Process for the preparation of 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide and for the purification thereof

6-Methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is prepared by cylcizing acetoacetamide-N-sulfonic acid or its salts with an at least approximately equimolar amount of SO.sub.3 in the presence of a water-immiscible, inert organic solvent and, if appropriate, also an inert, inorganic solvent. In the event that an equimolar amount of SO.sub.3 is employed, working up is effected by adding aqueous sulfuric acid when the cyclization reaction is complete; in the event that the amount of SO.sub.3 employed is more than equimolar, the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide obtained in the form of the SO.sub.3 -adduct is hydrolyzed by adding water or ice, whereby sulfuric acid is formed from the SO.sub.3 combined in the SO.sub.3 -adduct. The inert, organic solvent is then removed from the resulting multi-phase mixture by distillation, and the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is obtained in a pure form from the remaining aqueous sulfuric acid phase by crystallization. Additionally, quite generally, crude 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is purified by recrystallization from aqueous sulfuric acid. The non-toxic salts of 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2 dioxide--in particular the potassium salt--are valuable synthetic sweetening agents.

6-Methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is the compound of 
the formula 
##STR1## 
As a result of the acid hydrogen on the nitrogen atom, the compound is 
capable of forming salts (with bases). The non-toxic salts--such as, for 
example, the Na, K and Ca salt--can be used as sweetening agents in the 
food industry because of their sweet taste, in some cases intense sweet 
taste, the K salt ("Acesulfam K" or just "Acesulfam") being of particular 
importance. 
A number of different processes are known for the preparation of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide and its non-toxic 
salts; cf. Angewandte Chemie 85, issue 22 (1973), pages 965 to 973, 
corresponding to International Edition Volume 12, No. 11 (1973), pages 
869-876. Virtually all the processes start from chlorosulfonyl or 
fluorosulfonyl isocyanate (XSO.sub.2 NCO in which X=Cl or F). The 
chlorosulfonyl or fluorosulfonyl isocyanate is then reacted with 
monomethylacetylene, acetone, acetoacetic acid, tert.-butyl acetoacetate 
or benzyl propenyl ether (in a multi-stage reaction in most cases) to give 
acetoacetamide-N-sulfochloride or acetoacetamide-N-sulfofluoride, which 
cyclizes under the influence of bases (such as, forexample, methanolic 
KOH) and affords the corresponding salts of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide. The free 
oxathiazinone can, if desired, be obtained from the salts in a customary 
manner (by means of acids). 
A further process for the preparation of the oxathiazin-one intermediate 
stage acetoacetamide N-sulfofluoride starts from sulfamoyl fluoride 
H.sub.2 NSO.sub.2 F, the partial hydrolysis product of fluorosulfonyl 
isocyanate (German OffenLegungsschrift No. 2,453,063). The fluoride of 
sulfamic acid H.sub.2 NSO.sub.2 F is then reacted with an approximately 
equimolar amount of the acetoacetylating agent diketene in an inert 
organic solvent in the presence of an amine at temperatures between about 
-30.degree. and 100.degree. C.; the reaction proceeds in accordance with 
the following equation (using triethylamine as the amine): 
##STR2## 
The acetoacetamide-N-sulfofluoride is then cyclized to give the sweetening 
agent in a customary manner by means of a base, for example methanolic 
KOH: 
##STR3## 
Although the known processes give yields of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide and its non-toxic 
salts which are in some cases quite satisfactory (up to approx. 85% of 
theory, relative to the sulfamoyl halide starting materials), they are 
still in need of improvement, particularly for industrial purposes, 
because of the need to employ the starting materials chlorosulfonyl 
fluorosulfonyl isoxyanate which are not very easily accessible; this is 
because, owing to the starting materials (HCN, Cl.sub.2, SO.sub.3 and HF), 
some of which are rather unpleasant to handle, the preparation of 
chlorosulfonyl and fluorosulfonyl isocyanate requires considerable 
precautionary measures and safety precautions. The preparation of 
chlorosulfonyl and fluorosulfonyl isocyanate is based on the following 
equations: 
EQU HCN+Cl.sub.2 .fwdarw.ClCN+HCl 
EQU ClCN+SO.sub.3 .fwdarw.ClSO.sub.2 NCO 
EQU ClSO.sub.2 NCO+HF.fwdarw.FSO.sub.2 NCO+HCl 
The replacement of sulfamoyl fluoride in the process according to German 
Offenlegungsschrift No. 2,453,063 mentioned above, for instance by 
sulfamic acid H.sub.2 NSO.sub.3 H or salts thereof, which is considerably 
easier to obtain (for example from NH.sub.3 +SO.sub.3), hardly seemed 
promising for the simple reason that the reaction of Na sulfamate H.sub.2 
NSO.sub.3 Na with diketene in an aqueous alkaline solution does not give 
any reaction product which can be isolated in a pure state. On the 
contrary, it has only been possible to isolate the 1:1-adduct which is 
formed in this reaction, probably at least together with other products in 
the form of the coupling product with 4-nitrophenyuldiazonium chloride as 
a pale yellow dyestuff; cf. Ber. 83(1950), pages 551-558, in particular 
page 555, last paragraph before the description of the experiments and 
page 558, last paragraph: 
##STR4## 
Moreover, acetoacetamide-N-sulfonic acid has otherwise been postulated 
only, or also, as an intermediate product in the decomposition of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide when the latter is 
boiled in aqueous solution; cf. the literature quoted initially, Angew. 
Chemie (1973) (loc. cit.): 
##STR5## 
Because the processes of the state of the art for the preparation of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide and its non-toxic 
salts are not entirely satisfactory, above all for being carried out on an 
industrial scale, in particular as a result of the need to employ starting 
materials which are not readily accessible, it was, therefore, required to 
improve the known processes appropriately or to develop a new, improved 
process. 
In order to achieve this object, it has already been suggested that the 
process according to German Offenlegungsschrift No. 2,453,063 should be 
modified chiefly by replacing the sulfamoyl fluoride in the known process 
by salts of sulfamic acid and by subsequently cyclizing the resulting 
acetoacetylation product by means of SO.sub.3 (European Patent Application 
No. 85,102,885.2--Publication Number 0,155,634--with the priority of 
German Application No. P 3,410,439.9 dated 22.3.1984--U.S. Pat. No. 
4,607,100 (Clause et al.), issued Aug. 19, 1986. 
The patent application last mentioned relates particularly to a process for 
the preparation of 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide 
and its non-toxic salts by 
(a) reacting a sulfamic acid derivative with an at least approximately 
equimolar amount of an acetoacetylating agent in an inert organic solvent, 
if appropriate in the presence of an amine or phosphine catalyst, to give 
an acetoacetamide derivative and 
(b) cyclizing the acetoacetamide derivative; the process comprises using, 
as the sulfamic acid derivative in stage (a), a salt of sulfamic acid 
which is at least partly soluble in the inert organic solvent employed, 
cyclizing the acetoacetamide-N-sulfonate formed in this stage or the free 
acetoacetamide-N-sulfonic acid in stage (b) by the action of an at least 
approximately equimolar amount of SO.sub.3, if appropriate in an inert 
inorganic or organic solvent, to give 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, and then, if 
desired, also neutralizing with a base, in a stage (c), the product 
obtained here in the acid form. 
The following are indicated in the abovementioned patent application (using 
diketene as the acetoacetylating agent) as the reactions on which the 
process is based: 
##STR6## 
Stage (b) in this scheme of reactions is shown with an amount of SO.sub.3 
which is equimolar in respect of the acetoacetamide-N-sulfonate. It is 
preferable, however, to use an excess of SO.sub.3. An intermediate product 
is then formed, the chemical structure of which is not yet accurately 
known, but which possibly constitutes an SO.sub.3 adduct of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide--described below 
as the "SO.sub.3 -adduct"--and this adduct must then also be hydrolyzed. 
In this case the abovementioned reaction stage (b) thus comprises 2 
partial stages, namely: 
##STR7## 
The cyclization reaction (b1) is carried out in accordance with the 
abovementioned patent application at temperatures between about 
-70.degree. and +175.degree. C., preferably between about -40.degree. and 
+10.degree. C.; the reaction times are between about 1 and 10 hours. 
The hydrolysis (b2) is carried out after he cyclization reaction by adding 
water or ice. 
Working up is then carried out in a customary manner; working up is, 
however, only illustrated in detal for the preferred case in which 
methylene chloride is used as a reaction medium. 2 phases are formed after 
the hydrolysis in this case, the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide passing mainly 
into the organic phase. The fractions still present in the aqueous 
sulfuric acid can be obtained by extraction with a (water-immiscible) 
organic solvent, such as, for example, methylene chloride or an organic 
ester. 
Alternatively, after water has been added, the reaction solvent is removed 
by distillation and the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide remaining in the sulfuric acid of the reaction is extracted 
with a more suitable organic solvent. 
The combined organic phases are dried, for example with Na.sub.2 SO.sub.4, 
and are concentrated. Sulfuric acid which may have been carried over in 
the extraction can be removed by the controlled addition of an aqueous 
alkali solution to the organic phase. If it is intended to isolate the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, it is advisable 
also to purify it in a customary manner (preferably by recrystallization). 
The yield is between about 70 and 95% of theory, relative to the 
acetoacetamide-N-sulfonate (or the free acid). 
If, however, it is intended to isolate a non-toxic salt of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, the 
neutralization stage (c) is also carried out. This is effected by 
neutralizing, in a customary manner, by means of an appropriate base the 
oxathiazinone compound obtained in the acid form in stage (b). This is 
carried out, for example, by neutralizing, by means of an appropriate 
base--preferably a potassium base, such as, for example, KOH, KHCO.sub.3, 
K.sub.2 CO.sub.3, K alcoholates etc.--the combined, dried and concentrated 
organic phases at the end of stage (b) in suitable organic solvents, such 
as, for example, alcohols, ketones, esters or ethers or even water. Or the 
oxathiazinone compound is neutralized by direct extraction with an aqueous 
potassium base from the purified organic extraction phase (stage b). The 
oxathiazinone salt is then precipitated, if necessary after concentrating 
the solution, in a crystalline form, and can also be purified by 
recrystallization. The neutralization stage takes place in a virtually 
100% yield. 
Reference should be made to the detailed description in the patent 
application mentioned in regard to the further details of the process. 
The process starts from readily accessible and cheap starting materials and 
is extremely simple to carry out. The yields of the whole process are 
between about 65 and 95% of theory, relative to the sulfamate starting 
material. 
In the course of further work on this process it has also been suggested 
that both the cyclization reaction (b1) and the hydrolysis (b2) should be 
carried out within short to very short times (approx. 10 minutes down to 
the region of seconds and fractions of a second) (Patent Application No. P 
3,527,070.5 dated 29.7.1985 - HOE 85/F 134). The practical realization of 
the process is preferably effected in devices which are suitable and known 
for carrying out reactions of this type which proceed rapidly and with the 
evolution of heat (thin film reactors, falling film reactors, spray 
reactors, tubular reactors with and without internal fitments, etc.). The 
reaction mixture is worked up as described in the patent application 
mentioned above. This "short time variant" enables the technical procedure 
and, in particular, the space-time yield of the process to be considerably 
improved. 
Finally, it has also already been suggested that, instead of stages (a) and 
(b) of the process of the abovementioned European Patent Application No. 
85,102,885.2, acetoacetamide should be reacted with an at least about 
twice-molar amount of SO.sub.3, if appropriate in an inert inorganic or 
organic solvent (German Patent Application No. P 3,410,440.2 dated 
22.3.1984 - HOE 84/F 065). In this case acetoacetamide-N-sulfonic acid is 
probably first formed in one stage from one mole of acetoacetamide and one 
mole of SO.sub.3, and then undergoes cyclization with a further mole of 
SO.sub.3 to give 6-methyl-3,4-dihydro-1,2,3- oxathiazin-4-one 2,2-dioxide 
in accordance with the following scheme of reactions: 
##STR8## 
Here too the "SO.sub.3 adduct" is formed with excess SO.sub.3 and must 
also be hydrolyzed in order to liberate the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide. The working up of 
the hydrolyzed mixture and, if desired, the conversion of the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4one 2,2-dioxide into its non-toxic 
salts are effected, in principle, in the same way as that described in the 
above-mentioned European Patent Application No. 85,102,885.2. The yield 
figures for 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide are 
between about 30 and 90% of theory, relative to the acetoacetamide 
starting material. 
In all three of the abovementioned patent applications the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide which has been 
liberated in the hydrolysis of the "SO.sub.3 -adduct" is obtained from the 
organic phase which is formed, after adding water, when using a 
(water-immiscible) organic solvent for the reaction and/or which is formed 
if the reaction sulfuric acid is extracted with organic solvents. However, 
the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide thus obtained 
and also the non-toxic salts optionally obtained therefrom by reaction 
with appropriate bases are not always of the required purity, so that 
various purification operations--preferably recrystallization(s) are often 
also necessary--involving additional outlay and also associated with loss 
of substance. 
In developing the abovementioned processes further, it has now been found 
that a considerably purer 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide is obtained if it is isolated, not--as described above--from 
the organic phase, but from the aqueous sulfuric acid phase in a direct 
manner by crystallization. 
The invention relates to a process for the preparation of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide by cyclizing an 
acetoacetamide derivative; the process comprises using 
acetoacetamide-N-sulfonic acid or its salts--dissolved in a 
water-immiscible, inert organic solvent--as the acetoacetamide derivative, 
carrying out the cyclization by treatment with an at least approximately 
equimolar amount of SO.sub.3 --if appropriate dissolved similarly in a 
water-immiscible, inert, organic solvent or in an inert, inorganic 
solvent--adding aqueous sulfuric acid when the cyclization reaction is 
complete if an equimolar amount of SO.sub.3 has been employed or--in the 
event that a more than equimolar amount of SO.sub.3 has been employed, 
hydrolyzing the 6-methyl-3,4-dihydro-1,2,3- oxathiazin-4-one 2,2-dioxide 
obtained as the SO.sub.3 -adduct after the cyclization reaction, and 
removing the inert organic solvent from the resulting multi-phase mixture 
by distillation, and isolating the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide from the residual 
aqueous sulfuric acid phase by crystallization. 
The smooth success of the cyclization of acetoacetamide-N-sulfonic acid and 
its salts with SO.sub.3 is very surprising, because the elimination of 
water or bases which takes place with cyclization is not successful, or is 
in any case not successful for practical purposes, as is known, with other 
agents for eliminating water or bases, such as, for example, P.sub.2 
O.sub.5, acetic anhydride, trifluoroacetic anhydride, thionyl chloride 
etc., as it has already been possible to show in the abovementioned 
European Patent Application No. 85,102,885.2 by means of a comparison 
example (using P.sub.2 O.sub.5). 
Additionally, it is surprising that, when the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide crystallizes from 
sulfuric acid, a product is obtained which, apart from small amounts of 
adhering sulfuric acid (which can, however, easily be removed), contains 
virtually no impurities--at all events virtually no impurities of an 
organic nature--since it would have been entirely possible to expect that 
possible dissolved organic impurities--originating from the previous 
reaction--would crystallize out together with the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide. 
The preparation of the acetoacetamide-N-sulfonic acid starting material and 
its salts is preferably effected by stage (a) of the process of the 
abovementioned European Patent Application No. 85,102,885.2 by reacting 
the Li or ammonium salts of sulfamic acid with diketene in inert organic 
solvents. Solutions of the Li and ammonium salts of 
acetoacetamide-N-sulfonic acid which can be employed as such without 
further treatment for the cyclization reaction with SO.sub.3 are obtained 
in this process. 
It is, of course, also possible to use other salts of 
acetoacetamide-N-sulfonic acid--in particular alkali and alkaline earth 
metal salts--for the cyclization reaction mentioned. Compared with using 
the salts, the use of free acetoacetamide-N-sulfonic acid hardly affords 
any advantages. 
As in the case of the salts, it is also possible to employ the free 
acetoacetamide-N-sulfonic acid for the cyclization reaction directly, in 
the corresponding solution such as is obtained in its preparation. The 
solution of the free acetoacetamide-N-sulfonic acid which is probably 
formed as an intermediate in the process of German Patent Application No. 
P 3,410,440.2 (HOE 84/F 065) can also be regarded as a solution such as is 
obtained in its preparation. 
Inert organic solvents which are suitable for acetoacetamide-N-sulfonic 
acid or its salts are appropriately those solvents from the series of 
inert organic solvents listed in the abovementioned patent applications 
which are immiscible with water and which boil below 100.degree. C. (under 
normal pressure), i.e.: halogenated aliphatic hydrocarbons, preferably 
those having up to 4 carbon atoms, such as, for example, methylene 
chloride, chloroform, 1,2-dichloroethane, trichloroethylene, 
trichlorofluoroethylene etc., and also carbonic acid esters of lower 
aliphatic alcohols,ppreferably methanol. 
The organic solvents can be employed either on their own or as a mixture. 
Halogenated aliphatic hydrocarbons, especially methylene chloride, are 
particularly preferred solvents. 
The concentration of acetoacetamide-N-sulfonic acid or its salts in the 
inert solvent is not critical, but is limited on the one hand by the 
solubility and on the other hand by considerations of economy, since, at 
high dilution, a great deal of solvent has to be subsequently removed and 
worked up again. In general, concentrations between about 0.1 and 2 mole 
of acetoacetamide-N-sulfonic acid or its salts per liter are appropriate. 
The SO.sub.3 can be added either in a solid or liquid form or by condensing 
in SO.sub.3 vapor. It is preferable, however, to add it in dissolved form, 
in particular dissolved in a water-immiscible, inert organic solvent or in 
an inert inorganic solvent. 
The suitable water-imiscible, inert organic solvents are, in principle, the 
same as those which are also used for dissolving the 
acetoacetamide-N-sulfonic acid or its salts. 
Examples of inert inorganic solvents which can be employed are concentrated 
sulfuric acid or liquid SO.sub.2. The amount of inert solvent employed for 
the SO.sub.3 is, in principle, not critical either. If a solvent is 
employed, it is merely necessary to insure that the SO.sub.3 is adequately 
dissolved; an upper limit is set to the amount of solvent by 
considerations of economy. Advantageous concentrations are about 5 to 50% 
by weight, preferably about 15 to 30% by weight, of SO.sub.3. 
In a preferred embodiment of the invention the same inert solvent, 
preferably a solvent from the group of halogenated aliphatic hydrocarbons, 
in particular only methylene chloride, is used both for the 
acetoacetamide-N-sulfonic acid or its salts and for the SO.sub.3. 
Although the molar ratio of acetoacetamide-N-sulfonic acid or 
acetoacetamide-N-sulfonate to SO.sub.3 can be about 1:1, an excess of 
SO.sub.3 of up to about 20-fold, preferably a 3-fold to 10-fold and 
especially about 4-fold to 7-fold molar excess, is preferable. 
In other respects, the cyclization reaction is carried out, in principle, 
in the same manner and under the same conditions as is described in the 3 
patent applications mentioned above. 
As can be seen from the reaction schemes illustrated at the outset, no 
"SO.sub.3 -adduct" is formed if the acetoacetamide-N-sulfonic acid or its 
salts and the SO.sub.3 are employed in an equimolar ratio. Hydrolysis is 
therefore not necessary in this case. Sulfuric acid, preferably of a 
concentration between about 20 and 90%, in particular between about 50 and 
85%, is added in this case for the isolation of pure 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide. The amount of 
sulfuric acid should be such that good crystallization is possible, 
without too much product remaining in solution; the amount which is 
advantageous in a particular case can be determined easily by a few simple 
small-scale tests. 
In the preferred case of using acetoacetamide-N-sulfonic acid or its salts 
and SO.sub.3 in a molar ratio of 1: more than 1, an "SO.sub.3 -adduct" is 
formed in the cyclization reaction, and it is necessary to liberate the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide from this by 
hydrolysis. Hydrolysis is effected by adding water or ice, appropriately 
in a molar ratio of about 2-fold to 6-fold--in relation to the excess of 
SO.sub.3 used. 
A 2-phase or (if 6-methyl-3,4-dihydro-1,2,3-oxathiazin4-one 2,2-dioxide has 
already been precipitated) a 3-phase mixture is present after sulfuric 
acid has been added after the cyclization reaction using 
acetoacetamide-N-sulfonic acid or acetoacetamide-N-sulfonate and SO.sub.3 
in a 1:1 molar ratio and also after the hydrolysis subsequent to the 
cyclization reaction using acetoacetamide-N-sulfonic acid or 
acetoacetamide-N-sulfonate in a molar ratio of 1: more than 1. The bulk of 
the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide present is 
dissolved in the organic phase and in the sulfuric acid phase. If the 
inert organic solvent has already been removed, for example by evaporation 
in accordance with the "short-time variant" of Patent Application No. P 
3,527,070.5, HOE 85/F 134), the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide present is mainly 
dissolved only in the sulfuric acid phase. 
If, however, an organic phase is still present, the inert organic solvent 
is removed from the whole multi-phase mixture by distillation. The 
distillation can be carried out in vacuo, under atmospheric pressure or 
under an excess pressure. If, for example, methylene chloride, which is 
the particularly preferred solvent, is used, distillation is 
advantageously carried out under pressures of about 200 mbar to 1 bar and 
at temperatures of about 0.degree. to 42.degree. C. 
The distillation can be carried out either discontinuously or continuously. 
In the discontinuous procedure, the solvent can be distilled off directly 
from the reaction vessel. Continuous distillation with short dwell times 
is preferred, however, since this is the best way of avoiding possible 
thermal decomposition of the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide. The inert organic solvent used for the cyclization reaction 
is obtained as the distillate in an unpolluted form. 
The sulfuric acid phase remaining after the distillation contains all the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, but also all the 
byproducts and impurities. If the removal of the organic solvent by 
distillation has been carried out at elevated temperatures, the sulfuric 
acid phase is also at an elevated temperature. On cooling, the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide crystallizes out 
in a surprisingly high degree of purity--in particular free from organic 
impurities. 
If the organic solvent has been removed by distillation at a fairly low 
temperature, it is no longer possible, in some cases, to cool the sulfuric 
acid phase overmuch until solidification takes place. In this case it can 
be expedient to concentrate the sulfuric acid phase, as far as possible 
under reduced pressure, as a result of which crystals of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide are then 
precipitated. 
If a precipitate has already been formed at the end of the cyclization 
reaction and/or when the organic solvent is removed by distillation, it 
can also be expedient to cause this precipitate to dissolve by warming the 
sulfuric acid phase or, if appropriate, also by adding further sulfuric 
acid, and then to induce crystallization by means of cooling. 
The crystals are filtered off, and the filter cake is appropriately washed 
with fresh sulfuric acid, preferably sulfuric acid of about 20 to 30% 
strength, in order to displace the sulfuric acid mother liquor, and is 
thoroughly suction-drained. The filter cake obtained has a slight residual 
moisture of aqueous sulfuric acid, but is free, or at all events virtually 
free, from organic byproducts. If a product free from residual sulfuric 
acid is also desired, it is still possible to recrystallize the crystals. 
If, however, the intention is not to isolate pure 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, but to obtain 
non-toxic salts thereof--in particular the potassium salt--the crystals of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide which still 
contain a slight residual moisture of aqueous sulfuric acid can be 
converted directly into the corresponding salts by neutralization. In 
principle, the neutralization is carried out in the same manner as that 
described in European Patent Application No. 85,102,885.2. This is 
effected by dissolving the product to be neutralized in water or in 
organic solvents, such as, for example, alcohols, ketones, esters or 
ethers--preferably only in water--and neutralizing it with an appropriate 
base, in particular a potassium base, such as, for example, KOH, 
KHCO.sub.3, K.sub.2 CO.sub.3 or a K alcoholate. 
In a preferred embodiment of the neutralization, the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is dissolved in an 
approximately equal amount of water at temperatures between about 
0.degree. and 100.degree. C., preferably between about 20.degree. and 
80.degree. C., and is neutralized with potassium hydroxide solution of 
approximately 45 to 50% strength. After the neutralization, the aqueous 
suspension of the potassium salt is cooled and filtered. Owing to the 
relatively good solubility of the potassium salt in water, cooling to 
0.degree. to about 10.degree. C. is advisable. Since the 
non-recrystallized 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide 
still contains a slight residual moisture of sulfuric acid, a small amount 
of potassium sulfate is also formed together with the desired potassium 
salt in the neutralization with a potassium base. However, because of the 
good solubility of potassium sulfate in water, virtually all of the 
potassium sulfate remains in the neutralization mother liquor. 
After drying, the potassium salt of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide (Acesulfam K) has 
a purity of over 99.5%. The small residue is composed almost exclusively 
of potassium sulfate. 
The extent of isolation of Acesulfam K in this embodiment of neutralization 
is about 80-90% of theory, relative to the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide employed. The 
proportion of Acesulfam K remaining in the neutralization mother liquor 
corresponds to its solubility in water. The extent of isolation can be 
increased if less water is used to dissolve the 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide; the resulting 
suspension of Acesulfam K then becomes thicker, however. 
If an even lower content of potassium sulfate in the Acesulfam K is 
desired, a recrystallization from water can also be carried out 
subsequently. By this means the Acesulfam K is obtained in virtually 100% 
purity. The mother liquor from the recrystallization can ee recycled for 
dissolving the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, so 
that no loss in yield occurs. 
A further optimization of the yield can, if appropriate, also be carried 
out by extracting the product remaining dissolved in the sulfuric acid 
mother liquor after the 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide has been filtered off from the sulfuric acid phase with a 
water-immiscible, inert organic solvent--if possible the same solvent 
which was also used for carrying out the cyclization reaction--and 
combining the extract with the reaction product prior to the distillation 
of the solvent. 
Concentrating the neutralization mother liquor can also be regarded as a 
measure for optimizing the yield further. 
The yields from the cyclization process according to the invention and the 
subsequent isolation of the product are of the same order of magnitude as 
the yields indicated in the three patent applications mentioned above; a 
higher purity of product is, however, obtained by means of the invention. 
Finally, the isolation of the product carried out in accordance with the 
invention subsequent to the cyclization reaction can also be utilized to 
purify impure 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide 
prepared by other means. 
The invention also relates, therefore, to a process for the purification of 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide, which comprises 
recrystallizing crude 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide from aqueous sulfuric acid--preferably sulfuric acid of about 
20 to 90% strength, in particular about 50 to 85% strength. 
Any inorganic or organic impurities present in the crude 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide are completely or 
at all events virtually completely, removed by means of this 
recrystallization. 
The examples which follow are intended to illustrate the invention further. 
The examples of the invention (A) are followed by a comparison example 
(B), from which it can be seen that a less pure 
6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 2,2-dioxide is obtained if the 
latter is isolated, not from the sulfuric acid phase, but from the organic 
phase. In the examples 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one 
2,2-dioxide is abbreviated to "ASH", and its potassium salt to "ASK". 
Preparation of the acetoacetamide-N-sulfonate used as the starting material 
for the examples 
48.6 g (=0.5 mol) of sulfamic acid in 250 ml of CH.sub.2 Cl.sub.2 were 
neutralized with 52.5 g (=0.53 mol) of triethylamine while cooling so that 
the temperature did not exceed +30.degree. C. 3 g of acetic acid were 
added. 46.3 g (=0.55 mol) of diketene were added dropwise at 0.degree. C. 
in the course of 60 minutes. The mixture was then stirred for 60 minutes 
at 0.degree. C. and for 6 hours at room temperature. 
(A) Examples of the invention

EXAMPLE 1 
(a) Cyclization and hydrolysis: 
500 ml of CH.sub.2 Cl.sub.2 were initially placed in a reaction vessel and 
were cooled to -30.degree. C. The solution of triethylammonium 
acetoacetamide-N-sulfonate prepared as above and 120 ml (=2.8 mol) of 
SO.sub.3 were added dropwise simultaneously. The temperature in the 
reaction vessel did not exceed -30.degree. C. The mixture was then stirred 
for a further 30 minutes at -30.degree. C. 162 ml of water were then added 
dropwise at -15.degree. C. to -10.degree. C. and the mixture was stirred 
for 1.5 hours at 0.degree. C. 
The reaction product was a three-phase mixture composed of a CH.sub.2 
Cl.sub.2 phase, an H.sub.2 SO.sub.4 phase and solid ASH. 
(b) Working up: 
The CH.sub.2 Cl.sub.2 was distilled off from the reaction product by vacuum 
distillation at 500 mbar and a boiling point of 24.degree. C. In the 
course of this the maximum temperature in the bottom product did not 
exceed 40.degree. C. The suspension of ASH in sulfuric acid was cooled to 
0.degree. C. and filtered off by means of a glass suction filter. The 
filter cake was washed on the suction filter with 50 ml of 30% strength 
H.sub.2 SO.sub.4 and was suction-drained. 
The filter cake of ASH was analysed by potentiometric titration. It 
contained 92% of ASH and 2.5% of H2SO.sub.4. The remainder up to 100% was 
water. No organic byproducts could be found in HPLC (=high pressure liquid 
chromatography) analysis. 
The yield of ASH was 49.8 g (=0.31 mol)=61% of theory, relative to the 
sulfamic acid originally employed. 
The ASH was converted into the K salt by dissolving it in 61 ml of water at 
30.degree. C. and neutralizing the solution at 30.degree. C. with 50% 
strength KOH until pH 7 was reached. The suspension of ASK formed was 
cooled to 0.degree. C. and filtered. After being dried in a vacuum drying 
cabinet at 200 mm Hg and 60.degree. C., 52.3 g of ASK were obtained, 
corresponding to a yield of 52% of theory, relative to sulfamic acid. The 
ASK was white. Its purity was determined by means of potentiometric 
titration and HPLC. ASK: 99.9%; K.sub.2 SO.sub.4 : 0.1%; KCl: 20 ppm. 
Organic byproducts could not be detected by the HPLC. 
After the ASK had been recrystallized from 0.8 times its amount of water, 
its content of K.sub.2 SO.sub.4 was below 100 ppm. 
EXAMPLE 2 
The CH.sub.2 Cl.sub.2 was removed by distillation under normal pressure, 
boiling point 42.degree. C./1 bar, from the reaction product prepared in 
accordance with Example 1(a). The maximum temperature of the bottom 
product did not exceed 70.degree. C. toward the end of the distillation. 
On cooling, crystals were precipitated from the sulfuric acid phase, and 
were filtered off at 0.degree. C. and washed with 50 ml of 30% strength 
sulfuric acid. The filter cake contained 90% of ASH and 3.5% of H.sub.2 
SO.sub.4. 
The yield was 54 g (=0.33 mol) =66% of theory, relative to sulfamic acid. 
Neutralization was carried out as described in Example 1(b). This gave an 
ASK containing 99.8% of ASK and 0.2% of K.sub.2 SO.sub.4. 
The yield was 54 g (=0.27 mol) =54% of theory, relative to sulfamic acid. 
After one recrystallization the content of ASK was 100%. No byproducts were 
found in the HPLC analysis. 
EXAMPLES 3-6 
The reaction product prepared in accordance with Example 1(a) was worked up 
and neutralized as described in Example 2. After the ASH had been filtered 
off from the sulfuric acid phase and had been washed with 30% strength 
sulfuric acid, the sulfuric acid phase was extracted with twice 250 ml of 
methylene chloride. The extract was combined with the reation product from 
the following experiment in each case. The mother liquor obtained in the 
recrystallization of the ASK was employed to dissolve the ASH in the 
neutralization stage of the following experiment in each case. The results 
of Examples 3-6 can be seen in Table 1. 
TABLE 1 
__________________________________________________________________________ 
ASH filter cake ASK Recrystallized ASK 
Yield: 
Purity: 
Yield: 
Purity: 
Yield: 
Purity: % 
Example % % % ASK 
K.sub.2 SO.sub.4 
% ASK 
K.sub.2 SO.sub.4 
__________________________________________________________________________ 
3 Starting experiment 
69 91 56 99.5 
0.2 48 100 
0.03 
4 Recycling of extract 
67 90 63 99.8 
0.1 54 100 
0.03 
and mother liquor 
5 Recycling of extract 
71 91 67 99.8 
0.2 57 100 
0.01 
and mother liquor 
6 Recycling of extract 
71 93 66 99.5 
0.3 56 100 
0.03 
and mother liquor 
__________________________________________________________________________ 
EXAMPLE 7 
The reaction product prepared in accordance with Example 1(a). was metered 
from a stirred operation vessel into a thin film evaporator. The 
throughput rate and the heat input were adjusted so as to give an exit 
temperature of 60.degree. C. The ASH crystallized out on cooling from the 
emerging sulfuric acid phase. Inclusive of the extraction of ASH from the 
sulfuric acid mother liquor and the recycling of crystallization mother 
liquor, an ASH yield of 71% of theory was obtained. The ASH filter cake 
contained 94% of ASH and 2% of H.sub.2 SO.sub.4. 
Neutralization was carried out in accordance with Example 1. The resulting 
ASK contained 99.6% of ASK and 0.3% of K.sub.2 SO.sub.4. 
(B) Comparison example 
The methylene chloride phase in the reaction product prepared in accordance 
with inventive example (A)1 was separated off from the sulfuric acid 
phase, and the sulfuric acid phase was extracted by shaking with twice 250 
ml of CH.sub.2 Cl.sub.2. The combined methylene chloride phases were dried 
with sodium sulfate. When the CH.sub.2 Cl.sub.2 had been removed by 
distillation, the ASH was obtained in the form of a pale yellow oily 
residue. 
After being dissolved in 60 ml of H.sub.2 O, the ASH was neutralized with 
50% strength KOH to give ASK, and the latter was dried in vacuo. This gave 
a pale yellow ASK containing 85% of ASK and 5% of potassium sulfate. 
The yield was 66 g (=0.33 mol)=65% of theory, relative to sulfamic acid.