Synlett

We report the effect of samarium complexation of a modified flavin entity in displaying efficient photocatalytic activity toward sulfoxidation reactions. Single-crystal X-ray diffraction studies were performed for the structural characterization of the photocatalyst. Spectroscopic and electrochemical studies were undertaken to better understand the nature of the complex when compared with the nonmetalated flavin moiety. The catalytic efficiency was maximal in 2:8 water–2,2,2-trifluoroethanol using 1 mol% of the catalyst in the presence of blue LED light under an oxygen atmosphere. Insignificant product formation was observed with the nonmetalated flavin moiety. A wide range of sulfides were used to explore the substrate scope.

Many natural products and biologically important complex organic scaffolds have convoluted structures around their core skeleton. Interestingly, with just changing the outskirts, the core reflects new and unique degrees of various physical and chemical properties. A very common but intriguing core is a five-membered ring horning heaps of organic molecules crafts. The power of [3+2] cycloaddition reactions to generate five-membered ring systems allocate chemists to envision synthetic procedures of wonder molecules and if it is facilitating a cascade sequence, then the end product will imbibe significant level of complexity having applications in medicinal and pharmaceutical fields. This Account highlights the broad interest in assembling recent advances in cascade reactions involving [3+2] cycloaddition as the power tool in order to conceive breakthrough organic architectures reported in the last ten years. We foresee that our comprehensive collection of astonishing [3+2] cycloaddition enabled cascades will provide valuable insights to polycyclic molecular construction and perseverant approach towards nonconventional synthetic procedures to the organic community.1 Introduction2 Synthesis of Oxindoles Skeleton3 Synthesis of Oxazoles Skeleton4 Synthesis of Oxadiazoles Skeleton5 Synthesis of Nitrogen-Containing Heterocycles6 Synthesis via Formal [3+2] Cycloaddition7 Synthesis of Miscellaneous Scaffolds8 Conclusion

A simple, mild, and metal-free cascade reaction has been developed for the construction of highly functionalized olefins. The approach relies on the initial formation of [3+2]-cycloadducts from a pyridinium ylide generated in situ from polymer-bound DMAP (PS-DMAP) with an N-substituted maleimide or an α,β-unsaturated β-keto ester. The cycloadduct decomposes to regenerate supported DMAP and yield a functionalized itaconimide or olefin. The method has a broad substrate scope. The alkene product has been further transformed into trisubstituted furan. PS-DMAP could be reused for five cycles.

An organocatalytic asymmetric oxa-Michael–Michael reaction of 3-aryl-2-nitroprop-2-enols with alkylidene pyrazolones has been developed. This report describes the first use of a 3-aryl-2-nitroprop-2-enol as an O-nucleophile in enantioselective catalysis. With 10 mol% of a quinine-derived squaramide catalyst, a variety of spirotetrahydropyranopyrazolones were obtained in moderate yields, excellent diastereomeric ratios, and high to excellent enantioselectivities under mild reaction conditions.

C4-Functionalized indole scaffolds are ubiquitous in natural products, bioactive compounds, and pharmaceuticals. Much effort has thus been made to develop effective synthetic strategies for C4 functionalization of the indole core. Among them, chelation-assisted synthetic approaches using transition-metal catalysis for the C4-selective C–H functionalization of indole is attractive. This account highlights progress made in C4-carbon–carbon bond formation of indole using directing-group-assisted transition-metal-catalyzed C–H functionalization (up to May 2022). These studies have been performed using Ru, Rh, Pd and Ir-based catalytic systems, while attention has been focused on the use of first-row abundant catalytic systems.1 Introduction2 Alkylation3 Acylation4 Alkenylation5 Alkynylation6 Allylation7 Annulation8 Arylation9 Conclusion and Outlook

A bicyclo[4.1.0]heptane framework consisting of cis-fused cyclopropane and cyclohexane rings is found in several bioactive compounds. Given the symmetry of this core, catalytic desymmetrization can be considered as the most straightforward strategy for its enantioselective synthesis. Known desymmetrization reactions of meso-bicyclo[4.1.0]heptane derivatives proceed with opening of the cyclopropane ring. We now report the first ring-retentive desymmetrization of bicyclo[4.1.0]heptane derivatives, namely meso-cyclopropane-fused cyclohexene-1,4-diones, through a formal C(sp2)-H alkylation using a nitroalkane as the alkylating agent. This reaction is catalyzed by a dihydroquinine-derived bifunctional tertiary aminosquaramide and generates the products with up to 97:3 er. An application of this reaction is demonstrated by the first catalytic enantioselective synthesis of the natural product (–)-car-3-ene-2,5-dione.

A one-pot asymmetric aminoarylation reaction has been executed for the synthesis of trans-4-amino-5-aryltetrahydrobenzo[c]azepines with excellent diastereo- and enantioselectivity (dr > 99: 1; ee ≤97%). The reaction progresses through aziridination of prochiral N-tosyl-N-cinnamylbenzylamines, followed by an intramolecular 7-endo-tet Friedel–Crafts cyclization of the tethered aziridines generated in situ, where the combination of Cu(OTf)2 as a catalyst and PhINNs as a nitrene source was found to be effective. A chiral indenyl bis(oxazoline) was shown to be an efficient ligand for the catalytic enantioselective version of this one-pot transformation. This 7-endo-tet cyclization is contrary to the Baldwin cyclization rules.

A well-known squaramide-cinchonine organocatalyst was immobilized in a controlled way onto three types of commercial porous glass beads EziG™ (EziG OPAL, EziG Amber, and EziG Coral) and applied in asymmetric Michael reactions. The performance of the immobilized catalysts was evaluated under batch and continuous-flow conditions, showing promising results in both approaches. In batch reactions, 0.8 and 1.6 mol% of the immobilized cinchonine-squaramide provided the products with excellent yields (up to 99%) and enantioselectivities (up to 99% ee). These excellent results were also verified in the case of continuous-flow reactions, where also 0.8 and 1.6 mol% of the catalyst immobilized onto the glass beads afforded the product with extraordinary yields (up to 99%) and very high enantioselectivities (up to 97% ee). The immobilized catalysts could be recycled (up to seven cycles) using both approaches.

Stereoselective synthesis of key acyclic skeleton of structurally challenging and biologically active boscartin A possessing six stereogenic centers, among which three are tertiary, has been achieved for the first time. The synthetic study comprises the Sharpless asymmetric epoxidation (SAE) followed by stereoselective epoxide opening for installation of C-11 and C-12 centers, Meyer–Schuster rearrangement followed by intramolecular oxa-Michael addition for construction of C-1 center, SAE followed by CBS reduction and subsequent cycloetherification for stereoselective generation of C-3, C-4, and C-7 centers, Gilman reaction for introduction of C-9 olefin and Grignard reaction for installation of C-8 olefin.

A simple and transition-metal-free HFIP-induced allylation reaction of tertiary allylic alcohols with thiols or sulfonyl hydrazine derivatives was reported for the efficient and highly selective synthesis of allylic sulfides and allylic sulfones. Herein, HFIP played a vital role in not only activating the allylic alcohol, but also stabilizing the allylic cation intermediate to accelerate the subsequent transformations.