Process for the preparation of tubulysins and intermediates thereof

1. A method for preparing a composition comprising a tubuvaline compound of (R,R)-Formula 1a, or a salt thereof, having the structure of:

and one or more optical isomers thereof as optical impurities, wherein the (R,R)-Formula 1a tubuvaline compound, or salt thereof, is the predominate optical isomer, and wherein its corresponding enantiomer, (S,S)-Formula 1a, or a salt thereof, is the major optical impurity and has the structure of:

wherein:

the circled Ar is a 1,3-phenylene or a 5- or 6-membered nitrogen-containing 1,3-heteroarylene, optionally substituted at the remaining positions;

R 1 is phenyl, t-butyl, 9-fluorenyl or allyl, optionally substituted, or other moeity so that R 1 —OC(═O)— is a suitable nitrogen protecting group;

R 3 is optionally substituted saturated C 1 -C 8 alkyl, optionally substituted unsaturated C 3 -C 8 alkyl or optionally substituted C 3 -C 8 heteroalkyl;

R 6 is optionally substituted C 1 -C 8 alkyl; and

R 7 is optionally substituted saturated C 1 -C 20 alkyl, optionally substituted unsaturated C 3 -C 20 alkyl, optionally substituted C 3 -C 20 heteroalkyl, optionally substituted C 2 -C 20 alkenyl, optionally substituted C 3 -C 20 heteroalkenyl, optionally substituted C 2 -C 20 alkynyl, optionally substituted C 3 -C 20 heteroalkynyl, optionally substituted C 6 -C 24 aryl, optionally substituted C 5 -C 24 heteroaryl, optionally substituted C 3 -C 20 heterocyclyl, or other moeity so that R 7 —O— provides for a suitable carboxylic acid protecting group, the method comprising the steps of:

(a) contacting a compound of Formula A:

with a compound of Formula B:

R 3 NHC(O)OR 1   (B),

in a suitable polar, aprotic solvent in the presence of a suitable transition metal (II) or transition metal (III) catalyst, so as to form a composition comprising an enantiomeric mixture of tubuvaline intermediates represented by Formula AB:

(b) contacting the enantiomeric mixture with a suitable chiral reducing agent so as to form a composition comprising an equimolar mixture of diastereomers, wherein the diastereomeric mixture is represented by Formula R-1a,

or salts thereof, wherein the composition further comprises an equimolar mixture of optical impurities that are enantiomers of the diastereomers, and

(b′) separating the diastereomers from the composition of the Formula R-1a diastereomeric mixture so that the composition comprising (R,R)-Formula 1a as the predominate optical isomer and comprising (S,S)-Formula 1a as the major optical impurity is obtained, wherein the predominate optical isomer and the major optical impurities have the structures of:

or salts thereof, respectively.

2. The method of claim 1 , wherein the suitable polar, aprotic solvent is acetonitrile, dichloromethane, THF, dioxane, or a mixture of two or three of these solvents.

3. The method of claim 2 , wherein the chiral reducing agent is a chiral oxazaborolidine prepared from contacting BH 3 -DMS in THF with a suitable chiral ligand.

4. A method for preparing a composition comprising a tubuvaline compound of (R,R)-Formula 2, or a salt thereof, having the structure of:

and one or more optical isomers thereof as optical impurities, wherein the (R,R)-Formula 2 tubuvaline compound, or salt thereof, is the predominate optical isomer, and wherein its corresponding enantiomer, (S,S)-Formula 2, or salt thereof, is the major optical impurity and has the structure of:

wherein:

the circled Ar is a 1,3-phenylene or a 5- or 6-membered nitrogen-containing 1,3-heteroarylene, optionally substituted at the remaining positions;

R 1 is phenyl, t-butyl, 9-fluorenyl or allyl, optionally substituted, or other moeity so that R 1 —OC(═O)— is a suitable nitrogen protecting group;

R 3 is optionally substituted saturated C 1 -C 8 alkyl, optionally substituted unsaturated C 3 -C 8 alkyl or optionally substituted C 3 -C 8 heteroalkyl; and

R 6 is optionally substituted C 1 -C 8 alkyl, the method comprising the steps of:

(c) contacting the composition obtained from steps (a), (b) and step (b′) of claim 1 with a suitable hydrolysis agent, wherein the predominate optical isomer of the composition so obtained is (R,R)-Formula 2, having the structure of:

or a salt thereof, and the major optical impurity is (S,S)-Formula 2 having the structure of:

or a salt thereof.

5. A method for preparing a composition, comprising a tubuvaline compound of (R,R)-Formula 2a, or salt thereof, having the structure of:

and one or more optical isomers thereof as optical impurities, wherein the (R,R)-Formula 2a tubuvaline compound, or salt thereof, is the predominate optical isomer, and wherein its corresponding enantiomer, (S,S)-Formula 2a, or salt thereof, is the major optical impurity and has the structure of:

wherein:

the circled Ar is a 1,3-phenylene or a 5- or 6-membered nitrogen-containing 1,3-heteroarylene, optionally substituted at the remaining positions;

R 1 is phenyl, t-butyl, 9-fluorenyl or allyl, optionally substituted, or other moeity so that R 1 —OC(═O)— is a suitable nitrogen protecting group;

R 3 is optionally substituted saturated C 1 -C 8 alkyl, optionally substituted unsaturated C 3 -C 8 alkyl or optionally substituted C 3 -C 8 heteroalkyl;

R 6 is optionally substituted C 1 -C 8 alkyl; and

R 2B is saturated C 1 -C 6 alkyl, unsaturated C 3 -C 8 alkyl, C 2 -C 8 alkenyl or C 2 -C 4 alkynyl, optionally substituted or R 2B is —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 —CH 2 CH═CH 2 , —CH 2 CH(CH 3 ) 2 , —CH 2 C(CH 3 ) 3 , —CH 2 C(CH 3 )═CH 2 , —CH═CH 2 or —CHCCH, the method comprising the steps of:

(c) contacting the composition obtained from steps (a), (b) and step (b′) of claim 1 with a suitable hydrolysis agent to obtain a composition comprising (R,R)-Formula 2 having the structure of:

or salt thereof, as the predominate optical isomer and further comprising its enantiomer, having the structure of:

or a salt thereof, as the major optical impurity, and

step (d): contacting the composition so obtained with a suitable acylating agent to obtain a composition comprising (R,R)-Formula 2a, or a salt thereof, as the predominate optical isomer and (S,S)-Formula 2a, or a salt thereof, as the major optical impurity, wherein the predominate optical isomer and the major optical impurity, have the structures of:

respectively.

6. The method of claim 4 , wherein the optical purity of the composition from step (b′) is substantially or essentially retained by the composition obtained from step (c).

7. The method of claim 1 , wherein said step (b′) separation is by silica gel flash chromatography.

8. The method of claim 1 , wherein the circled Ar is a 5-membered nitrogen-containing 1,3-heteroarylene, optionally substituted at the remaining positions.

9. The method of claim 8 , wherein compound A and compound B of step (a) have the structures of:

respectively, wherein,

X 1 is ═N—; and

X 2 is S, O, or N(R X2 )—, or

X 1 is ═C(R X1 )—; and

X 2 is NR X2 ,

wherein R X1 and R X2 are independently selected from the group consisting of —H, —CH 3 or —CH 2 CH 3 ;

R 1 is phenyl, t-butyl, 9-fluorenyl or allyl, optionally substituted, or other moeity moiety so that R 1 —OC(═O)— is a suitable nitrogen protecting group;

R 3 is optionally substituted saturated C 1 -C 6 alkyl, optionally substituted unsaturated C 3 -C 6 alkyl or optionally substituted C 3 -C 6 heteroalkyl;

R 6 is C 1 -C 6 alkyl; and

R 7 is optionally substituted saturated C 1 -C 20 alkyl, optionally substituted unsaturated C 3 -C 20 alkyl, optionally substituted C 3 -C 20 heteroalkyl, optionally substituted C 2 -C 20 alkenyl, optionally substituted C 3 -C 20 heteroalkenyl, optionally substituted C 2 -C 20 alkynyl, optionally substituted C 3 -C 20 heteroalkynyl, optionally substituted C 6 -C 24 aryl, optionally substituted C 5 -C 24 heteroaryl or optionally substituted C 3 -C 20 heterocyclyl, or other moeity moiety so that R 7 —O— provides a suitable carboxylic acid protecting group.

10. The method of claim 1 , wherein the compound of (R,R)-Formula 1a, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 1a, or salt thereof, has the structure of:

wherein

X 1 is ═N—; and

X 2 is S, O, or N(R X2 )—,

or

X 1 is ═C(R X1 )—; and

X 2 is NR X2 , wherein

R X1 and R X2 are independently selected from the group consisting of —H, —CH 3 or —CH 2 CH 3 .

11. The method of claim 1 , wherein the compound of (R,R)-Formula 1a, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 1a, or salt thereof, has the structure of:

12. The method of claim 1 , wherein the suitable transition metal (II) or transition metal (III) catalyst comprises a transition metal selected from the group consisting of Cu(II), Zn(II) and Yb(III).

13. The method of claim 1 , wherein the transition (II) metal catalyst comprises Cu(II).

14. The method of claim 1 , wherein the transition (II) metal catalyst is Cu(OTf) 2 , Cu(SbF 6 ) 2 , or CuCl 2 .

15. The method of claim 1 , wherein the suitable polar, aprotic solvent is dichloromethane.

16. The method of claim 1 , wherein the chiral reducing agent is (S)-(−)-CBS.

17. The method of claim 4 , wherein the compound of (R,R)-Formula 2, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 2, or salt thereof, has the structure of:

wherein

X 1 is ═N—; and

X 2 is S, O, or N(R X2 )—,

or

X 1 is ═C(R X1 )—; and

X 2 is NR X2 , wherein

R X1 and R X2 are independently selected from the group consisting of —H, —CH 3 or —CH 2 CH 3 .

18. The method of claim 4 , wherein the compound of (R,R)-Formula 2, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 2, or salt thereof, has the structure of:

19. The method of claim 5 , wherein the compound of (R,R)-Formula 2a, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 2a, or salt thereof, has the structure of:

wherein

X 1 is ═N—; and

X 2 is S, O, or N(R X2 )—,

or

X 1 is ═C(R X1 )—; and

X 2 is NR X2 , wherein

R X1 and R X2 are independently selected from the group consisting of —H, —CH 3 or —CH 2 CH 3 .

20. The method of claim 5 , wherein the compound of (R,R)-Formula 2a, or a salt thereof, has the structure of:

and its corresponding enantiomer of (S,S)-Formula 2a, or salt thereof, has the structure of:

21. The method of claim 9 , wherein

R 1 is t-butyl;

R 3 is —CH 3 or —CH 2 CH 2 CH 3 ;

R 6 is —CH(CH 3 ) 2 ; and

R 7 is —CH 3 or —CH 2 CH 3 .

22. The method of claim 9 , wherein compound A and compound B have the structures of: