ethyl acetate-hexanes, 27 g, crop 1, mp ) 150-151 °C,
7.2 g, crop 2, mp 150-151 °C, and 1.9 g, crop 3, mp 149-
150 °C). Analysis of the product (crystal crop 1) by HPLC
using a chiral column showed it to be of 93% ee.20
The route we have described provides a very short
sequence to (+)-1 from simple heterocyclic precursors that
might readily be manufactured in multi-kilogram amounts.
While further process development will undoubtedly be
required to allow manufacture of 1 (and novel tetracyclines
thereafter), the quantities of 1 we have prepared as described
herein alone should allow for the synthesis and evaluation
(antimicrobial screening panels) of literally hundreds of novel
antibiotic candidates.
providing an epimeric mixture of secondary carbinol adducts
7 (1.3:1 ratio, 88%, 98 g, see Scheme 3 for assignments).15
In a key transformation, the mixture of epimeric alcohols 7
was heated in toluene with N,N-diisopropylethylamine (2
equiv) and 2,6-di-tert-butyl-4-methylphenol (0.004 equiv)
first at 95 °C for 105 h, then at 110 °C for 24 h, affording
a mixture of four diastereomeric intramolecular Diels-Alder
cycloadducts (combined yield 78%, 77 g). Detailed analysis
of the product mixture shows that cycloaddition occurs from
a single π-face of the allylic amine (dienophile) and that the
two major products are both endo-diastereomers, stereo-
chemically homologous with the target 1 at the AB juncture
(see Scheme 3). While both of the epimeric substrates 7
undergo endo-selective cycloaddition, the endo/exo ratios are
different (6.2:1 for the (S)-alcohol, and 1.6:1 for the (R)-
alcohol), as are the rates of cycloaddition, with the (S)-alcohol
7 reacting somewhat more rapidly.16 The endo selectivity
Acknowledgment. Financial support from the National
Institutes of Health (Grant AI048825) is gratefully acknowl-
edged. J.D.B. acknowledges an NSF predoctoral fellowship.
(14) Prepared by Villsmeier-Haack formylation (Jones, G.; Stanforth,
S. P. Org. React. 1997, 49, 1) of 3-methoxyfurfural, synthesized in hundred-
gram amountsswithout chromatographysin four steps from 3-chloro-1,2-
propanediol: Meister, C.; Scharf, H.-D. Synthesis 1981, 737.
(15) Allylic metalation (removal of the proton R to the dimethylamino
group) is competitive with metalation of the isoxazole ring at higher
temperatures and it is the predominant reaction pathway when bases other
than n-butyllithium are used for deprotonation. A preliminary experiment
with the 4-iodoisoxazole 9 (see Supporting Information) reveals that
magnesium-halogen exchange at -20 °C provides an efficient means of
anion formation; trapping with 3-methoxyfurfural (6) gives a higher
proportion of the (S)-alcohol 7.
Supporting Information Available: Detailed experi-
mental procedures, tabulated spectroscopic data (1H and 13
C
NMR, FT-IR, and HRMS) for all new compounds, and
complete ref 8d. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL071377D
(16) After 105 h at 95 °C cyclization of the (S)-alcohol 7 had proceeded
to 89% conversion whereas cyclization of the (R)-alcohol 7 had proceeded
to 77% conversion.
(17) Kappe, C. O.; Murphree, S. S.; Padwa, A. Tetrahedron 1997, 53,
14179.
(18) Pieniazek, S. N.; Houk, K. N. Angew. Chem., Int. Ed. Engl. 2006,
45, 1442.
Magnesium-halogen exchange: (a) Wakefield, B. J. Preparation of
Organomagnesium Compounds. In Organomagnesium Compounds in
Organic Synthesis; Academic Press, Inc.: San Diego, 1995; pp 51-59.
(b) Boudier, A.; Bromm, L. O.; Lotz, M.; Knochel, P. Angew. Chem., Int.
Ed. Engl. 2000, 39, 4414.
(19) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651.
(20) The optical rotation of synthetic 1 was [R]23D -138 (c 0.52, CHCl3);
literature: [R]23 -149 (c 0.265, CHCl3).1
D
Org. Lett., Vol. 9, No. 18, 2007
3525