Asymmetric [2,3]-Wittig Rearrangement: Synthesis of Homoallylic, Allenylic, and Enynyl α-Benzyl Alcohols

Experimental and Theoretical DFT Investigations in the [2,3]-Wittig-Type Rearrangement of Propargyl/Allyl-Oxy-Pyrazolones

Here we report the synthesis of interesting 3-alkyl-4-hydroxy-1-aryl-4-(propa-1,2-dienyl)1H-pyrazol-5(4H)-ones and 9-alkyl-7-aryl-1-oxa-7,8-diazaspiro[4.4]nona-3,8-dien-6-ones, starting from 1,2-diaza-1,3-dienes (DDs) and propargyl alcohol. The reaction proceeds through a sequence Michael-type nucleophilic attack/cyclization/[2,3]-Wittig rearrangement. In the same way, the reaction between the aforementioned DDs and allyl alcohol furnished 4-allyl-4-hydroxy-3-alkyl-1-aryl-1H-pyrazol-5(4H)-ones. A DFT study was also carried out, in order to have decisive clarifications about the mechanism.

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.

Sulfinyl-Mediated Stereoselective Functionalization of Acyclic Conjugated Dienes

The chemo- and stereocontrolled functionalization of conjugated sulfinyl dienes in a cascade process that involves a conjugate addition, diastereoselective protonation and a [2,3]-sigmatropic rearrangement is reported. Enantioenriched 1,4-diol and 1,4-aminoalcohol derivatives are obtained in a very straightforward manner. Further functionalization of these structures, including highly stereoselective epoxidation, dihydroxylation and the stereodivergent synthesis of several polyols in a controlled fashion is described.

Kinetic Resolution of Propargylic Ethers via [2,3]-Wittig Rearrangement to Synthesize Chiral α-Hydroxyallenes

An efficient kinetic resolution of propargyloxy dicarbonyl compounds via asymmetric [2,3]-Wittig rearrangement was achieved by using a chiral N,N'-dioxide/Ni^II complex catalyst. Various chiral α-allenyl alcohols were obtained in high enantioselectivities under mild conditions. The utility of this method was readily demonstrated in the asymmetric synthesis of the chiral 2,5-dihydrofuran derivative.

Stereocontrolled Addition of Scrambling ortho-Sulfinyl Carbanions: Easy Access to Homopropargylamines and α-Allenylamines

An unprecedented behavior of ortho-sulfinylpropargyl carbanions in the presence of optically active sulfinylimines affords two different families of compounds: this peculiar chemodivergency is importantly affected by the nature of the employed base, and assisted by the configuration of the electrophile, displaying no alteration in the stereocontrol of both reactions. α-Allenylamines are formed exclusively, using R-sulfinyl aldimines as electrophiles, while homopropargylamines result when S-sulfinyl aldimines are employed.

[2,3]-Wittig Rearrangement as a Formal Asymmetric Alkylation of α-Branched Ketones

The enantioselective [2,3]-Wittig rearrangement of cinnamyloxycyclopentanone derivatives was performed in the presence of a Cinchona-based primary amine. The described method provides synthetically valuable α-hydroxy ketones with quaternary stereogenic centers in excellent enantiomeric purities. Relying on the X-ray crystal structure of the product and the DFT calculations, we propose that the rearrangement is promoted by an intramolecular hydrogen bond between the substrate and the catalyst.