Migration hydroxyl groups

Fig. 18. The expected percentages of various labelled products of the dioldehydratase reaction using 25 as substrate. The calculation was based on the following facts and assumptions (1) The enzyme does not differentiate between the enantiotopic hydrogen positions (conclusion from experiments with species 17 and 18 shown in Fig. 14) (2) in the competition between vicinal hydrogen atoms there is an intramolecular kinetic deuterium isotope effect of 2.6 (Fig. 15) (3) this effect is 10 for geminal hydrogen atoms (4) the migrating hydroxyl group substitutes one of the hydrogen atoms in the vicinal position stereospecifi-cally (i.e., with inversion).

It must be pointed out that transformations of this type do not take place stepwise, as indicated above the ultimate change is equivalent to the simultaneous separation of the hydroxyl group and attachment of the migrating... 

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). 

Aminoalcohols are an important class of compounds in medicinal chemistry because many drugs contain this structure. For their resolution, there are two possibilities acylation of amino function or an enzymatic transesterification with vinyl esters through the hydroxyl group. However, the amino or hydroxyl group must be protected, because if the starting material is the free aminoalcohol, the O- and N-acylation can take place, and in addition, there are migrations obtaining... 

Thus, in most Beckmann rearrangements called anti) to the hydroxyl group migrates ... 

---