Preparation optically active

The present procedure is a convenient, one-step method of preparing optically active /raw.s -pin ocarveol. Although lower in yield than the lithium diethylamide procedure, it is more readily adaptable to large-scale work. Moreover, the two methods are complimentary in the conditions required (neutral vs. basic) and in the overall transformation accomplished ... 

Extensive work has been done by using buta-l,3-dienyl-glycosides of unprotected sugar to study aqueous Diels-Alder reactions and to prepare optically active oligosaccharides [20]. 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

The resolution of racemic esters via selective hydrolysis catalyzed by hydrolases is a practical way to prepare optically active pharmaceutical intermediates as shown in Figure 6.8 [53,54]. 

Chan et al. [38] prepared optically active atropoisomeric 2,2 -bipyridine by nickel(0)-catalyzed homo-couphng of 2-bromopyridylphenol derivatives (structure 28 in Scheme 16). Tested in the model test reaction, the copper catalyst led to frans-cyclopropanes as major products with up to 86% ee. 

Isopropyl (/ )-( —)-methylphosphinate (134) has been prepared" in > 90% optical purity by Raney nickel desulphurization of optically pure O-isopropyl (5)-(-f-)-methyIphosphonothioate (135). The phosphonate (134) is rapidly racemized by base, but not by acid, unlike secondary phosphine oxides"" [although whether these have been prepared optically active now seems doubtful (see Chapter 4)]. The phosphinate (134) can be reconverted into 89% optically pure (5)-( + )-(135) by addition of sulphur in dioxan. As shown in the Scheme, a series of interconversions has been used to establish the configurations. 

Although there were some trials to prepare optically active carboxylic acids via asymmetric decarboxylation, the optical yields of the products were not high enough for practical use. Thus, it is strongly desirable to find an enzyme which catalyzes asymmetric decarboxylation of arylmethylmalonates to give optically pure arylpropionates. 

Attempts to prepare optically active amines by replacing dppe with optically active diphosphines such as DIOP or DIPAMP in the Pd/dppe system [183] or by replacing PhP(Oi-Pr)2 with dimenthyl phenylphosphinite in the Ni/PhP(Oi-Pr)2 system [184] have met with little success. 

Several methods can be used to prepare optically active organotin compounds in which the tin atom is the only chiral center. 

In 1973, Taddei succeeded in preparing optically active (+)-benzyl-z-propylmethyl-phenyltin, (+)-(62), by a reaction of the optically unstable z -propylmethylphenyltin menthoxide with benzylmagnesium bromide U) (see Table 4). This asymmetric induction very probably does not yield an optically pure compound but is a very rapid and facile route to optically active organotin compounds of unknown optical purity. 

Pheromone synthesis was thoroughly reviewed in the past. In 1989, synthetic methods useful in preparing optically active pheromones were reviewed by Mori [1]. More comprehensive reviews are also available [2,3]. The present chapter follows the style adopted in Mori s encyclopedic review [3] to classify the pheromones according to the compound types. In comparison to 1229... 

Hu S, Tat D, Martinez C, Yazbeck D, Tao J (2005) An efficient and practical chemoenzymatic method for preparing optically active secondary amines. Org Lett 7 4329-4331... 

Among methods of preparing optically active cyclopropane compounds, the Simmons-Smith reaction, first reported in 1958, is of significance. This reaction refers to the cyclopropanation of alkene with a reagent prepared in situ from a zinc-copper alloy and diiodomethane. The reaction is stereospecific with respect to the geometry of the alkene and is generally free from side reactions in contrast to reactions involving free carbenes. 

In contrast to the allyltitaniums derived from acrolein cyclic acetals, such as 1,2-dicyclo-hexylethylene acetal shown in Scheme 9.8, those derived from acrolein acyclic acetals react with ketones and imines exclusively at the y-position. As shown in Eq. 9.29, the reaction with chiral imines having an optically active 1-phenylethylamine moiety proceeds with high diastereoselectivity, thus providing a new method for preparing optically active 1-vinyl-2-amino alcohol derivatives with syn stereochemistry [53], The intermediate allyltita-nium species has also found use as a starting material for a carbozincation reaction [54],... 

In summary, the asymmetric hydrogenation of olefins or functionalized ketones catalysed by chiral transition metal complexes is one of the most practical methods for preparing optically active organic compounds. Ruthenium and rhodium-diphosphine complexes, using molecular hydrogen or hydrogen transfer, are the most common catalysts in this area. The hydrogenation of simple ketones has proved to be difficult with metallic catalysts. However,... 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

The use of alkaloids in preparing optically active organic compounds has a venerable past. One hundred and thirty-two years ago Pasteur (1,2) used derivatives of quinine to effect the resolution of racemic acids, and even today the use of alkaloids in resolutions remains widespread (3). 

Finally, after a brief look at the structure and mechanism of action of enzymes, most of the chapter will be devoted to the innovative and promising field of catalytic antibodies or abzymes, which will probably be the method of choice for preparing optically active compounds in the future. 

Lipase has been used in organic solvents to produce useful compounds. For example, Zark and Klibanov (8) reported wide applications of enzymes to esterification in preparing optically active alcohols and acids. Inada et al (9) synthesized polyethylene glycol-modified lipase, which was soluble in organic solvent and active for ester formation. These data reveal that lipases are very useful enzymes for the catalysis different types of reactions with rather wide substrate specificities. In this study, it was found that moditied lipase could also synthesize esters and various lipids in organic solvents. Chemically moditied lipases can help to solve today s problems in esteritication and hopefully make broader use of enzymatic reactions that are attractive to the industry. 

Since Marshall s strategy55 in preparing optically active betweenanenes was asymmetric epoxidation by (-t-)-monoperoxycamphoric acid, this approach would be solely effective in [m.n]betweenanenes where the m ring is large enough to sway away, so exposing the buried unsaturated center. 

---