ISOFLANENE SYNTHETIC METHOD AND CATALYST
Introduction
The present invention relates to an improved process for the synthesis of isoflavene compounds. In particular the invention relates to the selective hydrogenation of hydroxy- substituted or phenolic starting isoflavones to afford correspondingly substituted isoflav-3- ene product compounds. The present invention further relates to a novel hydrogenation catalyst for effecting the methods of the present invention. The invention still further relates to products made by said processes of the invention.
Background of the Invention
Ready access to isoflav-3-ene compounds and derivatives thereof is a desirable target due to their important biological properties and potential therapeutic benefit to animals including humans, proven clinical efficacy of certain analogues and the isolation of various metabolites from ingestion of isoflavones.
Particular mention can be made of dehydroequol (4',7-dihydroxyisoflav-3-ene) and related compounds which show important and broad biological activity. Dehydroequol was first recognised as having health benefits in animals including humans in 1997 with patent application No. WO 98/08503 entitled "Therapeutic methods and compositions involving isoflavones". The patent specification teaches that dehydroequol belongs to a family of compounds based on a primary isoflavonoid ring structure, some members of which variously display estrogenic, anti-cancer, cardiovascular and anti-inflammatory health benefits in animals. More recent publications have expanded on the important biological activities of various isoflav-3-enes, including dehydroequol.
The synthesis of simple, unsubstituted isoflavene from isoflavone is relatively straightforward involving careful hydrogenation of the eneone system to stop at the secondary alcohol, followed by dehydration to isoflavene as depicted in Scheme 1.
Isoflavan Isofiavan-4-ol H
+ -H O
Scheme 1
However, it is known that when various substituents are present, complications can arise particularly during the hydrogenation step. In particular, the presence of free hydroxy or phenolic moieties complicate the reactions. This results in significantly lowered yields, the need to employ extensive chromatography and can often lead to intractable product mixtures.
For example, the presence of unprotected phenols in hydrogenation reactions involving daidzein has resulted in excessive production of the fully reduced isoflavan equol (4',7- dihydroxyisoflavan) amongst other products. Equol is a particularly undesirable contaminant to generate during these reactions, due to it being all but impossible to remove and, if it can be removed, its removal having a significant cost in terms of yield of the desired isoflavene product.
Further studies within the group have also found that deprotonation of the free phenolic hydroxy groups with base often prevents the hydrogenation reaction from proceeding to the isoflavan-4-ol intermediate, instead stopping at the isoflavanone intermediate.
In other cases, the addition of base leads to attack and ring opening of the pyran B ring of the isoflavone skeleton resulting in benzoin-like products.
hitemational patent application WO 00/49009 entitled "Production of Isoflavone Derivatives" describes for the first time an effective synthesis of isoflavene derivatives, and in particular hydroxy-substituted isoflavene compounds. The target products can be obtained in good to excellent yield without the need for chromatography and on sufficiently large scale to make them industrially useful. A general method is set out in Scheme 2.
Imidazole EtOH reflux
Scheme 2
The successful synthesis does have its drawbacks including the need to first protect any phenolic hydroxy groups prior to the hydrogenation step. As shown in Scheme 2, the starting isoflavone, daidzein, must first have its free phenolic hydroxy moieties protected, typically as acetoxy groups. The secondary alcohol product is isolated from the hydrogenation reaction mixture as an intermediate prior to dehydration, followed by a fourth step being removal of the acetoxy protecting groups to afford dehydroequol. The reaction, whilst being a marked improvement on what has gone before in the synthesis of dehydroequol, is time consuming, ungainly, expensive, requires many steps and can at times be unreliable.
There is a need for new, improved or at least alternative synthesis methodologies and also for new catalyst systems for the preparation of workable quantities of isoflavene compounds.
Accordingly it is a preferred object of the present invention to overcome, alleviate or at least provide an alternative synthetic methodology for arriving at isoflavene compounds. The present invention also seeks to provide a new catalyst system for use in hydrogenation reactions in general.
Summary of the Invention
Surprisingly a new and refined hydrogenation methodology has been found by the present inventors which enables ready access to isoflavene compounds from their corresponding parent isoflavones. The methods involve the use of an adapted, basified catalyst that allows for the hydrogenation of isoflavones without the need to first protect any free hydroxy or phenolic groups present on the ring systems. In general, the isoflavene products are obtainable in high to quantitative yield, without the need for chromatography. Importantly, the methods of the present invention lead to one-pot syntheses of isoflavenes without the need to conduct any isolation or purification steps of intermediates, thereby saving considerable time and expense.
Thus according to an aspect of the present invention there is provided a method for the preparation of a hydroxy-substituted isoflavan-4-ol comprising the step of hydrogenating a hydroxy-substituted isoflavone in the presence of a basified hydrogenation catalyst.
According to another aspect of the present invention there is provided a method for preparing a hydroxy-substituted isoflavene comprising the further step of dehydrating a hydroxy-substituted isoflavan-4-ol prepared according to the above method.
In a highly preferred embodiment, the isoflavan-4-ol is not isolated before being subjected to the dehydration step.
According to another aspect there is provided a method for preparing a hydroxy- substituted isoflav-3-ene comprising the steps of hydrogenating a hydroxy-substituted isoflavone in the presence of a basified catalyst to prepare a hydroxy-substituted isoflavan- 4-ol, and dehydrating the hydroxy-substituted isoflavan-4-ol to prepare the hydroxy- substituted isoflav-3-ene.
According to still another aspect of the present invention there is provided the use of a basified hydrogenation catalyst in the reduction of a hydroxy-substituted isoflavone to a hydroxy-substituted isoflavan-4-ol.
According to yet another aspect of the present invention there is provided isoflavan-4-ols and hydroxy-substituted isoflavenes obtainable by the methods above.
According to a further aspect of the present invention there is provided a basified hydrogenation catalyst comprising a hydrogenation catalyst in admixture with a base.
According to a still further aspect of the present invention there is provided a method for preparing a basified hydrogenation catalyst comprising the steps of treating a hydrogenation catalyst with an aqueous solution of a metal hydroxide, and filtering the resultant suspension to obtain the basified hydrogenation catalyst.
These and other aspects and preferred embodiments of the invention will become evident from the description and claims that follow.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
Brief Description of the Drawings
Fig. 1 shows the relative concentration of reactants and products over time for the hydrogenation of daidzein to cis- and trαns-tetrahydrodaidzein (47-dihydroxyisoflavan-4- ol) via the isoflavan-4-one intermediate according to the methods of the present invention.
Detailed Description of the Invention
Quantities of substantially pure, substituted isoflavenes are now reliably accessible directly from their corresponding isoflavones by hydrogenation with a modified catalyst for the first time. The methodology is particularly suited to the synthesis of hydroxy-substituted or phenolic isoflavenes which are acidic in nature. The basified catalyst and hydrogenation methods are applicable to the reduction of additional hydroxy-substituted and phenolic compounds.
The present inventors have found that pre-treatment of standard hydrogenation catalysts with base allows for reactions to be conducted on unprotected phenolic hydroxy substituted compounds, hi particular, substituted isoflavones can be simply and readily reduced to the corresponding isoflavan-4-ols, and in turn dehydrated to afford isoflavenes in good to high purity and yield. It has been found that it is possible to basify standard hydrogenation catalysts whilst retaining activity and selectivity.
Hydrogenation catalysts are basified by contacting the catalyst with a solution of base, such as an aqueous hydroxide solution. The resultant suspension of catalyst, such as palladium on alumina, and aqueous hydroxide is kept moving vigorously to ensure an even distribution of hydroxide through the catalyst particles. Simple filtration affords the basified catalyst as a clumping solid.
Note: care should be taken not to fully dry the catalyst as in the dried state it may ignite solvent fumes in air.
A hydrogenation reactor is then charged with the basified catalyst and the isoflavone added to the reaction pot, preferably as slurry in a lower alcohol or alkyl acetate. Reaction under a positive pressure of hydrogen is monitored until conversion of the isoflavone to the desired isoflavan-4-ol intermediate is observed.
The reaction mixture is filtered to remove the spent catalyst. The filtrate is then acidified to effect dehydration of the isoflavan-4-ol to isoflav-3-ene. The end product is quickly recovered to avoid prolonged contact of the isoflavene with acid and possible decomposition of the product.
The dehydration stage uses p-toluenesulfonic acid in preference to phosphorus pentoxide, giving a cleaner product without the need to change solvents, and the workup entailed in doing so.
Isoflavones for use in the methods of the present invention are preferably compounds of formula I
Ri, R2, R3, I , R5, R6, R7 and R8 are independently hydrogen, hydroxy, OR9, OC(O)R9, OS(O)R9, alkyl, haloalkyl, aryl, arylalkyl, thio, alkylthio, amino, alkylamino, dialkylamino, nitro or halo, and R9 is alkyl, haloalkyl, aryl, arylalkyl or alkylaryl.
Isoflavan-4-ol products of the present invention are preferably compounds of formula II
wherein
Ri, R , R , Ri, R5, Re, R7, R8 and R9 are as defined above.
Isoflavene products of the present invention are preferably compounds of formula III
Ri, R2, R3, R4, R5, Rό, R7, R8 and R are as defined above.
In the methods of the present invention, the starting isoflavone of formula I, the isoflavan- 4-ol hydrogenation product of formula II and the isoflav-3-ene dehydration product of formula III preferably have the following substituents wherein
Ri, R2, R3, R4, R5, R , R7 and R8 are independently hydrogen, hydroxy, OR , OC(O)R9, OS(O)R9, alkyl, aryl, arylalkyl, thio, alkylthio, bromo, chloro or fluoro, and R is alkyl, fluoroalkyl or arylalkyl;
more preferably they have the following substituents wherein Ri is hydroxy,
R2, R3, R4, R5, -5 and R7 are independently hydrogen, hydroxy, OR9, OC(O)R9, alkyl, aryl or arylalkyl,
R8 is hydrogen, and
R is methyl, ethyl, propyl, isopropyl or trifluoromethyl; and
most preferably they have the following substituents wherein Ri is hydroxy,
R2, R3, R4, R5 and R7 are independently hydrogen, hydroxy, OR , OC(O)R9, alkyl, aryl or arylalkyl,
R6 and R8 are hydrogen, and R9 is methyl.
The particularly preferred compounds of formula I are 4',7-dihydroxyisoflavone (daidzein) and 7-hydroxy-4'-methoxyisoflavone; the particularly preferred compounds of formula II are 4',7-dihydroxyisoflavan-4-ol (tetrahydrodaidzein) and 7-hydroxy-4,-methoxyisoflavan-4-ol; and the particularly preferred compounds of formula III are 4,,7-dihydroxyisoflav-3-ene (dehydroequol) and 7-hydroxy-4'-methoxyisoflav-3-ene.
Further compounds of formula II are:
4,,7,8-Trihydroxyisoflavan-4-ol 7,8-Dihydroxy-4'-methoxyisoflavan-4-ol 4',7-Dihydroxy-8-methylisoflavan-4-ol 3',7-Dihydroxy-8-methylisoflavan-4-ol 7-Hydroxy-4'-methoxy-8-methylisoflavan-4-ol 4',7-Dihydroxy-3'-methoxy-8-methylisoflavan-4-ol 4,,5,7-Trihydroxyisoflavan-4-ol 3',7-Dihydroxyisoflavan-4-ol 7-Hydroxy-3 '-methoxyisoflavan-4-ol
Further compounds of formula III are: Isoflav-3-ene-4',7,8-triol 4'-Methoxyisoflav-3-ene-7,8-diol
8-Methylisoflav-3-ene-4',7-diol 8-Methylisoflav-3-ene-3',7-diol 4'-Methoxy-8-methylisoflav-3-ene-7-ol 3'-Methoxy-8-methylisoflav-3-ene-4',7-diol Isoflav-3-ene-4',5,7-triol Isoflav-3-ene-3',7-diol 3 '-Methoxyisoflav-3 -ene-7-ol
The term "alkyl" is taken to mean both straight chain and branched chain alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secbutyl, tertiary butyl, and the like. Preferably the alkyl group is a lower alkyl of 1 to 6 carbon atoms. The alkyl group may optionally be substituted by one or more of fluorine, chlorine, bromine, iodine, carboxyl, Cι-C4-alkoxycarbonyl, Cι-C4-alkylamino-carbonyl, di-(Cι-C4-alkyl)-amino-carbonyl, hydroxyl, Cι-C4-alkoxy, formyloxy, Cι-C4-alkyl-carbonyloxy, Cι-C4-alkylthio, C3-C6- cylcoalkyl or phenyl.
The term "aryl" is taken to include phenyl and naphthyl and may be optionally substituted by one or more Cι-C4-alkyl, hydroxy, Cι-C4-alkoxy, carbonyl, Cι-C4-alkoxycarbonyl , d- C -alkylcarbonyloxy or halo.
The term "halo" is taken to mean one or more halogen radicals selected from fluoro, chloro, bromo, iodo and mixtures thereof, preferably fluoro and chloro, more preferably fluoro. Reference to for example "haloalkyl" includes monohalogenated, dihalogenated and up to perhalogenated alkyl groups. Preferred perhalogenated groups are trifluoromethyl and pentafluoroethyl.
The compounds of the invention include all salts, such as acid addition salts, anionic salts and zwitterionic salts, and in particular include pharmaceutically acceptable salts standard in the art.
The hydrogenation is ideally preformed with hydrogen in the presence of a reduction catalyst and a solvent. The reaction is preferably conducted under hydrogen at a pressure of 1-20 atmospheres, more preferably 1-5 atmospheres. The reaction may be performed from 10 to 60°C and is typically carried out at room temperature.
The reaction time may range from 12 hours to 96 hours or more and is typically about 24 hours or more. Generally better yields and cleaner reactions are achieved with longer reaction times and monitoring of the reaction products. It will be appreciated that reaction conditions may be varied depending on the individual nature of the compounds and the progress of the hydrogenation reaction.
The reduction catalysts suitable for the methods of the present invention include Raney nickel, palladium black, palladium hydroxide on carbon, palladium on activated carbon, palladium on alumina powder, palladium on various barium salts, sodium borohydride reduced nickel, platinum metal, platinum black, platinum on alumina, platinum on activated carbon, platinum oxide, rhodium salts, ruthenium salts and their chiral salts and zinc oxide.
Most highly preferably the catalyst is palladium on alumina (5% Pd or 10% Pd), more preferably about 10% palladium on alumina. Particular mention can also be made of palladium on carbon, typically having a content of 5% Pd or 10% Pd.
The solvents suitable for use in the present invention include but are not limited to Cι-C8 alcohols and polyols, alkyl acetates, tetrahydrofuran, ethers, dioxane and Cι-C3 acids. Preferably the solvent is a Cι-C6 alcohol or Cι-C6 alkyl acetate, more preferably methanol, ethanol or ethyl actate, as well as propanol, isopropanol, butanol, isobutanol, secbutanol, tertiary butanol, methyl formate, ethyl formate and methly acetate. Most preferably the solvent is methanol, ethanol or ethyl acetate. Particular mention is made of ethanol.
The base for synthesising the catalyst is preferably an alkali metal hydroxide such as sodium, potassium or lithium hydroxide. Particular mention is made of potassium hydroxide.
The present inventors have found that with a judicious choice of a basified catalyst and solvent in the presence of hydrogen that hydroxy-substituted isoflavones are reduced cleanly and in high yields to corresponding isoflavanols.
Whilst generally not required, it will be understood that some moieties on the isoflavone rings may require protection or derivatisation prior to being subjected to hydrogenation as would be understood by those skilled in the art. Protecting groups can be carried out be well established methods known in the art, for example as described in Protective Groups in Organic Synthesis, T. W. Greene.
Dehydration is most preferably effected with />-toluenesulfonic acid. Dehydration may also be effected by treatment with other catalysts including acids such as trifluoroacetic acid, sulfuric acid, hydrochloric acid, polyphosphoric acid, thionyl chloride and the like. The results may vary depending on the solvent system employed and the substrates being reacted.
Whilst this methodology is broad in its application across a wide range of substituted isoflavenes, particular mention is made herein of the important biologically active isoflavones with oxygen substitution (or precursors to oxygen substitution) at the 4'- and 7- positions. Reduction of these compounds leads to the biologically important dehydroequol and analogues thereof.
The production of dehydroequol most conveniently begins with the corresponding starting isoflavone daidzein. Daidzein is readily obtainable from commercial sources or alternatively it may be synthesised by established routes (see for example WO 98/08503). Likewise, access to other substituted isoflavone compounds including those disclosed above is obtainable by the methods of WO 98/08503.
Without wishing to be limited to theory, it is now thought that the phenolic proton is acidic enough to dissociate from the isoflavone molecule and attack the isoflavan-4-ol intermediates generated during the hydrogenation reaction. This may have the effect of dehydrating the 4-hydroxy intermediates in situ to 3 -ene products, which are themselves rapidly hydrogenated to isoflavan compounds. However, as noted above, the inventors observed that in general simple addition of base to the reaction mixtures introduces further problems including the reaction not going to completion, decomposition and ring opening of the pyran ring.
The present invention allows for ready access to isoflavene products from the hydrogenation of isoflavones in their native phenolic form. There is no need to go to the expense and time of protecting and deprotecting the phenolic groups, such as their respective acetates. Moreover, the basified catalyst and methods of the invention allow direct access to isoflavenes by hydrogenation of the corresponding isoflavones in effectively what is a one-pot process, without the need to isolate any intermediates.
The isoflav-3-ene compounds produced by the methods of the present invention can be end products themselves or used as intermediates in the synthesis of further derivatives, such as 4-alkyl or aryl substituted isoflavans.
The methodology described herein may be extended to the reduction of substrates other than just isoflavones. It will be appreciated that the basified catalyst can be employed in the hydrogenation of a wide range and variety of compounds. The hydrogenation of further substrates and compounds with the basified catalyst is a further aspect of the invention.
The invention is further described in and illustrated by the following Examples. The Examples are not to be construed as limiting the invention in any way.
Examples
The general "one-pot" synthesis of dehydroequol from daidzein with out the need to first protect the free hydroxy groups or isolate any intermediates is depicted in Scheme 3.
Scheme 3
1. Catalyst Pre-treatment
Catalyst pre-treatment was effected by stirring 10% palladium on alumina in a 1:1 w/v solution of potassium hydroxide. To this end, potassium hydroxide (10 g) was carefully dissolved in chilled, purified water (10 mL) in a flask standing in an ice- water bath. Once the basic solution returned to room temperature, 10% palladium on alumina catalyst (1 g) was added in a single portion and the suspension was stirred for 60 minutes. The suspension was filtered (Whatman Grade 114 paper) to give the basified catalyst as a damp clump. Once no more water was observed to be coming from the filter, the catalyst was immediately transferred to the hydrogenation vessel.
Safety Note: Care should be taken not to fully dry the basified catalyst as it may spontaneously react with flammable solvents such as ethanol.
Additional catalysts are also prepared by substituting other alkali hydroxides including sodium hydroxide or catalysts including platinum on alumina.
2. Hydrogenation
Daidzein (3 g) was suspended in degassed ethanol (47.5 mL) and added to the hydrogenation vessel. Two further aliquots of degassed ethanol (47.5 mL each) were used to rinse the daidzein remnants out of the transfer vessel into the hydrogenation vessel.
The hydrogenation vessel was then purged with nitrogen by sequentially evacuating the air from the vessel to -100 Kpa, and recharging with nitrogen gas for a total of 5 cycles.
Following this, the vessel was charged with hydrogen by sequentially evacuating the nitrogen from the vessel to -100 Kpa, and recharging with hydrogen gas again for a total of 5 cycles.
HPLC was used to monitor the formation of the 4',7-dihydroxyisoflavan-4-ol reaction product until such time as no daidzein or 4',7-dihydroxyisoflavan-4-one (reaction intermediate) were observed. Figure 1 depicts the relative proportions of the reactants and products over time for the hydrogenation. At completion the reaction vessel was purged with nitrogen. This was carried out by evacuating the hydrogen from the vessel to - lOOKpa and recharging with nitrogen for a total of 5 cycles.
The reaction mix was then filtered (celite/Whatman grade 114 paper combination) and the solid rinsed with fresh ethanol. The filtrate and rinsings were combined and then reduced in vacuo to one-third its original volume at 80°C.
3. Dehydration >-Toluenesulfonic acid (1.4 g) dissolved in ethanol (10 mL) was then added to the concentrated filtrate solution. The solution was again concentrated in vacuo to a viscous fluid of approximately 5 - 10 % of original volume. The resultant solution is immediately poured into stirred, chilled water to crash out the isoflavene product, dehydroequol.
The dehydration was also effected in the rotary evaporator maintained at 80°C and ambient pressure until HPLC indicated that the reaction had gone to completion. The isoflavene was again recovered after crashing out the product in ice water.
Dehydroequol was isolated in very good to excellent yields and was of high purity based on 1H n.m.r. analysis. Dehydroequol was recrystallised from ethanol/water or methanol/benzene. 1H NMR: δ 5.01 (s, 2H, H2), 6.24 (br s, 1H, H4), 6.33 (dd, 1H, J2, 8 Hz, H6), 6.72 (br s, 1H, H8), 6.76 (d, 2H, J 8 Hz, ArH), 6.93 (d, 2H, J 8 Hz, H5), 7.35 (d, 2H, J8 Hz, ArH), 9.5 (br s, OH).
The dehydration reaction is also effected with other acid catalysts including trifluoroacetic acid, sulfuric acid and thionyl chloride.
When the above "one-pot" method was scaled up, access to kilogram quantities of dehydroequol from daidzein was readily obtained in 60-65% yields and greater. The products were of high purity by n.m.r. Working quantities of dehydroequol were obtained in significaltly less time and with considerably less expense and effort with the new method, compared to the 4-step protection, hydrogenation, dehydration and deprotection method of WO 98/08503.
The other isoflavan-4-ol intermediates and isoflav-3-ene product derivatives described above were prepared by application of the same general method above to the corresponding substituted isoflavone starting materials. Spectral data were consistent with the structures of the named compounds.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour.
It is to be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that the generality of the preceding description is not to be
superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the invention.