Access to a Guanacastepene and Cortistatin-Related Skeleton via Ethynyl Lactone Ireland–Claisen Rearrangement and Transannular (4 + 3)-Cycloaddition of an Azatrimethylenemethane Diyl

Dual Proton/Silver-Catalyzed Serial (5 + 2)-Cycloaddition and Nazarov Cyclization of (E)-2-Arylidene-3-hydroxyindanones with Conjugated Eneynes: Synthesis of Indanone-Fused Benzo[cd]azulenes

A one-pot step-economic tandem process involving (5 + 2)-cycloaddition and Nazarov cyclization reactions has been reported for the facile synthesis of indanone-fused benzo[cd]azulenes from (E)-2-arylidene-3-hydroxyindanones and conjugated eneynes. This highly regio- and stereoselective bisannulation reaction is enabled by dual silver and Brønsted acid catalysis and opens up a new avenue for the construction of important bicyclo[5.3.0]decane skeletons.

Computational studies of Brønsted acid-catalyzed transannular cycloadditions of cycloalkenone hydrazones

The contribution to the energy barrier of a series of tethers in transannular cycloadditions of cycloalkenes with hydrazones has been computationally studied by using DFT. The Houk's distortion model has been employed to evaluate the influence of the tether in the cycloaddition reaction. That model has been extended to determine the contribution of each tether and, more importantly, the effect exerted between them. In addition to the distortion induced by the tethers, the entropy effects caused by them has also been studied. The analysis of the evolution of the electron localization function along the reaction revealed the highly concerted character of the reaction.

Recent Developments in Transannular Reactions

Transannular reactions have shown a remarkable performance for the construction of polycyclic scaffolds from medium- or large sized cyclic molecules in an unconventional manner. Recent examples of transannular reactions reported from 2011 have been reviewed, emphasizing the excellent performance of this approach when accessing the target compounds. This review also highlights how this methodology provides an alternative approach to other commonly used methodologies for the construction of cyclic entities such as cyclization or cycloaddition reactions

Secondary Products from Intramolecular Cycloadditions of Azidoalkyl Enol Ethers and Azidoalkyl Vinyl Bromides: 1-Azadienes, Their Reactions with Diphenylketene, and Radical Cyclizations To Form Bi- and Tricyclic Lactams

Azidoalkyl enol ethers undergo intramolecular 1,3-dipolar cycloaddition to generate stable triazolines; in contrast, the cycloadducts formed by heating analogous azidoalkyl vinyl bromides are unstable with respect to elimination of N₂ and HBr, affording 1-azadienes (2-alkenyl cyclic imines). These primary products may be isolated or treated directly with diphenylketene to produce bi- and tricyclic 3,4-dihydropyridin-2(1H)-ones; similar ring systems may also be produced from the azadienes by N-acylation and radical cyclization.

Development of a catalytic enantioselective synthesis of the guanacastepene and heptemerone tricyclic core

For nearly two decades, synthetic chemists have been fascinated by the structural complexity and synthetic challenges afforded by the guanacastepene and heptemerone diterpenoids.Numerous synthetic approaches to these compounds have been reported, but to date the application of enantioselective catalysis to this problem has not been realized. Herein we report an enantioselective synthesis of an advanced intermediate corresponding to the tricyclic core common to the guanacastepenes and heptemerones. Highlights of this work include sequential Pd-catalyzed decarboxylative allylic alkylation reactions to generate the two all carbon quaternary stereocenters, the use of ring-closing metathesis to close the A ring in the presence of a distal allyl sidechain, and a region and diastereoselective oxidation of an trienol ether to introduce oxygenation on the A ring.

Synthesis of Oxa-Bridged Medium-Sized Carbocyclic Rings via Prins Cyclization

Herein, we report a new method for the synthesis of oxa-bridged carbocyclic units based on intramolecular Prins reaction of dioxinones. Our new synthetic approach is flexible and practical and has been successfully applied to the preparation of highly functionalized seven-, eight-, and nine-membered carbocycles. The potential utility of this approach has also been demonstrated in a model study toward construction of the 7,8-fused ring system presented in neoabyssomicin D.

In(OTf)3-Catalyzed Cascade Cyclization for Construction of Oxatricyclic Compounds

A highly diastereoselective cascade cyclization reaction has been developed for establishing a series of oxatricyclic compounds using Chan's diene and simple keto alkynal substrates with only 1 mol % of In(OTf)₃ as the catalyst in 82-92% yields. The potential utility of this synthetic strategy has been demonstrated in model studies for the construction the core structures of 1α,8α:4α,5α-diepoxy-4,5-dihydroosmitopsin and cortistatin A.

Oxidative allene amination for the synthesis of nitrogen-containing heterocycles

The prevalence of stereochemically complex amines in natural products, pharmaceuticals and other bioactive compounds, coupled with the challenges inherent in their preparation, has inspired our work to develop new and versatile methodologies for the synthesis of amine-containing stereotriads ('triads'). The key step is a highly chemo-, regio-, and stereoselective transition-metal catalyzed nitrene transfer reaction that transforms one of the cumulated double bonds of an allene precursor into a bicyclic methyleneaziridine intermediate. This account summarizes our strategies to rapidly elaborate such intermediates into stereochemically rich, densely functionalized amine triads, nitrogen heterocycles, aminated carbocycles and other useful synthetic building blocks.

Improved synthesis of 8-oxabicyclo[3.2.1]octanes via tandem C-H oxidation/oxa-[3,3] Cope rearrangement/aldol cyclization

A tandem C-H oxidation/oxa-[3,3] Cope rearrangement/aldol reaction of allylic silylethers promoted by T⁺BF₄^-(tempo oxoammonium tetrafluoroborate)/ZnBr₂ has been successfully developed allowing the efficient construction of 8-oxabicyclo[3.2.1]octanes and their analogs with a wide substrate scope.