Destruxin analogues: Depsi peptidic bond replacement by amide bond

Combinatorial Solid-Phase Synthesis and Biological Evaluation of Cyclodepsipeptide Destruxin B as a Negative Regulator for Osteoclast Morphology

Combinatorial synthesis and biological evaluation of cyclodepsipeptide destruxin B have been achieved. The cyclization precursors were prepared by solid-phase peptide synthesis via a split and pool method utilizing SynPhase lanterns with colored tags and cogs, followed by cleavage from the polymer-support. Macrolactonization utilizing MNBA-DMAPO in solution-phase was successfully performed in parallel to afford the desired 64-member destruxin analogues in moderate to good yields. Biological evaluation of the synthesized analogues indicated that a MeAla residue for the building block A is required to induce the desired morphological changes in osteoclast-like multinuclear cells (OCLs), and introduction of the substituent at the R(4) position of a proline moiety is tolerated by the morphology and may enable the preparation of a molecular probe for the target identification in the osteoclasts.

Scalable solution-phase synthesis of the biologically active cyclodepsipeptide destruxin E, a potent negative regulator of osteoclast morphology

The scalable solution-phase synthesis of the cyclodepsipeptide destruxin E (1) has been achieved. Diastereoselective dihydroxylation of the terminal alkene in a 2-alkoxy-4-pentenoic amide, 7, was successfully accomplished utilizing (DHQD)2PHAL as the chiral ligand, and it was found that the use of the l-proline moiety in the substrate as a chiral auxiliary was essential for the induction of high diastereoselectivity to afford the key compound 4 on a gram scale. MNBA-mediated macrolactonization of 3 was also performed without formation of the dimerized product even under higher-dilution conditions, and it is noteworthy that the internal hydrogen bonds and s-cis configuration of the amide bond between N-methylalanine and N-methylvaline in the cyclization precursor 3 would assist in the macrolactonization to provide the macrolactone 2 without forming a dimerized product. Finally, epoxide formation in the side chain afforded destruxin E (1) on a gram scale in high purity (>98%).

Synthesis, structure determination, and biological evaluation of destruxin E

The total synthesis of destruxin E (1) has been achieved for the first time, and the stereochemistry of its chiral center at the epoxide has been determined to be (S). The cyclization precursor 3a was synthesized by solid-phase peptide synthesis. Macrolactonization of 3a utilizing MNBA-DMAPO, followed by formation of the epoxide, then furnished destruxin E. Its diastereomer, epi-destruxin E (2), was also synthesized in the same manner. Furthermore, the biological evaluation indicated that destruxin E exhibits V-ATPase inhibitory activity 10-fold greater than that of epi-destruxin E.

The cyclization of peptides and depsipeptides

Constricting the peptide backbone into a more defined conformational form through cyclization is an activity evolved in nature and in synthetic work, the latter straddling only the most recent decades. The resulting conformational constraints increase the probability of an optimum response with bio-receptors. The purpose of this review is to highlight developments that have proved to be reasonably efficient in the macrocyclization of linear precursors into cyclic peptides and depsipeptides.

A convenient protocol for selective cleavage of 2-hydroxy acid amides. Application to semisynthesis of the cyclic heptapeptide aza HUN-7293

A two-step protocol for the first chemoselective cleavage of 2-hydroxy acid amides has been developed. Mesylation of the model substrate 2-(hydroxypropionylamino)-4-methylpentanoic acid methyl ester (11) followed by treatment with N-ethylthiourea (13) allows cleavage of 2-hydroxy acid amides under smooth conditions. Successful application of this methodology to the open-chain transesterification product 15 (methylester) of the cyclic heptadepsipeptide HUN-7293, a potent inhibitor of inducible cell adhesion molecule expression, delivered the corresponding hexapeptide 18 with unprotected N-terminus in 70-75% yield. This result demonstrates that the protocol developed even works in the presence of an ester and several methylated and unmethylated amide bonds. Finally, a sequence of ligation of methyl D-dehydroglutaminate (20) to the C-terminus of the saponification product 21, followed by the degradation protocol and ring closure, allowed chemical "point mutation" at the DGCN site affording the aza analogue of HUN-7293 (24) in 15% overall yield. To the best of our knowledge this is the first report on chemoselective cleavage of 2-hydroxy acid amides.

The destruxins: synthesis, biosynthesis, biotransformation, and biological activity

Destruxins, secondary metabolites first reported in 1961, are cyclic hexadepsipeptides composed of an alpha-hydroxy acid and five amino acid residues. The name "destruxin" is derived from "destructor" from the species Oospora destructor, the entomopathogenic fungus from which these metabolites were first isolated. Individual destruxins differ on the hydroxy acid, N-methylation, and R group of the amino acid residues; where established, the configurations of the amino acid residues are S, and those of the hydroxy acids are R. Destruxins exhibit a wide variety of biological activities, but are best known for their insecticidal and phytotoxic activities. The great interest in destruxins derives from their potential role as virulence factors in fungi, whether such microorganisms are useful insect biocontrol agents or detrimental, causing great plant disease epidemics. Reports on isolation, chemical structure determination, total synthesis, transformation by diverse organisms, and biological activity of destruxins and related metabolites are reviewed for the first time.

Probing host-selective phytotoxicity: synthesis of destruxin B and several natural analogues

The syntheses of the host-selective phytotoxin destruxin B [cyclo(betaAla-Hmp-Pro-Ile-MeVal-MeAla), Hmp = (2R)-2-hydroxy-4-methylpentanoic acid], and the closely related natural analogues homodestruxin B (MeVal-->MeIle), desmethyldestruxin B (MeVal-->Val), hydroxydestruxin B (Hmp-->Dhmp, Dhmp = (2R)-2,4-dihydroxy-4-methylpentanoic acid), and hydroxyhomodestruxin B (MeVal-->MeIle, Hmp-->Dhmp) are described. In each case, the MeAla-betaAla linkage was formed by cyclization and the precursor linear hexadepsipeptides were formed by condensing two three-residue fragments. Radiolabeled samples of destruxin B, homodestruxin B, and hydroxydestruxin B were prepared by coupling [3-(14)C]-beta-alanine to the appropriate pentadepsipeptide followed by cyclization. A noteworthy feature of the synthesis involves the novel use of a Boc-hydrazide protecting group on dipeptides with a C-terminal N-methylalanine residue to inhibit the otherwise facile dioxopiperazine formation during peptide coupling.

Synthesis and biological evaluation of destruxin A and related analogs

This report describes the development of an efficient solid-phase synthesis protocol and adaptation of reported solution phase procedures for the synthesis of the cyclic depsihexapeptide destruxin A and related analogs. The solid-phase method described is based on standard Fmoc peptide chemistry, including a new synthetic method for the assembly of the depsi bond-containing unit. In order to select analogs of destruxin A for synthesis and evaluation of insecticidal activity, the work of Hellberg et al., describing a set of Z-descriptors for amino acid side-chains comparing their physicochemical properties, was utilized. Destruxin A and 27 different analogs with structural variations in four residues were synthesized and insecticidal activity was evaluated via injections into tobacco budworm (Heliothis virescens) larvae. Several destruxin A analogs were found to be at least as potent as the native compound.

Destruxin analogs: variations of the alpha-hydroxy acid side chain

This work describes the synthesis of three destruxin E cyclodepsipeptidic analogs. These compounds have an identical amino acid sequence but differ by the nature of the hydroxy acid residue with is 2-hydroxy-3-phenylpropionic (Hpp), 2-hydroxy-5-trimethylsilyl-4-pentynoic (Hpy-TMS) and 2-hydroxy-4-pentynoic (Hpy) acid, respectively. The insecticidal properties on the Galleria mellonella larvae (paralysis and lethal effect) of these analogs are presented in comparison with the natural destruxin E. All these compounds have toxic effects, the most potent being Hpy that induces the same effect as destruxin E.