Process for the preparation of (S)-4-fluoromethyl-dihydro-furan-2-one

1. A process for the preparation of (S)-4-fluoromethyl-dihydro-furan-2-one of the formula

comprising the steps:

a) conversion of a glycidyl ether of the formula

wherein R 1a is a protecting group, with a fluoride salt to provide a fluoro hydrin of formula

wherein R 1a is as defined above;

b) esterification of the hydroxy group in the fluoro hydrin of the formula IV with a sulfonic acid derivative;

c) ester substitution with a dialkylmalonate of the formula

wherein R 1b is lower alkyl or lower phenylalkyl, in the presence of a base to provide an intermediate of the formula

wherein R 1a and R 1b are as defined above, followed by

d) cyclization, hydrolysis and decarboxylation under acidic conditions.

2. The process according to claim 1 , characterized in that R 1a is tert-butyl.

3. The process according to claim 1 , characterized in that R 1b is methyl.

4. The process according to claim 1 , characterized in that the fluoride salt used for the conversion in step a) is potassium hydrogen difluoride.

5. The process according to claim 1 , characterized in that the conversion in step a) is performed in the presence of a phase transfer catalyst.

6. The process according to claim 5 , characterized in that the phase transfer catalyst is tetrabutylammonium hydrogen sulfate.

7. The process according to claim 1 , characterized in that the conversion in step a) is performed in the presence of triethyleneglycol.

8. The process according to claim 1 , characterized in that the sulfonic acid derivative used for the esterification in step b) is selected from trifluoromethanesulfonic acid anhydride, trifluoromethanesulfonyl chloride, methanesulfonic acid anhydride, nonafluorobutane-sulfonyl fluoride, p-toluene sulfonyl chloride, 2-nitrobenzene sulfonyl chloride and 4-nitrobenzene sulfonyl chloride.

9. The process according to claim 1 , characterized in that the esterification in step b) is performed with trifluoromethanesulfonic acid anhydride in the presence of an amine and an inert organic solvent at a temperature in the range of −40° C. to 20° C.

10. The process according to claim 1 , characterized in that the base used for the ester substitution in step c) is selected from an alkali metal hydride or an alkali metal alkoxide.

11. The process according to claim 10 , characterized in that sodium tert-butoxide is used.

12. The process according to claim 1 , characterized in that the ester substitution in step c) is performed in the presence of an organic solvent at a temperature range from −20° C. to 50° C.

13. The process according to claim 1 , characterized in that the acid used for the transformation in step d) is an aqueous mineral acid.

14. The according to claim 13 , characterized in that aqueous sulfuric acid is used.

15. A process for the preparation of pyrido[2,1-a]isoquinoline derivatives of the formula

wherein R 2 , R 3 and R 4 are each independently selected from the group consisting of hydrogen, halogen, hydroxy, lower alkyl, lower alkoxy and lower alkenyl, wherein lower alkyl, lower alkoxy and lower alkenyl may optionally be substituted by a group selected from lower alkoxycarbonyl, aryl and heterocyclyl,

comprising the process according to claim 1 followed by

e) coupling of the (S)-4-fluoromethyl-dihydro-furan-2-one of formula

with an amino-pyrido[2,1-a]isoquinoline derivative of formula

wherein R 2 , R 3 and R 4 are as defined above,

f) cyclization of the obtained amide of formula

wherein R 2 , R 3 and R 4 are as defined above,

in the presence of a base to obtain a compound of formula

wherein R 2 , R 3 and R 4 are as defined above and

g) deprotection of the amino group.

16. The process according to claim 15 for the preparation of (S)-1-((2S,3S,11bS)-2-amino-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a] isoquinolin-3-yl)-4-fluoromethyl-pyrrolidin-2-one, comprising the process according to claim 1 followed by

e) coupling of the (S)-4-fluoromethyl-dihydro-furan-2-one of formula

with (2S,3S,11bS)-3-amino-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a]isoquinolin-2-yl)-carbamic acid tert-butyl ester,

f) cyclization of the obtained (2S,3S,11bS)-3-(3-fluoromethyl-4-hydroxy-butyrylamino)-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a]isoquinolin-2-yl]-carbamic acid tert-butyl ester in the presence of a base, and

g) deprotecting the obtained (2S,3S,11bS)-3-((4S)-fluoromethyl-2-oxo-pyrrolidin-1-yl)-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a]isoquinolin-2-yl]-carbamic acid tert-butyl ester.