InCl3-promoted a novel Prins cyclization for the synthesis of hexahydro-1H-furo[3,4-c]pyran derivatives

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.

Stereo- and Regioselective Synthesis of 4-Vinylpyrrolidine from N-Tethered Alkyne-Alkenol

Indium(III) chloride can be efficiently used for the synthesis of 4-vinylpyrrolidine from N-tethered alkyne-alkenol in good yields. The reaction is highly stereo- and regioselective.

A tandem isomerization/prins strategy: iridium(III)/Brønsted acid cooperative catalysis

Working together: A mild and efficient isomerization/protonation sequence generates pyran-fused indoles by cooperative catalysis between cationic iridium(III) and Bi(OTf)3 . Three distinct cyclization manifolds lead to the corresponding bioactive scaffolds in good yields. In addition, N-substituted indoles can be synthesized enantioselectively in the presence of a chiral phosphate.

The Piancatelli rearrangement: new applications for an intriguing reaction

Nearly forty years ago, at the University of Rome, Giovanni Piancatelli and co-workers discovered the acid-catalyzed water-mediated rearrangement of 2-furylcarbinols into 4-hydroxycyclopentenones. These motifs are core components of several pharmacologically active compounds and precursors of many natural products. The main features of this reaction are the simple experimental conditions, the stereochemical outcome and the generality of the procedure. Consequently, a re-emergence of this reaction has been seen recently, including developments of the Piancatelli rearrangement with some interesting inter- and intramolecular variants. This review will mainly focus on the general aspects of the reaction along with its more recent applications.