Sodium bismuthate-induced oxidation of baccatin

Article abstract of DOI:10.1080/00397910500278842

Sodium bismuthate in acetic acid has proven very effective in the oxidation of allyic alcohols. An extension of this oxidation method using baccatin as the substrate has provided keto-baccatin, an important compound for the semisynthesis of Taxol and Taxotere. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500278842

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Sodium Bismuthate–Induced
Oxidation of Baccatin
a
a
Bimal K. Banik , Miguel Fernandez & Clarissa
a
Alvarez
a
Department of Chemistry , University of Texas—Pan
American , Edinburg, Texas, USA
Published online: 21 Aug 2006.
To cite this article: Bimal K. Banik , Miguel Fernandez & Clarissa Alvarez (2005)
Sodium Bismuthate–Induced Oxidation of Baccatin, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 35:23,
3065-3067
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Synthetic Communications , 35: 3065–3067, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500278842
Sodium Bismuthate–Induced Oxidation
of Baccatin
Bimal K. Banik, Miguel Fernandez, and Clarissa Alvarez
Department of Chemistry, University of Texas—Pan American,
Edinburg, Texas, USA
Abstract: Sodium bismuthate in acetic acid has proven very effective in the oxidation
of allyic alcohols. An extension of this oxidation method using baccatin as the substrate
has provided keto-baccatin, an important compound for the semisynthesis of Taxol and
Taxotere.
Keywords: Baccatin, oxidation, sodium bismuthate
We have been studying reactions mediated by metals or their salts with the
aim of developing several biologically active compounds, including anti-
[1]
[2]
cancer agents and antibacterial agents. We have also reported the use of
[3]
bismuth-derived reagents in several organic transformations. For example,
nitration of aromatic hydrocarbons, phenolic compounds, and b-lactams by
bismuth nitrate was performed in excellent yields. Facile deprotection of
hydrazones and oximes also worked well using bismuth nitrate. In continu-
ation of our research on bismuth salts–induced transformation, we describe
here the oxidation of commercially available baccatin to keto-baccatin, an
intermediate useful in the chemistry and biology of Taxol-related science.
Sodium bismuthate is a strong oxidizing agent, but the use of this reagent
[
4–8]
in organic synthesis is limited.
methylenes to the benzylic ketones by a novel oxidation reaction mediated by
We demonstrated the oxidation of benzylic
[
3a]
sodium bismuthate in acid media.
Oxidation of ally alcohol to ally
Received in the USA June 30, 2005
Address correspondence to Bimal K. Banik, Department of Chemistry, University
of Texas—Pan American, 1201 West University Drive, Edinburg, TX, USA 78541.
Tel.: 956-380-8741; Fax: 956-384-5006; E-mail: banik@panam.edu
3
065

3066
B. K. Banik, M. Fernandez, and C. Alvarez
Scheme 1.
aldehyde is an important synthetic reaction and widely used in the total
synthesis of natural products. Although there are many oxidizing agents
reported in the literature for the oxidation of saturated alcohols, the
methods available for the oxidation of allylic alcohol are very limited.
Swern oxidation and Dess–Martin oxidation of allyic alcohol to allyic
carbonyl compounds are the most useful. However, the complex reaction con-
ditions required for the successful Swern oxidation cause many chemists to
avoid this method unless it is absolutely necessary. In addition, manganese
dioxide has been used occasionally for the oxidation of the allylic alcohols.
At the beginning of the experiment, allylic alcohol 1 was oxidized to the
allylic aldehyde 2 by sodium bismuthate in the presence of acetic acid
(Scheme 1). The presence of acetic acid was crucial because the reaction
failed to produce any desired product in its absence. It was convenient to
use small amounts of acetone as the cosolvent.
The structure of the products was verified by comparison with authentic
samples. Realizing the importance of this method, we explored the possibility
of oxidizing baccatin 3 because it has an allylic alcohol functionality. To our
surprise, oxidation works well in this complex system and the keto-baccatin 4
was formed in 70% yield (Scheme 2). Although baccatin has a very labile
oxetane ring and many other sites of oxidation, the successful realization of
a selective oxidation at the C-13 center deserves credit. The 13-keto-
Scheme 2.

Sodium Bismuthate
3067
baccatin 4 is very well suited for a systematic structure–activity relationship
[
9,10]
study for the development of new Taxol and Taxotere analogues.
structure of 4 was confirmed by a direct comparison with respect to
reported compound (Mp and NMR).
The
[
10]
In conclusion, we have shown an operationally simple and readily
available oxidation of primary allylic alcohols and naturally occurring
baccatin to keto compounds. A representative procedure is given here:
Sodium bismuthate (8 mmol) was added to the baccatin (2 mmol) in
aqueous acetic acid (4 mL, 50% v/v) and acetone (2 mL). The mixture was
heated to reflux. At the end of reaction [indicated by TLC (30 min)] the
mixture was filtered through a pad of Celite and diluted with water (10 mL).
The mixture was extracted with methylene chloride (3 Â 20 mL). The
combined organic layer was washed with sodium bicarbonate solution
(
3 Â 10 mL, 10%) and brine (10 mL), dried over anhydrous sodium sulfate,
and concentrated under reduced pressure. The resultant crude product was
purified by column chromatography over silica gel using ethyl acetate–
hexanes to afford 13-keto-baccatin. Mp 210–2128C; IR (CHCl ): 3550
3
(
OH), 1725 (C 55 O), 1680 (conj C 55 O); NMR (CDCl ): 1.72 (s), 4.15 (d),
.35 (d, J ¼ 8), 7.58 (m), 8.10 (dd, J ¼ 2, 8), 2.22 (s), 2.33 (s).
3
4
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1