Process for preparing baccatin

Details Process for preparing baccatin Process for preparing baccatin

2000-07-20

2003-02-04

Marx, Irene (Department: 1651)

Chemistry: molecular biology and microbiology

Micro-organism, tissue cell culture or enzyme using process...

Preparing heterocyclic carbon compound having only o, n, s,...

C435S193000

Reexamination Certificate

active

06514732

ABSTRACT:

The invention relates to a process for preparing baccatin or baccatin derivatives by selective acetylation of the corresponding 10-deacetyl compounds, to an isolated enzyme which catalyses this acetylation reaction and to a process for preparing the enzyme.
Taxol is a promising agent for treating cancer which has antileukaemic and tumour-inhibiting activity (see, for example: M. Suffnes et al., in “The Alkaloids, Chemistry and Pharmacology”, A. Brossi, Ed., Academic Press: Orlando, Fla., 1985, Vol. XXV, Chapter 1). Originally, taxol was obtained from the bark of certain yew trees (
Taxus taxaceae
). However, the isolation of taxol from bark is difficult and expensive, and the desired taxol is obtained from the bark in only very poor yields (40 to 165 mg/kg) (see, for example, R. W. Miller et al., J. Org. Chem. 46 (1981) 1469-1474; V. Sénilh et al., J. Nat. Procl. 47 (1984) 131-137; N. Magri et al., J. Org. Chem. 51 (1986) 797-802). Moreover, the use of bark causes the yew trees, which grow back very slowly, to die, so that there are only limited supplies of starting materials.
Since the discovery of the properties of taxol which recommend it for use as a chemotherapeutic agent for cancer, numerous efforts have been made to prepare the compound by synthetic or semi-synthetic processes. Thus, it has been attempted to prepare the taxol structure by organic synthesis (see, for example, W. F. Berkowitz et al., J. Org. Chem. 52 (1987) 1119-1124). However, because of the complexity of the molecule, it has hitherto not been possible to prepare taxol in practically useful amounts by total organic synthesis.
A further route which was used to obtain taxol is partial synthesis starting from a precursor which is easily obtainable in large amounts. One of these routes starts with 10-deacetylbaccatin-III which can be extracted easily and in large amounts from the leaves of
Taxus baccata
L (G. Chauviere et al., Seances Acad. Sci., Ser. 2, 1981, 293, 501-503). Here, it is possible to isolate approximately 1 g of 10-deacetylbaccatin III per kilogram of leaves, the leaves growing back rapidly. Thus, it is possible without any problems to obtain large amounts of the precursor 10-deacetylbaccatin III.
The desired active compound taxol can be prepared from this precursor, obtained from biological material, by partial synthesis. However, it has been found that, as similar as the structures of 10-deacetylbaccatin III and taxol may be, this partial synthesis still entails significant problems and can for the most part be carried out successfully only by using specific protective groups, giving the desired product taxol in only poor yields.
Denis et al., (J. Am. Chem. Soc. 110 (1988), 5917-5919) describe the synthesis of 10-deacetylbaccatin III to give taxol in two steps. In the first step, 10-deacetylbaccatin III is acetylated chemically in the 10-position. In the second step, baccatin is converted into taxol. However, the first step is not regiospecific, so that acetylation of 10-deacetylbaccatin also occurs, in particular, in position 7. Because of this it is necessary to block the hydroxyl group at this position against acetylation by using a protective group. Exclusive acetylation in the 10-position could only be achieved by using a protective group. However, the use of a protective group entails two more process steps (introduction and removal of the protective group) which is, on the one hand, expensive and, on the other hand, considerably reduces the yield of the product obtained. A further disadvantage of using protective groups consists in the fact that, in particular when the product is used as a pharmaceutical active compound, complicated purification and analysis processes have to be carried out subsequently in order to ensure that there are no more molecules which still carry protective groups present in the product.
Zocher et al. (Biochem. Biophys. Res. Commun., 229 (1996), 16-20) describe a taxol biosynthesis. Here, in an intermediate step, the acetylation of 10-deacetylbaccatin III to give baccatin III was carried out with the aid of crude plant extracts from the roots of
Taxus baccata.
However, it was not possible to isolate or characterize substances which effect the acetylation. A disadvantage of using a crude extract is the fact that numerous other reactions, in particular acetylation at other positions, can also be initiated or influenced by substances present in the crude extract. Moreover, a crude plant extract has no defined and reproducible composition, so that the use of crude plant extracts results in uncontrollable and varying reactions and yields.
It was therefore an object of the present invention to provide a process for preparing baccatin and baccatin-like baccatin derivatives by selective acetylation of the corresponding 10-deacetyl compounds in position 10. It was a further object to provide an isolated substance which specifically catalyses this reaction.
According to the invention, these objects are achieved by a process for preparing baccatin or baccatin derivatives which is characterized in that 10-deacetylbaccatin or a 10-deacetylbaccatin derivative is reacted in the presence of an isolated enzyme and an acetyl donor, the enzyme being an acetyl transferase having a molecular weight of from 70 to 72 kD, determined by SDS-PAGE (sodium dodecylsulfate polyacrylamide gel electrophoresis, which acetyl transferase is obtainable from
Taxus chinensis
cell cultures. It has been found that regioselective acetylation in position 10 is catalysed by an isolated enzyme which can be obtained from suspended
Taxus chinensis
cell cultures. Surprisingly, it has been found that using the isolated and purified enzyme, it is possible to achieve high regiospecificity with respect to the acetylation in position 10. This specificity is preferably >80%, more preferably >90% and most preferably >95%. It has been found that using the enzyme used according to the invention, it is possible to achieve a specificity of >99%. Here, a specificity of >80% means that acetylation has taken place to more than 80% in position 10 and to less than 20% in other positions of the starting material. Consequently, other hydroxyl groups which are present in the starting material do not have to be blocked with a protective group, since acetylation of these other hydroxyl groups occurs to only a very limited extent, if at all, when the enzyme according to the invention is used.
Surprisingly, it has been found that the enzyme used according to the invention has a high substrate specificity. Thus, only 10-deacetylbaccatin or 10-deacetylbaccatin derivatives which have a 10-deacetylbaccatin III-like configuration at and in the vicinity of the C10-position are converted. In particular, baccatin derivatives where the access to position 10 is blocked by voluminous substituents, such as, for example, 10-deacetyltaxol and 10-deacetylcephalomannin, are not acetylated. A precondition for baccatin derivatives to be recognized as substrates by the enzyme according to the invention is therefore that these derivatives, which have a taxane ring structure, essentially correspond to 10-deacetylbaccatin III in positions 7, 8, 9, 10, 11, 12 and 13, i.e. that they do not carry any other substituents in these positions or only substituents having a small volume. The process is preferably suitable for baccatin derivatives which carry the same substituents as 10-deacetylbaccatin III in positions 7 to 13, or carry at least some substituents having a smaller volume than the substituents of 10-deacetylbaccatin III, in particular hydrogen. Voluminous substituents in the other positions do not interfere with the reaction. The process is particularly preferably employed for acetylating 10-deacetylbaccatin III. Furthermore, the process is particularly preferably used for selectively acetylating 14-hydroxy-10-deacetylbaccatin III in position 10.
In contrast, 10-deacetylbaccatin III derivatives whose hydroxyl group in position 7 is blocked by a voluminous protective group, such as, for example, 7-TES-10-DAB III or 7-BOC-

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