Abstract:
The present invention relates to the field of organic synthesis and more specifically to a process for the preparation of Michael-adducts, as defined below, by reacting a β,β- or a α,β-disubstituted, or a α,β,β-trisubstituted, α,β-unsaturated ketone (I) with a β-ketoester or a β-diketone (II) in presence of a suitable catalyst of formula M(X) n , according to scheme 1:

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to the field of organic synthesis and more specifically to a process for the preparation of Michael-adducts, as defined below, by reacting a β,β- or a α,β-disubstituted, or a α,β,β-trisubstituted, α,β-unsaturated ketone (I) with a β-ketoester or a β-diketone (II) in presence of a suitable catalyst of formula M(X) n , according to scheme 1:  
                         
 
         PRIOR ART  
         [0002]    To the best of our knowledge, no reaction according to scheme 1 involving a α,β-disubstituted, or a α,β,β-trisubstituted, α,β-unsaturated ketone with a β-ketoester or a β-diketone has been reported in the prior art.  
           [0003]    Various processes involving addition reactions between β,β-disubstituted α,β-unsaturated ketones and β-ketoester or a β-diketone in the presence of a base have been reported before. However, they all provide a product which is the result of a so-called Robinson annulation, e.g. as described in J. D. Surmatis et al.,  J Org. Chem.,  (1970), 1053.  
           [0004]    The coupling of a β,β-disubstituted enone with an alkyl β-ketoester in the presence of 5% of a metal/acac complex (acac being 2,4-pentanedione) and 5% of a Lewis or a Broënsted acid (P. Kocovsky et al.,  Tetrahedron Lett.,  (1986), 5015 or P. Kocovsky et al.,  Coll. Czech. Chem. Commun.,  (1988), 2667) has also been tried, but the β,β-disubstituted enones used by these authors proved to be inert under a variety of conditions.  
           [0005]    Similarly, the direct coupling of the same type of compounds under high pressures (W. G. Dauben et al,  Tetrahedron Lett.,  (1983), 3841), has shown that a Michael-adduct can only be obtained if a highly reactive β,β-disubstituted enone, such as the 3,4,5,6-tetrahydro-1(2H)-pentalenone, is used. Another example of the synthesis of a Michael-adduct, by using highly activated β,β-disubstituted α,β-unsaturated ketones, possessing a C═C double bond moiety as part of a bycyclic ring, is described in A. M. El-Gendy et al.  Asian. J. Chem.;  (1990), 2, 168.  
           [0006]    U.S. Pat. Nos. 4,939,143 and 4,900,754 report the synthesis of 3,3-dimethyl-2-(4-fluoro-3-methylbenzoyl)-5-oxohexanoate. In said synthesis a β,β-disubstituted enone is reacted with a β-ketoester in presence of a stoechiometric amount of BF 3 .OEt 2  at 0° C. However, this method has the major drawback to need a stoechiometric amount of an expensive, strong and reactive Lewis acid. Furthermore, said method, which has been reported only for the specific reaction described in the US patents, cannot be considered as a general method because if a β-diketone is used instead of β-ketoester then the reaction leads directly to the Robinson annulation product, as described in A. Fernandez-Mateos et al.  J. Org. Chem.;  (1998), 63, 9440.  
           [0007]    Y. L. Chow ;  Can. J. Chem.,  (1993), 71, 846 teaches about the photochemical reaction between a β,β-disubstituted enone and a B(acac)F 2  complex. Nevertheless, said reaction leads to the formation of several by-products and, additionally, needs a steochiometric amount of BF 3 .  
           [0008]    Although compounds of formula (III) are interesting intermediates in a number of synthesis, and can also be precursors of β,β-disubstituted-δ-diketonic or α,β-disubstituted-δ-diketonic compounds, to the best of our knowledge, none of the methods reported for their preparation is of general or of simple application.  
         DESCRIPTION OF THE INVENTION  
         [0009]    In order to overcome the difficulties aforementioned, the present invention relates to a simple and general process, aimed at the synthesis of the compounds of formula (III) in a single step.  
           [0010]    In this process, the preparation of a compound of formula (III):  
                         
 
           [0011]    wherein  
           [0012]    Q represents a R′ group, a OR′ group, or a NH 2 , NHR′ or NR′ 2  group;  
           [0013]    R 1 , R′ and R″ represent, independently from each other, an aromatic ring possibly substituted, or a linear or branched C 1 -C 8  alkyl or alkenyl group, possibly substituted;  
           [0014]    R′″ represents a hydrogen atom or a linear or branched C 1 -C 4  alkyl or alkenyl group;  
           [0015]    R 2 , R 3 , R 4 , represent, independently from each other, a hydrogen atom or an aromatic ring possibly substituted, or a linear, branched or cyclic C 1 -C 8  alkyl or alkenyl group, possibly substituted, provided that at least two of said R 2 , R 3  and R 4  groups do not represent simultaneously an hydrogen atom; or  
           [0016]    two of the groups R 1  to R 4  are bonded together to form a ring having 5 to 15 carbon atoms, said ring being possibly substituted;  
           [0017]    characterized in that a β,β- or a α,β-disubstituted, or a α,β,β-trisubstituted, α,β-unsaturated ketone (I)  
                         
 
           [0018]    wherein R 1 , R 2 , R 3  and R 4  have the same meaning as in formula (III),  
           [0019]    is reacted with a β-ketoester or a β-diketone (II)  
                         
 
           [0020]    wherein Y, R″ and R′″ have the same meaning as in formula (III),  
           [0021]    in the presence of a catalyst of formula M(X) n , M representing a metal or a group containing a metal, n representing an integer from 1 to 4 and X representing a weakly coordinating or non-coordinating mono-anion.  
           [0022]    As non-limiting examples, groups which are possible substituents of R 1 , R 2 , R 3 , R 4 , R′, R″ and of the ring, which two of said R′ to R 4  may form together, are C 1 -C 7  alkyl, alkenyl or alkoxy groups, C 5 -C 7  cycloalkyl or cycloalkenyl groups, or aromatic rings possibly substituted by a C 1 -C 8  alkyl or alkoxy group or a halide atom.  
           [0023]    Preferably,  
           [0024]    Q represents a R′ or a OR′ group;  
           [0025]    R′, R′ and R″ represent, independently from each other, a linear C 1 -C 5  alkyl or alkenyl group, possibly substituted;  
           [0026]    R′″ represents a hydrogen atom or a linear or branched C 1 -C 3  alkyl group;  
           [0027]    R 2 , R 3  and R 4  represent a hydrogen atom or a linear C 1 -C 5  alkyl or alkenyl group, possibly substituted, provided that at least two of said R 2 , R 3  and R 4  groups do not represent simultaneously an hydrogen atom; or  
           [0028]    two of the groups R 1  to R 4  are bonded together to form a ring having 5 to 8 carbon atoms, said ring being possibly substituted.  
           [0029]    As non-limiting examples, groups which are possible substituents of R 1 , R 2 , R 3 , R 4 , R′, R″ and of the ring, which two of said R 1  to R 4  may form together, are C 1 -C 4  alkyl, alkenyl or alkoxy groups, C 5 -C 6  cycloalkyl or cycloalkenyl groups or aromatic groups possibly substituted by a C 1 -C 6  linear or branched alkyl or alkoxy group or a halide atom.  
           [0030]    More preferably, the compound of formula (I) is 4-methyl-3-penten-2-one or 3-methyl-3-penten-2-one, and the compound of formula (JI) is 2,4-pentanedione or a C 1 -C 4  alkyl ester of the 3-oxo-butanoate.  
           [0031]    Preferred catalysts of formula M(X) n  are those wherein M is selected from the group consisting of the 3d transition metals, the lanthanides, the trimethylsilane group (Me 3 Si), the vanadyl group (VO 3+ ), the alkaline metals, Sc, Y, Sn, Pb, Al and Bi;  
           [0032]    n is an integer from 1 to 3; and  
           [0033]    X is selected from the group consisting of CF 3 SO 3   − , RSO 3   − , SbF 6   − , PF 6   − , ClO 4   − , [BF 3 (RCOCRCOR)] − , [BF 3 (RCOCRCO 2 R)] − , [BF 3 (RCOO)] − , [BF 3 (RO)] − , BZ 4   − , Z being a fluoride atom or an alkyl or aryl group possibly substituted, and R representing an C 1 -C 10  aromatic, alkylaromatic or alkyl group, possibly substituted.  
           [0034]    Groups which are possible substituents of Z and R are, for example, halides atomts, C 1  to C 6  alkyl or alkoxy groups or non-coordinatng nitrogen containing groups.  
           [0035]    More preferably M is selected from the group consisting of Cu, Zn, Y and Yb;  
           [0036]    n is an integer from 1 to 3; and  
           [0037]    X is selected from the group consisting of CF 3 SO 3   − , C 6 H 5 SO 3   − , CH 3 C 6 H 4 SO 3   − , CH 3 SO 3   −, SbF   6   − , PF 6   − , ClO 4   − , [BF 3 (acac)] −  (acac representing CH 3 COCHCOCH 3   − ), [BF 3 (CH 3 COO)] − , BF 4   −  and BPh 4   − .  
           [0038]    In general, the catalyst may be added to the reaction media as a pure and isolated chemical or it can be prepared in situ, by several methods, in the reaction medium, without isolation or purification, just before its use.  
           [0039]    One of the possible procedures to advantageously prepare in situ a catalyst according to the invention consists in reacting an appropriate anhydrous metal salt of formula M(acac) n  or M(RCOO) n , R being defined as herein above and n being an integer from 2 to 3, with n equivalents, in respect to the metal, of BF 3 .OEt 2  in a solvent, e.g. an ester. The mixture thus obtained is ready to be used in the process of the invention. Alternatively, it is possible to use a hydrate form of a catalyst of the invention, which is generally commercially available, and to proceed to a dehydration, using any of the conventional methods of the art, prior the use of said catalyst in the process.  
           [0040]    The catalyst can be added to the reaction medium in a large range of concentration. As non-limiting examples, one can cite as catalyst concentration values ranging from 0.001 to 0.1 molar equivalents, relative to the α,β-unsaturated ketone (I). Preferably, the catalyst concentration will be comprised between 0.005 and 0.05 molar equivalents. It goes without saying that the optimum concentration of catalyst will depend on the nature of the latter and on the desired time of reaction.  
           [0041]    The process of the invention can be carried out in presence or absence of solvent, but in any case it is advantageously performed in anhydrous conditions, wherein by anhydrous is intended a content in water below 1% by weight, preferably below 0.5%. When a solvent is required, it is possible to use a pure solvent or a mixture of solvents. Said solvent is chemically compatible with the reaction and does not deactivate the catalyst, e.g. a weakly or non-coordinating solvent. Preferred solvents for the process of the invention are selected from the group consisting of ethers, carboxylic acids, esters, ketones, aromatic solvent, linear or branched or cyclic hydrocarbons, chlorinated solvents and mixture thereof. More preferably, the solvent is selected from the group consisting of C 4 -C 6  ethers, C 2 -C 8  esters, C 3 -C 6  ketones, acetic acid, methylene chloride and mixture thereof.  
           [0042]    The temperature at which the process of the invention can be carried out is comprised between −20° C. and 100° C., preferably between 0° C. and 50° C. Of course a person skilled in the art is also able to select the preferred temperature as a function of the melting and boiling point of the starting and final products and/or an eventual solvent.  
           [0043]    The process of the invention may also be advantageously carried out under pressure of an inert gas such as nitrogen. In said eventuality, a pressure ranging from 1.5 bar to 20 Kbar, preferably from 2 to 200 bar, may be used. 
       
    
    
       [0044]    The invention will now be described in further details by way of the following examples, wherein the abbreviations have the usual meaning in the art, the temperatures are indicated in degrees centigrade (° C.) ; the NMR spectral data were recorded with a 360 MHz machine in CDCl 3 , the chemical displacement δ are indicated in ppm with respect to the TMS as standard, the coupling constant J are expressed in Hz and all the abbreviations have the usual meaning in the art.  
       EXAMPLE 1  
     Preparation of methyl 2-acetyl-3,3-dimethyl-5-oxohexanoate  
       [0045]    A suspension of 0.135 g of Cu(acac) 2  (0.517 mmole) is stirred at 25° C. in 0.7 g of ethylacetate. Boron trifluoride etherate (0.154 g; 1.08 mmole) is added over 30 min. at 30° C. and agitation is continued for 2 h. The resulting homogeneous solution is added as such to a mixture of 6 g of methyl 3-oxo-butanoate (51.7 mmole) and 5.07 g of 4-methyl-3-penten-2-one (51.7 mmole). The resulting solution is stirred at room temperature over 16 h and finally heated at 60° C. for an hour. After cooling down to 30° C., 1.5 g of cyclohexane is added and the solution is stirred vigorously in the presence of 2.25 g of a 20% aqueous solution of Na 4 EDTA. After the removal of the aqueous phase the resulting organic layer is washed with 2 g of 20% aqueous potassium carbonate and then with 2 g of a saturated sodium chloride solution.  
         [0046]    The solvent and the unreacted material (starting materials) are distilled out under reduced pressure. The oily residue is distilled, in a bulb to bulb apparatus, at 110-130° C. under 1-2 mbar. 4 g of methyl 2-acetyl-3,3-dimethyl-5-oxo-hexanoate are obtained (&gt;95% purity) corresponding to 36% yield.  
         [0047]    [0047] 1 H-NMR 1.16(s, 6H); 2.11(s, 3H); 2.21(s, 3H); 2.72(d.d, 2H); 3.71(s, 3H); 4.12(s, 1H).  
         [0048]    [0048] 13 C-NMR: 25.7(q); 26.1(q); 31.7(q); 31.9(q); 35.6(s); 51.9(q); 52.0(t); 63.7(d); 169.7(s); 203.4(s); 208.4(s).  
         [0049]    MS: 214(M + , 0.8); 172(6.0); 157(38.0); 139(10.0); 125(77.8); 116(58.5); 115(92.0); 99(80.0); 83(99.2); 55(42.5); 43(100.0); 29(34.0).  
       EXAMPLE 2  
     Preparation of 3-acetyl-4,4-dimethyl-2,6-heptanedione  
       [0050]    A suspension of 0.135 g of Cu(acac) 2  (0.517 mmole) is stirred at 25° C. in 0.7 g of ethylacetate. Boron trifluoride etherate (0.154 g; 1.08 mmole) is added over 30 min. at 30° C. and agitation is continued for 2 h. The resulting homogeneous solution is added as such to a mixture of 6 g of 2,4-pentanedione (60 mmole) and 5.88 g of 4-methyl-3-penten-2-one (60 mmole). The resulting solution is stirred at room temperature over 6 h and finally heated at 50° C. for an hour. After cooling down to 30° C., 1.5 g of cyclohexane is added and the solution is stirred vigorously in the presence of 2.25 g of a 20% aqueous solution of Na 4 EDTA. After the removal of the aqueous phase the resulting organic layer is washed with 2 g of 20% aqueous potassium carbonate and then with 2 g of a saturated sodium chloride solution.  
         [0051]    The solvent and the unreacted material (starting materials) are distilled out under reduced pressure. The oily residue is distilled, in a bulb to bulb apparatus, at 100-120° C. under 1-2 mbar. 3.8 g of 3-acetyl-4,4-dimethyl-2,6-heptanedione are obtained (90% purity) corresponding to 28% yield.  
         [0052]    [0052] 1 H-NMR: 1.16(s, 6H); 2.11(s, 3H); 2.21(s, 6H); 2.68(s, 2H); 4.37(s, 1H).  
         [0053]    [0053] 13 C-NMR: 26.3(q); 26.3(q); 31.9(q); 32.7(q); 32.7(q); 36.4(s); 52.2(t); 71.2(d); 204.9(s); 204.9(s); 208.6(s).  
         [0054]    MS: 198(M + , 0.1); 165(1.2); 156(4.3); 141(12.5); 123(7.6); 99(59.9); 83(12.8); 55(4.5); 43(100.0); 29(3.4).  
       EXAMPLE 3  
     Preparation of methyl 2-acetyl-3,4-dimethyl-5-oxohexanoate  
       [0055]    A suspension of 0.135 g of Cu(acac) 2  (0.517 mmole) is stirred at 25° C. in 0.7 g of ethylacetate. Boron trifluoride etherate (0.154 g; 1.08 mmole) is added over 30 min. at 30° C. and agitation is continued for 2 h. The resulting homogeneous solution is added as such to a mixture of 6 g of methyl 3-oxo-butanoate (51.7 mmole) and 5.07 g of 3-methyl-3-penten-2-one (51.7 mmole). The resulting solution is stirred at room temperature. After cooling down to 30° C., 1.5 g of cyclohexane is added and the solution is stirred vigorously in the presence of 2.25 g of a 20% aqueous solution of Na 4 EDTA. After the removal of the aqueous phase the resulting organic layer is washed with 2 g of 20% aqueous potassium carbonate and then with 2 g of a saturated sodium chloride solution.  
         [0056]    The solvent and the unreacted material (starting materials) are distilled out under reduced pressure. The oily residue is distilled, in a bulb to bulb apparatus, at 110-130° C. under 1-2 mbar. 6.5 g of methyl 2-acetyl-3,4-dimethyl-5-oxo-hexanoate (mixture of 4 diastereoisomers) are obtained, corresponding to 58.5% yield.  
         [0057]    [0057] 1 H-NMR: 0.78, 0.83, 0.95-1.05, 1.12(d, 6H); 2.15, 2.18(s, 3H); 2.21, 2.22, 2.23, 2.27(s, 3H); 2.60, 2.80(m, 2H); 3.50, 3.80(d, 1H); 3.68, 3.72, 3.74, 3.75(s, 3H).  
         [0058]    [0058] 13 C-NMR: 9.64, 10.91, 12.52, 12.63, 13.93, 14.36, 14.77, 15.09(q); 28.38, 28.68, 29.45, 29.49, 29.53, 29.66, 30.03, 30.37(q); 33.14, 33.47, 34.90, 35.71(d); 48.17, 48.50, 49.31, 49.47(d); 52.24, 52.38, 52.38, 52.55(q); 61.20, 61.95, 63.10, 63.53(d); 169.3(s); 202.3, 202.5, 202.9, 202.9(s); 210.6, 211.0, 211.3, 211.9(s).  
         [0059]    MS: 214(M + ); 196; 183; 171; 167; 159; 143; 139; 129; 116; 111; 101 ; 99; 97; 85; 72; 69; 43.