Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2003-02-19
2004-08-17
Dentz, Bernard (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06777562
ABSTRACT:
The invention relates to a process for synthesizing a trans-calanolide ketone intermediate, and for separating the two diastereomers of the product alcohol from one another to give racemic calanolide A.
(+)-Calanolide A is an HIV-1 specific reverse transcriptase inhibitor under investigation for the treatment of AIDS, Z. Q. Xu, M. T. Flavin, and D. Zembower; U.S. Pat. No. 6,277,879, Aug. 21, 2001. The current state of the art for synthesis of (+)-calanolide A is outlined in FIG. 1 below, Z. Q. Xu, M. T. Flavin, and D. Zembower, U.S. Pat. No. 6,277,879, Aug. 21, 2001, and W. A. Boulanger, M. T. Flavin, A. Kucherenko, A. K. Sheynkman; U.S. Pat. No. 5,489,697, Feb. 6, 1996. M. T. Flavin, et.al;
J. Med. Chem
. 39, 1303 (1996).
FIG. 1. Synthesis of (+)-Calanolide
In this route, intermediate 5 is resolved by a time-consuming enzyme-catalyzed kinetic resolution using an equivalent weight of lipase AK with vinyl acetate. A chromatographic separation is required to separate the acetate ester of (−)-5 from desired (+)-5. According to the current state of the at, the trans-calanolide ketone intermediate (+)-6 is produced by the treatment of (+)-5 with diethyl azodicarboxylate (DEAD) and triphenylphosphine under Mitsunobu reaction conditions. A second, more difficult chromatographic purification is required to remove the by-products diethyl hydrazinodicarboxylate and triphenylphosphine oxide from the desired trans-calanolide ketone. Reduction of the ketone function to the alcohol using sodium borohydride with cerium chloride (Luche conditions, A. L. Gemal and J. L. Luche;
J. Am. Chem. Soc
. 103, 5454 (1981)) gives crude (+)-calanolide A still containing by-products from the Mitsunobu reaction. The product of the Luche reduction also contains the diastereomer (+)-calanolide B. The crude (+)-calanolide A is purified by preparative chromatography to remove Mitsunobu by-products and (+)-calanolide B, to give an isolated yield of 17% from chromene intermediate 4.
Separation of (+)-calanolide A from the racemate by semipreparative chiral HPLC had been used previously to afford small quantities of the pure stereoisomer, W. A. Boulanger, M. T. Flavin, A. Kucherenko, A. K. Sheynkman; U.S. Pat. No. 5,489,697, Feb. 6, 1996. M. T. Flavin, et.al;
J. Med. Chem
. 39, 1303 (1996), and J. H. Cardellina II, H. R. Bokesh, T. C. McKee, M. R. Boyd;
Bioorg. Med. Chem. Lett
. 5, 1011 (1995). A larger scale separation has been proposed as an alternative method of preparation to avoid the lengthy enzymatic resolution step. To this end, a practical method of preparation of racemic calanolide A is desired.
The process of the invention comprises a new scaleable process for synthesizing the trans-calanolide A ketone intermediate. The process comprises the steps of 1) reacting a chromene intermediate in methylene chloride solution, stepwise, with titanium tetrachloride followed by diisopropylethylamine followed by acetaldehyde, 2) quenching the product into cold aqueous ammonium chloride, 3) washing the organic phase with an acidified water wash, 4) drying the organic phase with magnesium sulfate, 5) filtering, 6) evaporating the solvent, 7) treating the aldol intermediate obtained with dimethylformamide dimethylacetal in tetrahydrofuran, 8) treating with saturated brine and water, 9) removing the aqueous phase and solvent, 10) equilibrating with triethylamine containing t-amyl, alcohol, and 11) removing the crystalline ketone via filtration. The method of the invention eliminates the tedious enzymatic resolution and the difficult chromatographic purification steps required in the current process.
According to the synthesis process of the invention, the racemic trans-calanolide ketone may be produced by treatment of an aldol intermediate with dimethylformamide dimethyl acetal (DMFDMA) followed by equilibration using triethylamine in t-amyl alcohol. The desired trans-calanolide ketone crystallizes out of the solution and can be isolated by filtration. The intermediate does not require isolation for this procedure, and the byproducts produced by prior art processes are not present. Therefore, the process of the invention does not require any chromatographic purification to remove byproducts and gives similar isolated yields.
The invention further comprises a method for removing the racemic calanolide B diastereomer from a mixture of calanolide A and calanolide B diastereomers formed in the last step of the synthesis of calanolide A. The method comprises the steps of 1) repeatedly recrystallizing the calanolide B in the mixture from toluene, and 2) concentrating the combined mother liquors and recrystallizing the residue from aqueous 2-propanol to isolate purified racemic calanolide A.
The process of the invention comprises a new scaleable process for synthesizing the trans-calanolide A ketone intermediate used in the synthesis of racemic trans-calanolide A. The process comprises the steps of 1) reacting a chromene intermediate in methylene chloride solution, stepwise, with titanium tetrachloride followed by diisopropylethylamine followed by acetaldehyde, 2) quenching the product into cold aqueous ammonium chloride, 3) washing the organic phase with an acidified water wash, 4) drying the organic phase with magnesium sulfate, 5) filtering, 6) evaporating the solvent, 7) treating the aldol intermediate obtained with dimethylformamide dimethylacetal in tetrahydrofuran, 8) treating with saturated brine and water, 9) removing the aqueous phase and solvent, 10) equilibrating with triethylamine containing t-amyl alcohol, and 11) removing the crystalline ketone via filtration. The process of the invention eliminates the tedious enzymatic resolution and the difficult chromatographic purification steps required in the current process.
Prior art taught that base-catalyzed condensation of an aldol-type intermediate may be used for the preparation of a similar ring structure, T. Ishikawa, Y. Oku, K. I. Kotake, H. Ishii;
J. Org. Chem
. 61, 6484 (1996). However, use of cesium fluoride or triethylamine with the aldol condensation product 5 gave only partial conversion, along with decomposition products. Use of a dehydrating agent led to formation of the alkene 9, which was followed by cyclization to give the desired ring structure (FIG. 2).
FIG. 2. Dehydration and Cyclization of Aldol Intermediate.
According to the synthesis process of the invention, the racemic trans-calanolide ketone 6 may be produced by treatment of the aldol intermediate 5 with dimethylformamide dimethyl acetal (DMFDMA) followed by equilibration using triethylamine in t-amyl alcohol. The desired trans-calanolide ketone crystallizes out of the solution and can be isolated by filtration. The desired trans-calanolide ketone has a low solubility in solvents such as triethylamine and t-amyl alcohol, and so crystallizes from solution. Additionally, equilibration of the cis-calanolide ketone isomer 10 also formed in the cyclization reaction to the trans-calanolide ketone occurs readily in basic solution. The overall equilibrium is driven to produce the desired trans-calanolide form by removal of this form from solution by crystallization. The by-products produced by the prior art processes are not present in this process. Therefore, the process of the invention does not require any chromatographic purification to remove by-products, and gives similar isolated yields.
The aldol condensation of 4 with acetaldehyde in the presence of titanium tetrachloride and diisopropylethylamine in methylene chloride gives 5 as shown in FIG. 3 below. The reaction is mediated by initial complexation of the product with titanium tetrachloride. A decrease in either the titanium tetrachloride or the diisopropylethylamine unit ratio led to lower conversions. Reaction mixtures with lower conversions could not be driven to completion with additional quantities of titanium tetrachloride, diisopropylethylamine, or acetaldehyde. Although the conversion was reported to be approximately 90%, the isolated yield was only 47%, presumab
Dentz Bernard
Dow Global Technologies Inc.
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