Dihydropyrans, from ethyl vinyl ethers

More recently, Tietze s group adapted the well-known domino Knoevenagel-Hetero-Diels-Alder MCR to the efficient synthesis of the deoxyaminosugar (-l-)-D-Forosamine starting from nitroacetone, aqueous formaline, and ethyl vinyl ether (Scheme 65) [169]. The expected racemic dihydropyran was obtained in 37% yield and further transformed to the optically pure product in eight steps in chiral resolution with chiral HPLC. 

Atkins et al. [130] reported in 1977 that irradiation of mixtures of benzene and methyl acrylate or methyl methacrylate, both acceptors, yields mixtures of endo and exo adducts. A subsequent report from the same groups [120] describes the results of the irradiations of benzene in the presence of ethyl vinyl ether, //-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene. In all these cases, the major products were exo-ortho photocycloadducts. The orientations of these vinyl ethers with respect to benzene, in their loose ground-state associations, were inferred from NMR spectra. For ethyl vinyl ether, n-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene, the vinyl proton resonances were either unaffected by a solvent change from carbon tetrachloride to hexadeuterobenzene or appeared 4-10 Flz downfield, whereas the methyl and/or methylene signals all moved up-field by 10-25 Hz. This implies an endo arrangement of the molecules in the ground state. Thus, the ortho photocycloadducts of vinyl ethers with benzene show exo stereochemistry, even when the ground-state orientation is endo. 

Another landmark development in the area of hDA chemistry is the emergence of the tridentate chromium complexes 194 that can catalyze the reaction of njl-unsaturatcd aldehydes with vinyl ethers to afford dihydropyrans with high diastereo- and enantioselectivity (Equation 91) <2002AGE3059>. The same catalytic system can be used for the asymmetric synthesis of the 3,4-dihydropyran 195 from 3-boronoacrolein pinacolate and ethyl vinyl ether in quantitative yield (Equation 92) <2003JA9308, 2005JA1628>. 

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