Preparation of sufentanil citrate and sufentanil base

1. A process for preparing solid sufentanil citrate, the process comprising:

(a) forming a first mixture of sufentanil citrate by heating a first portion of sufentanil base with a molar excess of citric acid in water, the first portion of sufentanil base comprising about 10% to about 20% of the total amount of sufentanil base, wherein the volume to mass ratio of water to citric acid in the first mixture is about 8:1 to about 12:1;

(b) cooling the first mixture of sufentanil citrate, wherein the first mixture is substantially free of an oil phase comprising a sufentanil species;

(c) forming a second mixture of sufentanil citrate by heating a second portion of sufentanil base with an approximate molar equivalent of citric acid in water, the second portion of sufentanil base comprising about 80% to about 90% of the total amount of sufentanil base;

(d) adding the second mixture of sufentanil citrate, without cooling, to the first mixture of sufentanil citrate to form a combined mixture;

(e) cooling the combined mixture to form solid sufentanil citrate; and

(f) recovering the solid sufentanil citrate.

2. The process of claim 1 , wherein the molar excess of citric acid to sufentanil base at (a) is about 2.5:1 to about 4.0:1.

3. The process of claim 1 , wherein the molar excess of citric acid to sufentanil base at (a) is about 2.8:1.

4. The process of claim 1 , wherein the heating at (a) is to a temperature from about 70° C. to about 85° C.

5. The process of claim 1 , wherein the cooling at (b) is to a temperature of about 23° C. or less.

6. The process of claim 5 , further comprising maintaining the temperature of the first mixture of sufentanil citrate until (d).

7. The process of claim 1 , wherein the approximate molar equivalent of citric acid to sufentanil base at (c) is from about 0.9:1 to about 1.1:1.

8. The process of claim 1 , wherein the approximate molar equivalent of citric acid to sufentanil base at (c) is about 1:1.

9. The process of claim 1 , wherein the heating at (c) is to a temperature from about 90° C. to about 100° C.

10. The process of claim 1 , wherein the adding at (d) occurs over an extended period of time such that the combined mixture maintains a temperature from about 10° C. to about 23° C.

11. The process of claim 10 , wherein the extended period of time is from about 0.5 hour to about 4 hours.

12. The process of claim 1 , wherein the cooling at (e) is to a temperature from about 0° C. to about 5° C.

13. The process of claim 1 , wherein the recovering at (f) comprises filtering.

14. The process of claim 1 , further comprising drying the solid sufentanil citrate.

15. The process of claim 1 , wherein the first portion of sufentanil base comprises about 15% of the total amount of sufentanil base, and the second portion of sufentanil base comprises about 85% of the total amount of sufentanil base.

16. The process of claim 1 , wherein prior to forming the first mixture of sufentanil citrate, a first solution of citric acid is formed by mixing from about 4 mL to about 6 mL of water per gram of citric acid.

17. The process of claim 1 , wherein prior to forming the second mixture of sufentanil citrate, a second solution of citric acid is formed by mixing from about 12 mL to about 20 ml of water per gram of citric acid.

18. The process of claim 1 , further comprising:

(g) contacting the mixture remaining after recovery of solid sufentanil citrate with a proton acceptor to form a sufentanil base mixture;

(h) cooling the sufentanil base mixture to form solid sufentanil base; and

(i) recovering solid sufentanil base.

19. The process of claim 18 , further comprising contacting solid sufentanil base with a non-polar solvent to form crystalline sufentanil base.

20. The process of claim 18 , wherein the proton acceptor comprises a hydroxide and the non-polar solvent is an alkane.

21. The process of claim 1 , wherein step (a) is accomplished in an inert atmosphere under ambient pressure.