METHOD OF PREPARING 1-FORMYL CONGENERS OF INDOLE-DIHYDROINDOLE DIMERS

Abstract: A process of preparing one or more 1-formyl congeners of the equivalent indole-dihydroindole dimers which com­ prises contacting a solution in tetrahydrofuran (THF) of unseparated indole-dihydrondole dimeric alkaloids obtained from Vinca rosea leaves by extraction, with an aqueous chromate-sulfuric acid solution at a temperature in the range of between -80°C. and -50°C. until substantially all 1-methyl congeners initially present are converted to 1-formyl congeners; optionally, formylating any 1-desformyl congeners present; and isolating the same one or more 1-formyl congeners. This process eliminates a number of steps and is highly efficient.

Description: Method of Preparing l-Formyl Congeners of Indole-Dihydroindole Dimers This invention concerns a method of preparing l-formyl congeners of indole-dihydroindole dimers useful for the treatment of malignancies in animals. According to the present invention there is provided a process of preparing one or more l-formyl congeners of the equivalent indole-dihydroindole dimers which comprises contacting a solution in tetrahydrofuran (THF) of unseparated indole-dihydroindole dimeric alkaloids obtained from Vinca rosea leaves by extraction with an aqueous chromate-sulfuric acid solution at a temperature in the range of between -800C. and -50 C. until substantially all l-methyl congeners initially present are converted to l-formyl congeners; optionally, formylating any l-desformyl congeners present; and isolating the same one or more l-formyl congeners. To date, four methods of converting VLB to vincristine have appeared in the literature. These are: enzymatic oxidation with a peroxidase and H202, (Gorman-United States Patent 3,354,163); catalytic oxidation with molecular oxygen at ambient temperature in formic acid (Derwent Abstract 33812Y/l9 based on Soviet Union Patent 521,845); oxidation of VLB sulfate with chromic oxide in glacial acetic acid and acetone at -6O0C. (United States Patent 3,899,943); and the oxidation of VLB in tetrahydrofuran with an aqueous chromate-sulfuric acid solution of a temperature below -500C. (European Patent Office Publication 0 037 290.) The latter two processes can be used to prepare 1 formylleurosidene. The chromic acid - acetone - acetic acid oxidation process at -600C. is not without drawbacks, however. The maintenance of low reaction temperatures is difficult in a manufacturing plant, but higher temperatures produce increasing quantities of undesirable by-products. In addition, Barnett et al., United States Patent 4,110,330, have found that, in the VLBvincristine conversion, VLB reacts with acetone at C-5' under oxidizing conditions, even at -600C,, to yield 5!-acetonylvincristine and related products. Similar 5'-substitution products are produced in oxidizing leurosine or 41-deoxyleurosidine with this oxidizing system. These undesirable 5'-VCE derivatives en countered as by-products in the chromic acid oxidation of VLB sulfate in acetone are definite detriments to the use of that process in manufacturing vincristine from VLB. The 5'-acetonyl compounds are, of course, separable from vincristine by chromatography, but there is a consequent loss-of the desired product, vincristine. Furthermore, during this oxidation, there are produced substantial quantities of N-desformyl vincristine which must be reformylated in order to maximize vincristine yields. Reformylation is an added, expensive procedure when oxidation results in deformylation as well as oxidation. Particularly with regard to vincristine, this alternate process requires only one alkaloid separation step, that carried out after the oxidation procedure whereas, with the standard procedure, vincristine and VLB would both. be isolated from VRA, VLB would then be oxidized to vincristine and the vincristine again isolated from that reaction. The usual reformylation step would also be desirable to convert any desformyl congeners originally in the VRA and/or formed in the oxidation process to the corresponding N-formyl derivatives. It is apparent that my alternate oxidation procedure, utilizing a crude alkaloid mixture as substrate, is advantageous in that there is less handling required, a shorter time from leaf to product is attainable, etc compared to the standard oxidation procedure. My novel oxidation process utilizing a chromate-sulfuric acid oxidizing mixture in THF at about -600C is particularly useful in oxidizing the crude dimeric alkaloid mixture (VRA) to obtain substantial quantities of vincristine and formylleurosine in that there is less handling, sampling and recycling plus fewer operations and a shorter time period compared to the prior art procedures. These advantages have been encountered in trial runs in comparison with the CrO3-acetone-acetic acid process for converting VLB to vincristine. EMI4.1 In Formula II above VLB is represented when R is acetoxy, R1 is OH, R2 is C2H5, R3 is H and R4 is CH3 and it is oxidized to vincristine (R is acetoxy, R1 is OH, R2 is C2H5, R3 is H and R4 is CHO). My novel procedure also includes the oxidation of leurosidine (R is acetoxy, R1 is C2H5, R2 is OH, R3 is H and R4 is CH3) to l-formyl-l-desmethylleurosidine (R, R1, R2 and R3 are the same as before but R4 is CHO), the oxidation of 4'-deoxy VLB (R1 and R3 are H, R2 is C2H5, R is acetoxy and R4 is CH3) to 4'-deoxyvincristine (R1 and R are H, R is acetoxy, R is H5 and R is CHO), the oxidation of 4'-deoxyleurosidine (Rê and R are H, RÚ is C2H5, R is acetoxy and R is CH3 to l-formyl-4'deoxy-l-desmethylleurosidine (R2 and R3 are H, R1 is C2H5, R is acetoxy and R4 is CHO), the oxidation of 1. 2 3 leurosine (R is C2H5, R is acetoxy, R and R3 plus a bridging oxygen atom form an a-epoxide group and R4 is CH3) to formylleurosine and the oxidation of 4-des acetyl VLB (R and RÚ are OH, Rê is C2H5, R is H, and R4is methyl) to 4-desacetylvincristine. In naming those l-formyl compounds hereinafter which have no other trivial name, the term "l-desmethyl" will be omitted as understood; ie., if a formyl group is present at N-l, the methyl group must necessarily be absent. Sodium dichromate dihydrate has been found to be the preferred chromate for use in the above process although other dichromates or chromates such as 2Cr2O7, K2CrO4, Na2CrO4, CrO3 and the like can be employed. The term "chromates as used herein includes all such species. The ratio of sodium dichromate dihydrate to anhydrous alkaloid should be in the range 1.5-2.0 to 1 (w/w). The quantity of tetrahydrofuran (THF) customarily employed is about 100 to 200 times the amount (v/w) of VLB or leurosine or other alkaloid or mixture of alkaloids employed on an anhydrous basis with a ratio of about 150 to 1 (v/w) being preferred. Reaction temperatures can vary from about -800C. (dry ice/acetone) to about -50 C. Best results are obtained, however, by employing an acetone/dry ice bath (-77"C.) to maintain the temperature below about -7O0C. Temperatures below about -500C, while not optimal, are fully operative. As will be recognized by those skilled in the art, the lower temperature limit is one at which the reaction mixture begins to solidify, which temperature is about -800C. depending on the solutes present in THF. A reaction time of about 2 hours is optimal although reaction times varying from 1-3 hours are fully operative. A wash of the oxidation mixture after the reaction is substantially complete with a reducing agent such as ferrous sulfate or sodium metabisulfite is advisable in order to remove peroxides or residual oxidant. In the oxidizing reagent, sulfuric acid has ben specified and is preferred, although certain other strong acids such as perchloric acid are operative. It fiats been determined that, while THr is superior to acetone-acetic acid as a solvent in the -600C. chromic acid oxidation of V13 to vincristine, the combination of ThF and sodium dichromate-sulfuric acid is unexpectedly superior to the use of either THF with chromic acid or acetone with dichromate as disclosed in EPO Publication 0 037 290. Example 1 Four 10 g. samples of VRA (a crude alkaloid mixture produced by extraction from Vinca rosea leaves using the procedure of United States patent 3,932,417) were treated as follows: A. HPLC over alumina to produce VLB fraction; HPLC over silica to produce purified (98+%) VLB; conversion to VLB sulfate. B. Same as A but preceded by reformylation by the following procedure: To ten grams of VRA was added 60 ml. acetic anhydride and 60 ml. formic acid. The mixture was stirred for 1 hour at ambient temperature and then dried by vacuum. The dried residue was then neutralized with 150 ml. of 14 N aqueous ammonium hydroxide in 1,000 ml. water. The alkaline solution was then extracted three times with 1,000 ml. portions of methylene chloride. The extracts were then combined, dried over anhydrous sodium sulfate, filtered and the solvents removed by vacuum evaporation. C. Oxidation by the following procedure: Ten grams of VRA were dissolved in 1,200 ml. of THF. Thirty-seven-and-one-half ml. glacial acetic acid were added and the mixture was cooled to about -650C. by means of a chloroform/dry ice bath under a positive nitrogen blanket. Next, over a 1-2 minute period, a solution (prepared by dissolving 12.5 g. of sodium dichromate in 75 ml. of water and then adding 8.5 ml. of 18 M sulfuric acid) was added slowly with stirring to the VRA solution while maintaining the temperature at about -65 C. and was then poured into a solution containing 165 ml. of 14M aqueous ammonium hydroxide in 1,000 ml. of water. The solution was extracted 3 times with 1,000 ml. portions of methylene chloride. The organic extracts were combined and washed with 5% aqueous sodium metabisulfite at the rate of 2.5 g. per 50 ml. HPLC over alumina to produce vincristine fraction; HPLC over silica to produce purified vincristine; conversion to vincristine sulfate. D. Same as C but formylation step between oxidation and HPLC over alumina. Table 4 below summarizes these four runs. In the Table, column 1 gives the VLB sulfate yields, column 2 the yield of vincristine sulfate to be expected from the amount of l-desformylvincristine (DFVCR) present and column 3, the vincristine (VCR) sulfate yields, all as a percent of VRA free base. For the alkaloids isolated but present in quantities too small for practical subsequent purification, the final yields were extrapolated based on prior runs using larger amounts of the same type of material. Table 1 VCR SO4 Run VLB*SO4 from DFVCR VCR SO A 34.6% 1.7%* 4.0%* B 31.0% ND C ND 0.4%* 33.1% D ND ND 29.4% ND = none detectable * = extrapolated from an average of plant production and laboratory scale runs using larger quantities of VRA. Final yields of purified alkaloid are necessarily lower because of handling losses, purification losses and amount not recoverably present in other fractions. Formylleurosine, produced by oxidation of leurosine present in VRA (25-40% compared to VLB 40-45% by analysis of crude mixture), is obtained from the HPLC column as a fraction ahead of vincristine. The formylleurosine fraction is then further purified by a second HPLC procedure. The above figures would indicate that vincristine yields by my novel oxidation process are at least equal to those obtainable by standard procedures. For example, the yield of VCR expected from oxidation of VLB by my procedure would be about 27.7% in A (80% yield) plus desformylvincristine and vincristine = 33.48the actual yield was 33.1%. In order to study further the oxidation of a crude alkaloid mix, a fifth run (E) was carried out on a 4X scale (40 g of VRA) following the D protocol. The results of this run are given in Table 5, with column heading same as Table 4. Table 2 VCR SO4 Run VLB SO4 from DFVCR VCR- S04 E ND ND 43.0% VRA starting material* 31% 2.5% 3.5% * = expected yields based on prior runs ND = none detectable Here, the total projected yield of VCR would be 80% conversion X 31% VLB = 24.8% + 2.5$ DFVCR + 3.5% VCR=30. 88 total. Actual yield was 43.0%, far higher than the projected yield. Several possible explanations for this higher yield will spring to mind. First, oxidation of N-methyl vinca dimers to the corresponding N-formyl compounds simplifies the alkaloid mixture since the main components of the oxidation mixture are formylleurosine and vincristine whereas in VRA, these two desirable alkaloids (from the point of view of cancer chemotherapy) are grossly contaminated with VLB, leurosine and several minor N-methyl alkaloids. In other words, the HPLC purification mixture is much less complex than the VRA starting material. Secondly, when purifying YLB by chromatography, some VLB contaminates neighboring chromatographic fractions and is not available for oxidation when purified VLB is the substrate However, much of the VLB thus lost can be recovered by pooling neighboring chromatographic fractions , VLB sulfate formation mother liquors, etc, and isolating the VLB therefrom. This VLB can in turn be oxidized to vincristine in 80% yield. Thus, the increase in vincristine yield is really an increase in mainstream yield and not necessarily in overall yield. This increase in mainstream yield is particularly advantageous in that the cost of recovering VLB from mother liquors, adjacent chromatographic fractions, etc. and then converting the VLB to vincristine is far higher per gram of vincristine obtained than the cost per gram of mainstream material.

Claims: Claims 1. A process of preparing one or more l-formyl congeners of the equivalent indole-dihydroindole dimers which comprises contacting a solution in tetrahydrofuran (THF) of unseparated indole-dihydroindole dimeric alkaloids obtained from Vinca rosea leaves by extraction with an aqueous chromate-sulfuric acid solution at a temperature in the range of between -800C. and -50 C. until substantially all l-methyl congeners initially present are converted to l-formyl congeners; optionally, formylating any l-desformyl congeners present; and isolating the same one or more l-formyl congeners. 2. A process according to claim 1 for preparing vincristine and formyl-leurosine from the unseparated indole-dihydroindole dimeric alkaloids extracted from Vinca rcsea wherein the vincristine and formyl-leurosine are isolated. 3. A process according to claim 1 for preparing vincristine from the unseparated indoledihydroindole dimeric alkaloids extracted from Vinca rosea wherein the vincristine is isolated. 4. A process according to claim 1 for preparing formyl-leurosine from the unseparated indoledihydroindole dimeric alkaloids extracted from Vinca rosea wherein the formyl-leurosine is isolated. 5. A process according to any of claims 1 to 4 wherein the final step of isolation uses a chromatographic method.

Inventors: CONRAD, ROBERT ALLEN
Applicants: ELI LILLY AND COMPANY
Application year: 1982
Main classification: C07D519
IPC codes: C07D519, A61K31, A61P35, C07D219