Total Synthesis of the Reported Structure of Cahuitamycin A: Insights into an Elusive Natural Product Scaffold.

In a 2016 screen of natural product extracts, a new family of natural products, the cahuitamycins, was discovered and found to inhibit biofilm formation in the human pathogen Acinetobacter baumannii. The proposed molecular structures contained an unusual piperazic acid residue, which piqued interest related to their structure/function and biosynthesis. Herein we disclose the first total synthesis of the proposed structure of cahuitamycin A in a 12-step longest linear sequence and 18% overall yield. Comparison of spectral and biological data of the authentic natural product and synthetic compound revealed inconsistentancies with the isolated metabolite. We therefore executed the diverted total synthesis of three isomeric compounds, which were also found to be disparate from the isolated natural product. This work sets the stage for future synthetic and biochemical investigations of an important class of natural products.

Visible-Light/Nickel-Catalyzed Carboxylation of C(sp2) Bromides via Formate Activation.

A new visible-light-driven method for the carboxylation of (hetero)aryl/vinyl bromides has been developed using catalytic 4CzIPN, nickel, phenyl triflimide, and sodium formate as a carboxylation agent. Interestingly, we found catalytic phenyl triflimide plays an essential role in promoting the reaction. While many C(sp2) carboxylation reactions require harsh reagents or gaseous carbon dioxide, we demonstrate the mild and facile construction of carboxylic acids from readily available starting materials.

Dearomatization of Unactivated Arenes via Catalytic Hydroalkylation.

The dearomative cyclization of linear amides to complex spirocyclic butyrolactams has been enabled by photoredox catalysis through a reductive radical-polar crossover mechanism. This mechanism operates with precision on unactivated aromatic substrates to give a wide range of 1,4-hydroalkylation products. This method utilizes a simple organic catalyst/reductant pair to deliver products in a highly flexible manner with respect to substitution, and the products can be further functionalized under simple conditions to afford a collection of motifs. The mechanistic analysis performed here outlines the salient features of this strategy, which were applied to prepare a collection of complex scaffolds including the anticonvulsive agent gabapentin.

Radical Chain Reduction via Carbon Dioxide Radical Anion (CO2•-).

We developed an effective method for reductive radical formation that utilizes the radical anion of carbon dioxide (CO2•-) as a powerful single electron reductant. Through a polarity matched hydrogen atom transfer (HAT) between an electrophilic radical and a formate salt, CO2•- formation occurs as a key element in a new radical chain reaction. Here, radical chain initiation can be performed through photochemical or thermal means, and we illustrate the ability of this approach to accomplish reductive activation of a range of substrate classes. Specifically, we employed this strategy in the intermolecular hydroarylation of unactivated alkenes with (hetero)aryl chlorides/bromides, radical deamination of arylammonium salts, aliphatic ketyl radical formation, and sulfonamide cleavage. We show that the reactivity of CO2•- with electron-poor olefins results in either single electron reduction or alkene hydrocarboxylation, where substrate reduction potentials can be utilized to predict reaction outcome.

Unraveling Structures Containing Highly Conjugated Pyrrolo[4,3,2-de]quinoline Cores That Are Deficient in Diagnostic Proton NMR Signals.

This study began with the goal of identifying additional constituents from Zyzzya fuliginosa extracts obtained from an Indo-Pacific sponge (coll. no. 06132). The previous work identified several red and green pyrroloiminiquinones (aka pyrrolo[4,3,2-de]quinolines), and this reinvestigation provided two additional analogues, a blue compound named zyzzamine A (1) and a green compound named zyzzamine B (2). The relatively low ratio of H/[sum(CNO)] = 0.71 or 0.76 of this pair greatly complicated the final steps of compound characterization and required the use of five 2D NMR strategies and MS2 data sets.