Hemiacetals stereoisomeric

Dehydrations have been observed to be caused by water-accepting reagents. Thus cyclic hemiacetal 259 was dehydrated with oxalic acid to 2,2,4-trimethyl-2//-pyran (260) in a 50% yield289 (for similar examples see Section IV,G,1 and IV,G,4). The dehydration of easily accessible hemi-acetals 263 to appropriate condensed 4//-pyrans, e.g., 263a - 27 and 263b - 264, proceed smoothly in an acidic medium.59,290 Similarly, dehydration 28 - 27 has been mentioned in Section III,A. A more complex dehydration agent consisting of phthalimide, triphenylphosphine, and diethyl azodicar-boxylate was used for the transformation of stereoisomeric unsaturated monosaccharides 261 to 2//-pyran 262 (15 to 26%).291... 

Hemiacetal formation was first observed in optical studies on D-glucose. The optical rotation ([ ) of a freshly dissolved sample of D-glucose changes with time because it possesses two stereoisomeric hemiacetals (anomers) that are interconvertible in solution (fig. 12.5). One of these anomers, a-D-glucose, has [a]D=113 the other, a-L-glucose, has [a]D= 19. The optical rotation of a freshly prepared solution of either of these compounds eventually approaches an intermediate value that depends on the equilibrium between the two anomers. 

Caracurine V can be reduced catalytically over rhodium (93) to give two stereoisomeric bases, both of which are tetrahydro derivatives formed by saturation of the 19, 20, and 19, 20 double bonds. These bases can be iV-methylated, whereupon the hemiacetal rings open to afford the isomeric tetrahydrotoxiferine and isotetrahydrotoxiferine (LXXII R = OH with 19,20 and 19, 20 double bonds saturated). 

Scheme 9.4 Formation of cydic hemiacetals by the stereoisomeric 4-HNE adducts.

Stereoisomere Formen des Athyl-hemiacetals von aWehydo-D-Calaktosepen-taacetat, M. L. Wolfrom and William H. Decker, Liebigs Ann. Chem., 690, 163-165... 

Cyclization produces two stereoisomeric hemiacetals, called anomers. The newly created chirality center is called the anomeric carbon. 

Intramolecular hemiacetal formation is common in sugar chemistry (Chapter 24). For example, glucose, the most common simple sugar in nature, exists as an equilibrium mixture of an acyclic pentahydroxyaldehyde and two stereoisomeric cyclic hemiacetals. The cyclic forms constitute more than 99% of the mixture in aqueous solution. 

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