A Domino Diels–Alder Approach toward the Tetracyclic Nicandrenone Framework

A one-pot telescopic synthesis of benzo[b]carbazoles and exploration of their liquid crystalline properties

We describe a diversity-oriented one-pot telescopic synthesis of various benzo[b]carbazoles with the naphthannulation of indoles as the key step, enabled by an intramolecular furan-olefin Diels-Alder reaction. This strategy is general and efficient across a wide range of substrates. We applied this method to synthesize and characterize the first benzo[b]carbazole-based liquid crystalline materials, where the unique molecular design led to the formation of a rare nematic phase at room temperature.

Substituted Tetraethynylethylene-Tetravinylethylene Hybrids

A general synthetic approach to molecular structures that are hybrids of tetraethynylethylene (TEE) and tetravinylethylene (TVE) is reported. The synthesis permits the controlled preparation of many previously inaccessible structures, including examples with different substituents on each of the four branching arms. Most substituted TVE-TEE hybrids are found to be significantly more robust compounds than their unsubstituted counterparts, enhancing the prospects of their deployment in conducting materials and devices. Their participation in pericyclic reaction cascades, leading to sp³-rich polycycles, is demonstrated. The utilization of TEE-TVE hybrids as building blocks for larger acyclic, through/cross-conjugated hydrocarbon frameworks is also established. Aryl-substituted TEEs, TVEs, and their hybrids are fluorescent, with some exhibiting aggregation-induced emission enhancement. The structural requirements are defined and explained, setting the scene for applications as fluorescent probes and organic light-emitting diodes.

Synthetic Studies toward Parvistemoline Using Asymmetric Ir/Amine-Catalyzed Allylation

The common, key tricyclic core of stemona alkaloids parvistemonine (1) and parvistemoline (2), whose synthetic efforts have not reported yet, was constructed through a new strategy in which three contiguous stereogenic centers were set by using Carreira's asymmetric Ir/amine-catalyzed allylation of aldehyde with α-vinylfurfuryl alcohol and Ellman's sulfinamide chiral auxiliary, respectively. The furan ring was especially designed to act as the precursor of the butyrolactone while establishing the significant chirality.

An IMDAF approach to annellated 1,4:5,8-diepoxynaphthalenes and their metathesis reaction leading to novel scaffolds displaying an antiproliferative activity toward cancer cells

A 3,5a-epoxyfuro[2,3,4-de]isoquinoline scaffold, the product of ROCM of 1,4:5,8-diepoxynaphthalenes, is a promising antiproliferative agent toward breast and prostate human cancer cell lines.

Synthesis and ethylene-promoted metathesis of adducts of tandem [4+2]/[4+2] cycloaddition between bis-furyl dienes and maleic acid derivatives

A series of 1,4:5,8-diepoxynaphthalenes, annellated with carbo- and heterocycles, was synthesized based on the tandem intermolecular/intramolecular [4+2] cycloaddition of bis-furyl dienes with moderately to highly reactive cyclic dienophiles (maleic anhydride and maleinimides).

Experimental and Computational Studies Unraveling the Peculiarity of Enolizable Oxoesters in the Organocatalyzed Mannich-Type Addition to Cyclic N-Acyl Iminium Ions

γ− and δ-Oxoesters are easily available starting materials that have been sparingly used in some organocatalyzed reactions proceeding with a high enantioselectivity. In our experimentation we found that the use of these compounds as the enolizable (nucleophilic) component in organocatalyzed Mannich-type reactions using in situ-generated cyclic N-acyl iminium ions gave low diastereoselectivity and low to moderate values of enantioselectivity. This significant drop of facial selectivity with respect to simple aliphatic aldehydes has been rationalized by means of density functional theory (DFT) calculations.

Rapid Access to the Structural Core of Aflavinines via Stereoselective Tandem Intramolecular Diels-Alder Cycloaddition Controlled by the Allylic 1,3-Strain

An expedient route to access the functionalized structural core of aflavinines has been developed starting from three readily available fragments over 12 steps in 29.1% overall yield without using any transition metal catalysis. The key feature of this approach is a tandem intramolecular Diels-Alder cycloaddition to complete the hexacyclic framework with the correct stereochemistry and all the requisite structural elements in place to achieve the total synthesis of aflavinine and its congeners.

Strain-Activated Diels-Alder Trapping of 1,2-Cyclohexadienes: Intramolecular Capture by Pendent Furans

Intramolecular [4 + 2] cycloaddition reactions of substituted 1,2-cyclohexadienes with pendent furans enables the synthesis of complex tetracyclic scaffolds in a single step under mild conditions. All Diels-Alder cycloadducts were obtained as single diastereomers, assigned as the endo isomer. Substrates were easily assembled via Stork-Danheiser alkylation of 3-ethoxy-2-bromocyclohex-2-enone to accommodate a range of tethers and furan traps. Cleavage of enol acetate moieties resulted in room-temperature Diels-Alder cycloreversion to tethered furyl cyclohexenones.

A facile one pot synthesis of thiazolo[3,2-a]benzimidazole and pyran fused polyheterocyclic scaffolds

An efficient synthesis of dihydro-4H-benzo[4,5]imidazo[2,1-b]pyrano[2,3-d]thiazole by a multicomponent route starting from 2-((1H-benzo[d]imidazol-2-yl)thio)-1-phenylethan-1-one, an aromatic aldehyde and malononitrile is described. This protocol features a metal free approach with good substrate scope and decent yield. The heterocyclic skeleton obtained in this study may have interesting biological properties.

Regioselective Synthesis of Bicyclic and Polycyclic Systems by Cycloaddition Reactions of Alkenyl p-Benzoquinones

An efficient [3 + 2]/[4 + 2] or double [4 + 2] cycloaddition strategy has been established for the synthesis of heterocyclic systems under mild conditions. The reaction pathway is governed by the nature of reaction partner. Several dihydrofurocoumarin, furopyranocoumarin, dihydrofuran, dihydrobenzopyran, and dihydrobenzofuran derivatives were obtained as single diastereomers from cyclic or acyclic enol ethers and styrenes. This one-pot transformation constructed C-C and C-O bonds and generated molecular complexity by domino/tandem process to produce the heterocyclic systems in good yields. The ring closure of domino protocol was highly stereoselective and resulted in the formation of cis-fused systems.

Intramolecular Nitrofuran Diels-Alder Reactions: Extremely Substituent-Tolerant Cycloadditions via Asynchronous Transition States

Nitrofurans undergo intramolecular Diels-Alder reactions with tethered electron-poor dienophiles more rapidly and in higher yield than non-nitrated furans. Computational studies indicate that increased stabilization of a partial positive charge on the nitro-substituted carbon in both transition state and product is the driving force for these reactions. Frontier molecular orbital energy differences indicate a switch from normal to inverse electron demand upon nitration. There does not appear to be a contribution from any differences in aromatic stabilization energy between furans and nitrofurans. Calculations show that the nitrofuran reactions proceed via a highly asynchronous transition state allowing easier bond formation between two sterically hindered carbons.

Cytotoxicity of a Series of Norcantharidin-Inspired Tetrahydroepoxyisoindole Carboxamides

A series of 28 norcantharidin (NorC)-inspired analogues were accessed via a robust two-step Ugi intramolecular Diels-Alder (IMDA) sequence. Four analogues displayed whole-cell cytotoxicity equipotent to that of NorC and cisplatin against a number of cancer cell lines and a normal breast cell line (MCF10A). Notably, (3S,3aS,6R)-2-benzyl-7-methyl-N-(naphthalen-2-yl)-1-oxo-1,2,3,6-tetrahydro-3a,6-epoxyisoindole-3-carboxamide (trans-27) displayed superior whole-cell activity against breast (MCF-7, GI₅₀ =2.9 μm) and colon (HT29, GI₅₀ =6.4 μm) cancer cell lines relative to the control (cisplatin), which elicited respective GI₅₀ values of 6.5 and 11.3 μm against the aforementioned cell lines. This analogue also displayed improved activity relative to NorC across the breast (MCF-7, GI₅₀ =2.9 μm; NorC GI₅₀ =7.5 μm), ovarian (A2780, GI₅₀ =2.2 μm; NorC GI₅₀ =4.4 μm), and neuroblastoma (BE2-C, GI₅₀ =2.2 μm; NorC GI₅₀ =3.7 μm) cancer cell lines. Structure-activity relationship (SAR) investigations demonstrated that retention of sp² hybridized connections within the tetrahydroepoxyisoindole carboxamide scaffold is crucial, as aromatization to a phenolic functionality decreased activity, whereas removal of a single olefin bond abolished cytotoxicity. Nonetheless, with respect to the latter, use of crotonic acid as opposed 2-butynoic acid in the Ugi-IMDA sequence imparted a significant improvement to diastereoselectivity, with the cis/trans isomer ratio shifting from ≈1:1.2 to ≈0.5:9.5.

Masked Ketenes as Dienophiles in the Diels–Alder Reaction

The Diels–Alder reaction is one of the most powerful, well-established, and versatile reactions in organic chemistry; however, its application in certain settings remains a challenge as a result of functional group incompatibility. In this review, we examine the methods in which masked ketenes can be employed as dienophiles, taking particular note of applications in complex settings.

Pd(0)-catalyzed cross-coupling of allyl halides with α-diazocarbonyl compounds or N-mesylhydrazones: synthesis of 1,3-diene compounds

An efficient method for the synthesis of 1,3-butadiene derivatives based on Pd-catalyzed cross-coupling with allyl bromides or chlorides with α-diazocarbonyl compounds or N-mesylhydrazones is presented.

Thermal and Lewis acid promoted intramolecular Diels–Alder reaction of furanose tethered 1,3,9-decatriene systems: a synthetic and computational investigation

The intramolecular Diels–Alder (IMDA) reaction of furanose tethered 1,3,9-decatrienes (4a–4r) was investigated under thermal conditions and in the presence of a Lewis acid.