Abstract
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.
Highlights
Cycloaddition reactions provide quick and economic methods for the construction of monocyclic, polycyclic and heterocyclic systems
Inverse-electron-demand HDA reaction between a,b-unsaturated carbonyl compounds and electron-rich alkenes gives an enantioselective approach to chiral dihydropyrans which are precursors for the synthesis of carbohydrate derivatives
The HDA reactions of a,b-unsaturated carbonyl compounds are atom economic and they allow for regio, diastereo- and enantioselective synthesis of multifunctional pyran derivatives from relatively simple compounds
Summary
Keywords Hetero-Diels–Alder reactions Á 1-Oxa-1,3-butadienes Á Dihydropyrans Á Domino Knoevenagel hetero-Diels–Alder reactions Á Bioorthogonal cycloaddition. Abbreviations Ac iBu n-Bu t-Bu (S,S)t-Bu-box [bmim][NO3] Bn BPin Bz Cb DEA DIC DMAP DMF DMSO EDDA Eu(fod). 1-Butyl-3-methylimidazolium nitrate Benzyl 3-Boronopinacol Benzoyl Benzyloxycarbonyl Diethylamine N,N0-Diisopropylcarbodiimide N,N-Ddimethyl-4-aminopyridine Dimethylformamide Dimethyl sulfoxide Ethylene diammonium diacetate 6,6,7,7,8,8,8-Heptafluoro-2,2-dimethyl-3,5-octanedionato europium o-Iodoxybenzoic acid Modularly designed organocatalyst Molecular sieves Pyridinium chlorochromate Pyridinium dichromate Isopropyl tetra-n-Butylammonium bromide Tetrabutylammonium fluoride Tetrabutylammonium hydrogen sulfate tert-Butyldimethylsilyl tert-Butyldiphenylsilyl tert-Butyldimethylsilyl Trifluoromethanesulfonyl Tetrahydrofuran
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