Phosphine-catalyzed (3+3) annulation of cinnamaldehyde-derived Morita-Baylis-Hillman carbonates with dinucleophiles

The phosphine-catalyzed (3+3) annulation reaction of cinnamaldehyde-derived Morita-Baylis-Hillman (MBH) carbonates with 1,3-dicarbonyl compounds as dinucleophiles has been developed, giving hexahydrochromenone derivatives in high yields with moderate to good diastereoselectivities. The reaction worked through double conjugate addition of 1,3-dicarbonyl compounds to the phosphonium intermediates generated from the cinnamaldehyde-derived MBH carbonates.

Phosphine-Catalyzed (4 + 2) Annulation of Allenoates Bearing Acidic Hydrogen with 1,1-Dicyanoalkenes

α-succinimide-substituted allenoates were employed as phosphine acceptors in phosphine-catalyzed (4 + 2) annulation with 1,1-dicyanoalkenes. They served as C4 synthons in the annulation reaction under mild reaction conditions and produced hexahydroisoindole derivatives in moderate to high yields with good to excellent diastereoselectivities.

Phosphine-Catalyzed (3 + 2) Annulation of γ-Substituted Cinnamic Aldehyde-Derived Morita-Baylis-Hillman Carbonates through Remote Activation

A series of γ-substituted Morita-Baylis-Hillman (MBH) carbonates were synthesized and subjected to phosphine-catalyzed annulations with electrophilic exocyclic alkenes, giving various valuable spirocyclopentenes in moderate to excellent yields with moderate to excellent diastereoselectivities. A large scope of MBH carbonates bearing γ-hydrogen, alkenyl, and alkynyl substituents was well tolerated. The annulation unprecedentedly involves β-, γ-, and δ-carbons of MBH carbonates.

A review of the synthetic strategies toward spirobarbiturate-fused 3- to 7-membered rings

Spiro-connected hexahydropyrimidine-2,4,6-trione heterocyclic rings have received increasing attention from the viewpoint of biological activity and synthetic methods due to the importance of the rigid heterocyclic system and unique orientation of the functional substituents. Since 1900, numerous studies have exploited the synthesis of various spirobarbiturate-fused carbo- and heterocycles by applying different strategies. Due to the importance of barbiturate-based olefins as activated alkenes, these substrates have been utilized in many reactions. In addition, the nucleophilic addition of 1,3-disubstituted barbituric acid to different electrophiles has been applied to prepare highly functionalized spirobarbiturate scaffolds. This short review highlights various strategies for synthesizing spirobarbiturate-fused 3- to 7-membered carbo- and heterocyclic rings and some catalytic diastereoselective and enantioselective chemical reactions to access chiral compounds. The sections are divided by the incorporation of 3- to 7-membered heterocyclic or carbocyclic rings into a spirobarbiturate scaffold, and are presented in different subsections depending on the mechanistic details.

An Experimental and Theoretical Study of the 1,3-Dipolar Cyclo­addition of Alloxan-Derived Azomethine Ylides to Cyclopropenes

A diastereoselective synthesis of biologically interesting spirobarbiturates has been achieved via [3+2] cycloaddition of alloxan-derived azomethine ylides to 3-R-1,2-diphenylcyclopropenes. With this approach, a range of spirobarbiturate-3-azabicyclo[3.1.0]hexanes and spirobarbiturate-cyclopropa[a]pyrrolizines were obtained in moderate to good yields with excellent diastereoselectivities. DFT calculations (M11 density functional theory) were carried out to shed light on the molecular mechanism of 1,3-dipolar cycloaddition of alloxan-derived azomethine ylides to cyclopropenes. The cytotoxic activity of some obtained compounds against human erythroleukemia (K562) cell line was evaluated in vitro by MTS-assay.

Phosphine-catalyzed divergent reactivity of alkynoates with acid anhydrides: chemo- and stereoselective synthesis of polysubstituted olefins and dienes

Unprecedented phosphine-catalyzed chemoselective acylesterification, tandem addition and Michael type reaction between alkynoates and acid anhydrides have been developed, enabling efficient synthesis of either the polysubstituted olefins or the multifunctional dienes.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Chemoselective synthesis of 5,4'-imidazolinyl spirobarbiturates via NBS-promoted cyclization of unsaturated barbiturates and amidines

A selective cyclization of unsaturated barbiturates and amidines promoted by N-bromosuccinimide has been successfully developed to afford a vast variety of 5,4'-imidazolinyl spirobarbiturates in moderate to good yields. The present protocol features broad substrate scope, facile work-up procedure and mild reaction conditions, providing a novel strategy for the highly selective and efficient construction of structurally diverse spiroimidazolines.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

Synthesis of 2-chromanone-fused [3.2.0] bicycles through a phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction

A phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction of alkynone is described. 2-Chromanone-fused bicyclo[3.2.0]heptenones were synthesized in moderate to high yields with remarkably high regio- and diastereoselectivities.

Highly Enantioselective [3 + 2] Annulation of 3-Butynoates with β-Trifluoromethyl Enones Promoted by an Amine-Phosphine Binary Catalytic System

We report a highly enantioselective [3 + 2] annulation between 3-butynoates and β-trifluoromethyl enones, furnishing trifluoromethylated cyclopentenes with three contiguous stereogenic centers in good yields, high diastereoselectivities, and excellent enantioselectivities. A unique catalytic system consisting of a simple amine and a chiral phosphine was devised, and the synergistic play of Lewis basic amine and phosphine was crucial for alkyne isomerization and subsequent cyclization. The protocol disclosed herein allows facile activation of 3-butynoates in phosphine-mediated asymmetric transformations.

Synthesis of spirobarbiturate-pyrrolidinones via a domino aza-Michael/SN2 cyclization of barbiturate-derived alkenes with N-alkoxy α-haloamides

A highly efficient domino aza-MIRC (Michael Induced Ring Closure) reaction between barbiturate-derived alkenes and N-alkoxy α-haloamides has been achieved in moderate to excellent yields. This reaction proceeds smoothly under mild conditions via a domino aza-Michael addition/intramolecular SN2 sequence, providing a practical tool in the synthesis of bioactive molecules spirobarbiturate-3-pyrrolidinones.

Phosphine-Promoted [4 + 3] Annulation of Allenoate with Aziridines for Synthesis of Tetrahydroazepines: Phosphine-Dependent [3 + 3] and [4 + 3] Pathways

In this manuscript, phosphine-dependent [3 + 3] and [4 + 3] annulation reactions of allenoate with aziridines were disclosed. The alkyldiphenylphosphine-promoted [4 + 3] annulation of allenoate with aziridines has been achieved under mild conditions, providing biologically interesting functionalized tetrahydroazepines in moderate to excellent yield with moderate to excellent regioselectivity and diastereoselectivity.

Phosphine-dependent [3 + 3] and [4 + 3] annulation reactions of allenoate with aziridines were disclosed. The alkyldiphenylphosphine-promoted [4 + 3] annulation produced tetrahydroazepines.

Phosphine-Catalyzed α-Umpolung-Aldol Reaction for the Synthesis of Benzo[ b]azapin-3-ones

A novel phosphine-catalyzed intermolecular cyclization between 2-sulfonamidobenzaldehyes and ynones is reported. This methodology serves as a conduit for the construction of benzo[ b]azepin-3-ones in good to excellent yields under mild conditions. The resulting 2-benzylidene moieties are formed exclusively in the E-configuration. Mechanistically, this unusual annulation occurs through a phosphine-catalyzed α-umpolung addition, followed by an aldol reaction. One of the benzo[ b]azepin-3-one products was converted to the core structure of 3-amino-[ a]benzazepin-2-one-1-alkanoic acids, many of which function as angiotensin-converting enzyme inhibitors.

Chemoselective syntheses of spirodihydrofuryl and spirocyclopropyl barbiturates via cascade reactions of barbiturate-based olefins and acetylacetone

The Michael addition initiated ring closure reaction of barbiturate-based olefins and acetylacetone with NBS has been explored. The efficient and chemoselective approach for the synthesis of barbiturate-fused spirocycles was established. Spirodihydrofuryl barbiturates and spirocyclopropyl barbiturates were synthesized selectively via cascade reactions under different basic conditions in moderate to excellent yields. The structure of 2-(4-chlorophenyl)-1,1-diacetyl-5,7-dimethyl-5,7-diazaspiro[2,5]octane-4,6,8-trione was confirmed by single crystal X-ray diffraction analysis.

Enantioselective Catalytic Transformations of Barbituric Acid Derivatives

Since the beginning of the 20th century, numerous research efforts made elegant use of barbituric acid derivatives as building blocks for the elaboration of more complex and useful molecules in the field of pharmaceutical chemistry and material sciences. However, the construction of chiral scaffolds by the catalytic enantioselective transformation of barbituric acid and derivatives has only emerged recently. The specific properties of these rather planar scaffolds, which also encompass either a high Brønsted acidity concerning the native barbituric acid or the marked electrophilic character of alkylidene barbituric acids, required specific developments to achieve efficient asymmetric processes. This review covers the enantioselective catalytic reactions developed for barbituric acid platforms using an organocatalytic and metal-based enantioselective sequences. These achievements currently allow several unique addition and annulation reactions towards the construction of high valued chiral heterocycles from barbituric acid derivatives along with innovative enantioselective developments.

Efficient synthesis of (E)-2-nitromethylcinnamatesviaphosphine-catalyzed tandem α-addition and 1,3-rearrangement

An efficient synthetic approach for accessing (E)-2-nitromethylcinnamatesviaa phosphine-promoted tandem reaction between substituted nitromethanes and alkynes has been developed.

Chiral bifunctional bisphosphine enabled enantioselective tandem Michael addition of tryptamine-derived oxindoles to ynones

A chiral phosphine-catalyzed tandem Michael addition of tryptamine-derived oxindoles to ynones was developed to synthesize spiro[pyrrolidine-3,3′-oxindole] in good yields, good to excellent enantio- and diastereoselectivities.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Organocatalytic umpolung annulative dimerization of ynones for the synthesis of 5-alkylidene-2-cyclopentenones

A novel phosphine-catalyzed umpolung [3 + 2]-annulative dimerization of ynones was developed to furnish functionally rich 5-alkylidene-2-cyclopentenones. In this protocol, ynone acts as both C2 and C3 synthons, which undergo [3 + 2]-annulative dimerization.