Enantiomeric products, reaction pathway

This process only becomes possible when both enantiomers are converted by two independent enantioselective reactions to the same enantiomeric product. Both pathways must exhibit an opposite sense of enantioselectivity. For example, as shown in Scheme 5.58, whole-cell microbial transformation of a racemic epoxide using two different organisms, each harbouring a hydrolase that performs the enantioselective hydrolysis of the epoxide ring (with opposite stereocontrol), to give a single enantiomeric 1,2-diol as the sole product in high yield with excellent enantiomeric excess [148]. 

The catalytic enantioselective desymmetrization of meso compounds is a powerful tool for the construction of enantiomerically enriched functionalized products." Meso cyclic allylic diol derivatives are challenging substrates for the asymmetric allylic substitution reaction owing to the potential competition of several reaction pathways. In particular, S 2 and 5n2 substitutions can occur, and both with either retention or inversion of the stereochemistry. In the... 

In this reaction, two diastereoisomeric pathways are possible the catalyst, because of the chiral diphosphine ligand, can coordinate to the enamide in two diastereoisomeric ways. As a result, the two substrate complexes exhibit different chemical reactivity. One of the complexes is quite stable and relatively unreactive while the other is highly reactive towards molecular hydrogen. The high reactivity of the latter leads to a high enantiomeric excess of the one enantiomeric product generated by this complex. The two diastereoisomers have been termed major and minor by Halpern [30] and the rule of thumb here is that minor gives major and vice versa. 

Figure 4.8 Competing reaction pathway that leads to the formation of the two enantiomeric products.

Scheme 1.46 A revised catalytic cycle for the asymmetric transfer hydrogenation of aromatic ketones in propan-2-ol by the Noyori-Ikariya (pre)catalyst 2 demonstrates crossover of the reaction pathways the product is obtained via a H"/H+ outer-sphere hydrogenation mechanism and/or step-wise metal-ligand bifunctional mechanism (see text). Formation of the major enantiomeric product is shown. (Adapted from Dub, P. A. et al., /. Am. Chem. Soc., 135, 2604-2619. Copyright 2013 American Chemical Society.)...

With a common intermediate from the Medicinal Chemistry synthesis now in hand in enantiomerically upgraded form, optimization of the conversion to the amine was addressed, with particular emphasis on safety evaluation of the azide displacement step (Scheme 9.7). Hence, alcohol 6 was reacted with methanesul-fonyl chloride in the presence of triethylamine to afford a 95% yield of the desired mesylate as an oil. Displacement of the mesylate using sodium azide in DMF afforded azide 7 in around 85% assay yield. However, a major by-product of the reaction was found to be alkene 17, formed from an elimination pathway with concomitant formation of the hazardous hydrazoic acid. To evaluate this potential safety hazard for process scale-up, online FTIR was used to monitor the presence of hydrazoic acid in the head-space, confirming that this was indeed formed during the reaction [7]. It was also observed that the amount of hydrazoic acid in the headspace could be completely suppressed by the addition of an organic base such as diisopropylethylamine to the reaction, with the use of inorganic bases such as... 

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