The Heyns Rearrangement: Synthetic Routes Beyond Carbohydrate Chemistry.

This concept review describes the recent achievements on the Heyns rearrangement appeared in literature over the last decade and aims to provide the reader with a general overview of the fundamental synthetic advances in this research area.

Substrate-dependent regiodivergence in [3 + 2] annulation reactions of 2-(phenacylethylidene)cyclobutanones with thioureas.

The [3 + 2] annulation reaction between a thiourea, an ambident dinucleophile, and a 2-(phenacylethylidene)cyclobutanone, containing a novel pull-pull alkene system, could in principle proceed with several chemo- and regioselectivity profiles. Here we describe a convenient synthesis of the functionalized cyclobutanone substrates and show that they react with thioureas in a manner that is rationalized mechanistically in terms of the steric and electronic effects at play. The [3 + 2] annulation proceeds in mild, additive-free conditions to provide access to previously unknown cyclobutane-fused imidazolidine-2-thione and thiazolidine-2-imine derivatives in good yields.

Magnetic Aerogels for Room-Temperature Catalytic Production of Bis(indolyl)methane Derivatives.

The potential of aerogels as catalysts for the synthesis of a relevant class of bis-heterocyclic compounds such as bis(indolyl)methanes was investigated. In particular, the studied catalyst was a nanocomposite aerogel based on nanocrystalline nickel ferrite (NiFe2O4) dispersed on amorphous porous silica aerogel obtained by two-step sol-gel synthesis followed by gel drying under supercritical conditions and calcination treatments. It was found that the NiFe2O4/SiO2 aerogel is an active catalyst for the selected reaction, enabling high conversions at room temperature, and it proved to be active for three repeated runs. The catalytic activity can be ascribed to both the textural and acidic features of the silica matrix and of the nanocrystalline ferrite. In addition, ferrite nanocrystals provide functionality for magnetic recovery of the catalyst from the crude mixture, enabling time-effective separation from the reaction environment. Evidence of the retention of species involved in the reaction into the catalyst is also pointed out, likely due to the porosity of the aerogel together with the affinity of some species towards the silica matrix. Our work contributes to the study of aerogels as catalysts for organic reactions by demonstrating their potential as well as limitations for the room-temperature synthesis of bis(indolyl)methanes.

A Brønsted acid catalyzed tandem reaction for the diastereoselective synthesis of cyclobuta-fused tetrahydroquinoline carboxylic esters.

A novel Brønsted acid catalyzed tandem reaction provides highly functionalized cyclobuta-fused tetrahydroquinoline carboxylic esters from anilines and 2-alkylenecyclobutanones in good to high yield. During the reaction a dynamic diastereoselective cyclization is achieved, resulting in the formation of three contiguous stereocenters with high stereoselectivity.

Lithiated three-membered heterocycles as chiral nucleophiles in the enantioselective synthesis of 1-oxaspiro[2,3]hexanes.

The reaction between configurably stable α-lithiated oxiranes and 3-substituted cyclobutanones allows obtaining enantiomerically enriched cyclobutanols (er > 98 : 2). These adducts, subjected to base-mediated Payne rearrangement, lead to the synthesis of a new class of oxaspirohexanes, useful precursors of 2,4-disubstituted cyclopentanones.

Visible light promoted continuous flow photocyclization of 1,2-diketones.

The continuous flow Norrish-Yang photocyclization of 1,2-diketones has been developed and used for the synthesis of a large number of functionalized 2-hydroxycyclobutanones, under blue light irradiation and employing acetone as a solvent. This eco-friendly procedure represents a valid alternative to the reactions carried out in batches thus reducing the reaction times, the formation of secondary products and simplifying the purification steps. The use of differently substituted diketone compounds has allowed us to obtain a wide range of 2 and 3-functionalized cyclobutanones, thus allowing the evaluation of the scope and limitations of this procedure.

Synthesis of ß-sulfinyl cyclobutane carboxylic amides via a formal α to ß sulphoxide migration process.

An original tandem reaction consisting of a thermal elimination-addition process was developed. Highly substituted ß-sulfinyl cyclobutane carboxylic acid derivatives were obtained from isomeric α-sulfinyl derivatives in a single operation in good to high yields and with high trans diastereoselectivity.

Preparation of Cyclobutene Acetals and Tricyclic Oxetanes through Photochemical Tandem and Cascade Reactions.

We describe a photochemical reaction using two starting materials, a cyclopent-2-enone and an alkene, which are transformed in a controlled manner via the initial [2+2]-photocycloaddition adducts into cyclobutene aldehydes (conveniently trapped as stable acetals) or unprecedented angular tricyclic 4:4:4 oxetane-containing skeletons. These compounds are formed through tandem or triple cascade photochemical reaction processes, respectively. Small libraries of each compound class were prepared, thus suggesting that this photochemistry approach opens new opportunities for synthesis design and for widening molecular diversity.

Synthesis of 2,2-bis(pyridin-2-yl amino)cyclobutanols and their conversion into 5-(pyridin-2-ylamino)dihydrofuran-2(3H)-ones.

A two-step protocol is presented for the preparation of 5-(pyridin-2-ylamino)dihydrofuran-2(3H)-ones from 2-hydroxycyclobutanone and some 2-aminopyridines via a catalyst-free synthesis of 2,2-bis(pyridin-2-ylamino)cyclobutanols followed by Dess-Martin periodinane mediated ring expansion.

Acid-catalyzed reaction of 2-hydroxycyclobutanone with benzylic alcohols.

The acid-promoted syntheses of 2-(benzyloxy)cyclobutanones and bis(benzyloxy)dioxatricyclo decanes were achieved starting from 2-hydroxycyclobutanone and variously functionalized benzyl alcohols. The reaction sequences afforded the desired products in good to high yields and in a solvent-dependent chemoselective fashion.

Deracemizing organocatalyzed Michael addition reactions of 2-(arylthio)cyclobutanones with ß-nitrostyrenes.

Organocatalyzed Michael addition reactions of 2-(arylthio)cyclobutanones with trans-ß-nitrostyrenes have been carried out using a bifunctional thiourea-primary amine catalyst, providing diastereoisomerically and enantiomerically enriched 2-alkyl-2-(arylthio)cyclobutanones having two contiguous stereocenters of which one is a chiral quaternary center. The absolute configuration of these novel adducts was assigned by X-ray diffraction analysis and a transition-state model is proposed to explain the observed stereoselectivities.

Stereocontrolled synthesis and functionalization of cyclobutanes and cyclobutanones.

In the last decade a certain number of new cyclobutane and cyclobutanone synthesis and functionalization protocols have been published. Organo- and biocatalyzed eco-friendly approaches to cyclobutane-containing molecules have been developed with interesting results. Also, successful new total synthesis of bioactive compounds and drugs have been recently reported where a four membered ring represented the key intermediate. Therefore, the rising interest in this field represents a great point of discussion for the scientific community, disclosing the synthetic potential of strained four membered ring carbocyclic compounds. Herein we report a critical survey on the literature concerning the enantiocontrolled synthesis and functionalization of cyclobutane derivatives, with particular attention to metal-free, low impact methodologies, published during the period 2000-2013.

Organocatalytic asymmetric tandem condensation-intramolecular rearrangement-protonation: an approach to optically active α-amino thioester derivatives.

An unprecedented and conceptually novel chiral Brønsted base/Brønsted acid catalytic method for the enantioselective synthesis of α-amino thioesters through a tandem condensation-intramolecular rearrangement-protonation has been developed which provides a number of important synthetic building blocks in good yield and with moderate to good enantioselectivities.

Very high stereoselectivity in organocatalyzed desymmetrizing aldol reactions of 3-substituted cyclobutanones.

N-Phenylsulfonyl (S)-proline catalyzes the direct aldol reaction of 3-substituted cyclobutanones and aryl aldehydes in good yield and with excellent diastereoselectivity and enantioselectivity. This desymmetrization process provides highly functionalized cyclobutanones with control over three contiguous stereogenic centers.

Easy access to trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted 3,6-dihydro-2H-pyrans and cis-substituted phenylcyclopropylsulfones by using the highly versatile 1-phenylsulfenyl- or 1-phenylsulfonyl-cyclopropylketones.

The high versatility of 1-phenylsulfenyl- or 1-phenylsulfonyl-cyclopropylketones has been exploited for the regioselective synthesis of trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted 3,6-dihydro-2H-pyrans and cis-2-alkyl- or cis-2-aryl-cyclopropylphenylsulfones.

Catalytic Tandem Friedel-Crafts Alkylation/C4-C3 Ring-Contraction Reaction: An Efficient Route for the Synthesis of Indolyl Cyclopropanecarbaldehydes and Ketones.

A general strategy for the synthesis of indolyl cyclopropanecarbaldehydes and ketones via a Brønsted acid-catalyzed indole nucleophilic addition/ring-contraction reaction sequence has been exploited. The procedure leads to a wide panel of cyclopropyl carbonyl compounds in generally high yields with a broad substrate scope.

Tandem Wittig Reaction-Ring Contraction of Cyclobutanes: A Route to Functionalized Cyclopropanecarbaldehydes.

An original tandem reaction consisting of a Wittig reaction-ring contraction process between α-hydroxycyclobutanone and phosphonium ylides has been developed. Highly functionalized cyclopropanecarbaldehydes are obtained in good to high yield.

Acid-catalyzed synthesis of functionalized arylthio cyclopropane carbaldehydes and ketones.

A general strategy for the synthesis of arylthio cyclopropyl carbaldehydes and ketones via a Brønsted acid catalyzed arylthiol addition/ring contraction reaction sequence has been exploited. The procedure led to a wide panel of cyclopropyl carbaldehydes in generally high yields and with broad substrate scope. Mechanistic aspects and synthetic applications of this procedure were investigated.

Brønsted Acid Mediated Cascade Reaction To Access 3-(2-Bromoethyl)benzofurans.

A unified protocol for the construction of 3-(2-bromoethyl)benzofurans and 2-(benzofuran-3-yl)ethylamines from bis[(trimethylsilyl)oxy]cyclobutene has been developed. This mild and facile strategy was applied for the synthesis of a series of 5-HT serotonin receptor agonists, underlining its potential for the syntheses of bioactive compounds and natural products.

l-Proline-catalyzed direct intermolecular asymmetric aldol reactions of 1-phenylthiocycloalkyl carboxaldehydes with ketones. Easy access to spiro- and fused-cyclobutyl tetrahydrofurans and cyclopentanones.

[reaction: see text] Acid-catalyzed ring expansion of chiral cyclopropyl and cyclobutyl derivatives for the synthesis of carbo- and heterocyclic compounds is reported. The starting materials have been successfully prepared by l-proline-catalyzed direct asymmetric aldol reactions of 1-phenylthiocycloalkyl carboxaldehydes with ketones.