合成路线1

Alkylation of 3-hydroxy-4-methoxybenzyl alcohol (I) with 1-bromo-3-methoxypropane (II) gives ether (III). Subsequent conversion of benzyl alcohol (III) into bromide (IV) is carried out using bromotrimetylsilane. The chiral isovaleryloxazolidinone (V) is alkylated with bromide (IV) by means of LiHMDS to afford (VI), which is hydrolyzed to the (S)-2-aryl-2-isopropylpropionic acid (VII) by means of lithium peroxide. The reduction of acid (VII) to the corresponding alcohol with NaBH4/I2 reagent, followed by treatment with PPh3 and NBS, provides bromide (VIII). Alkylation of the chiral dimethoxydihydropyrazin (IX) with bromide (VIII) produces (X). Further hydrolysis of the pyrazine ring of (X) with HCl, followed by Boc protection of the resulting (S,S)-amino ester, yields compound (XI). Reduction of the ester group of (XI) with DIBAL gives aldehyde (XII). This compound is condensed with the Grignard reagent (XIII) to afford the diastereomeric mixture of amino alcohols (XIV). Treatment of mixture (XIV) with 2,2-dimethoxypropane (XV) and TsOH produces a mixture of oxazolidines, from which the required (S,S,S)-isomer (XVI) is isolated by flash chromatography. Hydrogenolitic deprotection of the benzyl ether of (XVI) gives alcohol (XVII).

合成路线2

The reduction of the chiral propionic acid (VII) with NaBH4 in THF gives the primary alcohol (XXIX), which by reaction with SOCl2 and pyridine in CHCl3 yields the chloride (XXX). Condensation of (XXX) with the butyrolactone intermediate (XXVII) by means of Mg and dibromoethane affords the ketonic adduct (XXXI). Reduction of the exocyclic carbonyl group of (XXXI) with NaBH4 in THF/ methanol provides a 3:1 diastereomeric mixture of the desired chiral alcohol (XXXII) and its OH-epimer (XXXIII) that are separated by flash chromatography. Reaction of (XXXII) with MsCl and TEA, followed by treatment with NaN3, affords the azido derivative (XXXIV), which is condensed with 4-amino-3,3-dimethylbutyramide (XIX) by means of 2-hydroxypyridine and TEA to provide the adduct (XXXV). Finally, the azido group of (XXXV) is reduced with H2 over Pd/ in MeOH to yield the target amino alcohol. An extensive study on the stereoselective reduction of the ketonic adduct (XXXI) has been performed. No better diastereoselectivity than 3:1 has been obtained for the synthesis of the chiral alcohol (XXXII). However, excellent diastereoselectivity (97:3) has been obtained for the synthesis of its OH-epimer, alcohol (XXXIII), with K-selectride in THF, which could potentially provide access to the target amino alcohol via a double inversion protocol.

合成路线3

The condensation of benzaldehyde (I) with ethyl isovalerate (II) by means of hexyl lithium and DIA in THF gives the hydroxyester (III), which is acylated with Ac2O and DMAP in THF to yield the acetoxy derivate (IV). The elimination reaction in (IV) by means of t-BuOK in THF affords the unsaturated ester (V), which is hydrolyzed with KOH in ethanol to provide the unsaturated free acid (VI). Finally, this compound is enantioselectively reduced with H2 over several chiral Rh catalysts {[Rh(NBD)2BF4, [Rh(NBD)(OCOCF3)2], [Rh(NBD)Cl2], etc} to give the target intermediate 2(R)-isopropyl-3-[4-methoxy-3-(3-methoxypropoxy)phenyl]propionic acid (VII). (see scheme 26758001a, intermediate (VII)).

合成路线4

合成路线5