Recent Chemical Methodology Advances in the Total Synthesis of Meroterpenoids.

Heterogeneity of meroterpenoids arising from their dual biosynthetic origins is constantly provoking synthetic chemists to utilize their ingenuity and revise their retrosynthetic logic. By studying recent publications on meroterpenoid synthesis,tremendous advances in the field of synthetic organic chemistry can be witnessed. This feature article covers some of the most intriguing total syntheses and synthetic studies towards the meroterpenoid class of natural products from the last five years

Arenophile-Mediated Photochemical Dearomatization of Nonactivated Arenes

Aromatic compounds are one of the most abundant classes of organic molecules and find utility as precursors for alicyclic hydrocarbon building blocks. While many established dearomatization reactions are exceptionally powerful, dearomatization with concurrent introduction of functionality, i.e. dearomative functionalization, is still a largely underdeveloped field. This review aims to provide an overview of our recent efforts and progress in the development of dearomative functionalization of simple and nonactivated arenes using arenophile-arene cycloaddition platform. These cycloadducts, formed via a visible-light-mediated [4+2]-photocycloaddition, can be elaborated in situ through olefin chemistry or transition-metal-catalyzed ring-opening with carbon-, nitrogen-, and oxygen-based nucleophiles, providing access to diverse structures with functional and stereochemical complexity. Moreover, the dearomatized products are amenable to further elaborations, which effectively install other functionalities onto the resulting alicyclic carbocycles. The utility of the arenophile-mediated dearomatization methods are also highlighted by the facile syntheses of natural products and bioactive compounds through novel disconnections

The Total Synthesis of Rhabdastrellic Acid A

The first total synthesis of rhabdastrellic acid A, a highly cytotoxic isomalabaricane triterpenoid, has been accomplished in a linear sequence of 14 steps from commercial geranylacetone. The prominently strained trans-syn-trans-perhydrobenz[e]indene core characteristic of the isomalabaricanes is efficiently accessed in a selective manner for the first time through a rapid, complexity-generating sequence incorporating a reductive radical polyene cyclization, an unprecedented oxidative Rautenstrauch cycloisomerization, and umpolung -substitution of a p-toluenesulfonylhydrazone with in situ reductive transposition. A late-stage cross-coupling in concert with a modular approach to polyunsaturated side chains renders this a general strategy for the synthesis of numerous family members of these synthetically challenging and hitherto inaccessible marine triterpenoids.</div

Diversity-oriented synthesis of nanographenes enabled by dearomative annulative π-extension

Nanographenes and polycyclic aromatic hydrocarbons (PAHs) are important classes of compounds with numerous applications, but challenging to access due to a lack of programmable and diversity-oriented methods. Here, the authors report a diversity-oriented, growth-from-template synthesis of nanographenes enabled by iterative annulative π-extension reactions from small PAH starting materials

Enantioselective Iridium-Catalyzed Allylic Cyclizations

ISSN:1523-7060ISSN:1523-705

Palladium-Catalyzed Dearomative <i>syn</i>-1,4-Carboamination

A dearomative 1,4-carbo­amination of arenes has been achieved using areno­phile cycloaddition and subsequent palladium-catalyzed substitution with non­stabilized lithium enolates. This protocol delivers products with exclusive <i>syn</i>-1,4-selectivity and can be also conducted in an asymmetric fashion. The method allows rapid dearomative difunctionalization of simple aromatic compounds into functional small molecules amenable to further diversification

Stereochemical Studies of the Opening of Chloro Vinyl Epoxides: Cyclic Chloronium Ions as Intermediates

A systematic study of the opening of a collection of chlorinated vinyl epoxides is reported, which includes experiments that implicate both five- and four-membered chloronium ions as plausible intermediates in this type of epoxide opening reaction

Structure Prediction and Synthesis of a New Class of Macrocyclic Peptide Natural Products

Owing to advances in genomic sequencing and bioinformatics, the breadth of natural product biosynthetic gene clusters (BGCs) has meteorically risen. This remains true for ribosomally synthesized and post-translationally modified peptides (RiPPs), where the rate of bioinformatically identifying clusters vastly outpaces characterization efforts. Uniting bioinformatics and enzymological knowledge to predict the chemical product(s) of a RiPP BGC with total chemical synthesis to obtain the natural compound is an effective platform for investigating cryptic gene clusters. Herein, we report the bioinformatic identification of a biosynthetically divergent class of RiPP bearing a subset of enzymes involved in thiopeptide biosynthesis. These natural products were predicted based on BGC architecture to undergo a formal, enzymatic [4+2]-cycloaddition with subsequent elimination of the leader peptide and water to produce a tri-substituted pyridine-based macrocycle. Bearing a pyridine similar to thiopeptides but lacking the ubiquitous thiazole heterocycles, these new RiPPs were termed pyritides. One of the predicted natural products was chemically synthesized using an 11-step synthesis. This structure was verified to be chemically identical by an orthogonal chemoenzymatic synthesis utilizing the precursor peptide and the cognate [4+2]-cycloaddition enzyme. The chemoenzymatic platform was used to synthesize a second in-cluster pyritide product as well as analogs from other bioinformatically identified pyritide BGCs. This work exemplifies complementary bioinformatic, enzymological, and synthetic techniques to characterize a structurally distinct class of RiPP natural product.</p

Iridium-Catalyzed Enantioselective Allylic Vinylation

The first example of Ir-catalyzed asymmetric substitution reaction with vinyl trifluoroborates is described. The direct reaction between branched, racemic allylic alcohols and potassium alkenyltrifluoroborates proceeded with high site selectivity and excellent enantioselectivity (up to 99%) mediated by an Ir-(P,olefin) complex. This method allows rapid access to various 1,4-dienes or trienes including the biologically active natural products (−)-nyasol and (−)-hinokiresinol