Flexible Synthesis of Metacycloprodigiosin and Functional Derivatives Thereof

Structure, Chemical Synthesis, and Biosynthesis of Prodiginine Natural Products

The prodiginine family of bacterial alkaloids is a diverse set of heterocyclic natural products that have likely been known to man since antiquity. In more recent times, these alkaloids have been discovered to span a wide range of chemical structures that possess a number of interesting biological activities. This review provides a comprehensive overview of research undertaken toward the isolation and structural elucidation of the prodiginine family of natural products. Additionally, research toward chemical synthesis of the prodiginine alkaloids over the last several decades is extensively reviewed. Finally, the current, evidence-based understanding of the various biosynthetic pathways employed by bacteria to produce prodiginine alkaloids is summarized.

Enantioselective synthesis of metacycloprodigiosin via the "Wasserman pyrrole"

A concise, nine-step enantioselective total synthesis of metacycloprodigiosin is reported. The synthesis provides increased step-efficiency over the previous racemic and enantioselective syntheses of this compound. Key features of the work include investigations into a convergent oxidative coupling reaction and subsequent ring-closing metathesis to deliver an advanced pyrrole intermediate we name the "Wasserman pyrrole" that can be converted to metacycloprodigiosin in one step.

Prodigiosin alkaloids: recent advancements in total synthesis and their biological potential

The present review article is focused on the medicinal potential and total synthesis of prodigiosins witnessed in the last decade. The aim will be to provide an inspiration to the marvels and pit falls of constructing the polypyrrole heterocycles with in the complex systems.

Elimination of butylcycloheptylprodigiosin as a known natural product inspired by an evolutionary hypothesis for cyclic prodigiosin biosynthesis

The cyclic prodigiosins are an important family of bioactive natural products that continue to be the subject of numerous structural, synthetic, and biosynthetic studies. In particular, the structural assignments of the isomeric cyclic prodigiosins butylcycloheptylprodigiosin (BCHP) and streptorubin B have been the cause of significant confusion. Herein, we report detailed studies regarding the electron impact (EI) mass spectra of synthetic BCHP and streptorubin B that have allowed us to distinguish the two compounds in the absence of quality historical isolation NMR data. On the basis of these fragmentation differences, the status of BCHP as a natural product is challenged. The proposed mechanism of fragmentation is supported by the EI mass spectra of synthetic pentyl-chain analogues of BCHP and streptorubin B, X-ray crystallography, and DFT calculations. Elimination of BCHP from the prodigiosin family supports a proposed evolutionary hypothesis for the surprising biosynthesis of cyclic prodigiosins.

Template-constrained macrocyclic peptides prepared from native, unprotected precursors

Peptide-protein interactions are important mediators of cellular-signaling events. Consensus binding motifs (also known as short linear motifs) within these contacts underpin molecular recognition, yet have poor pharmacological properties as discrete species. Here, we present methods to transform intact peptides into stable, templated macrocycles. Two simple steps install the template. The key reaction is a palladium-catalyzed macrocyclization. The catalysis has broad scope and efficiently forms large rings by engaging native peptide functionality including phenols, imidazoles, amines, and carboxylic acids without the necessity of protecting groups. The tunable reactivity of the template gives the process special utility. Defined changes in reaction conditions markedly alter chemoselectivity. In all cases examined, cyclization occurs rapidly and in high yield at room temperature, regardless of peptide composition or chain length. We show that conformational restraints imparted by the template stabilize secondary structure and enhance proteolytic stability in vitro. Palladium-catalyzed internal cinnamylation is a strong complement to existing methods for peptide modification.

Advances in the Krapcho Decarboxylation

Po. S. Poon, who received her Doctorate degree in 2007 from Venezuelan Institute of Scientific Research (IVIC), is working at IVIC as an Investigator in the Department of Chemistry. Her research encompases the synthesis of terpenoid and bioactive compounds. She was received the Paula Prize from IVIC for her high academic achievement.

Manuel S. Laya obtained his MSc degree in 1997 from IVIC and is now working as Research Associate in the Department of Organic Chemistry, IVIC. He has published more than 30 papers, most of which are related to synthetic studies on terpenoid compounds.

Ajoy K. Banerjee, who received his PhD in 1965 from the University of Kolkata (India), joined IVIC in 1968. Since 1982 he has been working as Senior Research Scientist. Most of his work is related to synthetic studies on terpenoid compounds (diterpenes and sesquitepenes). He teaches organic chemistry and supervises students who join IVIC for MSc or PhD degrees. Dr Banerjee is the author of two books on organic chemistry, written in Spanish, and several papers.

1. Modifications

2. General features

3. Mechanism

4. Synthetic applications

5. Conclusion

6. References

This review provides a brief description of the Krapcho dealkoxycarbonylation and its recent applications in the synthesis of organic compounds and natural products. The general features of the Krapcho reaction, its mechanism and several modifications of the Krapcho reaction are discussed.

Total Synthesis, Molecular Editing and Evaluation of a Tripyrrolic Natural Product: The Case of “Butylcycloheptylprodigiosin”

Conflicting reports are found in the literature on whether the ortho-pyrrolophane derivative 6, which has been named "butylcycloheptylprodigiosin" even though it is a cyclononane derivative, is a natural product or merely a mis-assigned structure. This dispute has now been resolved by an unambiguous total synthesis of this complex alkaloid which confirms the initial structure assignment. The chosen approach is largely catalysis-based, featuring the first application of a "Narasaka-Heck" reaction in natural product chemistry. This palladium-catalyzed transformation allows the unsaturated oxime ester 26 to be converted into the bicyclic dihydropyrrole 27. Other notable reactions of the reported approach to 6 are a regioselective Tsuji-Trost reaction of the doubly allylic acetate 21 with methyl acetoacetate, a base-induced aromatization of 27 to the corresponding pyrrole 28, a chemoselective oxidation of the benzylic methyl group in 33 with cerium ammonium nitrate in a biphasic reaction medium that does not affect the labile pyrrole nucleus, and a Suzuki cross-coupling for the completion of the heterocyclic domain. Diversification in the latter step leads to a set of analogues that differ from the natural product in the terminal (hetero)arene ring. This structural modification results in complete loss of the very pronounced ability of the parent compound 6 to induce oxidative cleavage in double stranded DNA in the presence of Cu(II). Several cyclononane-, cyclononene- and cyclononadiene derivatives prepared en route to 6 have been characterized by crystal structure analysis, allowing the conformational behavior of nine-membered carbocycles to be studied.

Asymmetric total synthesis of ent-(-)-roseophilin: assignment of absolute configuration

An asymmetric total synthesis of ent-(-)-roseophilin (1), the unnatural enantiomer of a novel naturally occurring antitumor antibiotic, is described. The approach enlists a room temperature heterocyclic azadiene inverse electron demand Diels-Alder reaction of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (7) with the optically active enol ether 6 bearing the C23 chiral center followed by a reductive ring contraction reaction for formation of an appropriately functionalized pyrrole ring in a key 1,2,4,5-tetrazine --> 1,2-diazine --> pyrrole reaction sequence. A Grubbs' ring closing metathesis reaction was utilized to close the unusual 13-membered macrocycle prior to a subsequent 5-exo-trig acyl radical-alkene cyclization that was used to introduce the fused cyclopentanone and complete the preparation of the tricylic ansa-bridged azafulvene core 32. Condensation of 32 with 33 under the modified conditions of Tius and Harrington followed by final deprotection provided (22S,23S)-1. Comparison of synthetic (22S,23S)-1 ([alpha](25)(D), CD) with natural 1 established that they were enantiomers and enabled the assignment of the absolute stereochemistry of the natural product as 22R,23R. Surprisingly, ent-(-)-1 was found to be 2-10-fold more potent than natural (+)-1 in cytotoxic assays, providing an unusually rewarding culmination to synthetic efforts that provided the unnatural enantiomer.

Analysis of the prodiginine biosynthesis gene cluster of Streptomyces coelicolor A3(2): new mechanisms for chain initiation and termination in modular multienzymes

BACKGROUND

Prodiginines are a large family of pigmented oligopyrrole antibiotics with medicinal potential as immunosuppressants and antitumour agents that are produced by several actinomycetes and other eubacteria. Recently, a gene cluster in Streptomyces coelicolor encoding the biosynthesis of undecylprodiginine and butyl-meta-cycloheptylprodiginine has been sequenced.

RESULTS

Using sequence comparisons, functions have been assigned to the majority of the genes in the cluster, several of which encode homologues of enzymes involved in polyketide, non-ribosomal peptide, and fatty acid biosynthesis. Based on these assignments, a complete pathway for undecylprodiginine and butyl-meta-cycloheptylprodiginine biosynthesis in S. coelicolor has been deduced. Gene knockout experiments have confirmed the deduced roles of some of the genes in the cluster.

CONCLUSIONS

The analysis presented provides a framework for a general understanding of the genetics and biochemistry of prodiginine biosynthesis, which should stimulate rational approaches to the engineered biosynthesis of novel prodiginines with improved immunosuppressant or antitumour activities. In addition, new mechanisms for chain initiation and termination catalysed by hitherto unobserved domains in modular multienzyme systems have been deduced.

Synthesis and biological evaluation of nonylprodigiosin and macrocyclic prodigiosin analogues

Nonylprodigiosin (4) and various of its analogues have been prepared by Suzuki cross-coupling reactions of a well accessible pyrrolyl triflate with (hetero)aryl boronic acid derivatives bearing alkenyl side chains. The resulting alkenes or dienes were subjected to metathesis dimerization or ring-closing metathesis (RCM) reactions, respectively, by using a ruthenium indenylidene complex as the catalyst. The biological activity of the products thus obtained was tested in two different assays monitoring i) the proliferation of murine spleen cells induced by lipopolysaccharides (LPS) and concanavalin A (Con A), and ii) the vacuolar acidification of baby hamster kidney (BHK) cells. Compounds 4 and 21 suppressed Con A-induced T-cell proliferation much more potently than LPS-induced B-cell proliferation. Furthermore, compounds 4 and 26 markedly inhibited vacuolar acidification, although other compounds exhibited no or only marginal effects. Thus, the immunosuppressive activity of prodigiosins toward T-cell proliferation seems to be mediated through cellular targets distinct from vacuolar acidification, and the prodigiosin analogues might be powerful tools to dissect these biological responses. The X-ray crystal structure of the macrocyclic product 25 has been determined, showing that the replacement of one pyrrole ring of the parent compound 4 by a phenyl group does not alter the overall electronic features of the remaining heterocyclic ring system of these alkaloids.