Alternate hydroxyl group positions

A novel approach was developed very recently by Kita et al. [15]. DKR of allylic alcohols was performed by combining a lipase-catalyzed acylation with a racemization through the formation of allyl vanadate intermediates. Excellent yields and enantioselectivities were obtained. An example is shown in Figure 4.4. A limitation with this approach for the substrates shown in Figure 4.4 is that the allylic alcohol must be equally disubstituted in the allylic position (R = R ) since C—C single bond rotation is required in the tertiary alkoxy intermediate. Alternatively, R or R can be H if the two allylic alcohols formed by migration of the hydroxyl group are enantiomers (e.g. cyclic allylic acetates). 

Monosaccharide residues containing vicinal hydroxyl groups are oxidized by periodate, and are subsequently removed in the reduction-hydrolysis step. Therefore, the positions to which such monosaccharide residues are linked can be located by methylation analysis performed before, and after, Smith degradation. Alternatively,59 the oxidized and reduced sample is methylated, the ether hydrolyzed, and the product realkylated with CD3I or CH3CH2I. This kind of procedure can have advantages over that first described. For example, methylation before the hydrolysis step hinders the acetal protection of hydroxyl groups that can occur in acid hydrolysis.7... 

Now, you may have noticed that the hydroxyl group in methylecgonine is oriented differently from that in tropine. In methylecgonine it is easy to define the position of the hydroxyl, since this is a chiral centre and we can use the R/S nomenclature. An alternative stereoisomer of tropine exists, and this is called pseudotropine. How can we define the configuration for the hydroxyl when the plane of symmetry of the molecule goes through this centre and means this centre is not chiral but can exist in two different arrangements ... 

Disubstituted alkenes undergo iodohydroxylation regioselectively when the allylic position bears a hydroxyl group [110]. The attack on the distal carbon of the iodonium intermediates is favored on the polarity alternation basis. 

X-ray crystallographic studies have shown that crystalline D-glucose as commonly isolated exists in the a-D-glucopyranose form. Furthermore the stable chair conformation (1), in which the hydroxyl groups on C-2, C-3 and C-4, and the hydroxymethyl group on C-5 are equatorial, is preferred to the alternative chair conformation (6) in which these groups occupy axial positions. 

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