1. A mixture free of polyphenol and steroid derivatives and which can be isolated by grinding EUGENIA JAMBOLANA LAMARCK seeds, maceration of the powder with a lower aliphatic alcohol with the use of heat, filtration, recovery of the insoluble part no longer containing polyphenol and sterol compounds, maceration of the insoluble part with a water-lower aliphatic alcohol solution, filtration and recovery of the insoluble matter no longer containing polyphenol and sterol compound, treatment of the insoluble part with an ammoniacal solution, treatment of the ammoniacal mixture with a lower aliphatic alcohol with the use of heat, filtration, recovery of the insoluble matter and drying this insoluble matter which constitutes the mixture I. 2. A mixture free of polyphenol and steroid derivatives and which can be isolated by grinding EUGENIA JAMBOLANA LAMARCK seeds, maceration of the powder with a lower aliphatic alcohol with the use of heat, filtration, recovery of the insoluble part no longer containing polyphenol and sterol compounds, maceration of the insoluble part with a water-lower aliphatic alcohol solution, filtration and recovery of the insoluble matter no longer containing polyphenol and sterol compound, treatment of the insoluble part with an ammoniacal solution, treatment of the ammoniacal mixture with a lower aliphatic alcohol with the use of heat, filtration, recovery of the insoluble matter and drying this insoluble matter and further treatment it with a water-lower aliphatic alcohol solution, decantation and then, on the one hand, drawing off the top phase which is filtered to give the filtrate 1 and, on the other hand, treating the bottom phase with water and filtration to give the filtrate 2, pooling of the filtrates 1 and 2 and concentration to aqueous phase, treatment with a nonpolar adsorbent resin and then filtration, partial concentration of the filtrate, centrifugation, filtration and then ultrafiltration, freeze-drying and isolation of the extract II. 3. Process for preparing the mixture I according to claim 1, characterized in that EUGENIA JAMBOLANA LAMARCK seeds are dried, finely ground, screened and the powder obtained is subjected to the following treatment: a - maceration, with stirring, in a lower aliphatic alcohol, at a temperature of between 40 and 70 degree C, b - filtration under vacuum and recovery of the insoluble matter, c - maceration, with stirring, of the insoluble matter with a lower aliphatic alcohol at a temperature of between 40 and 70 degree C, d - filtration under vacuum and removal of the alcoholic phases containing mainly the undesirable polyphenols and sterols and recovery of the insoluble matter, e - taking up the insoluble matter in an ammoniacal solution at a temperature of between 10 and 30 degree C, f - taking up the whole wet ammoniacal mass in a water-lower aliphatic alcohol solution, at a temperature of between 40 and 70 degree C, g - filtration and removal of the alcoholic solution and recovery of the insoluble matter, h - washing of the insoluble matter with a lower aliphatic alcohol, filtration and removal of the alcoholic solution and  i - drying the mixture I. 4. Process according to claim3, for which, in step a, the procedure is carried out by means of 2 to 10 litres of a lower aliphatic alcohol per 1 kg of screened powder. 5. Process according to either of claims 3 or 4, for which, in step a, 5 litres of ethanol with a titre of 93-95 degree  Gay Lussac are used at 60 degree C for 1 hour. 6. Process according to one of claims 3 to 5. for which the filtration of steps b and d are carried out under a vacuum of 40 kPa. 7. Process according to one of claims 3 to 6. for which, in step c, the procedure is carried out by means of 2 to 10 litres of a lower aliphatic alcohol per 1 kg of starting screened powder. 8. Process according to one of claims 3 to 7. for which, in step c, 4 litres of ethanol with a titre of 93-95 degree  Gay Lussac are used at a temperature of 60 degree C for 1 hour. 9. Process according to one of claims 3 to 8. for which, in step e, 750 to 1250 ml of an aqueous aoononiacal solution are used per 1 kg of starting screened powder. 10. Process according to one of claims 3 to 9. for which, in step e, per 1 kg of starting screened powder, 1 litre of aqueous ammoniacal solution containing 350 ml of 28% ammonium hydroxide is used and the procedure is carried out at a temperature close to 20 degree C for 10 to 30 hours hours. 11. Process according to one of claims 3 to 10. for which, in step f, the wet ammoniacal mass obtained from 1 kg of starting screened powder is taken up in 2 to 10 litres of a water-lower aliphatic alcohol solution. 12. Process according to one of claims 3 to 11. for which, in step f, the procedure is carried out in 5 litres of an ethanol-water mixture (75/25 by volume), at 60 degree C for 1 hour. 13. Process according to one of claims 3 to 12. for which, in step h, the washing is carried out with 500 to 1500 ml of a lower aliphatic alcohol per 1 kg of starting screened powder. 14. Process according to one of claims 3 to 13, for which, in step h, the washing is carried out with 1 litre of ethanol and the filtration is carried out on a cotton cloth and under a vacuum of about 80 kPa. 15. Process according to one of claims 3 to 13, for which, in step i, the drying is carried out in the open air and protected from light. 16. Process for preparing the mixture II according to claim 2, characterized in that EUGENIA JAMBOLANA LAMARCK seeds are dried, finely ground, screened and the powder obtained is subjected to the following treatment: a - maceration, with stirring, in a lower aliphatic alcohol, at a temperature of between 40 and 70 degree C, b - filtration under vacuum and recovery of the insoluble matter, c - maceration, with stirring, of the insoluble matter with a lower aliphatic alcohol at a temperature of between 40 and 70 degree C, d - filtration under vacuum and removal of the alcoholic phases containing mainly the undesirable polyphenols and sterols and recovery of the insoluble matter, e - taking up the insoluble matter in an ammoniacal solution at a temperature of between 10 and 30 degree C, f - taking up the whole wet ammoniacal mass in a water-lower aliphatic alcohol solution, at a temperature of between 40 and 70 degree C, g - filtration and removal of the alcoholic solution and recovery of the insoluble matter, h - washing of the insoluble matter with a lower aliphatic alcohol, filtration and removal of the alcoholic solution and  i - drying the mixture I, j - treatment of the mixture I by means of a water-lower aliphatic alcohol solution, k - decantation and then drawing off the top phase which is filtered to give the filtrate 1 and treating the bottom phase with water and filtration to give the filtrate 2, pooling of the filtrates 1 and 2 and concentration to aqueous phase, 1 - treatment with a nonpolar adsorbent resin and then filtration, m - concentration of the filtrate, centrifugation, filtration and then ultrafiltration, n- freeze-drying and isolation of the extract II. 17. Process according to claim 16, for which steps a to i are carried out according to any claim 3 to 15. 18. Process according to claim 16 and 17, for which, in step j, 10 to 25 litres of the water-lower aliphatic alcohol solution (95/5 to 90/10 by volume) are used per 1 kg of the mixture I per 1 kg of the mixture I. 19. Process according to either of claims 16 to 18, for which, in step j, 18 litres of a water -ethanol solution (17.7-0.93 by volume) are used. 20. Process according to one of claims 16 to 19, for which, in step k, the top phase is filtered on a cotton cloth and 10 to 25 litres of water are added to the bottom phase per 1 kg of the mixture I and the mixture is filtered on sintered glass. 21. Process according to one of claims 16 to 20, for which, in step k, the concentration is generally carried out in a thermosiphon concentrator at a temperature of 35 degree C under a vacuum of 0.4 kPa. 22. Process according to one of claims 16 to 19, for which, in step 1, S861 resin or XAD-type resins marketed by Rhom and Hass are used and the mixture is filtered on sintered glass. 23. Process according to one of claims 16 to 22, for which, in step m, the concentration is carried out in a thermosiphon concentrator at a temperature of 35 degree C under a vacuum of 0.4 kPa. 24. Process according to one of claims 16 to 23, for which, in step m, 3 successive ultrafiltrations are carried out on 10 kd, 3 kd and 1 kd cartridges. 25. Process for preparing sodium oxamate, for which, mixture II is subjected to the following operations according to claim 2: o - chromatography of the mixture II on an infusorial earth column, recovery of the fractions containing the 4 products and pooling of these fractions into a single fraction, p - chromatography of the fraction previously obtained on a Sephadex(RTM) column in order to obtain sodium oxamate, 26. Process according to claim 25, for which the chromatography of step o is carried out on a CHEM ELUT(RTM) column marketed by Prolabo, saturated with water and then eluted successively with heptane, a heptane-ethyl acetate mixture (50/50 by volume), an ethyl acetate-n-butanol mixture (95/5; 90/10; 80/20; 50/50;20/80 and then 0/100 by volume) and an n-butanol-water mixture (98/2 and then 95/5 by volume). 27. Process according to claim 25 or 26, for which the chromatography of step p is carried out by means of a water-ethanol mixture (50/50 by volume). 28. Process for preparing sodium oxamate and the compounds of formula:   in which either R1 represents a hydrogen atom and R2 represents a chain of formula:   or R1 represents a chain of formula (A) and R2 represents a hydrogen atom for which the mixture II according to claim 2 is subjected to the following operations: o - chromatography of the mixture II according to the invention on an infusorial earth column, recovery of the fractions containing the 4 products and