Asymmetric reactions allylic alcohol resolution

Stopping the reaction before completion. This method is very similar to the asymmetric syntheses discussed on page 132. A method has been developed to evaluate the enantiomeric ratio of kinetic resolution using only the extent of substrate conversion. An important application of this method is the resolution of racemic alkenes by treatment with optically active diisopinocampheylborane, since alkenes do not easily lend themselves to conversion to diastereomers if no other functional groups are present. Another example is the resolution of allylic alcohols such as (56 with one... 

Related catalytic enantioselective processes It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [27] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [28]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect the resolution of five- and six-membered allylic carbinols [29] in this instance, as with the Ti-catalyzed procedure, the presence of an unprotected hydroxyl function is required. Perhaps the most efficient general procedure for the enantioselective synthesis of this class of cyclic allylic ethers is that recently developed by Trost and co-workers, involving Pd-catalyzed asymmetric additions of alkoxides to allylic esters [30]. 

When the allylic alcohol is the desired product of the kinetic resolution process, the accompanying epoxy alcohol also may be converted to the desired allylic alcohol by the two-step sequence shown in Scheme 6A. 1. The epoxy alcohol, after separation from the allyl alcohol, is mesylated and then subjected to reaction with sodium telluride, which effects the transformation of epoxy mesylate to the allylic alcohol with inversion at the asymmetric carbinol center [ 115e]. Preliminary results suggest that the rearrangement follows this pathway only when the epoxy alcohol is unsubstituted at the 3-position. 

In order to reduce the time needed to perform a complete kinetic resolution Lindner et al53 reported the use of the allylic alcohol 30 in enantiomerically enriched form rather than a racemic mixture in kinetic resolution. Thus, the kinetic resolution of 30 was performed starting from the enantiomerically enriched alcohol (R) or (S)-30 (45%) ee obtained by the ruthenium-catalyzed asymmetric reduction of 32 with the aim to reach 100 % ee in a consecutive approach. Several lipases were screened in resolving the enantiomerically enriched 30 either in the enantioselective transesterification of (<5)-30 (45% ee) using isopropenyl acetate as an acyl donor in toluene in non-aqueous medium or in the enantioselective hydrolysis of the corresponding acetate (R)-31, (45% ee) using a phosphate buffer (pH = 6) in aqueous medium. An E value of 300 was observed and the reaction was terminated after 3 h yielding (<5)-30 > 99% ee and the ester (R)-31 was recovered with 86% ee determined by capillary GC after 50 % conversion. 

The ability of the Sharpless epoxidation catalyst to differentiate between the two enantiomers of an asymmetric allyl alcohol affords a powerful synthetic tool to obtain optically pure materials through kinetic resolution.43 Because the procedure relies on one enantiomer of a secondary allyl alcohol undergoing epoxidation at a much faster rate than its antipode, reactions are usually run to 50-55% completion.22 In this way, resolution can often be impressive.18 25 26 44-46 An increase in steric bulk at the olefin terminus increases the rate of reaction.46 47... 

The original report32 of the titanium-catalyzed asymmetric epoxidation of allylic alcohols in 1980 has been followed by hundreds of applications, the majority of which use the initially reported conditions. In the decade since the introduction of this reaction numerous improvements have been made41. The most complete discussion of the preparative aspects of both the asymmetric epoxidation and the kinetic resolution was presented by the Sharpless group42. This paper details the effects of reagent stoichiometry and concentration, substrate concentration, aging of the catalyst and variation of oxidant, solvent and tartrate as well as workup procedures. What is particularly noteworthy in this presentation is that significant amounts of unpublished work are drawn upon to develop recommendations for successful reaction. 

Walsh and co-workers have developed a one-pot method for the synthesis of hydroxyepoxides via an initial synthesis of an allylic alcohol followed by an asymmetric epoxidation <05JOC1262,05JA14668,05JA16416>. This reaction provides an improvement in overall yields over the typical kinetic resolution reaction. The method involves an initial asymmetric addition to the aldehyde followed by a diastereoselective epoxidation reaction. 

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. 

Analogously, we have tried to use the titanium/tartrate-catalyzed asymmetric epoxidation for the kinetic resolution of the other component of the reaction, namely the hydroperoxide when it is chiral. If the molar ratio of allylic alcohol and hydroperoxide is 1 2 and the reaction is quenched after the consumption at half the amount of hydroperoxide, in the ideal case the homochiral epoxy alcohol, the homochiral hydroperoxide and the homochiral carbinol should be expected. 

Redox reactions. With Pdl2 complexed to 3 kinetic resolution of secondary henzyhc and allylic alcohols can be carried out via enantioselective oxidation (O2, CS2CO3, PhMe, 80°). The acetoxydiiodo-Rh carbene complex of 3 is a catalyst for asymmetric reduction of aroylacetic esters with Ph2SiH2- ... 

Scheme 8.5. Proposed mechanism for the Sharpless asymmetric epoxidation reaction of allylic alcohols, shown here for a simple tran -allylic alcohol. For the AE reaction, Ra = Rb = H. When one (or occasionally both) of these substituents are alkyl groups, the Scheme pertains to the kinetic resolution sequence described in the next section.

Scheme 8.8. Reactions of a chiral allylic alcohol under Sharpless epoxidation conditions (Ti(0-i-Pr)4, /-BuOOH) using the chiral tartrates given (DIPT = diisopropyltartrate). (a) The matched case, in which the preferred approach of the asymmetric catalyst and the diastereoselectivity of the substrate are the same, (b) The mismatched case, (cj An example of a Sharpless kinetic resolution (KR).

Representative examples are shown in Scheme 9. The Sharpless AE of geraniol (57) with (+)-diethyl tartrate (DET) gave a-epoxide 58 with 95% ee. In a double asymmetric induction, epoxidation of allylic alcohol 59 with (—)- and (+)-DET provided a- and P-epoxides, 60 and 61, in ratios of 40 1 and 1 14, respectively [23]. It is noteworthy that high asymmetric selectivity was induced even in the mismatched case. The Sharpless AE is also effective for the kinetic resolution of racemic allylic alcohols. In the reaction of 62 with 0.6 equiv. of t-BuOOH and... 

The kinetic resolution reaction can be used for the asymmetric synthesis of chiral secondary allylic alcohols or their corresponding epoxides. The yield of either is, of course, limited to 50%, starting from the racemic allylic alcohol, but the methodology has found widespread use in organic synthesis. For example, epoxidation of the racemic allylic alcohol 54 gave the epoxide 55, used to prepare the anticoccidial antibiotic diolmycin A1 (5.63). ... 

There can be no doubt that the reliability of the Sharpless reaction amongst many different classes of allyl alcohol contributes to its success as a synthetic tool in asymmetric synthesis. Another remarkable attribute of the Sharpless asymmetric epoxidation is the very high level of discrimination between enantiomers of secondary allylic alcohols leading to the wide use of this system for the kinetic resolution of these substrates. The kinetic resolution (Figure 4.5) was first reported using stoichiometric amounts of titanium/tartrate, but catalytic amounts of titanium may also be employed. ... 

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