INTERMEDIATES AND THEIR USE IN THE SYNTHESIS OF ORGANIC COMPOUNDS

Abstract: Compounds of formula (wherein Ar, R, R₁ and X have the meanings reported in the de­ scription) are useful intermediates for the preparation of a variety of organic compounds, also optically active, like alpha-haloketones, alpha-hydroxyketones or ketals, alpha-arylalkanoic acids.

Description: The present invention concerns new intermediates for organic synthesis and more particularly it concerns new intermediates useful for the preparation of different organic compounds also of optically active organic compounds, such as alpha-haloketals, alpha-hydroxyketones or ketals (acetals) and alpha-arylalkanoic acids. Object of the present invention are the compounds of formula: wherein Ar represents an aryl or heteroaryl group having 5, 6 or 10 atoms in the ring and optionally substituted by from 1 to 3 substituents selected from C₁-C₄ alkyl, C₁-C₄ alkoxy C₁-C₄ haloalkyl, C₁-C₄ haloalkoxy, phenyl or heteroaryl having 5 or 6 atoms, phenoxy, benzoyl, heteroareyl and halogen atoms, each aromatic substituent being optionally further substituted by 1 or 2 substituents selected from halogen atoms, C₁-C₄ alkyl and C₁-C₄ alkoxy; R represents a methyl or phenyl; R₁ represents a C₁-C₄ alkyl; X represents a chlorine, bromine or iodine atom. The compounds of formula 1 are useful intermediates in organic synthesis as herebelow discussed. Specific examples of Ar are phenyl, napthyl, pyridyl, pyrrolyl, furyl, thienyl, thiazolyl, oxazolyl and isoxazolyl. Specific examples of substituted aryl groups include 4-chloro-phenyl, 4-isobutyl-phenyl, 4-difluoromethoxy-phenyl, 3-phenoxy-phenyl, 4-(2-thienylcarbonyl)-phenyl, 4-diphenyl, 6-methoxy-2-naphthyl, 5-chloro-6-methoxy-2-naphthyl, 5-bromo-6-methoxy-2-naphthyl, 6-hydroxy-2-naphthyl, 5-bromo-6-hydroxy-2-napthyl. Preferred meanings of R₁ are methyl and isopropyl. The preparation of compounds of formula 1 is carried out by halogenation of compounds of formula wherein Ar, R and R₁ have the meanings reported for formula (1). The halogenation is carried out by means of usual halogenating agents, in acidic conditions, in an inert solvent and at a temperature comprised between -50°C and room temperature. Preferred halogenating agents are ammonium or phosphonium perhalogenides, N-chloro, N-bromo or N-iodo-succinimide, cupric bromide, hexachloro-cyclohexadienone, alkyl or alkali hypochlorites, iodine, iodine chloride and bromine. The most preferred halogenating agent, due to its low cost, is bromine, which yields the compounds of formula I in which X=Br, The compounds of formula II can easily be prepared according to known procedures described for example in J. Am. Chem. Soc., 99 , 6038, (1977), Bull. Soc. Chim. Fr., 332, (1970), Zn. Org. Khim., 9 , 1145, 51973), (C.A., 79 :78654s). A general procedure for the preparation of the compounds of formula II comprises the condensation of a ketone of formula wherein Ar and R₁ have the above reported meanings, with a carboxylic acid or a reactive derivative thereof, of formula wherein R has the meanings reported for the compounds of formula (I). The condensation is carried out in an inert solvent, that in some cases may be an excess of compound III, and in the presence of an acid, for example sulphuric or p.toluensulphonic acid. The water formed during the reaction must be withdrawn from the reaction mixture, for example by distillation or by chemical means. The condensation may be carried out on compounds III and IV as such or they may be used in masked form. Examples of ketones of formula III in masked form are the compounds in which the carbonyl function is masked as dichloro-derivative or acetal or enolether. Instead of the acids of formula IV the analogs in which the alcoholic OH group is replaced by a halogen atom and their silylethers may be used. When, instead of the ketones of formula III, the alpha-functionalyzed ketones of formula are used, the compounds of formula I are obtained directly. The compounds of formula I can be used as intermediates in the synthesis of alpha-arylalkanoic acids. In fact, they can be rearranged, to give depending from the operative conditions, the alpha-arylalkanoic acids of formula (wherein Ar and R₁ have the meanings reported for formula I) or their precursors such as their esters. The hydrolysis of these affords the acids of formula V. The experimental conditions required for the rearrangement reaction are similar to those known for the rearrangement of ketals, for example, as disclosed in European patents 34871 and 35305 (Blaschim S.p.A.), 101124 and 151817 (Zambon S.p.A.). The compounds of formula I, are characterized by a chemical reactivity which is due to the presence of a carbon atom carrying both an ether and ester function. They can be defined as lactones. Compounds of formula I may be rearranged by treatment with a "soft" or "border line" Lewis acid (J. March, "Advanced Organic Chemistry" 3rd EA., J. Wiley & Sons, page 229) or by treatment with a mixture of a polar aprotic solvent with a protic solvent having a high dielectric constant. Alternatively, the compounds of formula I may be rearranged by treatment with water at acidic pH. A preferred embodiment of the invention comprises the halogenation of the compounds of formula II in optically active form. Said compound have two as asymmetric carbon atoms marked by an asterisk in the herebelow reported formula: wherein Ar, R and R₁ have the above reported meanings. The compounds of formula II-A are easily prepared, according to the same procedure described above for the preparation of the compounds of formula II, starting from the ketones of formula III and from single enantiomers, in respect of the carbon atom bound to R substituent of the compounds of formula IV. Examples of optically active alpha-hydroxy acids useful for the preparation of compounds of formula II-A are S(+)-lactic acid, R(-)-lactic acid, S(+)-mandelic acid and R(-)-mandelic acid. The single enantiomers of lactic and mandelic acid by reaction with the ketones of formula III afford the compounds of formula II-A wherein R is methyl or phenyl respectively. The compounds of formula II-A can be functionalized according to what is described above, to obtain compounds of formula wherein Ar, R, R₁ and X have the above reported meanings and the asterisks indicate the asymmetric carbon atoms. The rearrangement of compounds I-A according to the above described procedures affords the optically active alpha-arylalkanoic acids of formula (wherein Ar and R₁ have the above reported meanings and the asterisk indicates the asymmetric carbon atom) or their precursors such as the corresponding esters. It is worth noting that rearrangement of the compounds of formula I-A affords the optically active alpha-arylalkanoic acids V-A having an enantiomeric excess corresponding to the stereoisomeric excess in the starting compounds. The compounds of formula I-A are also useful intermediates for the synthesis of other optically active compounds. In fact, the hydrolysis of compounds I-A,carried out in acid as well as in basic conditions, yields optically active ketones of formula wherein Ar, R₁ and X have the same meanings as reported for formula I. The reaction of the compounds of formula I-A with an alkaline alcoholate affords optically active alpha-hydroxyketals of formula wherein Ar and R₁ have the above reported meanings and R₃ is the residue of the alkaline alcholate employed. Thus, for example, when the alcoholate is sodium methoxide,R₃ is methyl. The optically active ketones and ketals of formulae VI and VII, respectively, are intermediates for the synthesis of alpha-arylalkanoic acids according to known procedures that comprise, in the case of ketones VI, their transformation into optically active ketals and rearrangement of the latters [European patent applications 81993 (Syntex Co.), 154853 and 153701 (Zambon S.p.A.)]. The most characteristic feature of the compounds of formula I-A is the possibility of preparing them diastereoselectively. In fact the halogenation of the compounds of formula II-A (obtained from a single enantiomer of lactic or mandelic acid) is diastereoselective. Let us assume that the configuration of the two carbon atoms marked by an asterisk in formula II-A is established (e.g. both have S configuration). The halogenation of said compound affords the corresponding compound I-A as a mixture of two diastereoisomers (S,S,R and S,S,S), with reference to the carbon atom bound to the halogen atom X, in which one of the two diastereoisomers strongly prevails over the other. We have also found that the configuration of the tertiary carbon atom of the ring is the one that predominantly influences the configuration of the new asymmetric carbon atom that is created by the halogenation. Therefore the halogenation is diastereoselective and, accordingly, it is possible to have available compounds of formula I-A enriched in one of the possible diastereoisomers. When desired, although generally it is not necessary, it is possible to further enrich the mixture in the desired diastereoisomer by conventional separation methods. The diastereoselective halogenation is a particularly important feature because it allows to obtain the compounds of formula I-A from which various optically active intermediates and end-products may be obtained. Among these is worth citing the S(+)-2-(6-methoxy-2-naphthyl)-propionic acid (V-A, Ar=6-methoxy-2-naphthyl, R₁=CH₃), an antiinflammatory drug know as Naproxen, or its precursors like its esters₁ and the S(+)-2-(5-bromo-6-methoxy-2-naphthyl)-propionic acid and its esters. Among the optically useful compounds which can be prepared starting from the compounds of formula I-A the following acids may be cited which are intermediates in the synthesis of pyrethroid insecticides: S(+)-2-(4-chlorophenyl)-3-methyl-butirric acid (V-A, Ar=4-chlorophenyl, R₁=isopropyl) and S(+)-2-(4-difluoromethoxy-phenyl) -3-methyl-butirric acid (V-A, Ar=4-difluoromethoxyphenyl, R₁=isopropyl). In order to better illustrate the invention the following examples are given. Example 1 Preparation of 2-(1-bromoethyl)-2-phenyl-5(S)-methyl-1,3-dioxolane-4-one . The compound 2-ethyl-2-phenyl-5(S)-methyl-1,3-dioxolane-4-one was prepared starting from propiophenone and S(+)-lactic acid according to the procedure described in J. Am. Chem. Soc., 99 , 6038, (1977) or as described in Bull. Soc. Chim. Fr., 332, (1970). To a solution of 2-ethyl-2-phenyl-5(S)-methyl-1,3-dioxolane-4-one (1 mmol) and hydrogen bromide (0.1 mmol) in carbon tetrachloride (3 ml), bromine (1 mmol) was added and the temperature was kept at 15°C. After complete conversion, the reaction mixture was diluted with methylene chloride and washed with water. The organic phase was separated and dried on sodium sulphate. After evaporation of the solvents under vacuum, 2-(1-bromoethyl)-2-phenyl-5(S)-methyl-1,3-dioxolane-4-one was obtained as a mixture of 4 diastereoisomers one of which strongly prevails. Example 2 Preparation of 2-phenyl-propanoic acid . Silver tetrafluoroborate (0.974 g; 5 mmol) was added at 15°C to a solution of the diastereomeric mixture of 2-(1-bromoethyl)-2-phenyl-5-(S)-methyl-1,3-dioxolan-4-one (see example 1; 1.425 g, 5 mmol) in methanol (10 ml). The reaction mixture was kept at 15°C until complete conversion, diluted with water, and extracted with dichloromethane. The combined organic extracts were washed with water and dried over sodium sulfate. Evaportation of the solvent and purification of the crude by column chromatography gave the optically active 2-phenyl-propanoic acid methyl ester which by hydrolysis in acid conditions gave the optically active free acid.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, GR, IT, LI, LU, NL, SE A compound of formula wherein Ar represents an aryl or heteroaryl group having 5, 6 or 10 atoms in the ring and optionally substituted by from 1 to 3 substituents selected from C₁-C₄ alkyl, C₁-C₄ alkoxy, C₁-C₄ haloalkyl, C₁-C₄ haloalkoxy, phenyl or heteroaryl having 5 or 6 atoms, phenoxy, benzoyl, heteroaroyl and halogen atoms, each aromatic substituent being optionally further substituted by 1 or 2 substituents selected from the group consisting of halogen atoms, C₁-C₄ alkyl and C₁-C₄ alkoxy; R represents a methyl or phenyl; R₁ represents a C₁-C₄ alkyl; X represents a chlorine, bromine or iodine atom. A compound according to claim 1 in optically active form. A compound according to claim 1 in which R₁ is methyl or isopropyl. A compound according to claim 1 in which Ar is selected from 4-chloro-phenyl, 4-isobutyl-phenyl, 4-difluoromethoxy-phenyl, 3-phenoxy-phenyl, 4-(2-thienylcarbonyl)-phenyl, 4-diphenyl, 6-methoxy-2-naphthyl, 5-chloro-6-methoxy-2-naphthyl, 5-bromo-6-methoxy-2-naphthyl, 6-hydroxy-2-naphthyl, 5-bromo-6-hydroxy-2-naphthyl. A compound according to claim 1 having the formula wherein Ar, R, R₁ and X have the meanings reported in claim 1 and the asterisks indicate the asymmetric carbon atoms, in optically active form. A process for the preparation of the compounds of claim 1 comprising the halogenation of a compound of formula wherein Ar, R and R₁ have the meanings reported in claim 1; by treatment with a halogenating agent, in acidic conditions, in an inert solvent and at a temperature comprised between -50°C and room temperature. A process according to claim 6 in which the halogenating agent is selected from ammonium or phosphonium perhalogenides, N-chloro-, N-bromo-or N-iodo-succinimide, cupric bromide, hexachloro-cyclohexadienone, alkyl or alkali hypochlorites, iodine, iodine chloride and bromine. A process for the preparation of alpha-arylalkanoic acids of formula wherein Ar and R₁ have the meanings reported in claim 1, which comprises the rearrangement of a compound according to claim 1. A process for the preparation of optically active alpha-arylalkanoic acids according to claim 8, which comprises the rearrangement of a compound of claim 1 in optically active form. A process according to claim 8 in which the rearrangement of the compound of claim 1 is carried out by treatment with a "soft" or "border line" Lewis acid or with a mixture of a polar aprotic solvent and a protic solvent having a high dielectric constant. A process according to claim 8 in which the rearrangement of the compound according to claim 1 is carried out by treatment with water at an acidic pH. A process for the preparation of optically active ketones of formula wherein Ar, R₁ and X have the meanings reported in claim 1 and the asterisk indicates the asimmetric carbon atom, comprising the hydrolysis of a compound according to claim 5 in optically active form. A process for the preparation of optically active alpha-hydroxy-ketals of formula which comprises the treatment of a compound according to claim 5 with an alkali alcoholate to obtain the compounds of formula VII wherein Ar and R₁ have the meanings given in claim 1, R₃ is the residue of the employed alcoholate and the asterisk indicates the asymmetric carbon atom. Claims for the following Contracting State : ES A process for the preparation of a compound of formula wherein Ar represents an aryl or heteroaryl group having 5, 6 or 10 atoms in the ring and optionally substituted by from 1 to 3 substituents selected from C₁-C₄ alkyl, C₁-C₄ alkoxy, C₁-C₄ haloalkyl, C₁-C₄ haloalkoxy phenyl or heteroaryl having 5 to 6 atoms, phenoxy, benzoyl, heteroaroyl and halogen atoms, each aromatic substituent being optionally further substituted by 1 or 2 substituents selected from the group consisting of halogen atoms, C₁-C₄ alkyl and C₁-C₄ alkoxy; R represents a methyl or phenyl; R₁ represents a C₁-C₄ alkyl; X represents a chlorine, bromine or iodine atom, comprising the halogenation of a compound of formula wherein Ar, R and R₁ have the meanings reported above, by treatment with a halogenating agent, in acidic conditions, in an inert solvent and at a temperature comprised between -50 °C and room temperature. A process according to claim 1 in which the halogenating agent is selected from ammonium or phosphonium perhalogenides, N-chloro-, N-bromo- or N-iodo-succinimide, cupric bromide, hexachloro-cyclohexadienone. alkyl or alkali hypoclorites, iodine, iodine chloride and bromine. A process for the preparation of alpha-arylalkanoic acids of formula wherein Ar and R₁ have the meanings reported in claim 1, which comprises the rearrangement of the compound obtained according to the process of claim 1. A process for the preparation of optically active alpha-arylalkanoic acids according to claim 3, which comprises the rearrangement of the compound obtained according to the process of claim 1 when it is in optically active form. A process according to claim 3 in which the rearrangement of the compound obtained according to the process of claim 1 is carried out by treatment with a "soft" or "border line" Lewis acid or with a mixture of a polar aprotic solvent and a protic solvent having a high dielectric constant. A process according to claim 3 in which said rearrangement is carried out in water at an acidic pH. A process for the preparation of optically active ketones of formula wherein Ar, R₁ and X have the meanings reported in claim 1 and the asterisk indicates the asimmetric carbon atom, comprising the hydrolysis of the compound obtained according to the process of claim 1 when it is in an optically active form. A process for the preparation of optically active alpha-hydroxy-ketals of formula which comprises the treatment of a compound obtained according to the process of claim 1 with an alkali alcoholate to obtain the compounds of formula VII wherein Ar and R₁ have the meanings given in claim 1, R₃ is the residue of the employed alcoholate and the asterisk indicates the asymmetric carbon atom.

Inventors: CASTALDI, GRAZIANO, GIORDANO, CLAUDIO
Applicants: ZAMBON S.P.A.
Application year: 1987
Main classification: C07D317
IPC codes: C07C59, C07C43, C07B31, C07C45, C07C51, C07C41, C07C57, C07B41, C07B53, C07C49, C07D317, C07C67