Structural diversification of taxanes by whole-cell biotransformation
Graphical abstract
Introduction
While semisynthetic chemical methods have proven useful to approach many natural product derivatives, these techniques run into difficulties with molecules that are labile or those with functional groups that require protection. However, biocatalytic derivatization offers a number of advantages over chemical synthesis when working with complex molecules and offers a general approach toward a synthetic derivatization strategy.1 Accordingly, biocatalysis complements chemosynthesis. When biocatalysis is used alone or in combination with synthetic organic transformations, it provides access to derivatives not readily accessible by chemical synthetic means alone. In principle, the microbial/plant whole cells possess a variety of enzymes, which can catalyze various chemical reactions and can be used for derivatization of natural and non-natural compounds. Thus, an enzymatic approach could directly produce not only numerous compounds for pharmacological evaluation, but it could also yield potential alternatives for further chemical modification.
The treatment of cancer with chemotherapeutic drugs is frequently hindered by either intrinsic or acquired resistance of the tumor cells. In both cases, the tumor can become refractory to a variety of antineoplastic drugs of varying structures and mechanisms of action, a process termed multi-drug resistance (MDR). Although MDR can develop by several different mechanisms, a common cause is believed to be overexpression of an Mr 170,000 plasma membrane glycoprotein (P-gp), a transporter protein that acts as an energy-dependent drug efflux pump, preventing adequate intracellular accumulation of a broad range of cytotoxic drugs.2
Many natural taxanes and chemical derivatives with MDR-reversal activity have been reported.3, 3(a), 3(b), 3(c), 3(d) Inspired by these results, we have been investigated the MDR-reversal activity of our patented natural taxanes [sinenxan A (1) and its analogs, which are 4(20),11(12)-taxadienes with C-14 oxygenated substituents from cell cultures of Taxus in high yield] and their derivatives produced by systematic enzymatic and/or chemical modifications.4, 4(a), 4(b), 4(c), 4(d), 4(e), 4(f) Herein, we briefly report the structural diversification by enzymatic approach of two major metabolites of Taxus cells (1, and yunnanxane 2) and those derivatives of 1 lacking the C-14 or C-2 functional group (3 and 4). We also describe the MDR-reversal activity of one chemo-enzymatic derivative (58) toward three MDR cell lines with P-gp overexpression.
Section snippets
Results and discussion
Incubation of 1 [2α,5α,10β,14β-tetraacetoxy-taxa-4(20),11(12)-diene] with 15-day-old suspended cultured cells of Asparagus officinalis for 6 days afforded three products (Fig. 1, 5–7). Of them, 7 is a new compound identified as 6α-hydroxy-10-oxo-2α,5α,14β-triacetoxy-taxa-4(20),11-diene on the basis of its physical and spectroscopic data analyses. It is an unusual taxane, probably biosynthesized from 1 through specific 10-deacetylation (5) followed by regio- and stereospecific hydroxylation at
Conclusions
In summary, this paper reports the successful structural derivatization of four 4(20),11(12)-taxadienes by enzymatic synthesis, although the expected hydroxylations of C-1 and 13α did not occur. Totally, 53 derivatives have been obtained, and of them, 41 are new compounds. The reactions exhibited diversity, which resulted in structurally diverse products. In addition, the results also indicated that the addition of β-CD may lead to more structurally varied derivatives. Furthermore, a variety of
General experimental procedures
Optical rotations were obtained using a Horiba SEPA-200 polarimeter. IR spectra were taken on a Hitachi 270-30 spectrometer in CHCl3. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded with a Varian Unity-PS instrument using CDCl3 or CD3OCD3 as solvent and reference. 1H NMR and 13C NMR assignments were determined by 1H–1H COSY, DEPT, HMQC, and HMBC experiments. HRFABMS were carried out on a JEOL-HX 110 FAB-mate instrument and HRESIMS on a VG ZabSpec mass spectrometer. Semipreparative
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos. 30572243, 30630069, and 30772736), Beijing Natural Science Foundation (Grant No. 7062048), and the Program for New Century Excellent Talents in University (NCET-06-0155).
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