Method of preparing a DON prodrug from L-pyroglutamic acid

1. A method of preparing a compound of Formula I:

the method comprising:

(a) reacting a compound of Formula II:

with a compound of Formula III:

in a solvent in the presence of an alkyl cyano(hydroxyimino)acetate, a carbodiimide, and a base at a temperature of about 0° C. to about 40° C., wherein:

R 1 is C 1 -C 4 alkyl;

R 2 is C 1 -C 4 alkyl; and

R 3 is selected from the group consisting of C 1 -C 6 alkyl, (aryl)alkyl, and (heteroaryl)alkyl; and

(b) isolating the compound of Formula I.

2. The method of claim 1 , wherein the solvent is selected from the group consisting of N,N-dimethylformamide, N-methyl-2-pyrrolidinone, dimethylacetamide, dichloromethane, tetrahydrofuran, 2-methyltetrahydrofuran, and dioxane.

3. The method of claim 1 , wherein the carbodiimide is N,N′-diisopropylcarbodiimide or N,N′-dicyclohexylcarbodiimide.

4. The method of claim 1 , wherein the alkyl cyano(hydroxyimino)acetate is ethyl cyano(hydroxyimino)acetate.

5. The method of claim 1 , wherein the stereomutation of a compound having Formula II is less than about 2% and the stereomutation of a compound having Formula III is less than about 2%.

6. The method of claim 1 , wherein the compound of Formula II is obtained by reacting a compound of Formula IV:

with a deprotecting agent in a solvent at a temperature of about −5° C. to about 30° C., wherein:

R 1 is C 1 -C 4 alkyl; and

PG is a protecting group.

7. The method of claim 6 , wherein PG is selected from the group consisting of fluorenylmethyloxycarbonyl, carboxybenzyl, tert-butyloxycarbonyl, allyloxycarbonyl, and methyl carbamate.

8. The method of claim 6 , wherein the compound of Formula IV is obtained by reacting a compound of Formula V:

with a diazo reagent in the presence of an organolithium reagent in a solvent at a temperature of about −120° C. to about −40° C.,

wherein:

R 1 is C 1 -C 4 alkyl; and

PG is a protecting group.

9. The method of claim 8 , wherein PG is selected from the group consisting of fluorenylmethyloxycarbonyl, carboxybenzyl, and tert-butyloxycarbonyl.

10. The method of claim 8 , wherein the diazo reagent is trimethylsilyldiazomethane and the organolithium reagent is n-butyllithium.

11. The method of claim 8 , wherein the compound of Formula V is obtained by reacting a compound of Formula VI:

with compound of Formula VII:

PG-X  VII,

in a solvent at a temperature of about −100° C. to about −60° C.,

wherein:

R 1 is C 1 -C 4 alkyl;

X is selected from the group consisting of —Cl, —Br, —I, —O-succinimide, —O-1,2,3-benzotriazole, and —O-aryl; and

PG is a protecting group.

12. The method of claim 11 , wherein PG is fluorenylmethyloxycarbonyl, carboxybenzyl, or tert-butyloxycarbonyl.

13. The method of claim 11 , wherein the compound of Formula VI is obtained by esterifying L-pyroglutamic acid with R 1 OH, wherein R 1 is C 1 -C 4 alkyl.

14. The method of claim 1 , wherein R 1 is isopropyl.

15. The method of claim 1 , wherein R 2 is methyl.

16. The method of claim 1 , wherein R 3 is:

17. The method of claim 1 , wherein isopropyl (S)-2-((S)-2-acetamido-3-(1H-indol-3-yl)propanamido)-6-diazo-5-oxohexanoate is obtained in about 35-90% yield or more starting from a compound of Formula II.

18. The method of claim 1 , wherein isopropyl (S)-2-((S)-2-acetamido-3-(1H-indol-3-yl)propanamido)-6-diazo-5-oxohexanoate is obtained in about 98% chemical purity or more starting from a compound of Formula II.

19. The method of claim 1 , wherein isopropyl (S)-2-((S)-2-acetamido-3-(1H-indol-3-yl)propanamido)-6-diazo-5-oxohexanoate is obtained in about 98% ee or more starting from a compound of Formula II.

20. Crystalline (S)-2-((S)-2-acetamido-3-(1H-indol-3-yl)propanamido)-6-diazo-5-oxohexanoate characterized as having powder x-ray diffraction peaks at 6.6, 13.3, 18.3, 20.0, 23.4, and 25.8 degrees 2Θ.