合成路线1

合成路线2

Pinanediol leucine boronate (I) was coupled with N-Boc-L-phenylalanine (II) in the presence of TBTU to afford the dipeptide boronate (III). Acid cleavage of the Boc protecting group of (III), followed by acylation of the resultant amine (IV) with 2-pyrazinecarboxylic acid (V), furnished amide (VI). Finally, deprotection of the boronic acid (VI) was accomplished by two-phase transesterification with isobutylboronic acid.

合成路线3

Piperazine-2-carboxylic acid (VIII) was prepared by catalytic hydrogenation of pyrazine (VII). Sequential protection of the N-4 of (VIII) with 2-(t-butoxycarbonyloxyimino)-2-phenylacetonitrile and the N-1 with benzyl chloroformate provided the diprotected piperazinecarboxylic acid (X), which was subsequently esterified with diazomethane to give the methyl ester (XI). The N-benzyloxycarbonyl group of (XI) was selectively deprotected by hydrogenation in the presence of Pd/C to yield the N-Boc-piperazine (XII). Reductive alkylation of (XII) with isovaleraldehyde (XIII) afforded the corresponding N-isoamyl piperazine (XIV). After acid removal of the N-Boc group of (XIV) to yield (XV), a second reductive alkylation with acetone (XVI) produced the dialkyl piperazine (XVII). Hydrolysis of the methyl ester group of (XVII) under acidic conditions gave acid (XVIII). This was finally coupled with the intermediate piperidine (VI) in the presence of HBTU to provide the title compound.

合成路线4

Nalpha-(Benzyloxycarbonyl)-1,4-diaminobutyric acid (I) was protected as the Nomega-Boc derivative (II) and then coupled to glycine ethyl ester (III) by means of EDC and HOBt to produce dipeptide (IV). Hydrogenolysis of the benzyloxycarbonyl group of (IV) to give (V), followed by condensation with alpha-toluenesulfonyl chloride (VI) gave sulfonamide (VII). Side-chain deprotection of (VII) with HCl liberated amine (VIII), which was acylated with 2-pyrazinecarboxylic acid (IX) to afford amide (X). Saponification of the ethyl ester of (X) with LiOH gave carboxylic acid (XI).

合成路线5

Clauson-Kaas reaction between 2-fluoroaniline (I) and 2,5-dimethoxytetrahydrofuran (II) in refluxing HOAc affords pyrrolic compound (III), which is then converted into cyanopyrrole derivative (IV) through a one-pot sequence involving formylation with oxalyl chloride, oximation with NH2OH.HCl and dehydration with Ac2O. Cyclization of nitrile (IV) by treatment with KOH in ethylene glycol or in tert-butyl alcohol yields quinoxalinone derivative (V), which is then transformed into the hydrazine derivative (VI) by refluxing with POCl3 and catalytic N,N-dimethylaniline followed by treatment with hydrazine in MeOH. Finally, the desired product is obtained by reaction of (VI) with pyrazinecarboxylic acid (VII) in the presence of PPh3 and 2,2'-dipyridyl disulfide (Aldrithiol-2) in CH2Cl2.

合成路线6

Hydrogenolysis of the benzyloxycarbonyl protecting group in N-Cbz-L-cyclohexylglycyl-L-tert-leucine methyl ester (I) in the presence of Pearlman’s catalyst in MeOH gives the free dipeptide ester (II), which is acylated with pyrazinecarboxylic acid (III) by means of DCC in dichloromethane/THF to yield the N-acyl dipeptide (IV) (1). Alkaline hydrolysis of the methyl ester (IV) followed by coupling of the resulting carboxylic acid (V) with ethyl cis-perhydrocyclopenta[c]pyrrole-1-carboxylate (VI) by means of DCC provides the acyl tripeptide ester (VII), which is then hydrolyzed to the corresponding carboxylic acid (VIII) upon treatment with NaOH in EtOH (1, 2). Coupling of L-norvaline (IX) with cyclopropylamine (X) in the presence of EDC and N-hydroxysuccinimide followed by catalytic hydrogenolysis of the N-Cbz group provides N-cyclopropyl-norvalinamide (XI) (3). Subsequent condensation of aminoamide (XI) with the N-acyl tripeptide (VIII) using either PyBOP or EDC/HOBt gives the tetrapeptide derivative (XII). Finally, oxidation of the secondary alcohol (XII) by means of Dess-Martin periodinane or NaOCl and a catalytic amount of TEMPO furnishes the title α-ketoamide (1-3). Scheme 1.

合成路线7

An alternative route to the precursor N-acyl tripeptide (VIII) consists of coupling of N-Cbz-L-tert-leucine (XIII) with the bicyclic amino ester (XIV) to afford the protected dipeptide (XV), from which the N-Cbz group is removed by catalytic hydrogenolysis over Pearlman’s catalyst, yielding (XVI). The dipeptide ester (XVI) is then coupled with N-Cbz-L-cyclohexylglycine (XVII) to give (XVIII), which is further deprotected by catalytic hydrogenolysis to afford the tripeptide ester (XIX). After acylation of (XIX) with pyrazinecarboxylic acid (III) employing CDI, the resulting N-acyl tripeptide tert-butyl ester (XX) is treated with HCl in formic acid to provide the target intermediate (VIII) (3). Scheme 2.