Benzoin, deracemization

Figure 18.21. Deracemization of benzoin (62) by Rhizopus oryzae ATCC 9363.

It is well known that various microbial systems are able to deracemise racemic secondary alcohols via a process that generally involves two different alcohol dehydrogenases with complementary enantiospecificity. For example racemic benzoin may be deracemized using Rhizopus oryzae ATCC 9363 (Scheme 4.38). Interestingly, through control of the pH of the medium, it was possible to control the absolute configuration of the major enantiomer produced at pH 7.5-8.S, the (J )-enantiomer was produced in 75% yield and 97% ee whereas at pH 4-5, the (S)-enantiomer was produced in 71% yield and 85% ee [89]. 

However, not only CALB but also other lipases have been fruitfully coupled in the deracemization of a wide variety of substrates, such as diols [26] a- [27], p- [28], and 8-hydroxyesters [29] benzoins [30] hydroxynitriles [31] haloalcohols [32] hydroxyalkanephosphonates [33] y-hydroxyamides [34] hydroxyacids and hydroxy-aldehydes protected with bulky groups [35] or cyclic allylic alcohols [36]. From those pioneer examples, many efforts have been attempted in order to design other ruthenium catalysts, which could decrease the reaction time and temperature, and improve the reaction conditions, to extend the applicability of this strategy to the resolution of other substrates. Some of those ruthenium catalysts that have led to relevant results are shown in Figure 14.5. 

Demir AS. Hamamci H, Sesenoglu O, Neslihanoglu R, Asikoglu B. Capanoglu D. Fungal deracemization of benzoin. Tetrahedron Lett. 2002 43 6447-6449. 

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