Pt-Catalyzed Rearrangement of Oxaspirohexanes to 3-Methylenetetrahydrofurans: Scope and Mechanism

1,6-Dioxo-2-azaspiro[3.4]oct-2-enes and Related Spirocycles: Heterocycles from [3 + 2] Nitrile Oxide Cycloadditions with 2-Methyleneoxetanes, -Thietanes, and -Azetidines

Isoxazolines and 4-membered heterocycles are significant structural motifs in numerous synthetic intermediates and natural products. [3 + 2] Cycloadditions between enol ethers and nitrile oxides have been well studied; however, nitrile oxide cycloadditions with 4-membered heterocycles to give spiroisoxazolines are unreported. Here, we showcase the regio- and diastereoselective [3 + 2] nitrile oxide cycloadditions of 2-methyleneoxetanes, -azetidines, and -thietanes to give an array of 1,6-dioxo-2-azaspiro[3.4]oct-2-enes and related spirocycles. 2D NMR experiments suggested that most of the observed diastereoselectivities were dictated by steric interactions; however, dipolarophiles with H bonding donors reversed the stereochemical outcome. X-ray crystallography confirmed the structural assignments.

Unusual Transformations of Strain-Heightened Oxetanes

Oxetanes are important motifs for drug discovery and are valuable templates in organic synthesis. Much of their use as synthetic intermediates exploits their inherent strain, often resulting in chain extensions at the expense of the heterocycle. Modifications on the carbon alpha to the oxygen of oxetanes, such as the C═O of β-lactones, extend the modes of reactivity. Nevertheless, the outcomes are still largely predictable. On the other hand, other alpha modifications, such as a ═CH₂, a spiro-oxiranyl moiety, or a spiro-cyclopropyl group, increase strain and open pathways not available to simple oxetanes or β-lactones. Methods in generating 2-methyleneoxetanes, 1,5-dioxaspiro[3.2]hexanes, and 4-oxaspiro[2.3]hexanes have been developed by us and others. To date, reactions of these systems have sometimes been predictable, but often the outcomes have been unexpected. This has provided fertile ground for thinking about what controls reactivity and what other reaction pathways might be accessible to these strain-heightened oxetanes.This Account summarizes the published literature on the most straightforward approaches to 2-methyleneoxetanes, dioxaspirohexanes, and oxaspirohexanes and on their reactivity. In contrast to simple oxetanes, reactions of 2-methyleneoxetanes with nucleophiles at C4 release an enolate rather than an alkoxide. Also, 2-methyleneoxetanes can be converted to homopropargyl alcohols or undergo a silicon accelerated isomerization/electrocyclic ring opening, processes accessible only because of the exocyclic double bond. In addition, oxetane oxocarbenium ions, derived from protonation of the enol ether, can react with nucleophiles to provide 2,2-disubstituted oxetanes. Oxaspirohexanes are readily prepared by Simmons-Smith cyclopropanation of 2-methyleneoxetanes. These unusual systems undergo a variety of substituent dependent rearrangements in the presence of the Lewis acid BF₃·Et₂O. In addition, upon treatment with Zeise's dimer, oxaspirohexanes are transformed to synthetically useful 3-methylenetetrahydrofurans. Dioxaspirohexanes are easily accessed by dimethyldioxirane oxidation of 2-methyleneoxetanes. Predictably, dioxaspirohexanes react with many nucleophiles to give α-functionalized-β'-hydroxy ketones. Unexpectedly, 2,2-disubstituted oxetanes can also be selectively produced. This latter pathway has led to further unusual transformations, illuminating computational studies, and novel routes to biologically relevant molecules.

Theoretical Study on the Mechanism of Rearrangement Reactions of Bicyclic Derivatives of Cyclopropane to Monocyclic Derivatives under the Catalysis of Pt-Salt

In this paper, the mechanistic studies on the isomerization of hydroxyl and silyl derivatives of bicyclic cyclopropanes under the catalytic action of Zeise's salt have been reported. The catalytic activity of both the monomeric and the dimeric forms of Zeise's salt has been studied by applying the high-level quantum mechanical method. Results from this investigation reveal that the reaction goes favorably under the catalysis of the dimeric form of Zeise's salt. The calculated activation barrier for the catalytic process using Zeise's dimer reveals that the rearrangement occurs with an activation barrier of 19-25 kcal mol^-1. Depending on the nature of substituents present on the substrate, formation of various products has been explained. This study also includes the heteronuclear counter part of Zeise's dimer where one of the Pt-metals is replaced by palladium (Pd) and nickel (Ni) successively. The calculated activation barrier using these heteronuclear catalysts is found to be close enough to that calculated for the catalytic pathway using Zeise's dimer.

Pd-Catalyzed Acyl C-O Bond Activation for Selective Ring-Opening of α-Methylene-β-lactones with Amines

A Pd-catalyzed ring-opening of β-lactones with various types of amines (primary, secondary, and aryl) to provide β-hydroxy amides with excellent selectivity toward acyl C-O bond cleavage is reported. The utility of this protocol is demonstrated in an asymmetric kinetic resolution providing enantioenriched α-methylene-β-lactones.