Related Background Art A method for synthesizing 3,3-dimethylbutyraldehyde via hydrolysis of 1,1-dichloro-3,3-dimethylbutane at 300°C with a 60% yield is reported by D. G. Botteron and G. P.

Shulman in J. Am. Chem. Soc., vol. 86,1946, pp. 1650- 1654. This reaction would generate hydrochloric acid but was carried out in a closed system by heating a mixture of water and 1,1-dichloro-3,3-dimethylbutane without addition of a base. It is well known that hydrochloric acid is very corrosive to typical commercial reactors, which are constructed of metal alloys, especially at high temperature and pressure.

Such reaction conditions are highly undesirable because they may lead to structural failure of the reactors and may be very hazardous.

A method for synthesizing 3,3-dimethylbutyraldehyde via hydrolysis of 1,1-dichloro-3,3-dimethylbutane at 180°- 200°C is disclosed in Soviet Patent No. 721400. The method disclosed therein comprises heating 1,1- dichloro-3,3-dimethylbutane with water in the presence of catalytic amounts of magnesium oxide or triethylamine, at 180°-200°C. The yield is reported to be 88-90%.

However, repetition of this reported reaction was found to give a significant amount (30-40%) of cis and trans tert-butyl vinyl chloride and 43% of the starting material remained unreacted. This is highly undesirable because the boiling points of 3,3- dimethylbutyraldehyde and tert-butyl vinyl chloride are very close; hence, they can not be separated by distillation techniques.

Purification of aldehydes by their conversion into a solid aldehyde/bisulfite adduct has been described in the literature. "Purification-of Laboratory Chemicals" (Pergamon Press, 1988), pages 60-61, is incorporated by reference herein as a general reference for this methodology. Typically, the crude aldehyde is stirred with aqueous sodium bisulfite or sodium bisulfite in a mixture of water and alcohol. The precipitates are isolated, washed with water and/or alcohol, and then converted into free aldehyde with a base or acid.

3,3-Dimethylbutyraldehyde is an intermediate that is useful in the preparation of the sweetener N- N- (3,3- dimethylbutyl)-L-a-aspartyl-L-phenylalanine disclosed in U. S. Patent No. 5,480,668, U. S. Patent No. 5,510,508

and U. S. Patent 5,728,862. Thus, despite the techniques for the hydrolysis of 1,1-dichloro-3,3- dimethylbutane described in the prior art, it is clear that there is a need to economically produce 3,3- dimethylbutyraldehyde using more efficient methods than currently available.

SUMMARY OF THE INVENTION This invention is directed to the synthesis and purification of 3,3-dimethylbutyraldehyde with high purity. In particular, this invention is directed to a method for preparing 3,3-dimethylbutyraldehyde comprising the steps of heating 1,1-dichloro-3,3- dimethylbutane or 1-bromo-1-chloro-3,3-dimethylbutane in the presence of water and a base; mixing the resulting 3,3-dimethylbutyraldehyde with a first organic solvent and an aqueous solution of sodium bisulfite in order to form a bisulfite precipitate of the 3,3-dimethylbutyraldehyde; washing said bisulfite precipitate with a second organic solvent; and contacting, with heating, said washed bisulfite precipitate with an inorganic acid or base in order to yield purified 3,3-dimethylbutyraldehyde.

DETAILED DESCRIPTION In the present invention, 3,3-dimethylbutyraldehyde is formed by the hydrolysis of 1,1-dichloro-3,3- dimethylbutane or 1-bromo-1-chloro-3,3-dimethylbutane in the presence of water and a base as illustrated below: Hydrolysis CHO C2 base' or ci ----Br

The aldehyde product prepared via the above-described embodiment of this invention is purified through an aldehyde/bisulfite adduct followed by the regeneration of the aldehyde as illustrated below: OH , MHSO#H20, X M-Na. K, etc.'j'"S03M isolated as a solid, insoluble in organic solvent adduct add or base CHO The 1,1-dichloro-3,3-dimethylbutane precursor is commercially available, e. g., from Aldrich Chemicals, Milwaukee, Wisconsin. The 1-bromo-1-chloro-3,3-dimethylbutane precursor is formed by a method such as described by Arne Brandstrom, Acta Chem. Second, 13,610 (1959). In the practice of this invention, the 1,1-dichloro-3,3-dimethylbutane or 1-bromo-1-chloro-3,3-dimethylbutane is then heated in the presence of water and a base.

The base is selected from an inorganic or an organic base. A mixture of bases is also effective. The inorganic base may preferably be selected from sodium bicarbonate, potassium bicarbonate, sodium carbonate, potassium carbonate, zinc oxide, aluminum oxide, zinc carbonate, magnesium oxide, calcium oxide, calcium carbonate, magnesium carbonate, potassium phosphates

(monobasic, dibasic and tribasic), sodium phosphates (monobasic, dibasic and tribasic), ammonium phosphate, calcium phosphate, magnesium phosphate, ammonia or mixtures thereof. The organic base may preferably be a tertiary amine or a pyridine derivative.

Generally, the reaction time ranges from about 1 to about 48 hours. Preferably, the reaction time ranges from about 3 to about 24 hours. Most preferably, the reaction time ranges from about 4 to about 10 hours.

Generally, the reaction temperature ranges from about 170°C to about 350°C. Preferably, the reaction temperature ranges from about 200°C to about 300°C.

Most preferably, the reaction temperature ranges from about 230°C to about 280°C.

The aldehyde is collected and purified via its sodium bisulfite adduct using organic solvents. This purification preferably includes the following steps: A. A mixture of the aldehyde product, a first organic solvent, and an aqueous sodium bisulfite solution is stirred for a period of time.

B. The solid adduct is isolated by filtration and the solid is washed thoroughly with a second organic solvent.

C. The solid adduct is dried.

D. 3,3-dimethylbutyraldehyde is regenerated by heating a mixture of the dried solid aldehyde/bisulfite adduct and an aqueous inorganic base or acid, followed by distillation.

The stirring time in Step A is generally from about 10 minutes to about 1440 minutes, preferably from about 20 minutes to 600 minutes, most preferably from-30 minutes to 360 minutes.

The first organic solvent in Step A is preferably an ether, a hydrocarbon compound, an ester, an alcohol or a mixture of the above. In another preferred embodiment of the present invention, the first organic solvent is the same as the second organic solvent in Step B.

The second organic solvent in Step B is any organic solvent in which the adduct is insoluble. The organic solvent may be water-miscible or water-immiscible.

Preferably the organic solvent is isopropanol, MTBE, heptane, hexane, ethylacetate, cyclohexane, toluene, or a mixture thereof.

The inorganic acid or base in Step D is preferably hydrochloric acid, sulfuric acid, phosphoric acid, sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium carbonate, or mixtures thereof. ~ :-a preferred embodiment of the present invention, the inorganic base is a carbonate or bicarbonate.

The 3,3-dimethylbutyraldehyde produced and purified by the methods of this invention is sufficiently pure for use in the synthesis of highly pure N- N- (3,3- dimethylbutyl)-L-a-aspartyl] L-phenylalanine 1 methyl ester (neotame).

The Examples which follow are intended as an illustration of certain preferred embodiments of the invention, and no limitation of the invention is implied.

EXAMPLE 1 Preparation of 3,3-dimethylbutyraldehyde by hydrolysis of 1,1-dichloro-3,3-dimethylbutane with ZnO 30.0 g 1,1-dichloro-3,3-dimethylbutane, 15.0 g ZnO and 30.0 g water were loaded into a 150 ml Parr reactor.

The reactor was sealed, stirred and heated at 250°C for 7.5 hours. After the reactor was cooled to room temperature, the mixture was extracted with 100 ml methyl tert-butyl ether (MTBE), and the organic layer was separated. To this organic layer was added 10 g sodium bisulfide in 20 g water dropwise with agitation at 5°C, and the mixture was stirred for another 60 minutes before filtration. The solid was washed with 3x40 ml MTBE and dried in an 40°C oven. Weight of the solid: 14.9 g.

The above solid, 7.3 g sodium bicarbonate and 70 g water were loaded into a round bottom flask and the mixture was heated to reflux. 3,3- Dimethylbutyraldehyde was distilled off from this mixture. Yield: 7.0 g.

COMPARATIVE EXAMPLE 1 Preparation of 3,3-dimethylbutyraldehyde via Russian Patent 721400 Procedure 1,1-Dichloro-3,3-dimethylbutane (15 g), magnesium oxide (2 g), triethyl amine (3 g) and water (27 ml) were loaded into a Parr reactor. The reactor was sealed, stirred and heated at 200°C for 4 hours. 1H NMR of the product showed 21% of 3,3-dimethylbutyraldehyde, 36% cis and trans tert-butyl vinyl chloride and 43% starting material.

Other variations and modifications of this invention will be obvious to those skilled in this art. This invention is not to be limited except as set-forth in the following claims.