Kinetic resolution of allylic alcohols

Figure 6.4 Some successful examples of kinetic resolution of allylic alcohols by enantioselective epoxidation [21, 27].

Bellemin-Laponnaz S, Twedel J, Ruble JC, Breitling FM, Fu GC (2000) The kinetic resolution of allylic alcohols by a non-enzymatic acylation catalyst application to natural product synthesis. Chem Conunun 1009-1010... 

Similarly,110 (lR,2S )-ephedrine is an effective poison in the kinetic resolution of allylic alcohols using racemic BINAP instead of the expensive (R)-BINAP. (/ )-2-cycIohexenoI can thus be obtained in >95% ee using a racemic... 

Another interesting application of the deoxygenation reaction is shown in Scheme 12.6. Sharpless epoxides are transformed to enantiomerically pure allylic alcohols [14]. It should be noted that the disadvantage of the loss of one-half of the allylic alcohol, as in the case of kinetic resolutions of allylic alcohols, is not a problem when this protocol is employed. 

Sharpless "asymmetric epoxidation" has been used in the enantioselective synthesis of several natural products, including the kinetic resolution of allylic alcohols [11] and the creation of ... 

Substituted acrylates (which reseitible the enamide substrates employed 1n asymmetric hydrogenation) may be deracemized by reduction with an optically active catalyst, especially DIPAMPRh . Selectivity ratios of 12 1 to 22 1 have been obtained for a variety of reactants with compounds of reasonable volatility, separation of starting material and product may be effected by preparative GLC. Recovered starting material can then be reduced with an achiral catalyst to give the optically pure anti product. Examples of kinetic resolutions by this method are given in Table II. More recently very successful kinetic resolutions of allylic alcohols have been carried out with Ru(BINAP) catalysts. 

Lipases [BMIM PEj] Kinetic resolution of allylic alcohols 47... 

Kinetic and Dynamic Kinetic Resolution of Allylic Alcohols... 

J. H. Koh, and J. Park, Dynamic kinetic resolution of allylic alcohols mediated by ruthenium- and lipase-based catalysts,... 

Later, the chiral bicyclic phosphine catalyst 5a was also used for kinetic resolution of allylic alcohols with isobutyric anhydride [8, 9]. The best results were obtained for trisubstituted allylic alcohols - selectivity factors ranged from 32 to 82 at -40 °C. 

Akai S, Tanimoto K et al (2006) A dynamic kinetic resolution of allyl alcohols by the combined use of lipases and [VO(OSiPh3)3]. Angew Chem Int Ed 45 2592-2595... 

The PBO catalyst lc was recently applied to the kinetic resolution of allylic alcohols 2e-l by acylation with isobutyric anhydride [18]. The enantioselec-tivity depends strongly on the structure of allylic alcohols and increases with the conformational flexibility of the substrate. The dienyl alcohols 2i-l are good substrates for this kinetic resolution as shown by the high selectivity factors (30-80) obtained (Table 1). 

The lipase-catalysed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated by Itoh et al. [16] (Fig. 7.7). They found that the acylation rate was strongly dependent on the counter anion of the imidazolium salt, while the lipase-catalysed acylation proceeded with high enantioseleclivity in all ionic liquid tested. Good results were obtained when the reaction was carried out in [bmimT [PFg ] or [bmun" ][BF ]. Other examples of kinetic resolution of allylic alcohols catalysed by lipases in ionic liquids were also reported by these authors [71, 72]. The transesterification of 5-phenyl-l-penten-3-ol under reduced pressure at 27 hPa and 40°C was carried out using methyl phenylthioacetate as acyl donor in [bmim+] [PF ] and [bdmim ][BF ], for obtaining the corresponding acylated compound in optically pure form [71], The acetylation of methyl mandelate catalysed by immobilised P5L in [bdmim ][BF ] is another example reported by these authors about the successful application of ionic liquids as reaction media in racemic resolutions... 

We will see Sharpless epoxidation reactions in the Double Methods section towards the end of the chapter. Interestingly, Sharpless other famous asymmetric method - dihydroxylation - has not found widespread use in kinetic resolution. This is probably because the AD is just too powerful or, to be anthropomorphic, too wilful. In other words, it is not sensitive to the chirality of the substrate and charges ahead and reacts with both enantiomers. That is not to say there are not examples of kinetic resolution with dihydroxylation,19 but they are more rare. However, the dihydroxylation is even more useful and much more general than the kinetic resolution of allylic alcohols by asymmetric epoxidation and was discussed in Chapter 25. A slightly complicated case of kinetic resolution of alcohols by asymmetric dihydroxylation is in the Double Methods section. 

Another ruthenium catalyst was used for the dynamic kinetic resolution of allylic alcohols [reaction (24)] by acylation yielding allylic acetates. Again a redox process should be responsible for the racemization. 

Table 16.2-10. Substrates and products cholesterol oxidase122. in the kinetic resolution of allylic alcohols with...

Candida rugosa lipase 1-5 Kinetic resolution of allylic alcohols 64... 

Scheme 4.3 Sharpless kinetic resolution of allylic alcohols...

Bogar, K., Vidal, P.H., Leon, A.R.A., and Baeckvall, J.-E. (2007) Chemoenzymatic dynamic kinetic resolution of allylic alcohols a highly enantioselective route to acyloin acetates. Org. Lett., 9 (17), 3401-3404. 

Scheme 41.3 Kinetic resolution of allylic alcohols by Birman et al.

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