Total Synthesis of the Ramoplanin A2 and Ramoplanose Aglycon

Abstract

Full details of a convergent total synthesis of the ramoplanin A2 and ramoplanose aglycon are disclosed. Three key subunits composed of residues 3-9 (heptapeptide 15), pentadepsipeptide 26 (residues 1, 2 and 15-17), and pentapeptide 34 (residues 10-14) were prepared, sequentially coupled, and cyclized to provide the 49-membered depsipeptide core of the aglycon. Key to the preparation of the pentadepsipeptide 26 incorporating the backbone ester was the asymmetric synthesis of an orthogonally protected l-threo-beta-hydroxyasparagine and the development of effective and near-racemization free conditions for esterification of its hindered alcohol (EDCI, DMAP, 0 degrees C). The coupling sites were chosen to maximize the convergency of the synthesis including that of the three subunits, to prevent late stage racemization of carboxylate-activated phenylglycine-derived residues, and to enlist beta-sheet preorganization of an acyclic macrocyclization substrate for 49-membered ring closure. By altering the order of final couplings, two macrocyclization sites, Phe(9)-d-Orn(10) and Gly(14)-Leu(15), were examined. Macrocyclization at the highly successful Phe(9)-d-Orn(10) site (89%) may benefit from both beta-sheet preorganization as well as closure at a d-amine terminus within the confines of a beta-turn at the end of the H-bonded antiparallel beta-strands. A more modest, but acceptable macrocyclization reaction at the Gly(14)-Leu(15) site (40-50%) found at the other end of the H-bonded antiparallel beta-strands within a small flexible loop may also benefit from preorganization of the cyclization substrate, is conducted on a substrate incapable of competitive racemization, and accommodates the convergent preparation of analogues bearing depsipeptide modifications. Deliberate late-stage incorporation of the subunit bearing the labile depsipeptide ester and a final stage Asn(1) side-chain introduction provides future access to analogues of the aglycons which themselves are equally potent or more potent than the natural products in antimicrobial assays.

Chain Initiation in the Leinamycin-producing Hybrid Nonribosomal Peptide/Polyketide Synthetase from Streptomyces atroolivaceus S-140

“…A number of nonribosomal peptide natural products, such as HC toxin (9), cyclosporin (10), daptomycin (11), microcystin (12), dendroamide A (13), and ramoplanin (14), contain D-Ala. Sequence data suggest that the internal C-A-PCP-E module, which activates L-Ala and then epimerizes L-Ala into D-Ala, has been found in the NRPS genes for daptomycin (11) and microcystin (12) biosynthesis. The NRPS module for D-Ala incorporation in cyclosporine or HC toxin biosynthesis, however, does not contain an integrated E domain and cannot activate L-Ala; instead, D-Ala is among the pool of available amino acids and is directly activated.…”

Total synthesis and structure of the ramoplanin A1 and A3 aglycons: Two minor components of the ramoplanin complex

“…Soon thereafter, the stereochemistry of the 7-methyloctadi-2,4-enoic acid side chain of ramoplanin A2 was also revised to cis-trans by Kurz and Guba (19) in studies that served to establish the Hpg 6 and Hpg 7 absolute stereochemistry and provided the three dimensional, solution-phase conformation of the natural product. Recently, we described the total synthesis of the ramoplanin A2 aglycon (5) and confirmed the revised structure of ramoplanin A2 (20,21). Key to the strategic planning of the approach was the introduction of the lipid side chain onto the fully functionalized cyclic depsipeptide core, thereby potentially providing direct access to all natural aglycons from a common, late-stage intermediate.…”

“…Similarly, Fmoc deprotection, acylation of the free amine with anhydride 7b (3.0 eq, DMF, 25°C, 14 h) to provide 8b, and HF global deprotection provided the ramoplanin A3 aglycon (9b, 90%). Notably, Fmoc deprotection enlists conditions introduced and developed for ramoplanin that avoid competitive depsipeptide cleavage by ␤-elimination that was observed with typical base-catalyzed Fmoc deprotections (21). Similarly, the Orn-4 and Orn-10 2-(trimethylsilyl)ethylsulfonyl (SES) deprotections upon treatment with HF used conditions first introduced and developed to avoid competitive depsipeptide cleavage under strongly basic conditions (24).…”

6.11 N-Acylation Reactions of Amines

“…253 Figure 41 DEPTB-mediated amide bond-coupling reaction for the synthesis of ramoplanin A2. 182 used for amide synthesis. [254][255][256] Proteases have the advantage that they can operate as catalysts at ambient temperature in aqueous media on unprotected amino acids, whereas the use of immobilized enzymes facilitates biocatalyst recycling.…”