Hemiacetals cyclic hemiacetal formation

FIGURE 25 3 Cyclic hemiacetal formation in 4 hydroxybutanal and 5 hydroxypentanal... 

Furanose form (Section 25 6) Five membered nng ansing via cyclic hemiacetal formation between the carbonyl group and a hydroxyl group of a carbohydrate... 

Cyclic hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the a- and jS-furanose forms of D-arabinose. 

Addition to the Carbonyl Group — The internal, cyclic hemiacetal formation is one of the illustrations of such addition. The H2N-X nucleophiles, with X being NH2 (hydrazine), NHAr (arylhydrazines), OH (hydroxylamine), NHCONH2 (semi-carbazide), NHCSNH2 (thiosemicarbazide), or alkyl (primary amine), produce hydrazones, arylhydrazones, oximes, semicarbazones, thiosemicarbazones, and alkyl imines (Schiff bases), respectively, following the following path 5.4 + 5.15 — 5.16 ->. .. -> 5.21. 

Cyclic hemiacetal formation is most common when the ring that results is five- or six-membered. Five-membered cyclic hemiacetals of carbohydrates are called furanose forms six-membered ones are called pyranose forms. The ring carbon that is derived from the carbonyl group, the one that bears two oxygen substituents, is called the anomeric carbon. 

Aldol condensation between a 1,3-dicarbonyl component and a ketone with an a-methylene, under acidic, dehydrating conditions, prodnces pyrylinm salts.It is likely that the initial condensation is followed by a dehydration before the cyclic hemiacetal formation and loss of a second water molecule. The use of the bis-acetal of malondialdehyde, as a synthon for malondialdehyde, is one of the few ways available for preparing a-unsubstitnted pyrylinms. ... 

It had now become clear that if we wished to successfully methylenate the C(l)-carbonyl of 40, we would have to prevent cyclic hemiacetal formation from occurring. A synthetic retreat was therefore beaten, and a triethylsilyl (TES) group was positioned on the C(4)-OH of 63 (Scheme 15). Nucleophilic displacement of the tosyloxy unit now afforded the iodopyranoside 65, and Vasella reductive ring cleavage delivered the desired aldehyde 66 in 85% yield. 

The new stereocenter resulting from cyclic hemiacetal formation is referred to as an anomeric carbon. The stereoisomers thus formed are called anomers. 

FIGURE 21.3 Cyclic hemiacetal formation involves an intramolecular hydroxyl group acting as a neighboring group on a ketone or aldehyde. 

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