A concise synthesis of butylcycloheptylprodigiosin

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C−N bond‐forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Ion Mobility Mass Spectrometry as an Efficient Tool for Identification of Streptorubin B in Streptomyces coelicolor M145

Ion mobility spectrometry was utilized to corroborate the identity of streptorubin B (2) as the natural product produced by Streptomyces coelicolor. Natural product 2 was initially assigned as butylcycloheptylprodigiosin (3), and only relatively recently was this assignment clarified. We present additional evidence of this assignment by comparing collisional cross sections (Ω) of synthetic standards of 2, 3, and metacycloprodigiosin (4) to the cyclic prodiginine produced by S. coelicolor. Calculated theoretical Ω values demonstrate that cyclic prodiginines could be identified without standards. This work highlights ion mobility as an efficient tool for the dereplication of natural products.

The importance of asking “how and why?” in natural product structure elucidation

This review highlights why careful consideration of the biosynthetic origin (the how) and the biological function (the why) of a natural product can be so useful during the determination of its structure.

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.

Enantioselective total synthesis and confirmation of the absolute and relative stereochemistry of streptorubin B

The enantioselective total synthesis of the pyrrolophane natural product streptorubin B is described. Key steps in the concise route include the application of a one-pot enantioselective aldol cyclization/Wittig reaction and an anionic oxy-Cope rearrangement to forge the crucial 10-membered ring. Comparisons between CD spectra of synthetic and natural samples of streptorubin B coupled with X-ray crystallography allowed for the determination of the absolute stereochemistry of this natural product for the first time. These studies also provided unambiguous proof of the relative configuration between the butyl side chain and the bispyrrole subunit. Additional studies revealed a novel atropstereoselective Paal-Knorr pyrrole condensation and provided fundamental experimental insight into the barrier for atropisomerization of the natural product.

Development of a merged conjugate addition/oxidative coupling sequence. Application to the enantioselective total synthesis of metacycloprodigiosin and prodigiosin R1

A merged conjugate addition/oxidative coupling sequence that represents an efficient strategy for preparing structurally diverse pyrroles has been developed. Success of the method hinged upon the controlled oxidative coupling of unsymmetrical silyl bis-enol ether intermediates, formed by the 1,4-addition of a Grignard reagent with subsequent enolate trapping by a (chloro)silylenol ether. The process was applied to the first enantioselective syntheses of the biologically active pyrrolophane natural products, metacycloprodigiosin and prodigiosin R1.

Prodigiosin synthesis with electron rich 2,2′-bipyrroles

The prodigiosins possess a linear tripyrrolic scaffold with the A and B pyrrolic rings connected directly in a bipyrrole unit, and the B and C rings joined in a dipyrrin. These red streptomycete metabolites exhibit among other activities, antitumor, antimicrobial, and immunomodulating properties. A new prodigiosin synthesis has been developed that employs electron rich 2,2′-bipyrroles. In particular, undecylprodigiosin and PNU-156804 were synthesized by condensations of 4-methoxy and 4-benzyloxy 2,2′-bipyrroles, 20a and 20b, respectively, with 5-undecylpyrrole-2-carboxaldehyde in 75% and 83% yields. The 4-alkoxy-2,2′-bipyrroles were synthesized from 4-alkoxypyrrolyl butenyl ketones (15a and 15b), which were made by a novel one-pot process from 2-acyl-N-(PhF)pyrrolidin-4-one (13) [PhF = 9-(9-phenylfluorenyl)] using PhFNa in the presence of alkyl halide. A sequence of alkoxy pyrrole 15a and 15b protection, olefin oxidation, Paal–Knorr condensation, and deprotection gave the requisite 4-alkoxy-2,2′-bipyrroles (20). This route offers utility for making 4-alkoxybipyrroles as well as prodigiosins with diversity at the 6-position of the linear tripyrrolic skeleton.Key words: prodigiosin, pyrrole, bipyrrole, PNU-156804, undecylprodigiosin.