Access to Thiophene and 1H-Pyrrole via Amine-Initiated (3 + 2) Annulation and Aromatization Cascade Reaction of β′-Acetoxy Allenoate and 1,2-Bisnucleophile

Phosphine-catalyzed [5+1] annulation of β'-acetoxy allenoates: straightforward access to tetrahydroquinoline derivatives

An unprecedented phosphine-catalyzed [5+1] annulation of β'-acetoxy allenoates with 1,5-dinucleophiles has been developed, which provides novel and facile access to functionalized tetrahydroquinolines in good to high yields in the presence of PPh3 and K3PO4 under mild reaction conditions. Notably, it is the first report of β'-acetoxy allenoates acting as C1 synthons in Lewis base-catalyzed annulation reactions.

Synthesis of 1,2-Fused Benzimidazoles by Amine-Initiated [3 + 3] Annulations of β'-Acetoxy Allenoates with 1C,3N-Bisnucleophiles

The amine-catalyzed [3 + 3] annulations of β'-acetoxy allenoates with 1C,3N-bisnucleophiles have been established. Under the optimal reaction conditions, this operationally simple synthetic process works well with a wide substrate scope, delivering novel 1,2-fused benzimidazole derivatives in moderate to good yields. In addition, preliminary attempts on the asymmetric version of this reaction have been explored by using cinchona alkaloid-based tertiary amines.

Cu-Catalyzed decarboxylative annulation of N-substituted glycines with 3-formylchromones: synthesis of functionalized chromeno[2,3-b]pyrrol-4(1H)-ones

A novel protocol was developed for preparing functionalized chromeno[2,3-b]pyrrol-4(1H)-ones 3 (CMPOs) from 3-formylchromones with N-substituted glycine derivatives. The method entailed decarboxylative annulation of the acyl group of 3-formylchromones by simply heating a mixture of substrates 1-2 and toluene oxidized by 2-di-tert-butyl peroxide (DTBP) and catalyzed by CuBr. As a result, a series of CMPOs 3 were produced via a cascade reaction. This protocol can be used to synthesize functionalized CMPOs via combinatorial and parallel syntheses in a one-pot reaction rather than a tedious multi-step reaction.

Bi-Catalyzed 1,2-Reactivity of Spirocyclopropyl Oxindoles with Dithianediol: Access to Spiroheterocycles

The efficient Bi-catalyzed 1,2-reactivity of spirocyclopropyl oxindoles has been disclosed with dithianediols as the sulfur surrogate to furnish spiroheterocycles at moderate temperature. The procedure provides a potential approach for the construction of spirotetrahydrothiophene scaffolds with functional group diversity. The catalytic 1,2-reactivity of cyclopropanes, mechanistic studies using density functional theory studies, diastereoselectivity, and additive-free mild conditions are the important practical features.

Synthesis of Substituted Thiophenes through Dehydration and Heterocyclization of Alkynols

A protocol was described for obtaining a variety of substituted thiophenes with functional potential via metal-free dehydration and sulfur cyclization of alkynols with elemental sulfur (S₈) or EtOCS₂K in moderate-to-good yields. The method provides the base-free generation of a trisulfur radical anion (S₃^•-) and its addition to alkynes as an initiator. This research broadens the applications of S₃^•- in the synthesis of sulfur-containing heterocycles.

Visible-Light-Induced Formation of Thiavinyl 1,3-Dipoles: A Metal-Free [3+2] Oxidative Cyclization with Alkynes as Easy Access to Thiophenes

A visible-light-induced [3+2] oxidative cyclization of various alkynes with easily available ketene dithioacetals as the previously unknown thiavinyl 1,3-dipoles in the presence of an acridine photosensitizer is reported. A series of multisubstituted thiophenes were achieved regioselectively in ≤98% yields under very mild metal-free conditions without other additives. This reaction could tolerate a wide range of substrates and achieve good efficiency in large-scale syntheses. The reaction mechanism and their applications are described in detail to reveal the reactivity of the new 1,3-dipoles and the selectivity of the reactions.

Recent approaches in the organocatalytic synthesis of pyrroles

Organocatalysis has emerged as one of the most important tools for the synthesis of diverse structural scaffolds, and has become one of the most important hot topics of current research. Construction of the pyrrole ring has gained much attention from the last few decades due to its remarkable biological activities, pharmaceutical application, intermediate in the synthesis of many natural products, and material science application. With access to these 5-membered aza heterocycles, organocatalytic approaches have provided a new alternative from the perspective of synthetic efficiency, as well as from the green chemistry point of view, and a vast array of synthetic procedures has been developed. Enlightened by the significance of this growing research area, we aim to describe the recent organocatalytic approaches developed for the construction of pyrroles, and organized them based on substrates employed.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

Contrasting Carboannulation Involving δ-Acetoxy Allenoate as a Four-Carbon Synthon Using DABCO and DMAP: Access to Spiro-carbocyclic and m-Teraryl Scaffolds

Spiro-annulation involving δ-acetoxy allenoate and alkyl benzoisothiazole dioxide (N-sulfonyl ketimine) triggered by DABCO/MeCO₂H combination leads to an essentially single diastereomer via chemo- and regiospecific [4 + 2]-carboannulation and a new hydroxyl group is introduced. In contrast, DMAP-catalyzed benzannulation using the same reactants affords unsymmetrical m-teraryls via Mannich coupling, sequential proton transfers, and C-N bond cleavage. Here, δ-acetoxy allenoate serves as a 4C-synthon and the carboannulation is completely base dependent and mutually exclusive.

Phosphine-catalysed (4+1) annulations of β′-acetoxy allenoate with β,γ-unsaturated carbonyl compounds

While β,γ-unsaturated carbonyl compounds have been widely used as γC- or αC-nucleophiles, their potential αC,αC-bisnucleophilic reactivity is still underdeveloped.

Access to Substituted Thiophenes through Xanthate-Mediated Vinyl C(sp2)-Br Bond Cleavage and Heterocyclization of Bromoenynes

An environmentally sustainable strategy for the chemoselective heterocyclization of bromoenynes through a transition-metal-free sulfuration/cyclization process is reported. Using inexpensive and safe EtOCS₂K as a thiol surrogate and tetrabutylphosphonium bromide and H₂O as a mixed solvent, the reaction provided a range of substituted thiophenes in moderate to good yields. In addition, 2,3,4,5-tetrasubstituted thiophenes were able to be prepared under mild reaction conditions by electrophilic heterocyclization with NH₄I and EtOCS₂K in good yields.

Construction of CF3-Containing Tetrahydropyrano[3,2-b]indoles through DMAP-Catalyzed [4+1]/[3+3] Domino Sequential Annulation

A [4+1]/[3+3] domino sequential annulation reaction of o-aminotrifluoroacetophenone derivatives and β'-acetoxy allenoates enabled by DMAP has been reported. A variety of CF₃-containing tetrahydropyrano[3,2-b]indoles were obtained as a single diastereomer in high yields (≤98%) under mild conditions. The reaction can build one C-N bond, one C-C bond, and one C-O bond sequentially in a single step. The synthetic utility was demonstrated with gram-scale reactions and various transformations of the products.

Cascade cyclization versus chemoselective reduction: a solvent-controlled product divergence

A convenient controllable cascade cyclization and partial reduction of enones for the divergent construction of two types of valuable compounds including polysubstituted thiophenes and saturated ketones are developed.

Synthesis of tetrasubstituted thiophenes via a [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes

A [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes was developed to access tetrasubstituted thiophenes.

Thio-Michael addition of thioamides and allenes for the selective construction of polysubstituted 2-arylthiophenes via TBAI/H2O2 promoted tandem oxidative annulation and 1,2-sulfur migration

A novel TBAI-catalyzed tandem thio-Michael addition/oxidative annulation of allenes and thioamides for the construction of polysubstituted 2-arylthiophenes under a sulfur migration transformation protocol has been developed. The transition-metal-free protocol achieves the oxidative cyclization reaction of thioamides containing electron-rich substituents with allenes to construct polysubstituted thiophenes selectively by controlling oxidation conditions.

Assembly of fully substituted 2,5-dihydrothiophenes via a novel sequential multicomponent reaction

A sequential multicomponent reaction between ketoesters, isothiocyanates and 1,2-diaza-1,3-dienes to create 2,5-dihydrothiophenes that can be converted into thiophenes.

Organocatalytic asymmetric [3 + 2] annulation of 1,4-dithiane-2,5-diol with azlactones: access to chiral dihydrothiophen-2(3H)-one derivatives

Herein, the first organocatalyst-mediated enantioselective [3 + 2] annulations of 1,4-dithiane-2,5-diol with various azlactones utilizing C4 and C5 reactivity using squaramide as the catalyst in a ring-opening manner have been reported.

Phosphine-catalyzed [4 + 2] annulation of γ-benzyl allenoates: facile synthesis of benzothieno[3,2-b]pyran derivatives

An efficient tris(4-methoxyphenyl)phosphane catalyzed domino reaction between γ-benzyl allenoate and ethyl (Z)-2-(3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate has been developed, which produces a series of 2H-benzo[4,5]thieno[3,2-b]pyran derivatives in high yields.

PPh3-Catalyzed [3 + 2] Spiroannulation of 1C,3N-Bisnucleophiles Derived from Secondary β-Ketoamides with δ-Acetoxy Allenoate: A Route to Functionalized Spiro N-Heterocyclic Derivatives

A [3 + 2] annulation of α-substituted secondary β-ketoamides with δ-acetoxy-modified allenoate has been developed in the presence of phosphine catalyst. In this spiroannulation reaction, β-ketoamides were used as the bis-nucleophilic partner while the γ,δ-carbon of 5-acetoxypenta-2,3-dienoate participated as a C2 synthon, affording the desired functionalized five-membered N-heterocyclic derivatives in moderate to excellent yields and diastereoselectivities under mild conditions. Preliminary attempts on the asymmetric variant of this reaction have been also examined, giving the corresponding products with moderate ee values.

A metal-free cascade reaction of β-halo-α,β-unsaturated aldehydes and 1,4-dithiane-2,5-diols: synthesis of polycyclic 2-formylthiophenes

A cascade reaction of β-halo-α,β-unsaturated aldehydes and 1,4-dithiane-2,5-diols for the synthesis of 2-formylthiophenes has been described employing a polymer supported organic base.

Direct Csp2–H enolization: an allenoate alkylation cascade toward the assembly of multi-substituted furans

A new route toward tri- and tetrasubstituted furans has been realized via an allenoate alkylation pathway that employs the allenic moiety as a two-carbon unit in the construction of a furan scaffold.

Access to thiopyrano[2,3-b]indole via tertiary amine-catalyzed formal (3+3) annulations of β′-acetoxy allenoates with indoline-2-thiones

Amine-catalyzed formal (3+3) annulations of β′-acetoxy allenoates with indoline-2-thiones are described, which provide facile access to thiopyrano[2,3-b]indole.