2- Alkoxy-3,4-dihydro-2H-pyranes

Reduction of dichlorocarbene adducts. Reduction of the trans-cis mixture of the dichlorocarbene adducts (2) of 2-alkoxy-3,4-dihydro-2H-pyranes (1) gives... 

This reaction is an example of a hetero-DiELS-AiDER reaction with inverse electron demand [20]. 2-Alkoxy-3,4-dihydro-2H-pyrans 9 are employed as masked 1,5-dicarbonyl compounds (cf. p. 371). 

The 1,5-dicarbonyl functionality is also represented by 2-alkoxy-3,4-dihydro-2H-pyrans (e.g., 162, a masked 5-ketoaldehyde), which are obtained by hetero-DiELS-ALDER reaction of enones and vinyl ethers (c p. 315). On treatment with hydroxylamine, they afford pyridines (e.g., 163) ... 

Examination of Raman spectra shows that at room temperature 3,4-dihydro-2H-pyran exists predominantly in a halftwist-angle of 23°. According to H NMR spectra the conformers of 2-alkoxy- and 2-aryloxy-3,4-dihydro-2H-pyrans are in equilibrium. The conformer with the 2-OR in an axial position predominates due to an anomeric effect (see p. 317) ... 

Inverse type hetero-Diels-Alder reactions between p-acyloxy-a-phenylthio substituted a, p-unsaturated cabonyl compounds as 1-oxa-1,3-dienes, enol ethers, a-alkoxy acrylates, and styrenes, respectively, as hetero-dienophiles result in an efficient one step synthesis of highly functionalized 3,4-dihydro-2H-pyrans (hex-4-enopyranosides). These compounds are diastereospecifically transformed into deoxy and amino-deoxy sugars such as the antibiotic ramulosin, in pyridines having a variety of electron donating substituents, in the important 3-deoxy-2-gly-culosonates, in precursors for macrolide synthesis, and in C.-aryl-glucopyranosides. 

An anomeric effect is observed in 3,4-dihydro-2H- pyrans. For example, a 2-alkoxy group preferentially occupies an axial position (71DOKd96)367>. Indeed, a study of the NMR spectra of some 2-alkoxy-3,4-dihydro-2//-pyrans and their 4-methyl derivatives established that the anomeric effect was more important in the unsaturated heterocycles than in the corresponding tetrahydropyrans (72BSF1077). The axial preference of an alkoxy group is even more accentuated when the double bond is associated with a fused benzenoid ring, as in the 2-alkoxychromans. It is also of interest to note that the role of the polarity of the solvent on the conformational equilibrium is less important than for the saturated analogues. 

A. P. Kozikowski, K. L. Sorgi, and R. J. Schmiesing, Stereochemistry of the alkoxyselenation of substituted 3,4-dihydropyrans A useful process for the construction of 2-alkoxy-5,6-dihydro-2H-pyran, Chem. Commun., 477 (1980). 

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