Pericyclic reactions in nature: Evidence for a spontaneous [1.7]-hydrogen shift and an 8πe electrocyclic ring closure in the biosynthesis of olefinic hydrocarbons from marine brown algae (phaeophyceae)

Total Synthesis of Pulvomycin D

A synthetic route to the pulvomycin class of natural products is presented, which culminated in the first synthesis of a pulvomycin, pulvomycin D. Key elements of the strategy include a pivotal aldol reaction which led to bond formation between the C24‐C40 and the C8‐C23 fragment. The remaining C1‐C7 fragment was attached by a Yamaguchi esterification completing the assembly of the 40 carbon atoms within the main skeleton. Ring closure to the 22‐membered lactone ring was achieved in the final stages of the synthesis by a Heck reaction. The completion of the synthesis required the removal of six silyl protecting groups in combination with olefin formation at C26‐C27 by a Peterson elimination.

Biomimetic Approaches to the Synthesis of Natural Disesquiterpenoids: An Update

Natural disesquiterpenoids represent a small group of secondary metabolites characterized by complex molecular scaffolds and interesting pharmacological profiles. In the last decade, more than 400 new disesquiterpenoids have been discovered and fully characterized, pointing out once more the "magic touch" of nature in the design of new compounds. The perfect blend of complex and unique architectures and biological activity has made sesquiterpene dimers an attractive and challenging synthetic target, inspiring organic chemists to find new and biomimetic approaches to replicate the efficiency and the selectivity of natural processes under laboratory conditions. In this work, we present a review covering the literature from 2010 to 2020 reporting all the efforts made in the total synthesis of complex natural disesquiterpenoids.

Density Functional Theory Study of Mechanisms of [8 + 2] Cycloadditions of Dienylfurans/Dienylisobenzofurans with DMAD

The mechanisms of [8 + 2] cycloaddition reactions between dienylfurans/dienylisobenzofurans and the activated alkyne, DMAD (dimethyl acetylenedicarboxylate), have been investigated by DFT calculations. The former [8 + 2] reaction is stepwise, starting from attack of the diene substituent on furan, not the furyl moiety in dienylfurans, to DMAD to give a diradical intermediate, which then undergoes ring closure to form the second bond between DMAD and the furan moiety, generating the final [8 + 2] cycloadducts. In contrast, the latter [8 + 2] reaction starts from [4 + 2] cycloaddition of the diene in the furan ring of dienylisobenzofurans toward DMAD, followed by the rate-determining stepwise [1,5]-vinyl shift, forming the [8 + 2] products. The different mechanisms of [8 + 2] reactions are attributed to the facts that for dienylfurans, the reactive diene part is the diene substituent on furan, but in the case of dienylisobenzofurans, it is the diene in the furan ring (its reaction with DMAD to generate an aromatic benzene ring is the driving force for this regiochemistry). Consequently, the [8 + 2] reactions begin with the reaction of the most reactive part of tetraene (either the diene substituent on furan for dienylfurans or the diene in the furan ring for dienylisobenzofurans) with DMAD. FMO analysis and kinetic study have been carried out to gain more information of the reaction mechanisms. Two [8 + 2] reactions of dienylisobenzofurans with different substituents toward DMAD have also been further analyzed by DFT calculations in this paper.

Recent advances in the total synthesis of cyclobutane-containing natural products

Recent developments of strategies on the construction of cyclobutanes and their application in complex natural product synthesis are discussed.

Biomimetic Synthesis of Psiguajdianone Guided Discovery of the Meroterpenoids from Psidium guajava

Psiguajdianone (1), a novel caryophyllene-derived meroterpenoid dimer, was isolated from Psidium guajava. The structure of 1 was determined by X-ray analysis and confirmed by total synthesis. Our synthetic strategy involves biomimetic cascade Knoevenagel condensation/hetero-Diels-Alder reaction and dimerization. Notably, the caryophyllene-derived meroterpenoids obtained during our synthesis were first identified as artifacts in the laboratory, and five of them were proven to be natural products present in the plant. Moreover, these compounds show significant anti-inflammatory activity.

Ravynic acid, an antibiotic polyeneyne tetramic acid from Penicillium sp. elucidated through synthesis

Ravynic acid, an unsaturated tetramic acid possessing antibiotic activity, has been isolated from the fungus Penicillium sp. and identified through synthesis.

Biosynthesis and genomic analysis of medium-chain hydrocarbon production by the endophytic fungal isolate Nigrograna mackinnonii E5202H

An endophytic fungus was isolated that produces a series of volatile natural products, including terpenes and odd chain polyenes. Phylogenetic analysis of the isolate using five loci suggests that it is closely related to Nigrograna mackinnonii CBS 674.75. The main component of the polyene series was purified and identified as (3E,5E,7E)-nona-1,3,5,7-tetraene (NTE), a novel natural product. Non-oxygenated hydrocarbons of this chain length are uncommon and desirable as gasoline-surrogate biofuels. The biosynthetic pathway for NTE production was explored using metabolic labeling and gas chromatography time of flight mass spectometer (GCMS). Two-carbon incorporation (13)C acetate suggests that it is derived from a polyketide synthase (PKS) followed by decarboxylation. There are several known mechanisms for such decarboxylation, though none have been discovered in fungi. Towards identifying the PKS responsible for the production of NTE, the genome of N. mackinnonii E5202H (ATCC SD-6839) was sequenced and assembled. Of the 32 PKSs present in the genome, 17 are predicted to contain sufficient domains for the production of NTE. These results exemplify the capacity of endophytic fungi to produce novel natural products that may have many uses, such as biologically derived fuels and commodity chemicals.

Terminal substituent effects on the reactivity, thermodynamics, and stereoselectivity of the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes

M06-2X/6-31+G(d,p) computations are reported for the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6π) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8π electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6π electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8π electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6π ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6π electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The exo mode of ring closure is favored for these 6π electrocyclizations due to a steric interaction that destabilizes the endo transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically.

Total synthesis of the potent androgen receptor antagonist (-)-arabilin: a strategic, biomimetic [1,7]-hydrogen shift

The first total synthesis of (-)-arabilin, a Streptomyces metabolite that inhibits hormone activation of the androgen receptor, has been completed. The key step, a [1,7]-hydrogen shift, establishes the enol ether-containing skipped-tetraene substructure. This nonenzymatic pericyclic reaction is considered to be biomimetic.

Synthesis and cannabinoid receptor activity of ketoalkenes from Echinacea pallida and nonnatural analogues

Despite its popularity and widespread use, the efficacy of Echinacea products remains unclear and controversial. Among the various compounds isolated from Echinacea, ketoalkenes and ketoalkenynes exclusively found in the pale purple coneflower (E. pallida) are major components of the extracts. In contrast to E. purpurea alkamides, these compounds have not been synthesized and studied for immunostimulatory effects. We present a practical and useful synthetic approach to the ketoalkenes using palladium-catalyzed cross-coupling reactions and the pharmaceutical results at the human cannabinoid receptors. The synthetic route developed provides overall good yields for the ketoalkenes and is applicable to other natural products with similar 1,4-diene motifs. No significant activity was observed at either receptor, indicating that the ketoalkenes from E. pallida are not responsible for immunomodulatory effects mediated via the cannabinergic system. However, newly synthesized non-natural analogues showed micro-molar potency at both cannabinoid receptors.

Cascade Reactions in Total Synthesis

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Multicomponent coupling approach to (+/-)-frondosin B and a ring-expanded analogue

[reaction: see text] A recently discovered multicomponent coupling reaction is used to give direct access to a late intermediate in the synthesis of frondosin B. This intermediate can also be efficiently converted to a ring-expanded analogue of frondosin B by sustained heating of the reaction mixture. An unprecedented tandem 1,7-hydrogen shift, 8pi-electrocyclization is proposed to explain the formation of this ring-expanded species.

"Formal" ruthenium-catalyzed [4+2+2] cycloaddition of 1,6-diynes to 1,3-dienes: formation of cyclooctatrienes vs vinylcyclohexadienes

[reaction: see text] A new "formal" Ru-catalyzed [4+2+2] cycloaddition of 1,6-diynes to 1,3-dienes giving conjugated 1,3,5-cyclooctatrienes and vinylcyclohexadienes is described. This formal cycloaddition is really a tandem process, the Ru(II)-catalyzed formation of (Z)-tetraenes or vinyl-(Z)-trienes followed by a pure thermal conrotatory 8 pi- or disrotatory 6 pi-electrocyclization. The proposed mechanism allows the differences in product ratio to be explained in terms of steric and stereochemical considerations.

Asymmetric syntheses of new functionalized beta-amino alcohols via diastereoselective addition of organometallic reagents onto oxazolidines

Diastereoselective reactions between (S)-phenylglycinol-derived oxazolidines and two unsaturated organolithium reagents afforded chiral beta-amino alcohols having vinyl and alkynylsilane moieties. When the same reactions were performed in the presence of titanium isopropoxide, a dramatic change of regioselectivity was observed, from the alpha- to the gamma-position, thus producing beta-amino alcohols with allyl or allenylsilane functions. A rationalization of the observed diastereoselectivity was suggested for each case.

Combined Addition of Alkenyl and Allenic Anions to Squarate Esters. Direct Competition between Six-Ring and Eight-Ring Electrocyclization of 1,2,4,6,8-Cumulenic Pentaenes

The sequential addition of 1 equiv each of a lithioallene and an alkenyllithium to diisopropyl squarate has been studied. The product compositions resulting from the ensuing reaction cascades were not at all comparable to those resulting from the reverse mode of nucleophile addition. These results require that different intermediates intervene. When the alkenyl anion is the lead reagent, the lithioallene enters predominantly trans and two sequential electrocyclic steps ensue. Initial introduction of methoxyallene causes the second nucleophile to enter preferably from the cis direction, presumably because of oxygen coordination to lithium. The proximal orientation of the unsaturated moieties leads to operation of the dianionic oxy-Cope rearrangement. The second electrocyclic event that transpires following formation of the trans dialkoxide is not of the customary 8pi conrotatory type. Rather, a 6pi disrotatory process occurs exclusively. This "allene effect" is shown to originate from strain, orbital alignment, and conjugative factors operating at the transition state.