154 Non-Aromatic Heterocycles
3,4-Dihydro-2H-pyran is an enol ether. It is widely used to protect alcohols, with which it reacts readily under acidic
catalysis, producing acetals known as THP derivatives, which are stable to even strongly basic conditions but easily
hydrolysed back to the alcohol under mildly acidic aqueous conditions.
A great deal is known about hydroxylated tetrahydrofurans and tetrahydropyrans because such ring systems occur in
sugars. Glucose has two cyclic forms (-D-glucopyranose and -D-glucopyranose), hemiacetals containing a tetrahydro-
pyran. However, in solution many sugars, including glucose, exist in equilibrium with a small concentration of ring-opened
hydroxy aldehydes, which are involved in many of the important chemical reactions of sugars. Sugars such as glucose, which
form these aldehydes, are known as ‘reducing sugars’ due to the typical reactivity of the aldehyde functional group, whereas
sugars that cannot form aldehydes, for example fructose, are ‘non-reducing’. All 5-hydroxy-aldehydes, 5-hydroxy-ketones
and 5-hydroxy-acids easily close to form six-membered oxygen-containing rings – lactols and lactones, respectively.
Five-membered rings, too, are relatively easy to form: depending on conditions, glucose derivatives can easily be formed
in the furanose form, that is, based on tetrahydrofuran.
Tetrahydropyran, like piperidine, adopts a chair conformation. 2-Alkoxy-tetrahydropyrans prefer a conformation in
which the alkoxy group is axial – this is referred to as the ‘anomeric effect’.