Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

A Base-Catalyzed, Domino Aldol/hetero-Diels-Alder Synthesis of Tricyclic Pyrano[3,4-c]chromenes in Glycerol

The domino aldol/hetero-Diels-Alder synthesis of some new tricyclic pyrano[3,4-c]chromene derivatives has been achieved successfully after assembling a variety of acyclic or cyclic monoketones with prenyl ether-tethered aldehydes in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene in glycerol at 120 °C. The hitherto unreported stereochemical outcome of this synthetic sequence was studied and established on the basis of single-crystal X-ray diffraction data and 2D NMR NOESY spectroscopy along with the isolation and characterization of the intermediate Aldol condensation product.

Recent Advances in Inverse-Electron-Demand Hetero-Diels-Alder Reactions of 1-Oxa-1,3-Butadienes

This review is an endeavor to highlight the progress in the inverse-electron-demand hetero-Diels-Alder reactions of 1-oxa-1,3-butadienes in recent years. The huge number of examples of 1-oxadienes cycloadditions found in the literature clearly demonstrates the incessant importance of this transformation in pyran ring synthesis. This type of reaction is today one of the most important methods for the synthesis of dihydropyrans which are the key building blocks in structuring of carbohydrate and other natural products. Two different modes, inter- and intramolecular, of inverse-electron-demand hetero-Diels-Alder reactions of 1-oxadienes are discussed. The domino Knoevenagel hetero-Diels-Alder reactions are also described. In recent years the use of chiral Lewis acids, chiral organocatalysts, new optically active heterodienes or dienophiles have provided enormous progress in asymmetric synthesis. Solvent-free and aqueous hetero-Diels-Alder reactions of 1-oxabutadienes were also investigated. The reactivity of reactants, selectivity of cycloadditions, and chemical stability in aqueous solutions and under physiological conditions were taken into account to show the potential application of the described reactions in bioorthogonal chemistry. New bioorthogonal ligation by click inverse-electron-demand hetero-Diels-Alder cycloaddition of in situ-generated 1-oxa-1,3-butadienes and vinyl ethers was developed. It seems that some of the hetero-Diels-Alder reactions described in this review can be applied in bioorthogonal chemistry because they are selective, non-toxic, and can function in biological conditions taking into account pH, an aqueous environment, and temperature.

Efficient and regioselective synthesis of bridged ring systems by domino Knoevenagel – hetero-Diels–Alder reaction

The bridged heterocycles 10–12 were efficiently prepared by a domino Knoevenagel – hetero-Diels–Alder reaction of N,N'-dimethylbarbituric acid 2 and the aldehydes 6–8 which contain an allyl moiety as dienophile. In contrast, reaction of 2 and aldehyde 5 with a dimethylallyl moiety leads to the annulated product 9. For the formation of 10–12, an exo-Z-syn and for that of 9 an endo-E-syn transition structure is proposed.Key words: bridged compounds, domino reaction, Diels–Alder reaction, heterocycles, Knoevenagel reaction.

Multicomponent domino reactions for the synthesis of biologically active natural products and drugs

A main issue in modern synthetic organic chemistry, which deals with the preparation of natural products, pharmaceuticals, diagnostics, agrochemicals, and other important materials, is the improvement of efficiency, the avoidance of toxic reagents, the reduction of waste, and the responsible treatment of our resources. One of the ways to fulfill these goals is the development and use of domino processes, which consist of several bond-forming reactions and which allow the highly efficient synthesis of complex molecules starting from simple substrates. Herein, the combination of several catalytic bond-forming transformations is clearly most appropriate. The synthesis of the enantiopure alkaloid (-)-hirsutine 22, which has a strong inhibitorial effect on influenza A viruses, was accomplished using a biomimetic domino Knoevenagel-hetero-Diels Alder-solvolysis-hydrogenation process. In a similar way the alkaloids (+)-dihydrocorynantheine 23 and (-)-dihydroantirhine 24 as well as heterosteroids 62, D-homosteroids 65 and 68, and azasteroids 25 are prepared. In addition, novel steroid alkaloids 26 are accessible by a combination of the formation of an iminium salt, a hydride shift, and an alkylation. The anti-leukemic pentacyclic (-)-cephalotaxine 27 is obtained by a combination of two Pd-catalyzed reactions.