Oxygen-containing six-membered rings

Descottes, J. C. Martin, and N. Mathicolonis, Bull. Soc. chim. France, 1972, 1077. 

The carbonyl group of aromatic aldehydes with electron-withdrawing substituents on the aromatic ring may, contrary to previous reports, act as a dienophile in a Diels-Alder reaction, giving dihydropyran derivatives, e.g. (128). Toluene-4-sulphonic acid catalyses several of these reactions and it is clear that two mechanisms must operate for this type of cycloaddition (thermal [4 +2], and acid-catalysed). 

The reaction between cyclohexanone enamines and a variety of benzylidene ketones gives rise to some bicyclic dihydropyran derivatives (129).  

The photosensitized oxygenation of simple acyclic 1,3 dienes gives moderate to good yields of 3,6-dihydro-l,2-dioxins (130).  

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The phthaleins derived from di- or polyhydric phenols are all anhydrides formed by the elimination of water from two phenolic hydroxyls, attached to two different benzene rings. These anhydride phthaleins are known as pyronines, since they contain, like the pyrones, a six-membered oxygen-containing ring. 

Derivatives with five- and six-membered oxygen-containing rings have been isolated when vindoline and deacetylvindoline were treated under superacid conditions.686 Formation of the unexpected oxolane derivatives (R = Ac 4%, R = H 18%) was accounted for by rearrangement through ethyl and hydride shifts (Scheme 5.66). 

Glucose is an equilibrium mixture of cyclic forms (hemiacetals containing a tetrahydropyran), and a small concentration of acyclic polyhydroxyaldehyde, which is responsible for many of the observed chemical reactions. This illustrates the inherent stability of chair conformers of saturated six-membered systems. The propensity for cyclisation is a general one 5-hydroxy-aldehydes, -ketones and -acids all easily form six-membered oxygen-containing rings - lactols and lactones respectively. 

There are more than 170 known trichothecenes which can be classified in four categories (A, B, C, and D) according to their chemical structure (Krska et al., 2001). All trichothecene mycotoxins have a basic tetracydic sesquiterpene structure with a six-membered oxygen containing ring, an epoxide group on the 12,13 position, and an olefinic bond on the 9,10... 

Cyclic Conformation of Monosaccharides. The crystalline form of glucose was shown as cyclic (six-membered, oxygen-containing ring) from X-ray diffraction (15) infrared spectroscopy detects no carbonyl group, confirming a... 

Figure 11.17. Examples of unsaturated fatty acids. Large numbers of such alkenes both as ( )- and (Z)-isomers exist. The intermediate in the oxidation of arachidonic acid to (only one shown member of the) prostaglandins is part of a cascade of reactions to an entire family of compounds that vary in oxidation state. Leukotrienes (three conjugated double bonds and no five-membered ring) and thromboxanes with six-membered oxygen-containing rings (oxa-cyclohexane, pyran, oxane) are also derived from C20 polyalkenes.

The anthocyanins, the blue, red or violet pigments found in many flowers, are glucosides in which there is a six-membered oxygen-containing ring. Their aglucones (non-sugar portion) or anthocyanidins are hydroxy-lated derivatives of a benzopyrilium nucleus ... 

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