Process for the preparation of 4,4-dimethyl-6-ethynylthiochroman

1. A process for the preparation of 4,4-dimethyl-6-ethynylthiochroman comprising:

(a) reacting 4,4-dimethyl-6-acetylthiochroman of the formula

 with an acid chloride and an amido-group containing compound of the general formula

 wherein R is hydrogen or a hydrocarbyl of from 1 to about 15 carbon atoms and R 1 and R 2 can be the same or different and are hydrocarbyls of from 1 to about 15 carbon atoms or R 1 and R 2 together with the nitrogen atom to which they are bonded are joined together to form a heterocyclic group, optionally containing one or more additional heterocyclic atoms, or one of R 1 and R 2 together with the nitrogen atom to which it is bonded are joined together with the carbonyl radical to form a heterocyclic group, optionally containing one or more additional heterocyclic atoms to form a β-chloro vinyl carbonyl compound intermediate of the general formula

 wherein R has the aforestated meanings; and

(b) reacting the β-chloro vinyl carbonyl compound intermediate with an alkali metal hydroxide to provide the 4,4-dimethyl-6-ethynylthiochroman.

2. The process of claim 1 , wherein the acid chloride is selected from the group consisting of phosphorous oxychloride, thionyl chloride, phosgene, oxalyl chloride and mixtures thereof.

3. The process of claim 1 , wherein the R group of the amido-group containing compound is hydrogen and R 1 and R 2 each are hydrocarbyl groups of from 1 to about 15 carbon atoms.

4. The process of claim 1 , wherein the R group of the amido-group containing compound is hydrogen and R 1 and R 2 each are hydrocarbyl groups of from 1 to about 12 carbon atoms.

5. The process of claim 1 , wherein the R group of the amido-group containing compound is hydrogen and R 1 and R 2 each are hydrocarbyl groups of from 1 to 6 carbon atoms.

6. The process of claim 1 , wherein the R group of the amido-group containing compound is hydrogen and R 1 and R 2 together with the nitrogen atom to which they are bonded are joined together to form a heterocyclic group.

7. The process of claim 1 , wherein the R group of the amido-group containing compound is hydrogen and one of R 1 and R 2 together with the nitrogen atom to which it is bonded are joined together with the carbonyl radical to form a heterocyclic group.

8. The process of claim 1 , wherein the R group of the amido-group containing compound is a hydrocarbyl group of from 1 to about 15 carbon atoms and R 1 and R 2 each are hydrocarbyl groups of from 1 to about 15 carbon atoms.

9. The process of claim 1 , wherein the R group of the amido-group containing compound is a hydrocarbyl group of from 1 to about 12 carbon atoms and R 1 and R 2 each are hydrocarbyl groups of from 1 to 6 carbon atoms.

10. The process of claim 1 , wherein the R group of the amido-group containing compound is a hydrocarbyl of from 1 to about 15 carbon atoms and R 1 and R 2 together with the nitrogen atom to which they are bonded are joined together to form a heterocyclic group.

11. The process of claim 1 , wherein the R group of the amido-group containing compound is a hydrocarbyl of from 1 to about 15 carbon atoms and one of R 1 and R 2 together with the nitrogen atom to which it is bonded are joined together with the carbonyl radical to form a heterocyclic group.

12. The process of claim 1 , wherein the amido-group containing compound is selected from the group consisting of dimethyl formamide, N-methyl formanilide, N-formyl piperidine, N-formyl morpholine, dimethyl acetamide, N-methyl pyrrolidone, N,N-dimethyl benzamide and mixtures thereof.

13. The process of claim 1 , further comprising adding a chlorinated alkane solvents to the reaction mixture formed in step (a).

14. The process of claim 13 , wherein the chlorinated alkane solvents are selected from the group consisting of dichloromethane, chloroform, carbon tetrachloride and mixtures thereof.

15. The process of claim 1 , wherein the reaction in step (a) is carried out at a temperature of about −10° C. to about 100° C.

16. The process of claim 1 , wherein the concentration of the amido-group containing compound is from about 45 to about 90 weight percent, based on the total weight of the reaction mixture of step (a).

17. The process of claims 1 , wherein the concentration of the amido-group containing compound is from about 55 to about 75 weight percent, based on the total weight of the reaction mixture of step (a).

18. The process of claim 1 , wherein the acid chloride is present in a ratio of about 1:3 (w/v) to about 1:5 (w/v) with respect to 4,4-dimethyl-6-acetylthiochroman.

19. The process of claim 1 , wherein the alkali metal hydroxide is selected from the group consisting of potassium hydroxide, sodium hydroxide and mixtures thereof.

20. The process of claim 1 , wherein the alkali metal hydroxide is added in a molar ratio of about 1:4 to about 1:8 with respect to the β-chloro vinyl carbonyl compound intermediate.

21. The process of claim 1 , wherein in step (b) further comprises adding an ether type solvent selected from the group consisting of a dialkyl ether, dioxane, tetrahydrofuran, pyran and mixtures thereof.

22. The process of claim 1 , wherein the 4,4-dimethyl-6-ethynylthiochroman is thereafter converted to tazarotene.

23. The process of claim 1 , wherein the acid chloride is phosphorous oxychloride and the amido-group containing compound is dimethyl formamide.