Process for the preparation of substituted oxazoles
The present invention relates to a process for preparing 5-alkoxy-substituted oxazoles, in particular for preparing 4-methyl-5-alkoxy-substituted oxazoles and also a process for preparing pyridoxine derivatives.
1. ) A process for preparing 5-alkoxy-substituted oxazoles of the formula I,
where
R 1 is an unsubstituted or substituted C 1 -C 6 -alkyl radical and
R 2 is hydrogen or an unsubstituted or substituted C 1 -C 6 -alkyl radical,
which comprises
converting α-isocyanoalkanoate esters of the formula II
in the presence of assistants at temperatures above 80° C. into the 5-alkoxy-substituted oxazoles of the formula I
and, simultaneously with the conversion, removing the 5-alkoxy-substituted oxazoles of the formula I from the reaction mixture, wherein said assistants are cyclizing assistants selected from the group consisting of tertiary amines, alcohols and substituted or unsubstituted C 1 -C 6 -alkanoates.
2. A process as claimed in claim 1 , wherein the process is carried out batchwise.
3. A process as claimed in claim 2 , wherein the process is carried out in a batchwise or semi-batchwise reactor equipped with an emplaced reaction column and, simultaneously with the conversion, removing the 5-alkoxy-substituted oxazoles of the formula I from the reaction mixture by rectification.
4. A process as claimed in claim 3 , wherein the rectification parameters are set in such a way that
D the α-isocyanoalkanoate esters of the formula II are converted to the 5-alkoxy-substituted oxazoles of the formula I in the reactor and/or on the internals of the emplaced reaction column and
E the 5-alkoxy-substituted oxazoles of the formula I resulting from the conversion are removed via the emplaced reaction column.
5. A process as claimed in claim 2 , wherein the conversion is carried out in the presence of an inert solvent.
6. A process as claimed in claim 3 , wherein the reaction column used is a dividing wall column.
7. A process as claimed in claim 4 , wherein the top pressure of the column is set to from 5 to 800 mbar and the resulting bottom pressure, which depends on the type of column used and, if used, the type of column internals, is from 10 mbar to atmospheric pressure.
8. A process as claimed in claim 1 , wherein the process is carried out continuously.
9. A process as claimed in claim 8 , wherein the process is carried out in a reaction column and, simultaneously with the conversion, the 5-alkoxy-substituted oxazoles of the formula I are removed from the reaction mixture by rectification.
10. A process as claimed in claim 9 , wherein the rectification parameters are set in such a way that
A the α-isocyanoalkanoate esters of the formula II are converted to the 5-alkoxy-substituted oxazoles of the formula I on the internals and, if present, in the liquid phase of the reaction column,
B the 5-alkoxy-substituted oxazoles of the formula I resulting from the conversion are continuously removed with the top stream or side stream of the reaction column and
C the assistant and any high-boilers resulting from the conversion are removed continuously and independently of each other with the bottom stream or side stream of the reaction column.
11. A process as claimed in claim 8 , wherein the conversion is carried out in the presence of an inert solvent and the reaction parameters are set in such a way that
A the α-isocyanoalkanoate esters of the formula II are converted to the 5-alkoxy-substituted oxazoles of the formula I on the internals and, if present, in the liquid phase of the reaction column,
B1 when the solvent has a higher boiling point than the 5-alkoxy-substituted oxazoles of the formula I resulting from the conversion, the 5-alkoxy-substituted oxazoles of the formula I are continuously removed with the top stream and the solvent is continuously removed via the side stream or bottom stream of the reaction column,
B2 when the solvent has a lower boiling point than the 5-alkoxy-substituted oxazoles of the formula I resulting from the conversion, the 5-alkoxy-substituted oxazoles of the formula I are continuously removed with a side stream and the solvent is continuously removed with the top stream of the reaction column and
C the assistant and any high-boilers resulting from the conversion are removed continuously and independently of each other with the bottom stream or side stream of the reaction column.
12. A process as claimed in claim 9 , wherein the reaction column used is a dividing wall column.
13. A process as claimed in claim 9 , wherein, when the assistant forms an azeotrope with the 5-alkoxy-substituted oxazoles of the formula I, the top pressure of the column is set in such a way that the fraction of the assistant in the azeotrope in the top stream is as low as possible.
14. A process as claimed in claim 9 , wherein the top pressure of the column is set to from 5 to 800 mbar and the resulting bottom pressure, which depends on the type of column used and, if used, the type of column internals, is from 10 mbar to atmospheric pressure.
15. A process for preparing pyridoxine derivatives of the formula IX
where
R 2 is hydrogen or an unsubstituted or substituted C 1 -C 6 -alkyl radical,
which comprises converting amino acids of the formula III
to amino esters of the formula IV,
where
R 1 is an unsubstituted or substituted C 1 -C 6 -alkyl radical,
converting the latter into formamido esters of the formula V
converting the latter into α-isocyanoalkanoate esters of the formula II,
converting the latter in the presence of assistants at temperatures above 80° C. to the 5-alkoxy-substituted oxazoles of the formula I
and, simultaneously with the conversion, removing the 5-alkoxy-substituted oxazoles of the formula I from the reaction mixture,
reacting the 5-alkoxy-substituted oxazoles of the formula I with protected diols of the formula VI,
where
R 3 and R 4 independently or R 3 and R 4 together are a protecting group of the hydroxyl function,
to give the Diels-Alder adducts of the formula VII,
and converting the latter by acid treatment and detachment of the protecting group to the pyridoxine derivatives of the formula IX.
16. A process as claimed in claim 15 , wherein the assistants used are cyclizing assistants selected from the group consisting of bases, alcohols and esters.
17. A process of claim 1 wherein the tertiary amines are triethylamine, triisopropylamine, tri-n-buytlamine, di-methylcyclohexylamine, tris(2-ethylhexy)amine, N-methylpyrrolidone, 5 N,N,N′,N′-tetramethyl-1,3-propanediamine, N, N-diethylaniline or N,N-dibutylaniline.
18. The process of claim 17 wherein the tertiary amine is tri-n-butylamine.
19. The process of claim 16 wherein said bases are tertiary amines and said esters are substituted of unsubstituted C 1 -C 6 -alkyl C 1 -C 6 -alkanoates.
20. The process of claim 19 wherein said triethylamine, triisopropylamine, tri-n-butylamine, tertiary amines are di-methyl-cyclohexylamine, tris(2-ethylhexyl)amine, N-methylpyrrolidone, 5 N,N,N′,N′-tetramethyl-1 ,3-propanediamine, N,N-diethylaniline or N,N-dibutylaniline.
21. The process of claim 1 wherein the cyclizing assistants are alcohols.
22. The process of claim 16 wherein the cyclizing assistants are alcohols.