Methods for producing beraprost and its derivatives
The present invention is directed to methods for preparing Beraprost and novel synthetic intermediates for Beraprost. In one aspect, a process is provided to produce a pharmaceutical compound represented by the general Formula (I) via a radical cyclization route. The process is completed in fewer steps than the known synthetic methods and may be conducted to prepare commercially useful quantities. In another aspect, synthetic methods are provided for producing Beraprost and its derivatives, which are stereoselective, efficient, scalable and economical. In another aspect, substantially isomerically pure compounds and intermediates are produced by the above processes.
1. A method of preparing a beraprost ester diol compound represented by the structural Formula (IX):
comprising:
(i) reacting a substituted halophenol with 6-oxabicyclo[3.1.0]hex-2-ene in presence of a palladium catalyst to form an ether compound represented by structural Formula (III):
(ii) allylating the compound of Formula (III) with an allylating reagent in presence of a radical initiator to form the allylation product (VA)
(iii) subjecting the terminal alkene compound of Formula (VA) to terminal olefin isomerization to form a compound of Formula (VIA); and
(iv) ozonolyzing and reducing the compound of Formula (VIA) to provide a diol of Formula (IX)
wherein:
X is F, Cl, Br or I;
R 1 is an alkyl, cycloalkyl or TBDMS; and
R 2 is independently H or an alcohol protecting group.
2. The method of claim 1 , further comprising converting the beraprost ester diol compound of Formula (IX) to a compound represented by the structural Formula (I):
or a salt thereof; comprising:
(v) converting the diol of Formula (IX) to a protected aldehyde of Formula (VIIIA); and
(vi) reacting the protected aldehyde of Formula (VIIIA) with a phosphonate of Formula (X) to obtain a compound of Formula (I)
wherein:
R 1 is an alkyl, cycloalkyl or TBDMS;
R 3 is a straight or branched alkyl group having 1-4 carbon atoms; and
R 6 is an alcohol protecting group.
3. The method of claim 1 , wherein R 1 is methyl or ethyl.
4. The method of claim 1 , wherein the palladium catalyst is PdCl 2 (PPh 3 ) 2 .
5. The method of claim 1 , wherein the terminal olefin isomerization reaction is carried out in the presence of a transition metal catalyst.
6. The method of claim 1 , wherein the palladium catalyst is PdCl 2 (PPh 3 ) 2 , Pd(Ph 3 ) 4 , and PdCl 2 (dppf) 2 , Pd(OAc) 2 , Pd 2 (dba) 3 .
7. The method of claim 1 wherein the allylating reagent is allyltributyltin (allyl-BusSn), allylmagnesium bromide (allyl-MgBr); allyl halide; allyltrihalosilane; allyl carboxylate; allyl carbonate; diallyl carboxylate; diallyl sulfate; metalallyl reagent or a combination thereof.
8. The method of claim 1 , wherein the allylating reagent is allyltributyltin.
9. The method of claim 1 , wherein the allylating is Keck radical allylation.
10. The method of claim 1 , wherein the radical initiator is azobis-iso-butyronitrile, di-tert-butyl, dilauroyl, or dibenzoyl peroxide.
11. The method of claim 1 , wherein the allylating reagent is allyltributyltin and the radical initiator is azobis iso butyronitrile.
12. The method of claim 1 , wherein X is Br or I, R 1 is C 1 -C 4 alkyl and R 2 is H.
13. The method of claim 1 , wherein X is Br, R 1 is methyl and R 2 is H.