Efficient highly selective synthesis of methyl 2-(ethynyl)alk-2(E)-enoates and 2-(1'-chlorovinyl)alk-2(Z)-enoates from 2-(methoxycarbonyl)-2,3-allenols

The Rearrangement of Alkylallenes to 1,3-Dienes

1,3-Dienes are vital building blocks in organic synthesis. They underpin many fundamental synthetic transformations and are present in numerous natural products and drug candidate molecules. The rearrangement of an alkylallene to a 1,3-diene is an atom efficient, redox neutral, transformation that provides a straightforward synthetic route to functionalized 1,3-dienes. Herein, we provide an account of this transformation using allenes that are not predisposed by the presence of heteroatoms or electron-withdrawing groups directly attached to the allene. Early reports of this skeletal rearrangement are acid-mediated approaches, with limited substrate scope, but they provide valuable mechanistic insights. More recent transition metal-mediated approaches that exhibit improved substrate scope are described, together with isolated examples that have utilized this rearrangement.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Modern Synthetic Methods for the Stereoselective Construction of 1,3-Dienes

The 1,3-butadiene motif is widely found in many natural products and drug candidates with relevant biological activities. Moreover, dienes are important targets for synthetic chemists, due to their ability to give access to a wide range of functional group transformations, including a broad range of C-C bond-forming processes. Therefore, the stereoselective preparation of dienes have attracted much attention over the past decades, and the search for new synthetic protocols continues unabated. The aim of this review is to give an overview of the diverse methodologies that have emerged in the last decade, with a focus on the synthetic processes that meet the requirements of efficiency and sustainability of modern organic chemistry.

2,3-Allenoic acids via palladium-catalyzed carboxylation of propargylic alcohols

The first example of Pd-catalyzed carboxylation of propargylic alcohols in the presence of CO (1 atm) and water affording 2,3-allenoic acids has been developed.

[3,3]-Sigmatropic rearrangement of allenic alcohols: stereoselective synthesis of 1,3-diene-2-ol sulfonates

An efficient synthetic pathway to 1,3-diene-2-ol sulfonates involving the [3,3]-sigmatropic rearrangement of allenic alcohols with sulfonic acids under mild reaction conditions is described. These products can easily undergo reduction or transition-metal catalyzed cross-coupling reactions to yield a series of stereodefined multisubstituted 1,3-dienes.

Stereoselective preparation of conjugated (Z)-1,3-enynes by dehydration reactions of allenic bromohydrins and the use of the enynes in base-mediated tandem allylation ene-carbocyclization reactions with β-ketoesters

A procedure has been developed for the concise, stereoselective synthesis of (Z)-5-bromo-4-aryl-pent-3-en-1-ynes through Sc(OTf)₃ catalyzed dehydration reactions of allenic bromohydrins. (Z)-1,3-Enynes are transformed to methylenecyclopentenes when subjected to a sequential, one-pot process involving base mediated allylation reactions with ethyl acetoacetate followed by ene-carbocyclization reactions. An unprecedented rearrangement reaction involving 1,5-acyl migration takes place when the methylenecyclopentenes are treated with acid to form highly substituted cyclopentadienes.

Cu-Catalyzed hydrophosphorylative ring opening of propargyl epoxides: highly selective access to 4-phosphoryl 2,3-allenols

A CuI-catalyzed cross-coupling of propargyl epoxides with P(O)H compounds to produce 4-phosphoryl 2,3-allenols with excellent selectivity under mild conditions is disclosed.

Regio- and stereoselective synthesis of 2-amino-dienes via decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO

A metal-free decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO via base-induced aza-Michael addition/elimination has been developed. A variety of substituted N-tosyl 1,3-dien-2-yl amines were obtained in good yields and excellent regio- and stereoselectivity. Moreover, this transformation could be applied in preparation of 2-amino-trienes.

The metal tin promoted cascade reaction of ketones in aqueous media for the construction of 2-bromo-4-aryl-1,3-pentadiene

A novel type of transformation was discovered serendipitously during the Barbier-type allenylation reaction of aromatic ketones promoted by the metal, tin, in aqueous media. Additionally, a series of new, highly functionalized 2-bromo-4-aryl-1,3-pentadienes could be obtained with good yields in this reaction. This cascade reaction shows the unique properties of the metal, tin. Furthermore, it is actually a cascade reaction which involves two steps: one is the Barbier-type allenylation of the carbonyl compound, and the other is an SN2' type addition-elimination reaction. Notably, this reaction has the advantages of simple, mild conditions and is easy to operate. Furthermore, the corresponding product could be applied to various coupling reactions or other diversified transformations.

Subtle electronic effects in metal-free rearrangement of allenic alcohols

A general and stereoselective rearrangement of allenic alcohols to (E,E)-1,3-dien-2-yl triflates and chlorides was developed under metal-free conditions. Subtle electronic effects of the alkyl and aryl substituents on the carbon bearing the hydroxyl group has a profound impact on the reaction rate and efficiency such that vinyl triflates were obtained from electron-deficient substrates and trimethylsilyl triflate whereas vinyl chlorides were generated with an electron-rich substrate and trimethylsilyl chloride.

Direct FeX3-based stereocontrolled access to (Z)-3-alkenyl-oxindoles from allenols

Iron trihalides (FeCl(3) and FeBr(3)) smoothly promote the halogenation/rearrangement of 2-indolinone-tethered allenols to efficiently afford 3-halodienyl-oxindoles with good yield and total selectivity. Also, 2-halo-1,3-dienes are synthetically interesting building blocks for the preparation of functionalized 3-alkenyl-oxindoles through a Suzuki-Miyaura reaction.

Recent advances in stereoselective synthesis of 1,3-dienes

The aim of this review is to present the latest developments in the stereoselective synthesis of conjugated dienes, covering the period 2005-2010. Since the use of this class of compounds is linked to the nature of their appendages (aryls, alkyls, electron-withdrawing, and heterosubstituted groups), the review has been categorized accordingly and illustrates the most representative strategies and mechanisms to access these targets.