Method for synthesis of 2,5-dioxane-1,4-diones

The present invention relates to a novel method for the synthesis of 2,5-dioxane-1,4-diones.

PLGAs are generally obtained by ring-opening (co)polymerization of lactide and glycolide. These monomers derived from lactic acid and glycolic acid are the prototypes of 2,5-dioxane-1,4-diones. Modification of the properties of the PLGAs is of great importance, in particular in their use as a biodegradable and bioassimilable matrix for the trapping and controlled release of active ingredients. Somewhat surprisingly, the approach which consists of modifying the substituents of the 2,5-dioxane-1,4-dione backbone has only been slightly developed up to the present, which can in practice be explained by the somewhat low accessibility of these units.

Symmetrical monomers such a lactide or glycolide are generally prepared from the corresponding α-hydroxy acids. This approach is difficult as it requires the elimination of the water formed and the distillation under vacuum of the monomer. In order to access asymmetrical monomers, two different precursors must be used, typically an α-hydroxy acid and a mono- or di-halogenated derivative (C.-M. Dong et al., J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 4179-4184; M. Leemhuis et al., Eur. J. Org. Chem. 2003, 3344-3349).

In practice, the major limitation of all these synthesis strategies is probably the final stage of closing the ring with 6 members which is inherently in competition with the formation of dimers and oligomers, by intermolecular rather than intramolecular route. The applicant has therefore envisaged a novel synthesis route for 2,5-dioxane-1,4-diones.

FIG. 1 is a crystal structure of 3,3-dimethyl-2,5-dioxane-1,4-dione synthesized according to Example 1.

As subject of the present invention is therefore a process for preparing 2,5-dioxane-1,4-diones of formula (I)

in which R 1 , R 2 , R 3 and R 4 represent, independently, the hydrogen atom; halo; (C 2 -C 6 )alkenyl; (C 3 -C 7 )cycloalkyl; cyclohexenyl; a radical of formula —(CH 2 ) m —V—W

V represents a covalent bond, the oxygen atom or the —C(O)—O— radical; W represents the hydrogen atom, a (C 1 -C 18 )alkyl radical optionally substituted by one or more identical or different halo radicals; the aryl or aralkyl radical, the aryl and aralkyl radicals being optionally substituted by one or more identical or different substituents chosen from: —(CH 2 ) n —Y-Z, halo, nitro and cyano;

Y represents —O—, —S— or a covalent bond; Z represents the hydrogen atom or a (C 1 -C 6 )alkyl radical optionally substituted by one or more identical or different halo radicals; or aralkyl; m and n represent independently an integer from 0 to 4;

by oxidation of the ketone function of a cyclic compound of formula (II)

In the definitions indicated above, the expression halo represents the fluoro, chloro, bromo or iodo radical, preferably chloro, fluoro or bromo. The expression (C 1 -C 6 )alkyl represents a linear or branched alkyl radical having from 1 to 6 carbon atoms, such as the methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl, pentyl or amyl, isopentyl, neopentyl, 2,2-dimethyl-propyl, hexyl, isohexyl or 1,2,2-trimethyl-propyl radicals. The term (C 1 -C 18 )alkyl designates a linear or branched alkyl radical having 1 to 18 carbon atoms, such as the radicals containing from 1 to 6 carbon atoms as defined above but also heptyl, octyl, 1,1,2,2-tetramethyl-propyl, 1,1,3,3-tetramethyl-butyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl. By the expression alkyl substituted by at least one radical halo is meant any linear or branched alkyl chain, containing at least one radical halo positioned along the chain such as for example —CHCl—CH 3 but also —CF 3 .

In the present Application also, the (CH 2 ) i radical (i being an integer which can represent m and n as defined above), represents a linear or branched hydrocarbonated chain, of i carbon atoms. Thus the —(CH 2 ) 3 — radical can represent —CH 2 —CH 2 —CH 2 — but also —CH(CH 3 )—CH 2 —, —CH 2 —CH(CH 3 )— or —C(CH 3 ) 2 —.

By (C 2 -C 6 )alkenyl, is meant a linear or branched alkyl radical containing from 2 to 6 carbon atoms and having at least one unsaturation (double bond), such as for example vinyl, allyl, propenyl, butenyl or pentenyl.

The term (C 3 -C 7 )cycloalkyl designates a saturated carbon monocyclic system comprising from 3 to 7 carbon atoms, and preferably the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl or cycloheptyl rings.

The expression aryl represents an aromatic radical, constituted by a condensed ring or rings, such as for example the phenyl, naphthyl, fluorenyl or anthryl radical. The term aralkyl (arylalky) preferably designates the radicals in which the aryl and alkyl radicals are as defined above such as for example benzyl or phenethyl.

Thus, during the conversion process of compound (II) to compound (I)

the competitive dimerization and oligomerization reactions which are observed during the synthesis of lactide or glycolide by condensation, are completely avoided.

For conversion of the ketone function of compound (II) to an ester function, several types of oxidation can be utilized; the oxidation can thus be carried out for example in the presence of an oxidizing agent such as a peracid or a peroxide (according to the Baeyer Villiger oxidation reaction), in the presence of a metallic catalyst (S. I. Murahashi et al., Tetrahedron Lett. 1992, 33, 7557-7760 and C. Bolm et al., Tetrahedron Lett. 1993, 34, 3405-3408) or by enzymatic route (M. D. Mihovilovic et al., Eur. J. Org. Chem. 2002, 3711-3730).

Preferably, a process according to the invention is carried out in the presence of an oxidizing agent according to the Baeyer Villiger oxidation reaction. In this case, the oxidation reaction is carried out very preferentially on the more encumbered side of the ketone in such a manner that 2,5-dioxane-1,4-diones can be obtained very selectively. In a preferable way, the oxidizing agent is used in the presence of a catalyst.

The oxidizing agent (or oxidation agent) used for implementing the process according to the invention, can be a peracid or a peroxide. As an example of a peracid, there can be mentioned trifluoroperacetic acid (TFPAA), peracetic acid (PAA), metachloroperbenzoic acid (m-CPBA), preferably in combination with Lewis acids (SnCL 4 , Sn(OTf) 3 , Re(OTf) 3 ) or strong acids (sulphonic acids, Nafion-H, CF 3 COOH etc.). As an example of a peroxide, there can be mentioned hydrogen peroxide (H 2 O 2 ); the hydrogen peroxide is used alone or in the presence of a catalyst which can be a Lewis acid (such as BF 3 ) or a metallic complex either in homogeneous phase (Mo, Re, Pt) or in heterogeneous phase (tin zeolite, tin hydrotalcite); there can also be mentioned bis(trimethylsilyl)peroxide Me 3 SiOOSiMe 3 which is used in the presence of a Lewis acid (Me 3 SiOTf, SnCl 4 or BF 3 ·OEt 2 ).

A more particular subject of the present invention is a process as defined above, characterized in that the oxidation agent is a peracid or a peroxide.

Preferably, the oxidizing agent is a peracid. The peracid is preferably used in the presence of a Lewis acid or a strong acid, and more particularly in presence of a strong acid selected from sulphonic acids.

More preferably the peracid is metachloroperbenzoic acid (m-CPBA). The metachloroperbenzoic acid is preferably used in the presence of trifluoromethanesulfonic acid.

Preferably also, the oxidizing agent is a peroxide.

The oxidation agents mentioned above are in general commercially available. The non-commercial agents can be synthesized according to methods known to a person skilled in the art. Thus, trifluoroperacetic acid which is not commercial can be easily obtained by the action of hydrogen peroxide H 2 O 2 on trifluoroacetic acid or anhydride CF 3 CO 2 H and (CF 3 CO) 2 O respectively (R. Liotta et al., J. Org. Chem. 1980, 45, 2887-2890; M. Anastasia et al., J. Org. Chem. 1985, 50, 321-325; P. A. Krasutsky et al., J. Org. Chem. 2001, 66, 1701-1707). Similarly, bis(trimethylsilyl)peroxide is not commercially available but it is easily accessible from the H 2 O 2 -1,4-diazabicyclo[2,2,2]octane [DABCO, N(CH 2 CH 2 ) 3 N] and Me 3 SiCl complex (P. G. Cookson e al., J. Organomet. Chem. 1975, 99, C31-C32; M. Taddei et al., Synth. Comm. 1986, 633-635).

The cyclic keto-esters of formula (II), used as precursors for the synthesis of 2,5-dioxane-1,4-diones (I) as defined above, are easily accessible by standard methods known to a person skilled in the art (E. B. Reid et al., J. Org. Chem. 1950, 15, 572-582).

A more particular subject of the present invention is also a process as defined above, characterized in that the aryl radical is the phenyl radial and the aralkyl radical is the benzyl radical.

A more particular subject of the present invention is also a process as defined above, characterized in that R 1 , R 2 , R 3 and R 4 represent, independently, the hydrogen atom; or a radical of formula —(CH 2 ) m —V—W with V which represents a covalent bond and W a (C 1 -C 6 )alkyl radical, and preferably R 1 , R 2 , R 3 and R 4 represent, independently, the hydrogen atom, the methyl radical or the ethyl radical.

A more particular subject of the present invention is also a process as defined above, characterized in that R 1 and R 2 represent, independently, a radical of formula —(CH 2 ) m —V—W with V which represents a covalent bond, m=0 and W a (C 1 -C 6 )alkyl radical, and R 3 and R 4 represent, independently, the hydrogen atom or a radical of formula —(CH 2 ) m —V—W with V which represents a covalent bond, m=0 and W a (C 1 -C 6 )alkyl radical.

A more particular subject of the present invention is also a process as defined above, characterized in that R 1 and R 2 represent, independently, the methyl or ethyl radical, and R 3 and R 4 represent, independently, the hydrogen atom, the methyl or ethyl radical.

A subject of the present invention is also compounds of formula (I) as obtained according to the method defined above.

Experimental Part

3,3-dimethyl-2,5-dioxane-1,4-dione

Stage 1: Synthesis of the Precursor (II)

The synthesis of compound (II) is carried out according to the following reaction diagram:

The formation of compound (2) from compound (1) can be carried out according to H. C. Brown et al., J. Am. Chem. Soc. 1988, 110, 1539-1546. The synthesis stages of compounds (3) and (4) can be carried out according to M. Conrad et al., Ber. 1898, 31, 2726-2731. Finally, the final stage of formation of compound (II) from compound (4) can be carried out according to E. B. Reid et al., J. Org. Chem. 1950, 15, 572-582.

Stage 2: Synthesis of 3,3-dimethyl-2,5-dioxane-1,4-dione

Conditions 1:

A solution of 5 g of cyclic keto-ester (39 mmol) and 13.5 g of metachloropebenzoic acid (2 eq.) in 100 ml of dichloromethane is heated under reflux for 48 hours. NMR 1 H monitoring of an aliquot of the reaction medium reveals the complete conversion of the ring with 5 members and the formation of mostly 3,3-dimethyl-2,5-dioxane-1,4-dione (spectroscopic yield: 85%).

Conditions 2:

A solution of 5 g of cyclic keto-ester (39 mmol) and 8.1 g of metachloroperbenzoic acid (1.2 eq.) in 40 ml of dichloromethane is heated under reflux for 24 hours. The complete conversion of the ring with 5 members is monitored by NMR 1 H on a sample. The reaction medium is then cooled down to −18° C. overnight then filtered on frit in order to eliminate the metachlorobenzoic acid formed. The filtrate is concentrated under vacuum. The residue is recrystallized from ethyl acetate at −18° C. 3.9 g of analytically pure, 3,3-dimethyl-2,5-dioxane-1,4-dione are thus obtained (70% of isolated product yield). The product is characterized by NMR 1 H [4.97 (s, 2H), 1.70 (s, 6H)] and 13 C [167.7 and 163.9 (C═O), 79.8 (C q ), 65.8 (CH 2 ), 25.8 (CH 3 )], RX (cf. FIG. 1 ), mp (84-85° C.) and elementary analysis. Calculated C: 50.00, H: 5.56. Found C: 49.98, H: 5.33.

Conditions 3:

A solution of 1 g of cyclic keto-ester (7.8 mmol), 2.7 g of metachloroperbenzoic acid (2 eq.) and 70 μl of trifluoromethanesulphonic acid (0.1 eq.) in 20 ml of dichloromethane is left under stirring at ambient temperature for 3 hours. The solvent is eliminated under vacuum, then the medium is analyzed. NMR 1 H reveals the complete conversion of the ring with 5 members and the formation of mostly 3,3-dimethyl-2,5-dioxane-1,4-dione (spectroscopic yield: 60%).

3-ethyl-3-methyl-2,5-dioxane-1,4-dione

Stage 1: Synthesis of Precursor (II)

The synthesis of compound (II) is carried out according to the same reaction diagram as in Example 1:

Stage 2: Synthesis of 3-ethyl-3-methyl-2,5-dioxane-1,4-dione

A solution of 0.5 g of cyclic keto-ester (3.5 mmol) and 1.21 g of metachloroperbenzoic acid (2 eq.) in 10 ml of dichloromethane is heated under reflux for 48 hours. After returning to ambient temperature, the solvent is eliminated under vacuum. NMR 1 H analysis reveals the complete conversion of the ring with 5 members and the formation of mostly 3-ethyl-3-methyl-2,5-dioxane-1,4-dione (spectroscopic yield: 75%). NMR 1 H characteristics [4.97 (s, 2H), 1.95 (q, 2H, 3 J HH =7.5 Hz), 1.67 (s, 3H), 1.03 (t, 3H, 3 J HH =7.5 Hz)].