(i) The formation of a 16-membered depsipeptide: For the first step, L-valine methyl ester (I) was coupled to N-Fmoc-L-threonine using the BOP reagent [(benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate], removing the N-Fmoc group with Et2NH and coupling to N-Fmoc-(S-triphenylmethyl)-D-cysteine to yield the tripeptide (II). The tetrapeptide (III) was prepared by deprotection of the tripeptide (II) and BOP-mediated peptide coupling of the resulting amine with N-Fmoc-D-valine. The secondary hydroxyl group was activated as the tosylate and eliminated by treatment with DABCO to produce the internal alkene. After removal of the Fmoc protecting group by addition of Et2NH to the reaction mixture, the 16-membered depsipeptide (IV) was formed.

(ii) The asymmetric construction of the hydroxymercaptoheptenoic acid: For construction of the hydroxymercaptoheptenoic acid (VIII), cesium triphenylmethylthiolate anion was added to methyl 2,4-pentadienoate (V), which resulted in a beta,gamma-unsaturated methyl ester that was converted to an alpha,beta-unsaturated ester after exposure to Cs2CO3. DIBAL reduction to the primary alcohol followed by Swern oxidation yielded the alpha,beta-unsaturated aldehyde (VI). In the presence of the ligand derived from (R)-(-)-binaphthyl amino alcohol, Ti(IV)-catalyzed addition of O-benzyl, O-TMS ketene acetal to the alpha,beta-unsaturated aldehyde (VI) resulted in the aldol product (VII). Hydrolysis of the benzyl ester with LiOH in aqueous methanol formed the beta-hydroxymercaptoheptenoic acid (VIII).

(iii) An intramolecular oxidative coupling of the thiols to form a 15-membered disulfide-containing ring: To complete the synthesis of FR901228, the beta-hydroxymercaptoheptenoic acid (VIII) was coupled to the 16-membered cyclic depsipeptide (IV) using the BOP reagent, followed by LiOH-mediated hydrolysis, resulting in the free acid product (IX). Cyclization of the hydroxy acid (IX) with TsOH, DEAD and PPh3 and oxidation of the bis(S-triphenylmethyl)lactone (X) with iodine in methanol solution yielded the depsipeptide FR901228.

(i) The formation of a 16-membered depsipeptide: For the first step, L-valine methyl ester (I) was coupled to N-Fmoc-L-threonine using the BOP reagent [(benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate], removing the N-Fmoc group with Et2NH and coupling to N-Fmoc-(S-triphenylmethyl)-D-cysteine to yield the tripeptide (II). The tetrapeptide (III) was prepared by deprotection of the tripeptide (II) and BOP-mediated peptide coupling of the resulting amine with N-Fmoc-D-valine. The secondary hydroxyl group was activated as the tosylate and eliminated by treatment with DABCO to produce the internal alkene. After removal of the Fmoc protecting group by addition of Et2NH to the reaction mixture, the 16-membered depsipeptide (IV) was formed.

(ii) The asymmetric construction of the hydroxymercaptoheptenoic acid: For construction of the hydroxymercaptoheptenoic acid (VIII), cesium triphenylmethylthiolate anion was added to methyl 2,4-pentadienoate (V), which resulted in a beta,gamma-unsaturated methyl ester that was converted to an alpha,beta-unsaturated ester after exposure to Cs2CO3. DIBAL reduction to the primary alcohol followed by Swern oxidation yielded the alpha,beta-unsaturated aldehyde (VI). In the presence of the ligand derived from (R)-(-)-binaphthyl amino alcohol, Ti(IV)-catalyzed addition of O-benzyl, O-TMS ketene acetal to the alpha,beta-unsaturated aldehyde (VI) resulted in the aldol product (VII). Hydrolysis of the benzyl ester with LiOH in aqueous methanol formed the beta-hydroxymercaptoheptenoic acid (VIII).

(iii) An intramolecular oxidative coupling of the thiols to form a 15-membered disulfide-containing ring: To complete the synthesis of FR901228, the beta-hydroxymercaptoheptenoic acid (VIII) was coupled to the 16-membered cyclic depsipeptide (IV) using the BOP reagent, followed by LiOH-mediated hydrolysis, resulting in the free acid product (IX). Cyclization of the hydroxy acid (IX) with TsOH, DEAD and PPh3 and oxidation of the bis(S-triphenylmethyl)lactone (X) with iodine in methanol solution yielded the depsipeptide FR901228.