Synthesis of Chiral γ,γ-Disubstituted γ-Butenolides via Direct Vinylogous Aldol Reaction of Substituted Furanone Derivatives with Aldehydes

Organocatalyzed Synthesis of γ-Alkenyl Butenolides via Asymmetric Direct Vinylogous Conjugate Addition-Elimination of Substituted Furanone Derivatives to β-Phenylsulfonylenones

A diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric direct vinylogous conjugate addition of α-angelica lactone derivatives to β-phenylsulfonylenones, affording the corresponding γ-alkenyl γ-butenolides in high yields with excellent enantioselectivities (up to 97% ee) after the elimination of the phenylsulfonyl group. This study reports the first successful example of a stereoselective reaction using β-phenylsulfonylenone as the direct alkenyl donor.

Enantioselective Synthesis of Fused Butyrolactones via Organocatalytic Formal [2 + 2 + 2] Annulation of γ-Butenolides, Methylene Indolinones, and Nitroolefins

An organocatalyzed asymmetric synthesis of fused butyrolactones via formal [2 + 2 + 2] annulation between γ-butenolides, methylene indolinones, and nitroolefins in a one-pot process has been established. Products containing six contiguous stereocenters could be obtained in good yields (up to 95%) with excellent enantioselectivities (up to >99% ee) catalyzed by a l-tert-leucine-derived bifunctional thiourea catalyst.

5-Substituted-furan-2(3H)-ones in [8 + 2]-Cycloaddition with 8,8-Dicyanoheptafulvene

This study demonstrates the use of organocatalytic Brønsted base activation of 5-substituted-furan-2(3H)-ones to generate 2π-components for the diastereoselective [8 + 2]-cycloaddition involving 8,8-dicyanoheptafulvene as an 8π-component. The use of dienolates in a higher-order cycloaddition reaction leads to the formation of biologically relevant polycyclic products bearing a γ-butyrolactone structural motif, thus broadening the synthetic potential of Brønsted base activated higher-order cycloadditions.

Organo-catalysis as emerging tools in organic synthesis: aldol and Michael reactions

Organocatalysis has occupied sustainable position in organic synthesis as a powerful tool for the synthesis of enantiomeric-rich compounds with multiple stereogenic centers. Among the various organic molecules for organocatalysis, the formation of carbon–carbon is viewed as a challenging issue in organic synthesis. The asymmetric aldol and Michael addition reactions are the most significant methods for C–C bond forming reactions. These protocols deliver a valuable path to access chiral molecules, which are useful synthetic hybrids in biologically potent candidates and desirable versatile pharmaceutical intermediates. This work highlighted the impact of organocatalytic aldol and Michael addition reactions in abundant solvent media. It focused on the crucial methods to construct valuable molecules with high enantio- and diastereo-selectivity.

[Development of Green Asymmetric Organocatalytic Synthesis]

Organocatalysts, which are less toxic than metal catalysis, as well as inexpensive, environmentally benign, and stable against moisture and oxygen compared to metal-based catalysts, have received considerable attention for being efficient and clean catalysts. With respect to green chemistry, the development of organocatalysis is a significant research subject for a sustainable society. This article reviews studies on the development of novel organocatalysts and the reactions achieved from using them. Focusing on the push-pull ethylene moiety, in which two electron-withdrawing groups (EWGs) were introduced, we proposed that the vinylogous amide proton (N-H) will lead to the design of organocatalysts. We have developed the diaminomethylenemalononitrile (DMM) organocatalysts, which are push-pull ethylenes having two cyano groups as EWGs, and proved that they are effective for highly stereoselective hydrophosphonylation with aldehydes. The catalytic ability of the DMM organocatalyst was demonstrated in the development of the first asymmetric hydrophosphonylation of ketones using organocatalysts. The DMM organocatalyst can be applied to the selective 1,4-addition asymmetric hydrophosphonylation of enones. In addition, we designed and synthesized novel organocatalysts bearing squaramide-sulfonamide motif as multiple hydrogen bond donors. Squaramide-sulfonamide organocatalysts efficiently catalyzed the asymmetric direct vinylogous aldol reactions of furan-2(5H)-one with aldehydes. Successively, we achieved the synthesis of γ,γ-disubstituted-δ-hydroxy-γ-butenolide via asymmetric direct vinylogous aldol reaction of furanone derivatives using the squaramide-sulfonamide organocatalysts.

Organocatalytic β,γ-Selective Activation of Deconjugated Butenolides Access to Chiral Tricyclic Chroman-butyrolactones

A highly efficient method for the β,γ-selective activation of deconjugated butenolides has been developed through an organocatalytic asymmetric vinylogous cascade reaction. This protocol enables the construction of a broad range of substituted tricyclic chroman-butyrolactones by vinylogous Michael/oxa-Michael pathways in good yield (up to 89%) with good to high enantioselectivity (up to 97:3 er) and excellent diastereoselectivity. The ring-opening esterification of butyrolactones was also demonstrated.

Computational Study on the Synergic Effect of Brønsted Acid and Hydrogen-Bonding Catalysis for the Dearomatization Reaction of Phenols with Diazo Functionality

Catalytic dearomative transformations of phenol variants via an ipso-Friedel-Crafts reaction could provide a straightforward method for the rapid assembly of functionalized spiromolecules as versatile synthetic scaffolds. We previously reported a dearomative spirocyclization reaction by merging Brønsted acid and hydrogen-bonding catalysis. However, it was unclear how the reaction proceeded and how the synergic effect was triggered. Described herein are the computational studies used to elucidate the reaction mechanism. Such calculations indicated that the applied catalysts, maleic acid and Schreiner's thiourea, work cooperatively. The synergic effect enabled the chemoselectivity to interconvert between phenol dearomatization and O-H insertion, which is a major side reaction. This investigation also revealed that not only does the Schreiner's thiourea catalyst serve as a hydrogen bonding donor, but the sulfur atom in thiourea possesses a general base function. The dual functional support of the thiourea along with maleic acid would thus realize the chemoselective prioritization of dearomatization over the O-H insertion reaction under mild conditions.

Catalyst-Controlled Regioselective Nitrosocarbonyl Aldol Reaction of Deconjugated Butenolides

An unprecedented regiodivergent nitrosocarbonyl aldol reaction of γ-substituted deconjugated butenolides is described. While Lewis base catalyst quinidine leveraged O-selective nitrosocarbonyl aldol reaction exclusively at the γ-position of deconjugated butenolides to produce γ-aminoxylation products, Lewis acid catalyst Cu(OTf)₂ steered the competitive N-selective nitrosocarbonyl aldol reaction at the β-position, resulting in heterodifunctionalized butenolides. Both processes were amenable to a broad range of substrates and scalable, while the latter one represents a rare example of one-pot hetero-β,γ-difunctionalization of butenolide scaffolds.

Deconjugated butenolide: a versatile building block for asymmetric catalysis

Deconjugated butenolides have emerged as a popular synthon for the enantioselective synthesis of γ-lactones. This review provides a comprehensive overview on the catalytic asymmetric reactions of deconjugated butenolides reported till date.

Asymmetric vinylogous Michael addition of 5-substituted-furan-2(3H)-ones to an α,β-unsaturated-γ-lactam

Asymmetric vinylogous Michael addition involving an α,β-unsaturated-γ-lactam as an acceptor is described.

Organocatalytic enantioselective direct vinylogous Michael addition of γ-substituted deconjugate butenolides to azadienes

The diastereoselective and enantioselective direct vinylogous Michael addition of γ-substituted β,γ-unsaturated γ-lactones to 2-arylidene-N-tosylbenzofuran-3(2H)-imines has been achieved with the aid of a quinine-derived squaramide.

Highly enantioselective sequential vinylogous aldol reaction/transesterification of methyl-substituted olefinic butyrolactones with isatins for the construction of chiral spirocyclic oxindole-dihydropyranones

A highly stereoselective sequential vinylogous aldol reaction/transesterification of methyl-substituted olefinic butyrolactones and isatins was developed with 10 mol% Takemoto's amine–thiourea catalyst.