The synthesis of 8-substituted tetracycline derivatives, the first 8- position carbon-carbon bond

Article abstract of DOI:10.1016/S0040-4039(00)00768-1

The synthesis of C8-functionalized tetracyclines using Stille coupling is described. These derivatives are the first reported with a carbon-carbon bond in the 8-position of a tetracycline negating total synthesis routes to 8-substituted tetracyclines. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00768-1

Tetrahedron Letters 41 (2000) 5017±5020
The synthesis of 8-substituted tetracycline derivatives,
the ®rst 8-position carbon±carbon bond
Darrell J. Koza*
Department of Science and Allied Health, Mount Ida College, 777 Dedham St., Newton, Massachusetts 02459, USA
Received 29 March 2000; revised 5 May 2000; accepted 8 May 2000
Abstract
The synthesis of C8-functionalized tetracyclines using Stille coupling is described. These derivatives are
the ®rst reported with a carbon±carbon bond in the 8-position of a tetracycline negating total synthesis
routes to 8-substituted tetracyclines. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: antibacterials; antibiotics; coupling reactions; polycyclic aromatic compounds.
Since 1944 tetracyclines have been important for the treatment of bacterial infections when
chlortetracycline was introduced into clinical use. The advantages of these compounds were that
they inhibited a much wider spectrum of gram-positive and gram-negative organisms. In sub-
sequent years, lengthy programs directed at the chemical modi®cation of tetracyclines yielded few
medically useful derivatives. As a result, numerous chemical modi®cations of tetracyclines have
been reported at all conceivable positions except for the C8 position.1 3 Many researchers feel
that the synthesis of 8-substituted tetracylines falls strictly within the realms of total synthesis.4 In
1979 Glatz et al. reported the total synthesis of racemic 6-demethyl-6-deoxy-8-hydroxy-
tetracycline.5 The reportedly poor antimicrobial activity attributed to this compound was likely
due to the epimerization at C4 and not the substitution at C8.6 In fact, dactylocyclines are a class
of clinical isolates bearing a C8 methoxy substituent and show potent antimicrobial activity
against tetracycline resistant bacteria.7,8 These molecules are the ®rst known tetracycline glyco-
sides and the ®rst naturally occurring C6 epi tetracyclines. Unfortunately, e€orts directed toward
chemical modi®cation of tetracyclines subsided in the early 1980's due to bacterial resistance and
the advent of new antibiotics leaving this interesting class of compounds relatively unexplored.
Recently, access to 6-demethyl-6-deoxy-8-chloro-tetracyclines has been described in a com-
munication by Sum et al. via a diazonium rearrangement reaction (Schemes 1 and 2).9 However,
there has been no report utilizing this halogenated substrate as an intermediate for further mod-
i®cation. The absence of reported modi®cations to the 8-halotetracyclines is surprising in light of
* Corresponding author. E-mail: dkoza@mountida.edu
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00768-1

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the number of cross-coupling reactions which will tolerate a polyhydroxylated substrate such as
tetracycline. Research in our laboratory has focused on utilizing transition metal-catalyzed carbon±
carbon formation for preparing a number of novel C7- or C9-substituted tetracycline deriva-
tives.10 Fortuitously, these reactions are very high yielding, easy to carry out, and in most cases
improve the antimicrobial activity of the compound. This chemistry was easily extended to the
preparation of C8-substituted tetracyclines in good yield. This methodology eliminates the need
for total synthesis routes to C8-substituted tetracyclines and allows suitable quantities of material
to easily be prepared for biological evaluation.
Scheme 1.
Scheme 2.
Treatment of doxycyline 1 with potassium nitrate in concentrated sulfuric acid followed by
reduction with H2 and Pd/C a€ords 9-aminodoxycycline 2 in 79% yield. Diazotization with n-butyl-
nitrite in 1.0N methanolic HCl followed by treatment with NaN3 a€ords 9-azidodoxycycline 3.
Sum et al. have described the preparation of 9-amino-8-chlorodoxycycline by bubbling HCl gas
through a methanolic solution of 3. However, Stille coupling performed on the chloro compound
resulted in low yields presumably due to the diculty of oxidative addition to the active palladium
species.11 Since it is well known that aryl bromides undergo oxidative addition much more readily
than the corresponding aryl chlorides, the reaction conditions were modi®ed to give the C8

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bromodoxycycline. Thus, the treatment of 3 with HBr/HOAc a€orded 9-amino-8-bromodoxy-
cycline 4 in 56% isolated yield.12 The reaction of 4 with 1.1 equivalents of phenyl tri-n-butyltin or
4-nitrophenyl tri-n-butyl tin in the presence of a palladium catalyst gives good yields of the cross-
coupled product 5a and 5b, respectively, with no signi®cant side products or homo-coupling
products observed.13 Similarly, the reaction of 4 with tri-n-butylstannylacetylene gave 9-amino-8-
ethynyldoxycycline 5c in excellent yield. The C9 amino group may be removed via diazotization
followed by heating in methanol to a€ord the 8-substituted doxycyclines 6a±c (Scheme 3). In
addition, the potential for 8,9-disubstituted tetracyclines opens up the exciting possibility of
extending the aromatic ring system by tandem cyclization reactions.
Scheme 3.
In summary, we have demonstrated a concise approach to the synthesis of novel 8-substituted
tetracycline derivatives which previously were only available by total synthesis. This method should
be advantageous for several reasons. This general method allows the synthesis of a large number
of derivatives which are critical for structure±activity relationships and biological evaluation.
Extension of this methodology to other tetracyclines such as minocycline, sancycline, and chlor-
tetracycline may be easily accomplished under similar conditions. These compounds are amenable
to further structural modi®cation which may lead to a compound with antimicrobial activity
against multi-antibiotic resistant bacteria. Presently, biological assessment of these compounds is
being undertaken and the synthesis of derivatives with unique pharmacophores may shed light on
the importance of functionalization in this position.
References
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York, 1978.
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4. Hlavka, J. J.; Boothe, J. H. In Handbook of Experimental Pharmacology, Vol. 78; Born, G. V. R.; Farah, A.;
Herken, H.; Welch, A. D., Eds.; Springer-Verlag: New York, 1985.
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D. R. J. Am. Chem. Soc. 1979, 101, 1198.

5020
6. Doerschuk, A. P.; Bitler, B. A.; McCormick, J. R. D. J. Am. Chem. Soc. 1956, 78, 4687.
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12. 9-Amino-8-bromodoxycyline: 1H NMR (methanol d-4, 400 MHz) ꢀ 6.94 (s, 1H), 4.47 (s, 1H), 3.77 (s, 1H), 3.54 (m,
1H), 3.16 (s, 1H), 2.96 (s, 7H), 2.81 (m, 2H), 2.58 (m, 3H), 1.47 (d, J=6.13 Hz, 3H). MS (FAB) 538 and 540 amu
(isotopic pattern con®rming the presence of bromine).
13. Typical coupling procedure: 25 mg of 9-amino-8-bromodoxycycline (4), 3 mg of Pd(PPh3)2Cl2, 1 mg of AsPh3, and
1 mg of CuI is dissolved in 5 ml of toluene with stirring. Then 8.0 ml of phenyl tri-n-butyltin is added and the
solution re¯uxed for 6 h under nitrogen. The solution is cooled to room temperature, ®ltered, the solvent removed,
and the crude residue puri®ed by preparative thin layer chromatography. Yield: 23 mg (93%); 1H NMR
(methanol d-4, 400 MHz) ꢀ 7.78 (m, 5H), 6.71 (s, 1H), 3.95 (s, 1H), 3.65 (s, 1H), 2.79 (s, 7H), 2.52 (m, 3H), 1.53
(d, J=6.41 Hz, 3H).