GB2215331A - Improvements in or relating to the synthesis of vinblastine and vincristine and related compounds - Google Patents

Description

IMPROVEMENTS IN OR KELATING TO THE SYNTHESIS OF VINBLASTINE AND VINCRISTINE AND RELATED COMPOUNDS The present application relates to an improved method for producing dimer alkaloid compounds especially of the Catharanthus (Vinca) alkaloid group and, in particular provides an improved iminium/enamine reduction procedure applicable thereto. The general field of the invention is more specifically the production of anti-viral, anti-leukemic (antineoplastic) compounds, vincristine and vinblastine of Formula I.

OH OH 7 Indote Unit N 18 (Catharanthine) H H N 14 Unit 10hH1 (Vindotine) H3CO 1 N H H3C The above compound, when R is COOCH2 and R1 is OCH3, is vinblastine (NSC 49482) and when R is COOCH3, R1 is OCH3 and N1 is N-CHO (N-formyl) is vincristine (NSC 67574).

Compounds of the series of dimeric alkaloids encompassed by Formula I which include important antitumor agents, are formed from an indole, such as catharanthine (Formula II, R = COuCH3), and a dihydroindole unit, e.g. vindoline (Formula III), in which the halves are linked via a carbon-carbon bond involving an aliphatic centre C18 in the indole unit and an aromatic carbon C15 in the vindoline portion

Our copending Patent Application No. 8801296 (Serial No. ) describes and claims a process for the production of dimeric compounds represented by the following formula

wherein in formula XXI and where indicated in any of the substituting groups defined below alkyl = CH3 or (CH2)nCH3 where n = 1-5 R1 = CH3, cao or COR5, R5 being alkyl or aryl, R2 = H or CO-alk R3 = H R4 = COO-alk, CONH-NH2 or CONR13 R14 where R13 and H14 can be any member of the group consisting of hydrogen, alkyl, substituted alkyl, aryl or substituted aryl functions Z = -CH=CH- or -CH2-CH2 H = one of formulae XXII or XXII I

joined at the carbon atom of the latter carrying K7 or K11 respectively and wherein R7 = H or COO-alk R8 = H, OH, O-alk, OCO-alk or alkyl R9 = H, OH, O-alk, OCO-alk or alkyl R10 = H, OH, O-alk, OCO-alk R@@ = H or COO-alk R12 = H or alkyl from an appropriate indole unit of the natural Iboga alkaloid family containing an aza bicyclo-octane portion as depicted in Formula XXI and an appropriate dihydroindole unit of the natural Aspidosperma and Vinca alkaloid families having the general structure depicted in Formula XXII or XXIII; the stereochemistry of the carbon-carbon linkage between these two units being identical with that of vinblastine which consists of (a) forming an N-oxide intermediate at a temperature of -70 to +400C from said indole unit by oxidizing the bridge nitrogen and without isolating said intermediate;; (b) treating said N-oxide indole intermediate in the presence of one member of the group consisting of acetic anhydride, halogenated acetic anhydride, and acetyl chloride to effect a Polonovski-type fragmentation reaction; (c) without isolating the product of step (b), coupling said reaction product with a dihydroindole unit in the presence of acetic anhydride, halogenated acetic anhydride, and acetyl chloride at a low temperature of from -700C to +400C under inert conditions;; (d) the product of step (c) is reduced by 1,4-dihydropyridine compounds represented by formula IX

where K' 3 and R'5 in formula IX are carboxylic esters (COO-alk) and R'1, R'2, '4 and '6 are members of the group consisting of H, alkyl, aryl (Hantzch ester series) or N-substituted 1,4-dihydronicotinamides where R' is a substituted alkyl or substituted aryl function, for example, benzyl and '3 is CONK7H8 where R7 and R8 can be any member of the group consisting of hydrogen, alkyl or aryl functions;; wherein said reduction is conducted under an inert atmosphere at -600 to +60 C and preferably in the temperature range 4 0C to 20 0C and utilizing solvents selected from the group consisting of lower alkyl alkanols, acetonitrile, dimethyl sulfoxide, dimethylformamide, dioxane, tetrahydrofuran, and chlorinated lower hydrocarbons; (e) the enamine obtained in step (d) is used to prepare a corresponding iminium intermediate by an oxidative process including: (1) controlled aeration/oxygenation in the ambient atmosphere; or (2) controlled aeration/oxygenation in presence of a metal ion; or (3) controlled aeration/oxygenation in presence of a flavin coenzyme; or (4) controlled aeration/oxygenation in presence of a treating agent selected from hydrogen peroxide and hydroperoxides; utilizing an organic solvent at pH 5-9 and a reaction temperature of -600C to + 600C; (f) the product obtained in step (e) is converted to the target compound of formula XXI by reduction with alkali metal borohydride.

The process including the formation of the final product of formula XXI may be conducted in a one-pot operation. Alternatively, the products of steps (c), (d) and (e) may be isolated before use in subsequent steps, isolation occurring in each case by solvent evaporation preferably at low temperature in the range of -20 0C to OOC, According to one aspect of the present invention, there is provided a process for effecting the conversion of an iminium compound of formula VIa

to an enamine of formula Villa

wherein in formulae VIa and Villa and where indicated in any of the substituting groups defined below alkyl = CH3 or (CH2)nCH3 where n = 1-5 K1 = CH3, CHO or COR5, H5 being alkyl or aryl, R2 = H or CO-alk K3 = H R4 = COO-alk, CONH-NH2 or CONR13 R14 where R13 and H14 can be any member of the group consisting of hydrogen, alkyl, substituted alkyl, aryl or substituted aryl functions Z = -CH=CH- or -CH2-CH2- K7 = H or COO-alk K8 = H, OH, O-alk, OCO-alk or alkyl R10 = H, OH, O-alk, OCO-alk R12 = H or alkyl which comprises reducing the iminium compound with a 1,4-dihydropyridine compound of formula IXa

wherein K1' is an electron rich functional group comprising a carbon skeleton branching in the l-position and R3' @ is selected from cyano, 2'-carbamoyl-pyrrolidinyl and a group of formula CONH' 7H' 8 wherein H' 7 and '8 which, can be the same or different, are selected from hydrogen, alkyl and aryl, or H1' is benzyl or diphenylmethyl and H3' is selected from cyano and 2 '-carbamoyl-pyrrolidinyl.

Thus, we have now devised an improved procedure which is based on (1) improvement of the novel reduction method of the aforementioned copending application, in which, in the production of vinblastine, conversion of the iminium intermediate (Formula VI) ) to the enamine (Formula VIII)

I.e. N vin I vg o-oCH 0 I RCO2CH ccM a 1RN/ w i.e. ~ < 0 N1 vd ;' 0 N OH | CH3O + C'H3R o11 CH-I VIII R=cO2CH3 as described in step (d) of the synthesis procedure of the aforementioned copending application is achieved by the use of various new 1,4-dihydropyridines of the aforesaid formula IXa.Of special importance is the presence of electron rich functional groups such as carboxylic esters and carboxylate salts in K1 of formula IX capable of coordination with the positive charged iminium intermediate (formula VI). In general the electron rich functional group is selected from diphenylmethyl, l-carboxyalkyl in the form of an alkyl ester or salt and wherein the alkyl moiety has two or more carbon atoms and sugar moieties, C-N linkage attaching the moiety to the dihydropyridine ring taking place at a dehydro carbon atom of the sugar moiety which is generally in ring form and is preferably a 2' ,3' ,4' ,6'-tetracetyl-beta-D-glucopyranosidyl group.

Specific examples of preferred 1,4-dihydropyridine compounds for use in the reduction are set out in Tables I to III which follow from which it will be seen that particularly advantageous results are obtained with compounds wherein K1' is benzyl and K3 ' is cyano or 2'-carbamoylpyrrolidinyl-amide).

T a b l e I NADH Models for 1,2-Versus 1,4-Reduction of Iminium Intermediate VI

I MiN% ~ ..oNHi (Enamine) (3' 4,-dehydro NIROCIlCH3 vinbiastine) VIII CH3Qvi CH3 0 VII R=C02CH3 Reducing Agents Procedure

Reference has been made to coordination of electron rich functional groups with the positively charged iminium intermediate. Such coordination increases both the regioselectivity (1,4-reduction over 1,2-reduction) and the rate of the reduction of iminium intermediate (Formula VI), leading to an improved yield of the enamine (Formula VIII) (see Table II).Further improvement in the formation of the latter compound is achieved by performing the reduction of iminium intermediate (Formula VI) ) and maintaining the reaction mixture before any subsequent manipulation at a low temperature (0 to -70 C) preferably below -40 C (see Table III).

More particularly, specific examples related to the above improvements for the synthesis of the enamine (Formula VIII) by the 1,4-reduction of the iminium intermediate (Formula VI) are given in Examples 1-9 which follow (Procedures C to K). Résults of these examples are summarized in Tables 2 and 3. Table 2 indicates that reduction procedures (Procedures I and J as in Examples 7 and 8), employing reducing agents Formula XXXI and Formula XXXII, respectively afford the best yields of the 1,4-enamine (Formula VIII). Table 3 shows the effect of temperature in the reduction of iminium intermediate (Formula VI) by the reducing agent Formula XXXII.

Lowering the reduction temperature to -40 0C (Procedure K(ii) Example 9) resulted in an increase in both the ratio of 1,4-reduction production, enamine (VIII), versus 1,2-reduction product, 3,4-dehydrovinblastine (VII), (4,2:1), as well as in the overall yield of the reaction (85%).

Table II:Effect of Reducing Agent on 1,4- vs. 1,2-Reduction of Iminium VI l a 2 3 Reduction 1,4 1,4:1,2-Reduction Yield ( %) Procedure Products 5 A 1:1 75 B5 1:1 60 C 0.9:1.0 60 D 1:1 40 E 1.1:1 60 F 2:1 65 G 1.5:1 70 H 1.1:1 65 I 2.3:1 70 J 2.2:1 70 1 Typical Procedure: 100 mg iminium VI in 6 ml methanol reducing agents C-J (1-6 eq.) dissolved in 6 ml methanol were added. Full details in Experimental Section.

2 By reverse phase HPLC quantitation.

3 Combined 1,2-reduction (3,4-dehydrovinblastine, VII) 1,4-reduction (enamine VIII) products.

4 All of these reactions were conducted at 20 OC.

5 Procedures A and B are presented in the parent application and presented here for comparison.

Table III: Effect of Temperature on 1,4- vS. 1,2 Reduction of Iminium VI 3 Reduction Temp ( C) 1,4:1,2 Yield Procedure Reduction ( %) Products D 20 2.2:1 70 K (i) -20 3.2:1 80 K (ii) -40 4.2:1 85 1 Typical procedure as in Table 2.

2 Quantitation by HPLC.

3 Combined 1,2-reduction (3 ,4-dehydrovinblastine, VII) + 1,4-reduction (enamine VIII) products.

The production of compounds of the aforesaid general Formula XXI has now been further improved in other respects applicable to the overall procedure of the aforementioned copending application. Thus this invention also provides in a second aspect, a process for the production of dimeric compounds represented of the aforementioned formula XXI from an indole unit of the natural Iboga alkaloid family containing an aza bicyclo-octane portion and a dihydroindole unit of the natural Aspidosperma and Vinca alkaloid families, the stereochemistry of the carbon-carbon linkage between these two units being identical with that of vinblastine, which consists of (a) forming an N-oxide intermediate at a temperature of -70 to +400C from said indole unit by oxidizing the bridge nitrogen and without isolating said intermediate;; (b) treating said N-oxide indole intermediate in the presence of one member of the group consisting of acetic anhydride, halogenated acetic anhydride, and acetyl chloride to effect a Polonovski-type fragmentation reaction; (c) without isolating the product of step (b), coupling said reaction product with a dihydroindole unit in the presence of acetic anhydride, halogenated acetic anhydride, and acetyl chloride at a low temperature of from -700C to +400C under inert conditions; (d) the product of step (c) is reduced by the process of the first aspect of this invention; (e) the enamine obtained in step (d) is used to prepare a corresponding iminium intermediate of formula XVI or XVIa set out herein by an oxidative process including: : (1) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough; (2) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of a metal ion; (3) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of a flavin coenzyme; (4) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of hydrogen peroxide and/or a hydroperoxide of formula K-00H where H is alkyl or aryl;; (f) the product obtained in step (e) is converted to the target compound of formula XXI by reduction with alkali metal borohydride.

In Formula XXI pictured above alk represents a lower alkyl group of C1-C6 and preferably C1-C3; aryl (see R4) is mono-aryl, for example benzyl, styryl, and xylyl; K1 is selected from hydrogen, alk, CHO and CO1 < 5 where K5 is alkyl or aryl; R2 and K3 are selected from hydrogen and -CO-alk; R4 is preferably selected from CO-alk, CONH-NH2, CUNH2, CONHH6, and CON(R6)2 wherein K6 is alkyl; Z is selected from -CH2-CH2 - and -CH=CH- and R is a member of the indole family represented by either Formula XXII where R7 is selected from hydrogen and COO-alk;H8 is selected from hydrogen, OH, O-alk, OCo-alk or alkyl; K9 is selected from hydrogen, OH, O-alk, OCO-alk and alk; R10 is selected from hydrogen, OH, 0-alk, OCO-alk, or Formula XXIIL where R11 is selected from hydrogen and COO-alk; K12 is H or alkyl.

The overall process for producing a compound of formula XXI has been further improved at (e), the oxidative transformation of the enamine (Formula VIII) to the iminium intermediate (Formula XVI). Various parameters for this oxidative transformation have been studied to optimize the yield of the production, in particular, of vinblastine (Formula I). Their results are shown in Tables 4 to 7 set out hereinafter.

Table IV Effect of Ferric Chloride gn Production of Vinblastine (I) from Enamine VIII Amount of FeCl3 Oxidation Conditions % Yield (Equivalents) (Temp., time) of Vin blastine 10 0 Air 1 C. 5 min 0 1 AiR11 .0 C, 5 min 13.3 2 Air 0 C, 5 min 19,0 3 Air . 0 C, 5 min 10.4 1 At a rate of 60 ml/min.

2 By reverse-phase HPLC quantitation, after reductive work-un with NaBH4.

3 Enamine VIII generated at -40 C (Procedure K (ii)).

TableV:Effect of Time of Oxidation on Production of Vinblastine (I) from Enamine VIII@ Time1 (min) Yield of Vinblastine 1 8.2 5 15.4 10 15.5 15 15.7 45 6.5 1 Reaction conditions: - 2 eq. ferric chloride added, air bubbled through the solution at 60 ml/min at 00 C.

2 By reverse-phase HPLC quantitation after reductive work-up with NaBH 4 3 Enamine VIII gererated at -40 C (Procedure K (it)).

Table VI:Effect of Oxidation Temperzture on Production of Vinblastine (I) from Enamine VIII Temp., C %Yield of 2 vinblastine -40 3.7 -23 6.2 0 19.6 20 20.6' 45 16.0 1 Reaction conditions: 2 eq. ferric chloride added1 air bubbled through the solution at 60 ml/min for 15 min.

2 By reverse-phase HPLC quantitation after reductive work-up with NaBH4 3 Enamine generated at -40@ C (Procedure K (ii)).

Table VII Effect of Dilution on Production of Vinblastine (I) from Enamine VIII Dilution Factor 1 4 %Yield of 2 Vinblastine 1 19.6 5 25.2 10 30.1 20 29.6 50 24.7 1 Reaction conditions: 2 eq. ferric chloride added, air bubbled through the solution at 60 ml/min for 15 min. at 0 C.

2 By reverse-phase HPLC quantitation after reductive work-up with NaBH 4 3 Enamine generated at -40 C (Procedure K (ii)).

4 Dilution Factor 1 - 100 mg Iminium VI in 6 ml methanol to which reducing agent Formula XXXII (6 eq.) in 6 ml methanol was added. (Total volume - 12 ml) Dilution Factor 5 - Total volume of 60 ml, etc.

Table 4 indicates the effect of ferric chloride concentration. The use of two equivalents of ferric chloride provides the highest yield of vinblastine (I).

Table 5 indicates results relating to yield of vinblastine versus time of oxidation. The maximum yield of vinblastine (I) is reached after 5 to 20 minutes of aeration in the presence of 2 equivalents of ferric chloride.

Table 6 shows the results of employing various oxidation temperatures on the yield of vinblastine (I) when oxidation is effected, in the presence of two equivalents of ferric chloride. The temperature range of OOC to 200C provide the highest yield of vinblastine (I) after a reductive work-up with NaBH4, as quantified by reverse-phase HPLC.

The effect of dilution of the enamine (VIII) solution on the production of vinblastine (I), is indicated in Table 7. A dilution factor of 10 to 20 on the enamine (VIII) solution before aeration in the presence of ferric chloride (2 equivalents) at 0 C, affords the best yield of vinblastine (I) as quantified by reverse-phase HPLC after reductive work-up with NaBH4.

In summary, the specific improvements in the present application as they relate to the oxidative transformation of the enamine (VII1), involve the dilution (5 to 50 fold) of the enamine (Formula VII1) solution, obtained in the above reduction of iminium intermediate (Formula VI), by the same solvent used in the reduction. For practical purposes, the preferred dilution factor is usually in the range of 8 to 12 fold.

The dilution procedure is conducted at a low temperature (0 C to -70 C), preferably below -40 C and under cover with inert conditions such as nitrogen or an inert gas of Group Zero of the Periodic Table (argon, helium, neon, etc.). After this dilution process, the oxidative transformation of the enamine (Formula VIII) can be carried out as described above, typically involving aeration at 60 ml/min for 15 min at 0 C in the presence of ferric chloride (2 eq.), to afford the corresponding iminium intermediates (Formula XVI).

A final area of improvement of the overall procedure for producing the compound of formula XXI lies in the reduction of the iminium intermediates (Formula XVI) by alkali metal borohydride (NaBH4, KBH4, LBH 4 etc.) to yield for example vinblastine (Formula I) and/or leurosidine (Formula XXXV).

Formula XXXV (Leurosidine) The iminium intermediates (Formula XVI and Formula XVla where leurosidine production is concerned)

produced by the oxidative transformation of the enamine (Formula VIII) are reduced by the addition of alkali metal borohydride at low temperature (4 C to -20 C), preferably at 0 C. The reduction is conducted at a pH lower than 8.5 and preferably at 7.5 to 8. The total reaction mixture is then concentrated in vacuo at low temperature (OOC to 100C) before extraction and isolation of alkaloid products using techniques for example as described in the Examples of Application No. 8801296.

(Serial No.

For practical purposes, all the above improvements can be incorporated, as indicated, directly into the overall process conducted in a one-pot operation from the indole unit-providing compound (Formulas II, XXII or XXIII) and the dihydroindole unit-providing compound (Formula XXI, K = H) to the final products of Formula XXI. isolation of the various intermediates (Formulas XXIV, VI, VIII, XVI and XVIa) can be omitted. The overall procedure is summarised in Scheme 1 which follows: Scheme 1. Optimum procedure for one-pot process vinblastine from catharanthine and vindoline.

0 XXIV cOzc + L(cF3CO)2O -600C cH,o' O-CH3 H3 t} tl3 R QOH VI cH3O CHO CH3 R 6 CONH2 R = co2CH3 Y tcX C 1,2-Reduction + M;i CtO % R t-CHD CH D + -CH3 Vlll Enamine Anhydrovinbiastine Ratio 4.2: 1 Scheme 1 - continued

%};Ni;x.o1 R: COH3 CH30 Rz O2CH3 Vlil 3 R FtC13 (2 cq(tl \ nO C Aír, ka ml7min 15 min Dilution Factor 10 00e / \ ItT / N1 N1 OH co R CHO OH - aVI H3 R NaBH9 XVla 1 | NaBH4 ii A$ A 5 I :H3 R H3 R O Vinbbstine XXXV Leurosidine Overall Yield: Vinbiastine (42%); Leurosidine (17%);Anhydrpvinblastine (18% AS already stated herein, of special importance in step (d) of the overall process is the presence of electron rich functional groups in the 1,4-dihydropyridine compound used (e.g., carboxylic esters and carboxylate salts), more specifically in s1 of Formula IXa. The groups are capable of coordination with the positively charged iminium intermediate (Formula VI).

Some examples of these 1,4-dihydropyridine compounds are given in Table 1 (e.g., Formulas XXXI and XXXII). Such coordination increases both the regioselectivity (1,4-reduction over 1,2-reduction) and the rate of the reduction of iminium intermediate (Formula VI), leading to an improved yield of the enamine (Formula VIII).

The reduction is conducted under an inert atmosphere at -600C to +600C but preferably in the temperature range -200C to -40 0C. The solvents employed are alcohols, acetonitrile or higher members of this series, dimethyl sulphoxide, dimethylformamide, various ethers such as dioxane and tetrahydrofuran, and chlorinated hydrocarbons.

The product of step (d) above, an enamine, Formula VIII, is isolated by solvent evaporation, preferably at low temperature in the range of -200C to 0 0C.

The oxidative processes in step (e) (1)-(4) are conducted in organic solvents such as alcohols, acetonitrile or higher members of this series, dimethyl sulphoxide, dimethylformamide, various ethers such as dioxane, tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene, etc. An aqueous buffer (e.g., phosphate, Tris-HCl, MES buffers) at pH 5-9, but preferably in the range of 6-8, can be used as co-solvent. The reaction temperature may vary from -6OOC to +60 C.

The oxidative processes of step (e) for oxiding the enamine to an iminium intermediate (Formula XVI and XVIa) are (1) controlled aeration/oxygenation in which a solution of the enamine is stirred in open air with a stream of air/oxygen bubbled through the solution; (2) as in step (e)ll) but with the addition of a ferric chloride; (3) as in step (e)(l) but with the addition of a flavin coenzyme, generally as represented by Formula XII:

to generate, in situ, the corresponding 1,5-dihydroflavin coenzyme, as represented by Formula XIII

The oxidative processes are conducted in organic 'solvents such as alcohols, acetonitrile or higher members of this series, dimethyl sulphoxide, dimethylformamide, ethers such as dioxane, tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene, etc.

The following Examples illustrate this invention.

Each of Examples 1-8 was conducted at 200 C.

EXAMPLE 1 Reduction of Iminium Intermediate (Formula VI) with l-diphenylmethyl 1,4-dihydronicotinamide (Formula IX, K1 = diphenyl methyl; R2 = R4 = R5 = R6 = H; R3 = CONH2 (Formula XXVII) - Synthesis of Enamine (Formula VIII) Procedure C) To a stirred solution of 100 mg of iminium intermediate (VI) in degassed ethanol (6 ml) was added l-diphenylmethyl-1, 4-dihydronicotinamide (Formula XXVII) (76 mg, 2.5 equivalents) in methanol (6 ml) under a positive atmosphere of argon, the reducing agent being added portionwise at a rate of 1 equivalent each 60 min.

After completion of the reaction, reverse phase HPLC analysis (Waters Kadial-Pak C18 or CN cartridge, methanol-X2U-Et3N as solvent system) was effected and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 0.9:1 (60% yield).

EXAMPLE 2 Keduction of Iminium Intermediate (Formula VI) with l-Benzyl-3-cyano-1,4-dihydropyridine (Formula IX, K1 = benzyl; K, K 5 K6 H; H, H3 = CN) (Formula XXXIII) - Synthesis of Enamine (Formula VIII) - (Procedure D).

To a stirred solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) was added l-benzyl-3-cyano-1,4-dihydropyridine (Formula XXXIII) (206 mg, 10 equivalents) in methanol (10 ml) under a positive atmosphere of argon, the reducing agent being added portionwise at a rate of 1 equivalent each 60 min.

After completion of the reaction, reverse phase HPLC analysis (as described above) was effected and indicated, among other products, formation of enamine (VIrI) and 3',4'-dehydrovinblastine in a ratio of 1:1 (40% yield).

EXAMPLE 3 Heduction of Iminium Intermediate (Formula VI) with l-Benzyl-1,4-dihydronicotinyl-2'-carbamoyl-pyrrolidinyl)amide (Formula IX, 1 = benzyl; 1 < 2 = K4 = K5 = K6 = H; 1 < 3 = (2'-carbamoyl-pyrrolidinyl) carbonyl) (Formula XXXIV) Synthesis of Enamine VIII) - (Procedure E).

To a stirred solution of 100 mg iminium intermediate (VI) in degassed methanol (6 ml) was added l-benzyl-1,4-dihydronicotinyl-(2'-carbamoyl-pyrrolidinyl)amide (Formula XXXIV) (163 mg, 5 equivalents) in methanol (5 ml) under a positive pressure of argon, the reducing agent being added portionwise at the rate of 1 equivalent each 30 min. After completion of the reaction, reverse-phase HPLC analysis (as described above) was effected and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 1.1:1 (60% yield).

EXAMPLE 4 Reduction of Iminium Intermediate (Formula VI) with l-(l-methoxycarbonylisobutyl) - l,4-dihydro-nicotinamide (Formula IX, K1 = l-methoxycarbonyl isobutyl; K2 = a4 = R5 6 1 < 6 = H; K3 = CONH2) (Formula XXVIII) - Synthesis of Enamine (Formula VIII) - (Procedure F).

To a stirred solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) was added l-(l-methoxycarbonylisobutyl)-1,4-dihydro-nicotinamide (Formula XXVIII) (150 mg, 6 equivalents) in methanol (6 ml) under a positive pressure of argon, the reducing agent being added portionwise at the rate of 1 equivalent each 30 min. After the reaction, reverse-phase HPLC analysis (as described above) was carried out and indicated, among other products, formation of enami (VIII) and 3',4'-dehydrovinblastine in a ratio of 2:1 (65% yield).

EXAMPLE 5 Keduction of Iminium Intermediate (Formula VI) with 1-(2',3',4',6'-Tetraacetyl-(beta)-D-glucopyranosidyl)-1,4 dihydronicotinamide (Formula IX, H1 - (2' ,3' ,4' ,6'-Tetraacetyl-(beta)-D-glucopyranosidyl; H2 = R4 = R5 = R6 = H; R3 = CONH2) (Formula XXIX) Synthesis of Enamine (Formula VIII) - (Procedure G) To a stirred solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) was added 1-(2',3',4',6'-tetra-acetyl-(beta)-D-glucopyranosidyl)-1,4 -dihydronicotinamide (Formula XXIX) (238 mg, 5 equivalents) in methanol (10 ml) under a positive atmosphere of argon, the reducing- agent being added portionwise at the rate of 1 equivalent each 60 min.

After the reaction reverse-phase HPLC analysis (as described above was effected and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 1.5:1 (70% yield).

EXAMPLE 6 Reduction of Iminium Intermediate (Formula VI) with 1-(2'-methoxycarbonyl isopropyl)-1,4-dihydro-nicotinamide (Formula IX, K1 = 2'-methoxy carbonylisopropyl; R2 = R4 = K5 = s6 = H; s3 = CONH2) (Formula XXX) - Synthesis of Enamine (Formula VIII) - (Procedure H) To a stirred solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) was added 1-(2'-methoxy-carbonylisopropyl)-1,4-dihydro-nicotinamide (Formula XXX) (82 mg, 3.5 equivalents) in methanol (7 ml) under a positive atmosphere of argon, the reducing agent being added portionwise at the rate of 1 equjivalent each 30 min.After the reaction, reverse-phase HPLC analysis (as described above) was carried out and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 1.1:1 (65% yield).

EXAMPLE 7 Heduction of Iminium Intermediate (Formula VI) with 1-(1',2'-dimethoxycarbonyl ethyl)-1,4-dihydro nicotinamide, K5 = 1 < 4 6 R5 = 1 < 6 = H; K3 = CONH2) (Formula XXXI) - Synthesis of Enamine (Formula VIII) - Procedure I) To a solution of 100 mg of iminium intermediate (VI) in degassed methanol. (6 ml) was added 1-(1',2'-dimethoxycarbonyl ethyl)-1,4-dihydro-nicotinamide (Formula XXXI) (148 mg, 5 equivalents) in methanol (10 ml) under a positive atmosphere of argon, the reducing agent being added portionwise at the rate of 1 equivalent each 30 min.After the reaction, reverse-phase HPLC analysis (as described above) was carried out and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 1.1:1 (70% yield).

EXAMPLE 8 Reduction of Iminium Intermediate (Formula VI) with 1-(sodium-isobutyl-1-carboxylate)-1,4-dihydro-nicotinamide (Formula IX, K1 = sodium-isobutyl-1-carboxylate; R2 =R4 = R5 =1 < 6 = H; K3 = CONH2 (Formula XXXII) - Synthesis of Enamine (Formula V1II) - (Procedure J) lo a solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) was added 1-(sodium-isobutyl-1-carboxylate)-1,4-dihydro-nicotinamide (Formula XXXII) (130 mg, 5 equivalents) in methanol (6 ml) under a positive atmosphere of argon, the reducing agent being added portionwise at the rate of 1 equivalent each 30 min. After the reaction, reverse-phase HPLC analysis (as described above) was carried out and indicated, among other products, formation of enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 2.2:1 (70% yield).

EXAMPLE 9 Reduction of Iminium Intermediate (Formula VI) with l-(sodium-isobutyl-l-carboxylate)-1,4-dihydro-nicotinamide (Formula IX, R1 = sodium-isobutyl-1-carboxylate; R2 = R4 = R5 = R6 = H; R3 = CONH2) (Formula XXXII) at low temperature - Synthesis of Enamine (Formula VIII) (Procedure K).

(i) To a solution of 100 mg of iminium intermediate (VI) in degassed methanol (6 ml) at -20 0C was added l-(sodium-isobutyl-l-carboxylate)1,4-dihydro-nicotinamide (Formula XXXII) (155 mg, 6 equivalents) in methanol (6 ml) under a positive atmosphere of argon, the reducing agent being added in one portion. After 45 min. at this temperature reverse-phase HPLC analysis (as described above was carried out and indicated, among other products, formation of enamine (VIII) and 3'4'dehydrovinblastine in a ratio of 3.2:1 (80% yield).

(ii) Carrying out the reaction above at -400C gave, after 60 minutes, enamine (VIII) and 3',4'-dehydrovinblastine in a ratio of 4.2:1 (85% yield).

EXAMPLE 10 Synthesis of Vinblastine (Formula e I) encompassing oxidation of Enamine (Formula VIII) to Iminium Intermediate (Formula XVI) with Air in the Presence of Ferric Chloride at high dilution.

The solution containing enamine (VIII) obtained in procedure (ii) of Example 9, which had been obtained from 200 mg of iminium intermediate (VI), was diluted five-fold with methanol before oxidation (total vol.: 120 ml). Ferric chloride (75 mg, 2 equivalents) was then added, and air was bubbled through the solution, at 0 0C, for 20 min. Sodium borohydride (200 mg) was added, and the solution obtained was concentrated in vacuo before adding water (100 ml) and extracting with ethyl acetate (3 x 200 ml). the combined organic extract was dried over Na2 SO4 and the solvent evaporated in vacuo. The crude product was purified by column chromatography (silca gel, TLC grade, 15 g). Elution with ether: chloroform (10:7) gave 3'4'-dehydrovinblastine (18 mg, 11%).Further elution with ether: chloroform: methanol (10:7:0.5) gave vinblastine (I, 62 mg, 37%).

EXAMPLE 11 One-pot Conversion of Catharanthine (Formula II) and Vindoline (Formula III) to Vinblastine (Formula I) and Leurosidine (Formula XXXV) - Overall Procedure To a solution of 500 mg (1.5 mmol) of catharanthine (II) in dry dichloromethane (4.5 ml) at -15 C under a positive atmosphere of argon was added m-chloroperbenzoic acid (330 mg, 1.9 mmol) in one portion, and the mixture was stirred at -10 to -15 C for 5 minutes. After this time the reaction mixture was cooled to -400C and a solution of 450 mg 1 mmol) vindoline (III) in dry dichloromethane (1 ml) was added, followed immediately by trifluoroacetic anhydride (1 ml, 7.1 mmol). After 2 h at -600C, volatiles were removed in vacuo (high vacuum pump) and dry, degassed methanol (12 ml) was added after flushing the system with argon.The resulting orange solution was cooled to -40 C and a solution of l-(sodium-isobutyl-l-carboxylate)1,4-dihydro-nicotinamide (Formula XXXII) (1.5 g, 6 mmol) in dry degassed methanol (12 ml) was added under a positive atmosphere of argon. After reduction was seen to be completed by reverse-phase HPLC monitoring, cold methanol (about 300 ml) was added to keep the temperature of the solution between -5 and 0 C. Ferric chloride (330 mg, 2 mmol) was then added and dry air was bubbled through the solution at a rate of 60 ml/min for a period of 20 min. Sodium borohydride (1 g) was added and the solution was concentrated in vacuo (water aspirator) before adding water (100 ml) and extracting with ethyl acetate (3 X 150 ml). The combined organic extract was dried over Na2 SO4 and the solvent evaporated in vacuo to give a crude product which was purified by chromatography as previously described to give 3',4'-dehydrovinblastine (VIII, 95 mg, 12%), vinblastine (I, 315 mg, 39%) and leurosidine (XXXV, 130 mg, 16%).

Claims (25)

Claims:

1. A process for effecting the conversion of an iminium compound of formula VIa

to an enamine ot formula VIIIa

wherein in formula VIa and VIlla and where indicated in any of the substituting groups defined below alkyl = CH3 or (CH2)nCH3 where n = 1-5 H1 = CH3, CHO or COR5, R5 being alkyl or aryl, R2 = H or CO-alk R3 = H R4 = COO-alk, CONH-NH2 or CONR13 R14 where R13 and R14 can be any member of the group consisting of hydrogen, alkyl, substituted alkyl, aryl or substituted aryl functions Z = -CH=CH- or -CH2-CH2- R7 = H or COU-alk R8 = H, OH, O-alk, OCO-alk or alkyl R10 = H, OH, @=O-alk, OCO-alk K12 = H or alkyl which comprises reducing the iminium compound with a 1,4-dihydropyridine compound of formula LXa

wherein R1' is an electron rich functional group comprising a carbon skeleton branching in the 1-position and K3' is selected from cyano, 2'-carbamoyl-pyrrolidinyl and a group of formula CONR7'R8' wherein R7' and R8', which can be the same or different, are selected from hydrogen, alkyl and aryl, or R1' is benzyl or diphenylmethyl and K3' is selected from cyano and 2 '-carbamoyl-pyrrolidinyl.

2. A process as claimed in claim 1, wherein the electron rich functional group is selected from diphenylmethyl, l-carboxyalkyl in the form of an alkyl ester or salt and wherein the alkyl moiety has 2 carbon atoms, and sugar moieties, the C-N linkage attaching the moiety to the dihydropyridine. -ring taking place at a dehydro carbon atom of the sugar moiety.

3. A process as claimed in claim 2, wherein the 1,4-dihydropyridine compound is l-diphenylmethyl 1, 4-dihydro-nicotinamide.

4. A process as claimed in claim 2 wherein the 1,4-dihydropyridine compound is l-(l-methoxycarbonylisobutyl)-1,4-dihydro-nicotinamide.

5. A process as claimed in claim 2, wherein the 1,4-dihydropyridine compound is 1-(2',3',4',6'- tetracetyl-beta-D-glucopyranosidyi)-l,4-dihydro-nicotin- amide.

6. A process as claimed in claim 2, wherein the 1, 4-dihydropyridine compound is l-(2'-methoxy- carbonylisopropyl)-1,4-dibydro-nicotinamide.

7. A process as claimed in claim 2, wherein the 1,4-dihydropyridine compound is l-(l' ,2'- dimethoxycarbonylethyl)-1,4-dihydro-nicotinamide.

8. A process as claimed in claim 2, wherein the 1,4-dihydropyridine compound is l-(sodium isobutyl-l-carboxylate)-1,4-dihydro-r.lcotinamide.

9. A process as claimed in claim 1, wherein the 1,4-dihydropyridine compound is 1-benzyl-3-cyano-1,4- dihydropyridine.

10. A process as claimed in claim 1, wherein the 1,4-dihydropyridine compound is 1-benzyl-1,4-dihydro nicotinyl- (2' -carbamoyl-pyrrolidinyl ) -amide.

11. A process as claimed in any one of the preceding claims, wherein the reduction is carried out under an inert atmosphere at from -60 to +600C and utilising a solvent selected from C14 alkanols, acetonitrile, dimethylsulphoxide, dimethylformamide, dioxane, tetrahydrofuran and chlorinated C14 hydrocarbons.

12. A process as claimed in claim 11 which is carried out at a temperature of from 4 to 200C.

13. A process for the conversion of an iminium compound to an enamine compound, substantially as described in any one of the foregoing Examples 1 to 9.

14. An enamine compound whenever produced by the process claimed in any one of the preceding claims.

15. A process for the production of dimeric compounds represented by the following formula

wherein in formula XXI and where indicated in any one of the substituting groups defined below alkyl = CH3 or (CH-,)nCH3 where n = 1-5 R1 = CH3, CHO or COR5, K5 being alkyl or aryl, H2 = H or CO-alk R3 = H R4 = COO-alk, CONH-NH2 or CONR13 R14 where R13 and H14 can be any member of the group consisting of hydrogen, alkyl, substituted alkyl, aryl or substituted aryl functions Z = -CH=CH- or -CH2-CH2 H = One of formulae XXII or XXIII

joined at the carbon atom of the latter carrying H7 or R11 respectively and wherein R7 = H or COO-alk R8 = H, OH, O-alk, OCO-alk or alkyl R9 = H, OH, O-alk, OCO-alk or alkyl R10 = H, OH, O-alk OCO-alk R11 = H or COO-alk R12 = H or alkyl from an indole unit of the natural Iboga alkaloid family containing an aza bicyclo-octane portion and a dihydroindole unit of the natural Aspidosperma and Vinca alkaloid families, the stereochemistry of the carbon-carbon linkage between these two units being identical with that of vinblastine which consists of (a) forming an N-oxide intermediate at a temperature of -70 to +400C from said indole unit by oxidizing the bridge nitrogen and without isolating said intermediate; (b) treating said N-oxide indole intermediate in the presence of one member of the group consisting of acetic anhydride, halogenated acetic anhydride, and acetyl chloride to effect a Polonovski-type fragmentation reaction;; (c) without isolating the product of step (b), coupling said reaction product with a dihydroindole unit in the presence of acetic anhydride, halogenated acetic anhydride, and acetyl chloride at a low temperature of from -70 0C to +400C under inert conditions; (d) the product of step (c) is reduced by the process claimed in any one of claims 1 to 13; (e) the enamine obtained in step (d) is used to prepare a corresponding iminium intermediate of formula XVI or XVIa set out herein by an oxidative process including:: (1) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough; (2) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of a metal ion; (3) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of a flavin coenzyme;; (4) controlled aeration/oxygenation in which a solution of the enamine is stirred exposed to the atmosphere or with a stream of air/oxygen bubbled therethrough and in the additional presence of hydrogen peroxide andior a hydroperoxide of formula H-00H where K is alkyl or aryl; (f) the product obtained in step (e) is converted to the target compound as of formula XXI by reduction with alkali metal borohydride.

16. process as claimed in claim 15, wherein, in step (e), the enamine product of step (d) is used in a dilution ratio of 5 to 50:1 and reaction is carried out at a temperature of from OOC to -70 C.

17. A process as claimed in claim 16, wherein the dilution ratio is 8 to 12:1.

18. A process as claimed in claim 15, 16 or 17, Wherein the reaction temperature in step (e) is below -40 C.

19. A process as claimed in any one of claims 15 to 18, wherein the metal ion is a ferric ion and two equivalent of ferric ion are present in relation to the enamine.

20. A process as claimed in claim 19, wherein the oxidation of the iminium compound to the enamine is carried out for from 5 to 20 minutes.

21. A process as claimed in any one of claims 15 to 18, wherein the flavin coenzyme is represented by formula XII

to generate in situ the corresponding .1,5-dihydro flavin coenzyme as represented by formula XIII

22. A process as claimed in any one of claims 15 to 21, wherein after one or more of steps (c), (d) and (e), the product of the respective step is isolated by solvent evaporation at a temperature in the range of -20 0C to 0 0C.

23. A process as claimed in any one of claims 15 to 21, wherein the entire process is conducted in a one-pot operation.

24. A process for the production of a compound represented by the formula XXI set out in claim 15, substantially as described in the Examples.

25. A compound represented by formula XXI as set out in claim 15, whenever produced by the process claimed in any one of claims 15 to 24.