Enantioselective synthesis, optical

Apart from enantioselective synthesis, optically pure compounds can be obtained by chiral resolution. This involves the isolation of one enantiomer from a racemic mixture by a number of methods. This route remains cost-effective when the time and money required for making racemic mixtures are low, or both enantiomers have a separate use. 

The second system studied was the separation of the chiral epoxide enantiomers (la,2,7,7a-tetrahydro-3-methoxynaphth-(2,3b)-oxirane Sandoz Pharma) used as an intermediate in the enantioselective synthesis of optically active drugs. The SMB has been used to carry out this chiral separation [27, 34, 35]. The separation can be performed using microcrystalline cellulose triacetate as stationary phase with an average particle diameter greater than 45 )tm. The eluent used was pure methanol. A... 

Since the addition of dialkylzinc reagents to aldehydes can be performed enantioselectively in the presence of a chiral amino alcohol catalyst, such as (-)-(1S,2/ )-Ar,A -dibutylnorephedrine (see Section 1.3.1.7.1.), this reaction is suitable for the kinetic resolution of racemic aldehydes127 and/or the enantioselective synthesis of optically active alcohols with two stereogenic centers starting from racemic aldehydes128 129. Thus, addition of diethylzinc to racemic 2-phenylpropanal in the presence of (-)-(lS,2/ )-Ar,W-dibutylnorephedrine gave a 75 25 mixture of the diastereomeric alcohols syn-4 and anti-4 with 65% ee and 93% ee, respectively, and 60% total yield. In the case of the syn-diastereomer, the (2.S, 3S)-enantiomer predominated, whereas with the twtf-diastereomer, the (2f ,3S)-enantiomer was formed preferentially. 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

Brunner, H., Enantioselective Synthesis of Organic Compounds with Optically Active Transition Metal Catalysts in Substoichiometric Quantities, 18, 129. 

The optically active propargylic and allylic alcohols thus obtained are important synthetic intermediates in the enantioselective synthesis of insect pheromones, prostaglandins, prostacyclins, and many other bioactive compounds (Scheme 6-26).53... 

Scheme 6.11. Tandem Zr-catalyzed kinetic resolution and Ru-catalyzed conversion of the resulting optically pure ethers in the enantioselective synthesis of dihydrofurans. The efficiency of the catalytic resolution requires the presence of the pendant acyclic alkene and depends on its substitution.

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]. 

Attempts have been made for the enantioselective synthesis of (—)-yohim-bane. Kametani et al. (216) reported the total synthesis of key intermediate (-)-368 starting from L-tryptophan, using enamide photocyclization. However, the optical purity of (—)-17-methoxyhexadehydroyohimbane obtained was only 17% owing to the partical racemization of intermediates throughout the reaction sequence. 

The main lines of this approach were later embodied in an enantioselective synthesis of (—)-a-allokainic acid (Scheme 34) (179). The sole stereo center of die ene reaction starting material was derived from a glutamic acid derivative (132) to avoid loss of optical activity via double bond migration (see Scheme 33), the a acid function of kainic acid had to be reduced before the pyrolysis step... 

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

These reflections are of special interest in the case of industrial syntheses in which the economic aspects are important. In these syntheses there is another factor to be kept in mind that may be illustrated by considering the industrial syntheses of steroids developed by Velluz and his coworkers in 1960 [18]. In contrast with other syntheses in which the intermediates are racemates and are only resolved into their optical active forms in the last step, the industrial syntheses require the resolution of the racemic mixture at the first possible opportunity, in order to exclude the unwanted isomer and thus avoiding the expenses of its processing. For recent advances in enantioselective synthesis see Heading 9.3. 

Ammonia lyases catalyze the enantioselective addition of ammonia to an activated double bond. A one-pot, three-step protocol was developed for the enantioselective synthesis of L-arylalanines 50 using phenylalanine ammonia lyase (PAL) in the key step (Scheme 2.20). After formation of the unsaturated esters 48 in situ via a Wittig reaction from the corresponding aldehydes, addition of porcine Ever esterase and basification of the reaction mixture resulted in hydrolysis to the carboxylic acids 49. Once this reaction had gone to completion, introduction of PAL and further addition of ammonia generated the amino acids 50 in good yield and excellent optical purity [22]. 

A useful method for the diastereoselective and enantioselective synthesis of trans-and m-l,2-disubstituted cycloalkanecarboxaldehydes was devised by Koga et al.1990 starting from cycloalkanecarboxaldehydes. (S)-/er/.-Leucine ter/.-butyl ester, a highly effective chiral auxiliary reagent, could be recovered for recycling without any loss of optical purity in a reaction sequence similar to that in the acyclic synthesis of (202). 

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