Patent Application: US-24808394-A

Abstract:
a short route to the total synthesis of the core skeleton of the taxol ring system is described . the same sequence of transformations can be carried out to make the 7 - hydroxy series , and connect the additional carbon atoms required for the a - ring . the number of steps to the taxane skeleton is 13 , making it the shortest route from readily available inexpensive starting materials .

Description:
there are two important structure activity relation ( sar ) features of taxol that should be taken into consideration in the synthetic design . the amino - alcohol side chain at c - 13 is essential for antitumor activity ( ojima , et al ., 1992 ; farina , al ., 1992 ; georg , et al ., 1992 ). methods are known for the conversion of the c - 13 alcohol into taxol ( denis , et al ., 1988 ; holton , 1991 ; holton , 1992 ; holton , 1993 ). the oxetane , c - 20 , c - 4 and c - 5 , also appears to be essential for full biological activity ( samaranayake , et al ., 1991 ; denis , et al ., 1988 ). simple model studies demonstrate that the oxetane functionality is readily constructed ( magee , et al ., 1992 ; berkowitz , et al ., 1987 ; lin , et al ., 1987 ; nicolaou , et al ., 1992 ) and recent work has also described more complicated compounds with the oxetane intact ( kingston , et al ., 1990 ). in the following examples , two series of compounds are described ; one with all the oxygen functionality and the other as the 7 - desoxy series . this example shows the strategy for the synthesis of a hydroxypyranenone which provided the basis for the stereoselective synthesis of the taxol core structure . the reactions are shown in schemes 1 through 9 with numbers in the figure corresponding to the compounds whose synthesis is described . the starting material was ethyl 6 - methyl - 5 - oxo - 6 - heptanoate 3 , comprising carbon atoms c - 9 , 8 , 19 , 7 , 6 , 5 , 4 and 20 of the taxol core . the ester 3 is a known compound and is readily prepared on a large scale ( 354 g ., 92 %) by the bis ( triphenylphosphine ) palladium ( ii ) chloride catalyzed coupling of methacryloyl chloride in toluene with the organozinc reagent 2 ( smith , et al ., 1973 ; tamaru , et al ., 1988 ). ethyl 4 - bromobutyrate may be converted directly into ethyl 6 - methyl - 5 - oxo - 6 - heptanoate 3 ( jubert and knochel , 1992 ), by conducting the above reaction in the presence of n - bu 4 n + i - . the c - 5 carbonyl group is selectively reduced with sodium borohydride / cerium ( iii ) chloride heptahydrate in ethanol to give (±)- ethyl 5 - hydroxy - 6 - methyl - 6 - heptenoate 4 ( 265 g ., 80 %)( luche , 1978 ). both enantiomers of 4 are readily synthesized . reduction of the c - 5 carbonyl group with borane - dimethylsulfide in tetrahydrofuran in the presence of a catalytic amount of the chiral reagent ( s )-(-)- diphenyl - 2 - pyrrolidino methanol oxaazaborole gave the 5r enantiomer of 4 . ( 89 %)( corey , et al ., 1987 ; corey and bakshi , 1990 ; mathre , et al ., 1991 ; jones , et al ., 1991 ). the enantiomeric excess is & gt ; 93 % as judged from the 1 h nmr spectrum of the derived mosher ester . the ( r )-(-)- diphenyl - 2 - pyrrolidino methanol oxaazaborole catalyst gave the 5s enantiomer of 4 , which corresponds to the natural absolute configuration of taxol at the c - 7 secondary hydroxyl group . this route is not dependent upon a naturally occurring chiral starting material , thereby giving access to both enantiomers of the precursors . the chirality is introduced by reagents that are used in catalytic quantities . the secondary hydroxyl group in 4 was protected as the t - butyl dimethylsilyl ether ( tbs ) by treatment with tbsotf / netpr i 2 , to give 5 ( 98 %); for example , other protecting groups are also suitable , e . g ., methoxymethyl ( mom ) ethers . treatment of 5 with lithium diisopropylamide in tetrahydrofuran at - 78 ° c . followed by either 2 - furfural or 3 - methyl - 2 - furfural gave the aldol products 7 ( 98 %) and 8 ( 100 %) respectively . the aldol reaction gives two sets of diastereomers which can be separated . in the optically active series the stereochemistry at the furan carbinol is not important , but the newly created asymmetric carbon atom adjacent to the ethyl ester ( c - 4 , taxol numering ) controls the absolute stereochemistry at the quaternary methyl ( c - 8 , taxol numbering ). to control the absolute stereochemistry at c - 4 ( and eventually at c - 8 ) the ( 1s , 2r )-(+)- norephedrine was used as a recyclable chiral auxiliary ( gage and evans , 1989 ; evans , et al ., 1981 ; evans , et al ., 1981 ). the same type of condensation at a higher oxidation level ( claisen - condensation ) was conducted . treatment of 5 and ethyl 3 - methyl - 2 - furoate with lithium bis ( trimethylsilyl ) amide in treathydrofuran at 25 ° c . gave the β - keto ester 9 (& gt ; 95 %) as a mixture of diastereomers . both diastereoisomers of 7 , 8 and 9 were taken through the subsequent reactions for two purposes ; first , to assess the stereochemical requirements of the intramolecular pyrylium - ylide cyclization in a relative and absolute stereochemical sense ; secondly , to have access to unnatural seteroisomers of the taxol precursors for biological evaluation . only the natural absolute taxol stereochemistry is shown in fig2 but it should be understood that all the diastereomers and their mirror images are readily separated to allow use in pure form rather than mixtures . the separation of the pure diastereomers in the optically active series is best carried out after the key pyrylium - ylide cyclization . the use of the chiral auxiliary approach in the 7 - oxy series as described in example 3 makes separation unnecessary , and a single optically active natural absolute stereochemistry versions of 20 and 21 becomes available . reduction of 7 using lithium aluminum hydride in tetrahydrofuran added at - 78 ° c . and warmed to 25 ° c . gave 10 ( 99 %). similarly , reduction of 8 gave the methyl analog 11 ( 100 %). the primary hydroxyl group in 10 and 11 was protected as the triphenyl methyl ether ( trityl ether ). treatment of 10 with triphenylmethyl chloride in dichloromethane followed by triethylamine and 4 - dimethylaminopyridine ( dmap , 0 . 1 equiv ) gave the trityl ether 12 ( 97 %). similarly , the methyl analog 11 gave the corresponding trityl ether 13 (& gt ; 90 %). other protecting groups for the primary alcohol can be used ; for example , the primary t - butyldimethylsilyl ethers . oxidation of the furan carbinol 12 in dichloromethane with tert - butyl hydroperoxide in the presence of a catalytic amount of vanadyl acetyl - acetonate ( ho and sapp , 1983 ) at - 10 ° c . gave the rearranged hydroxypyranenone 14 ( 98 %). the methyl analog 13 gave 15 in 98 % yield . the hydroxypyranenone 14 was converted into its derived acetate 16 by treatment with acetic anhydride in dichloromethane containing pyridine or triethylamine and a catalytic amount of 4 - dimethylamino pyridine ( dmap ). it is not necessary to purify the acetate since it can be used directly in the pyrylium ylide cyclization reaction . the key reactions for the synthesis of the core taxol structure are shown in scheme 9 . results of this work indicated that the c - 4 substituent stereochemistry controls the stereochemistry of the angular methyl group at c - 8 . the numbering system for taxol is shown in fig1 . the acetate 16 is a single stereoisomer at c - 7 and a mixture at c - 4 . the other two stereogenic centers are destroyed in the conversion of 16 into the pyrylium ylide intermediate a / b . therefore , in principle , heating 16 should give rise to the two pyrylium ylides a and b which can undergo intramolecular cycloaddition to give four products 18 , 20 , 22 and 23 . in the racemic series the same compounds would be formed along with their mirror - image antipodes . the same situation prevails for the methyl analog series . unexpectedly , heating 16 in dichloromethane or more rapidly in toluene in the presence of dbu gave only two cycloaddition productions 18 and 20 ( 65 - 85 %) ( katritzky and dennis , 1989 ). this result indicated that the c - 4 substituent stereochemistry controls the stereochemistry of the angular methyl group c - 8 . the c - 8 methyl group is always trans to the c - 4 substituent . the methyl group at c - 8 is also always trans to the oxygen bridge in the newly formed tetrahydrofuran ring in order to maintain the thermodynamically preferred cis fusion of this ring to the cyclohexane ring . this result was significant because it showed that of all the possible stereoisomers of 16 , only one , namely the 4r , 7s diastereoisomer , gives the correct absolute stereochemistry at both c - 7 and c - 8 . in the racemic series the relative stereochemistry of 18 was established by single crystal x - ray crystallography . in the optically active series the absolute stereochemistry of the mirror image of 18 was established by single crystal x - ray crystallography of a derived camphanic acid ester . similarly in the methyl series heating 17 in the presence of dbu in dichloromethane or toluene gave 19 and 21 ( 65 - 85 %). the isomers 18 / 20 and 19 / 21 can be separated by fractional crystallization or by chromatography of the detritylated derivatives . the tricyclic enone 21 contains carbon atoms c - 2 , 3 , 4 , 20 , 5 , 6 , 7 , 8 , 19 , 9 , 10 , 11 , 15 and 16 ( or 17 ) of the taxol core . the c - ring is intact , and the b - ring requires a one carbon ring expansion from a seven - membered ring to an eight - membered ring to complete the carbon skeleton . based on these results , a series of transformations was performed using a more elaborate furan that incorporates an extra carbon atom destined to become part of the eight - membered ring . these reactions are shown in schemes 10 - 17 . the known furan 24 was reduced with lithium aluminum hydride to give the derived alcohol 25 . the alcohol 25 was treated with n - butyl lithium followed by carbon dioxide to give the acid 26 . treatment of 26 with benzyl bromide / sodium hydride / dimethylformamide gave 27 which was directly hydrolyzed to the acid 28 using sodium hydroxide in isopropanol . the acid 28 was converted into the acid chloride 29 by treatment with oxalylchloride ( sodium salt ). the acid chloride 29 was treated with the lithium enolate derived from the amide 30 to give the adduct 31 which was reduced with lithium borohydride to give the diol 32 and the recoverable chiral auxiliary . the diol 32 was protected as the t - butyl dimethylsilyl ether 33 or trityl ether . ( scheme 11 ). the ether 33 was oxidized using tert - butyl hydroperoxide in the presence of a catalytic amount of vanadyl acetylacetonate ( ho and sapp , 1983 ) at - 10 ° c . to give the rearranged hydroxypyranenone 34 . the hydroxypyranenone 34 was converted into its derived acetate by treatment with acetic anhydride in dichloromethane containing pyridine or triethylamine and a catalytic amount of 4 - dimethylamino pyridine ( dmap ). it was not necessary to purify the acetate since it could be used directly in the next stage . heating the acetate in dichloromethane or more rapidly in toluene in the presence of dbu gave only two cycloaddition products 35 ( 79 % from 31 ) and the diastereomer corresponding to 18 / 19 . only the diastereomer with the correct absolute configuration , namely 35 is shown . in the series with a chiral auxiliary , described in example 3 , the compound 35 was obtained as a single diastereomer and did not require any separation from unnatural stereoisomers . a unique approach to synthesis of the taxol core structure is illustrated in this example . the results demonstrate that transannular interactions across the eight membered b ring of taxol are an intrinsic property of these structures . taxol is not a particularly potent antitumor drug , and it is suspected that it is extensively degraded before it reaches the target cells . a plausible pathway for its degradation is shown in fig4 . it involves the known reversible retro - aldol process that causes c - 7 epimerization to give 65 via the aldehyde 64 , and irreversible β - elimination of the c - 5 oxetane c - o bond to the α , β - unsaturated aldehyde 66 . since it is known that the oxetane is necessary for activity this degradation would greatly reduce the potency of taxol . a way to prevent this process is to remove the c - 7 hydroxyl group . previous efforts to carry out this deoxygenation on taxol have failed . the only identifiable materials were ring b cleavage products . consequently , 7 - desoxytaxol 67 is a very important target for total synthesis . using the same strategy employed for the synthesis of 20 / 21 the methallyl chloride was converted into ethyl 6 - methylhept - 6 - enoate 68 ( 70 %). conversion of 68 into 72 via 69 , 70 and 71 utilizes the same chemistry as in the 7 - hydroxy series . the reaction scheme is shown in scheme 19 . the same sequences of transformations can be carried out with the c - 4 carbinol protected as a t - butyldimethylsilyl ether or a triisopropylsilyl ether . other protecting groups can be used . ## str13 ## using the more elaborate furan 27a , the ester 68 was treated with lithium diisopropylamide and quenched with 27a to give 73 . scheme 20 illustrates the reaction scheme . reduction of 73 using lithium aluminum hydride followed by protection of the primary hydroxyl group with t - butyldimethylsilyl chloride gave 74 ( 80 % from 68 ). oxidation of the furan carbinol 74 in dichloromethane with tert - butyl hydroperoxide in the presence of a catalytic amount of vanadyl acetyl - acetonate at - 10 ° c . gave the rearranged hydroxypyranenone 75 ( 90 %). treatment of 75 with acetic anhydride followed by heating the derived acetate in toluene in the presence of dbu gave 38 ( 64 %). the additional methyl of the gem methyl group was introduced by treatment of 38 with methyl magnesium bromide in the presence of a catalytic amount of cubr . sme 2 in tetrahydrofuran to give 39 ( 75 %). ## str14 ## hydrogenolysis and mild acid hydrolysis of 39 removed both the benzyl and t - butyldimethylsilyl ether protecting groups resulting in the diol 40 . treatment of 40 with zn / hgcl 2 / h 3 o + resulted in reductive cleavage of the oxido bridge and intramolecular ketalization to give 41 ( 80 %). the carbinol 41 was converted into the trifluoromethanesulfonate derivative 42 by standard methods . solvolysis of 42 in methanol resulted in ketal fragmentation , followed by ring expansion and proton loss , to give the eight - membered ring ketone 43 . exposure of 43 to aqueous acid gave the hemiketal hydrate 44 , whose structure was confirmed by x - ray . heating 44 converted it back into the ketone 43 . these results revealed the advantage of differentiation of the two ketone carbonyl groups in 43 ( scheme 13 ). the rearrangement may be carried out at a higher oxidation level to give a more functionalized eight - membered ring . oxidation of the carbinol 44 using tpap ( cat )/ nmno ( griffith and ley , 1990 ) gave the aldehyde 45 , which upon treatment with bf 3 oet 2 resulted in rearrangement to give 46 . the compound 46 has the c - 10 ketone thereby enabling functionalization at c - 9 , and the correct trans b / c ring fusion ( scheme 14 ). to control the absolute stereochemistry at c - 4 ( and eventually at c - 8 ), ( 1s , 2r )-(+)- norephedrine and r -(+)- phenylalanine were employed as recyclable chiral auxiliaries . compound 30 ( ee & gt ; 90 %) was synthesized using chiral claisen technology ( evans , 1981 ). the amide in 31 was readily reduced using lithium borohydride in tetrahydrofuran to give the diol 32 and the recoverable chiral auxiliary . the reactions are shown in scheme 11 . an identical sequence of transformations in the series with the c - 7 hydroxyl substituent absent was carried out . ( scheme 18 ). consequently , all of the compounds described in both antipodal forms ( i . e . both natural and unnatural absolute stereochemistry ) are accessible . scheme 21 outlines a total synthesis of the a / b / c ring system of taxol with the correct stereochemistry . the approach is based partly on the &# 34 ; cyclopropane &# 34 ; approach to ring expansion to the b / c rings . the entire sequence is 13 steps , starting from the relatively simple allyl ester shown in scheme 4 . ## str15 ## the overall route in scheme 21 comprises thirteen steps from the readily available heptenoate ester . hydrolysis to an acid and activation of the acid by conversion into an acid chloride , followed by addition of the lithium salt of the chiral auxiliary resulted in the precursor in three steps . claisen condensation and reduction in the same reaction vessel produced the diol which is protected on the primary alcohol and oxidatively rearranged to provide the pyrylium ylide precursor in three steps . acetylation of the pyrylium ylide precursor , and heating in the presence of dbu , gave the cycloheptenone in two steps . bromination of the cycloheptenone and treatment with cyanide anion resulted in the activated cyano - enone in two steps . gem - methyl cyclopropanation and reductive cleavage of the internal cyclopropane bond resulted in ring expansion to form the eight - membered ring b of the taxol core skeleton in two steps . finally , treatment of the eight membered - ring cyano compound with vinyl lithium followed by acidic workup formed the a - ring of taxol in one step . the inventor has discovered that the fourth step in the synthesis providing the precursor diol is readily performed as a &# 34 ; one - pot &# 34 ; reaction . the claisen condensation shown after step 3 need not be isolated but is treated directly with a reducing agent such as lithium borohydride to give the diol . while the compositions and methods of this invention have been described in terms of preferred embodiments , it will be apparent to those of skill in the art that variations may be applied to the compositions , methods and in the steps or in the sequence of steps of the method described herein without departing from the concept , spirit and scope of the invention . more specifically , it will be apparent that the synthesis allows for a route to a wide variety of novel taxol related compounds and that modifications of the overall synthesis by well known procedures and reactions may be employed . all such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit , scope and concept of the invention as defined by the appended claims . all claimed matter and methods can be made and executed without undue experimentation . the following references to the extent that they provide exemplary procedural or other details supplementary to those set forth herein are specifically incorporated herein by reference . berkowitz , w . f ., amarasekara , a . s . and perumattam , j . j ., j . org . chem ., 52 , 1119 , 1987 . blechert , s . and guenard , d ., &# 34 ; 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the chemistry of taxol , a clinically useful anticancer agent &# 34 ;, j . nat . prodt ., vol . 53 , 1 , 1990 . krucoff , c ., &# 34 ; unlocking the secrets of taxol &# 34 ;, the saturday evening post , ( sep ./ oct ., 1991 ). lin , j ., nikaido , m . m . and clark , g ., j . org . chem ., 52 , 3745 , 1987 . lipshutz , b . h ., comprehensive organic synthesis , ed ., b . m . trost , vol . 1 , 1991 . pergamon press . lythgoe , b ., &# 34 ; the taxus alkaloids &# 34 ;, the alkaloids , ed . r . h . f . manske , vol . 10 , ch . 14 , 1968 , academic press , new york . magee , t . v ., bornmann , w . g ., isaacs , r . c . a . and danishefsky , s . j ., j . org . chem ., 57 , 3274 , 1992 . mangatal , l ., adeline , m . t ., gumnard , d ., gueritte - voedelein , f . and potier , p ., tetrahedron , 45 , 4177 , 1989 . mathre , d . j ., jones , t . k ., xavier , l . c ., blacklock , t . j ., reamer , r . a ., mohan , j . j ., jones , e . t . t ., hoogsteen , k ., baum , m . w . and grabowski , e . j . j ., j . org . chem ., 56 , 751 , 1991 . nicolaou , k . c ., hwang , c . k ., sorensen , e . j . and clairborne , c . f ., j . chem . soc . chem . commun ., 1117 , 1992 . ojima , i ., habus , i ., zhao , m ., georg , g . i . and jayasinghe , l . r ., j . org . chem ., 56 , 1681 , 1991 . ojima , i ., habus , i ., zhao , m ., zucco , m ., park , y . h ., sun , c . h . and brigaud , t ., tetrahedron , 48 , 6985 , 1992 . ojima , i ., fenoglio , i ., park , y . h ., sun , c . m ., appendino , g ., pera , p . and bernacki , r . j ., j . org . chem . 59 , 515 , 1994 . palomo , c ., arrieta , a ., cossio , f ., aizpurua , j . m ., mielgo , a . and aurrekoetxea , n ., tetrahedron letters , 31 , 6429 , 1990 . rowinsky , e . k ., cazenave , l . a . and donebower , r . c ., j . national cancer inst . vol . 82 , 1247 , 1990 . samaranayake , g ., magri , n . f ., jitrangsri , c . and kingston , d . g . i ., j . org . chem ., 56 , 5114 , 1991 . sammes , p . g ., street , l . j ., j . chem . soc ., chem . comm ., 1056 , 1982 . sammes , p . g ., street , l . j ., j j . chem . soc ., chem . comm ., 666 , 1983 . smith , r . d ., simmons , h . e ., org . synth ., collected vol . 5 , 855 , 973 . swindell , c . s ., &# 34 ; taxane diterpene synthesis strategies . a review &# 34 ;, organic preparations and procedures int ., 23 ( 4 ), 465 , 1991 . tamaru , y ., ochai , h ., nakamura , t . and yoshida , z ., org . synth ., 67 , 98 , 1988 . wender , p ., lee , h . y ., wilhelm , r . s ., williams , p . d ., j . am . chem . soc ., 117 , 8954 , 1989 . ed . winkler , j . d ., &# 34 ; total and semi - synthetic approaches to taxol &# 34 ;, tetrahedron , vol . 48 , no . 34 , 1992 . &# 34 ; bark for cancer &# 39 ; s bite &# 34 ;, time , jul . 1st , 1991 . &# 34 ; a new cancer drug may extend lives - at cost of rare trees &# 34 ;, the wall street journal , tuesday , apr . 9th , 1991 .