An overview of the recent synthetic studies toward penifulvins and other fenestranes

Rh(I)-Catalyzed [4+3]/[4+1] Cycloaddition of Diene-Vinylcyclopropanes and Carbon Monoxide to Access Angular 5/7/5 Tricycles

Report here is a Rh-catalyzed [4+3]/[4+1] cycloaddition of diene-vinylcyclopropanes (diene-VCPs) and carbon monoxide to access compounds with angular 5/7/5 tricyclic skeleton found in natural products. The reaction has broad scope and further transformation of the [4+3]/[4+1] cycloadduct was also investigated. How this [4+3]/[4+1] reaction occurs and why its competing [4+3] reaction is disfavored have been investigated computationally.

Application of Pauson-Khand reaction in the total synthesis of terpenes

The Pauson-Khand reaction (PKR) is a formal [2 + 2 + 1] cycloaddition involving an alkyne, an alkene and carbon monoxide mediated by a hexacarbonyldicobaltalkyne complex to yield cyclopentenones in a single step. This versatile reaction has become a method of choice for the synthesis of cyclopentenone and its derivatives since its discovery in the early seventies. The aim of this review is to point out the applications of PKR in the total synthesis of terpenes.

A Unifying Bioinspired Synthesis of (-)-Asperaculin A and (-)-Penifulvin D

The first syntheses of the isomeric dioxafenestrene natural products (−)-asperaculin A and (−)-penifulvin D are reported. Each target is formed selectively by choice of oxidant in a final divergent bioinspired Baeyer–Villiger (BV) reaction. Density functional theory calculations reveal that electrostatic interactions between the oxidant leaving group and the lactone motif accounts for a reversal of selectivity with H₂O₂/H₃O⁺ compared to peracids. Synthetic features include forging the polycyclic carbon framework with a diastereoselective meta-photocycloaddition biased by an ether substituent at the aryl α-position. The encumbered tertiary alcohol was installed by cyanation of a ketone intermediate followed by nonaqueous hydrolysis of the resulting delicate cyanohydrin.

Unprecedented [5.5.5.6]Dioxafenestrane Ring Construction in Fungal Insecticidal Sesquiterpene Biosynthesis

Fenestranes, a specific class of natural products, contain four fused rings that share a central quaternary carbon atom. The fungal natural product penifulvin A (1) is a potent insecticidal sesquiterpene that features the [5.5.5.6]dioxafenestrane ring. Although the chemical synthesis of 1 has been achieved recently, the enzymes catalysing the cyclization and oxidation of FPP to 1 remain unknown. In this work, we identified a concise pathway that uses only three enzymes to produce 1. A new sesquiterpene cyclase (PeniA) generates the angular triquinane scaffold silphinene (6). A cytochrome P450 (PeniB) and a flavin-dependent monooxygenase (PeniC) catalyse a series of oxidation reactions to transform 6 into 1, including oxidation of the C15 methyl group to a carboxylate moiety, oxidative coupling of the C15 carboxylate and the C1-C2 olefin to form a γ-lactone, and Baeyer-Villiger oxidation to form a δ-lactone. Our results demonstrate the highly concise and efficient ways in which fungal biosynthetic pathways can generate complex sesquiterpene scaffolds.

Stereoselective Homocrotylation of Aldehydes: Enantioselective Synthesis of Allylic-Substituted Z/ E-Alkenes

Cyclopropanated allyl- and crotylboron reagents participate in homoallylation and homocrotylation reactions that enable enantioselective access to motifs that otherwise require many steps to synthesize. In this study, we investigated the effect of substituents α- to boron, predicted either to counteract or reinforce the 1,3- selectivity of the parent reagents. We then investigated the transformation of the substituted homocrotylation products in intramolecular photocycloadditions to produce stereochemically complex natural-product-like scaffolds, finding that flow conditions enhanced the regioselectivity and yield.

Intramolecular [2 + 2] Cycloadditions of Alkyl(phenylthio)ketenes: Total Synthesis of (+)-Sphaerodiol

Asymmetric total synthesis of (+)-sphaerodiol (2) has been achieved. A key step is an intramolecular [2 + 2] cycloaddition of alkyl(phenylthio)ketene for rapid assembly of the decalin ring.