Clean, high-yield preparation of S,S and R,S amino acid isosteres
The present invention provides compounds and methods that can be used to convert the intermediate halomethyl ketones (HMKs), e.g., chloromethyl ketones, to the corresponding S,S- and R,S-diastereomers. More particularly, the present invention provides: (1) reduction methods; (2) inversion methods; and (3) methods involving the epoxidation of alkenes. Using the various methods of the present invention, the R,S-epoxide and the intermediary compounds can be prepared reliably, in high yields and in high purity.
1. A method for preparing an R,S-epoxide compound having the following general formula:
said method comprising:
(a) contacting an S,S-halomethyl sulfonyl (S,S-HMS) compound having the following general formula:
with a carbamate-forming acetate containing a sufficient leaving group to form a cyclic carbamate having the following general formula:
and
(b) contacting said cyclic carbamate with an alkali metal base to form said R,S-epoxide;
wherein:
R 1 is an amino acid side chain;
R 2 is a blocking group;
R 3 is a member selected from the group consisting of arylsulfonyls and alkylsulfonyls; and
X 1 is a leaving group.
2. The method in accordance with claim 1 , wherein R 3 is a member selected from the group consisting of methylsulfonyl and toluenesulfonyl.
3. The method in accordance with claim 1 , wherein said carbamate-forming acetate is a member selected from the group consisting of sodium trichloroacetate, potassium trichloroacetate, tetrabutylammonium trichloroacetate, sodium tribromoacetate, potassium tribromoacetate sodium trifluoroacetate and potassium trifluoroacetate.
4. The method in accordance with claim 1 , wherein said carbamate-forming acetate is sodium trichloroacetate.
5. The method in accordance with claim 1 , wherein step (a) is carried out in toluene.
6. The method in accordance with claim 1 , wherein said alkali metal base is a member selected from the group consisting of NaOH, KOH, LiOH, NaOCH 3 , NaOCH 2 CH 3 and KOtBu.
7. The method in accordance with claim 1 , wherein said sufficient leaving group is selected from the group consisting of trichlormethyl, tribromomethyl and trifluoromethyl.
8. The method in accordance with claim 1 , wherein R 1 is a member selected from the group consisting of a side chain from any of the naturally occurring amino acids and amino acids mimetics; R 2 is a blocking group selected from the group consisting of BOC, MOC and CBZ; and X 1 is a leaving group selected from the group consisting of Cl, Br and F.
9. The method in accordance with claim 8 , wherein R 1 is a side chain from any of the naturally occurring amino acids.
10. The method in accordance with claim 1 , wherein R 1 is a member selected from the group consisting of a benzyl, a substituted benzyl, an S-phenyl, an alkyl and a para-nitrobenzene.
11. The method in accordance with claim 10 , wherein R 1 is benzyl.
12. The method in accordance with claim 8 , wherein R 2 is BOC.
13. The method in accordance with claim 1 , wherein X 1 is a chloro or bromo group.
14. The method in accordance with claim 1 , wherein R 1 is benzyl; R 2 is BOC; R 3 is methylsulfonyl; and X 1 is a bromo or chloro group.