C-13 taxol side chain

Another key precursor for the taxol semisynthetic process is the preparation of chiral C-13 taxol side chain. Two different stereoselective enzymatic processes were developed for the preparation of chiral C-13 taxol side chain syntiion [51-54]. In one process, the stereoselective microbial reduction of 2-keto-3-(iV-benzoylamino)-3-phenylpropionic acid ethyl ester 40 to yield (2/ ,5S)-(—)-iV-benzoyl-3-phenylisoserine ethyl ester 41a was dem-... 

In an alternate process for the preparation of C-13 taxol side chain, the stereoselective enzymatic hydrolysis of racemic ci5-3-(acetyloxy)-4-phenyl-2-azetidinone 42 to the corresponding (S)-(—)-alcohol 43 has been demonstrated [53,54]. Lipase PS-30 from Pseudomonas cepacia (Amano International Enzyme Co.) and BMS lipase (extracellular lipase derived from the fermentation of Pseudomonas sp. SC 13856) catalyzed hydrolysis of the undesired enantiomer of racemic 42, producing S-(—)-alcohol 43 and the desired i -(+)-acetate 44 (Fig. 12). Reaction yields of more than 48% (theoretical maximum yield is 50%) and e.e. of more than 99.5% were obtained for the desired R-(+)-acetate. For a very efficient enzyme source (BMS lipase), a lipase fermentation using Pseudomonas sp. SG 13865 was developed. In a fed-batch process using soybean oil, the fermentation resulted in 1500 U/ml of extracellular lipase activity. Crude BMS lipase (1.7 kg containing 140,000 U/g) was recovered from the filtrate by ethanol precipitation. BMS lipase and lipase PS-30 were immobilized on Accurel polypropylene (PP). These immobilized lipases were reused (10 cycles) without loss of enzyme activity, productivity, or the e.e. of the product in the resolution process. The enzymatic process for the resolution of racemic acetate 42 was scaled up to 150 L at 10 g/L substrate concentration using inunobilized BMS lipase and lipase PS-30, respectively. From each reaction batch, 3-(R)-acetate 44 was isolated in 45 M% yield (theoretical maximum yield is 50%) and 99.5% e.e. 3-(R)-acetate 44 was chemically converted to 3-(R)-alcohol 45. The C-13 taxol side chain (41a or 45) produced... 

Walker, K., Fujisaki, S., Long, R. and Croteau, R. (2002) Molecular cloning and heterologous expression of the C-13 phenylpropanoid side chain-CoA acyltransferase that functions in Taxol biosynthesis. Proceedings of the National Academy of Sciences of the United States of America, 99, 12715-12720. 

In a series of reports Gennari et al. demonstrated the synthesis of taxol side chain thioesters protected as oxazolines and oxazolidines. Thus reaction of the enolate of thioester 8.1.8 and TMS-imine 8.1.9 in the presence of chiral borate 8.1.10 gave the side chain thioester 8.1.11 in 60% yield. Compound 8.1.11 was then protected as its acetonide 8.1.12. The oxazoline analog 8.1.13 was synthesized by a similar strategy with an inversion to achieve the desired stereochemistry at C-2 during the formation of the oxazoline ring (299). 

Standard methods, and this was protected as the cyclic 1,2-carbonate and oxidized to 12.21. Removal of the cinnamoyl side chain was effected by osmylation followed by mild base to give the triol 12.22 and, in a nice touch, the diol 12.23 the latter compound was obtained in optically pure form and could be converted to the taxol side chain by known chemistry. Formation of the oxetane 12.24 from 12.22 followed the same pathway used by Scheeren and his coworkers 406), and ring-opening of the carbonate followed by reduction at C-13 gave 7-deoxybaccatin III (12.25). The synthesis was completed by attachment of the side chain to give 7-deoxytaxol (12.26). 

Tiecco M, Testaferri L, Temperini A, Marini F, Bagnoli L, Santi C (1999) Selenium Promoted Stereospecific One-Pot Conversion of Cinnamyl Derivatives into Oxazo-lines. A Simple Synthetic Route to Racemic Taxol Side Chain Syn Comm 29 1773 Lee K-Y, Kim Y-H, Park M-S, Ham W-H (1998) A Highly Stereocontrolled Asymmetric Synthesis of the Taxol C-13 Side Chain (45, 5/ )-2,4-Diphenyloxazo-line-5-carboxylic Acid. Tetrahedron Lett 39 8129... 

Ethanolamine, IPA, reflux, 21 h, >50% yield. These conditions did not affect the C-10 acetate or the C-13 side chain of a taxol derivative. ... 

The total synthesis of taxol (52) has been described in Chapter 34. Clearly, total synthesis cannot hope to meet the demand for taxol at the present time, and supplies are currently procured by semisynthesis. This approach uses baccatin III (derived from yew tree needles) and the C-13 side chain 51, made synthetically (Scheme 13). A practical synthesis of the side chain is necessary,... 

Scheme 13. Sharpless s asymmetric synthesis of the C-13 side chain 51 of taxol (52).

Gou, D.M., Liu, Y.C., Chen, C.S. (1993) A Practical Chemoenzymatic Synthesis ofthe Taxol C-13 Side Chain N-Benzoyl-(2R,3S)-3-phenylisoserine. Journal of Organic Chemistry, 58, 1287-1289. 

Taxol (Paclitaxel) 137, a natural product derived from the bark of the Pacific yew, Taxus brevifolia [213-215], and the hemisynthetic analogue Docetaxel (Taxotere) 138, two recent and promising antitumour agents, have been the matter of extensive in vivo and in vitro animal metabolic studies. The major metabolites of taxol excreted in rat bile [216] were identified as a C-4 hydroxylated derivative on the phenyl group of the acyl side chain at C-13 (139), another aromatic hydroxylation product at the mefa-position on the benzoate group at C-2 (140) and a C-13 deacylated metabolite (baccatin III, 142) the structure of six minor metabolites could not be determined. The major human liver microsomal metabolite, apparently different from those formed in rat [217], has been identified as the 6a-hydroxytaxol (141) [218, 219]. A very similar metabolic pattern was... 

The metabolism of taxol by Eucalyptus perriniana cell suspension cultures has been recently reported to induce hydrolyses of ester bonds at C-13, C-10 and C-2 [222]. At this moment only very few data have been published about the microbial metabolism of taxoid compounds only site specific hydrolyses of acyl side-chains at C-13 or C-10 by extracellular and intracellular esterases of Nocar-dioides albus SC13,911 and N. luteus SC13,912, respectively, have been reported [223]. On the other hand, Hu et al. [224-226] have recently described some fungal biotransformations of related natural taxane diterpenes extracted from Chinese yews or their cell cultures, in order to obtain new active substances or precursors for hemisynthesis. The taxadiene 145, a 14 -acetylated derivative... 

Baker s yeast reduction of organic compounds, especially carbonyl compounds, is an extremely useful method of obtaining chiral products255-257. Recently, much effort has been expended to improve the ee obtained in this process. In one very useful example, l-acetoxy-2-alkanones have been reduced enantioselectively into (5 )-l-acetoxy-2-alkanols in 60-90% yields and with 95-99% ee258. The reaction readily occurs in a variety of solvents, both aqueous and nonaqueous. The reduction is fairly selective and so may be brought about in the presence of a-amide, ether, ester and other acid functional groups, in reasonable yields and with excellent ee (equation 65)259 -261. Thus, in the synthesis of the C-13 side chain of taxol, the key step was the reduction of a w-ketoester to the corresponding alcohol in 72% overall yield (equation 66)262. 

The 1,3-dipolar cycloaddition of the carbonyl ylide (31) to the aldimine (32) produces the adduct (33), which has been used to synthesize the taxol C(13) side-chain (34), which is known to be required for the antitumour activity of taxol (Scheme 9).35 The dirhodium tetraacetate-catalysed decomposition of l-diazo-5-phenylpentane-2,5-dione (35) yields the carbonyl ylide (36), which cycloadds to methylenecyclopropanes (37) to produce spirocyclopropanated 8-oxabicyclo[3.2.1]octan-2-ones [(38)-(40)] in 6-75% yields (Scheme 10).36 The 1,3-dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides with thiobenzophenone produce both regioisomeric 1,3-dithiolanes as expected. However, in the case of highly sterically hindered thiocarbonyl ylides, methylene transfer leads to the formation of 4,4,5,5-tetraphenyl-l,3-dithiolane.37,38... 

Etiolates. Asymmetric addition of enolates to enantiopure sulfinimines is an important method for the preparation of P-amino esters.21,84,85 For example, treatment of (Ss)-sulfinimine 47 with the sodium enolate of methyl acetate in ether afforded P-amino ester 149 in 84-85% yield and in >98% de.86,87 After removal of the N-sulfinyl group, P-amino esters 150 were obtained in >90% yield.84 The P-amino esters were further elaborated into the Taxol C-13 side chain 151a,21 its fluoro analogue 151b,85 (+)-2-phenylpiperidine (152a), and (+)-dihydropinidine (152b).87... 

The incorporation of the C-13 side chain into Taxol and its analogs, roughly speaking, includes two stages—syntheses of side chains with various substituents, and... 

Jayasinghe, L. R. Datta, A. Ali, S. M. Zygmunt, J. Vander Velde, D. G. Georg, G. I. Structure-activity studies of antitumor taxanes synthesis of novel C-13 side chain homologated taxol and taxotere analogs. J. Med. Chem., 1994, 37 2981-2984. 

Early work on taxanes has shown that there are several natural taxanes in which the structures analogous to the taxol C-13 side chain are esterified to... 

A second approach to the hemisynthesis of taxol, published in 1988 (45), relies on the esterification of the hydroxyl group at position 13 of 13a (selectively protected at C-7 and acetylated at C-10) with the separately synthesized side chain 30 (51). This coupling reaction was performed under conditions previously... 

The asymmetric hydroxylation of ester enolates with N-sulfonyloxaziridines has been less fully studied. Stereoselectivities are generally modest and less is known about the factors influencing the molecular recognition. For example, (/J)-methyl 2-hydroxy-3-phenylpropionate (10) is prepared in 85.5% ee by oxidizing the lithium enolate of methyl 3-phenylpropionate with (+)-( ) in the presence of HMPA (eq 13). Like esters, the hydroxylation of prochiral amide enolates with N-sulfonyloxaziridines affords the corresponding enantiomerically enriched a-hydroxy amides. Thus treatment of amide (11) with LDA followed by addition of (+)-( ) produces a-hydroxy amide (12) in 60% ee (eq 14). Improved stereoselectivities were achieved using double stereodifferentiation, e.g., the asymmetric oxidation of a chiral enolate. For example, oxidation of the lithium enolate of (13) with (—)-(1) (the matched pair) affords the a-hydroxy amide in 88-91% de (eq 15). (+)-(Camphorsulfonyl)oxaziridine (1) mediated hydroxylation of the enolate dianion of (/J)-(14) at —100 to —78 °C in the presence of 1.6 equiv of LiCl gave an 86 14 mixture of syn/anti-(15) (eq 16). The syn product is an intermediate for the C-13 side chain of taxol. 

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