Alkylation of N-acetyl-D-cysteine (I) with 1-fluoro-5-methyl-2-nitrobenzene (II) gave adduct (III). After hydrolysis of the acetamido group of (III) with aqueous sulfuric acid, the resulting amine (IV) was protected as the benzyl carbamate (V). Reduction of the nitro group of (V) provided amino acid (VI), which was cyclized to the benzothiazepinone (VII) using EDC. Subsequent alkylation of (VII) with ethyl bromoacetate under phase-transfer conditions yielded (VIII) (1). Cleavage of both N-Cbz group and ethyl ester by HBr in AcOH, followed by introduction of the N-Boc group afforded intermediate (X) (2). Optionally, hydrogenolysis of the N-Cbz group of (VIII) provided amino lactam (XI). This was coupled with N-Boc-O-benzylserine (XII) to give amide (XIII). Then, basic hydrolysis of the ethyl ester produced carboxylic acid (XIV).
An alternative access to intermediate (XX) consisted in coupling of arginine-resin (XVI) with building block (XIV) by means of BOP reagent, followed by trifluoroacetic acid deprotection of the Boc group.
Peptide resin (XX) was subjected to sequential coupling-deprotection cycles with the following amino acids: N-Boc-(2-thienyl)alanine (XXI), N-Boc-glycine (XXIII), N-Boc-4-hydroxyproline (XXV), N-Boc-proline (XXVII), Nalpha-Boc-Ngamma-tosyl-arginine (XXIX) and Nalpha-Boc-Ngamma-nitro-D-arginine (XXXI) to produce peptide resins (XXII), (XXIV), (XXVI), (XXVIII), (XXX) and (XXXII), respectively. The title peptide was finally obtained by deprotection and cleavage from the resin by means of treatment with HF.
Peptide resin (XX) was subjected to sequential coupling-deprotection cycles with the following amino acids: N-Boc-(2-thienyl)alanine (XXI), N-Boc-glycine (XXIII), N-Boc-4-hydroxyproline (XXV), N-Boc-proline (XXVII), Nalpha-Boc-NG-tosyl-arginine (XXIX) and Nalpha-Boc-NG-nitro-D-arginine (XXXI) to produce peptide resins (XXII), (XXIV), (XXVI), (XXVIII), (XXX) and (XXXII), respectively. The title peptide was finally obtained by deprotection and cleavage from the resin by means of treatment with HF.
Construction of the peptide chain was carried out by solid phase synthesis on a Merrifield resin. The protected arginine bound to the resin (XV) was treated with trifluoroacetic to cleave the N-Boc group. The resulting NG-arginine-resin (XVI) was coupled with benzothiazepineacetic acid (X) by means of BOP reagent to produce (XVII). After deprotection of the Boc group of (XVII) with trifluoroacetic acid, further coupling with protected serine (XII) gave peptide resin (XIX). The Boc group of (XIX) was then deprotected with trifluoroacetic acid as above to furnish intermediate (XX).