Chiral alkoxide

An increasing number of examples of ring formation through 1,5-electrocyclization of appropriate carbanions are illustrated in Scheme 27. In the last example the use of a chiral alkoxide (R = menthyl or bornyl) results in the formation of chiral indolines with optical purities ranging from 17 to 31%. 

The mechanism of this one-pot reaction is proposed to be as follows (Figure 4.3) firstly, a chiral alkoxide ethylzinc is prepared from diethylzinc and the chiral alcohol with the evolution of a gas, which is probably ethane (I). The chiral ethylperoxyzinc alkoxide is formed by insertion of oxygen into the carbon zinc... 

Chiral active pharmaceutical ingredients, 18 725-726. See also Enantio- entries Chiral additives, 6 75—79 Chiral alcohols, synthesis of, 13 667-668 P-Chiral alcohols, synthesis of, 13 669 Chiral alkanes, synthesis of, 13 668-669 Chiral alkenes, synthesis of, 13 668—669 Chiral alkoxides, 26 929 Chiral alkynes, synthesis of, 13 668-669 Chiral ammonium ions, enantiomer recognition properties for, 16 790 Chiral ansa-metallocenes, 16 90 Chiral auxiliaries, in oxazolidinone formation, 17 738—739... 

The above two quinazolinones were prepared as intermediates in the synthesis of the chiral nonnucleoside reverse transcriptase inhibitor DPC 961 441, although compounds of this type can also be formed directly by the addition of lithium cyclopropylacetylide to the N-unsubstituted 2(l//)-quinazolinone 440, in the presence of a chiral alkoxide moderator <20000L3119, 2004JA5427>. 

JCS Perkin I 197 (stereochemistry using chiral alkoxide ligand), 979 (stereochemistry) (1982)... 

A stereoselective enolate protonation has been achieved by changing the counterion of the chiral alkoxide base employed the lithium alkoxide-generated enolate gives close to 90% of the /i-cpimcric ketone product, whereas the use of the potassium cation gives 99% a-epimer.293... 

Okuda and Arnold have reported a series of 5- and 6-coordinate chiral indium complexes (e.g. 12), analogous to those previously discussed with yttrium (Table 4), which show varying activities for the polymerization of rac-lactide [71]. The homoleptic species was fluxional, interconverting between a 5-coordinate and fac- and mer-isomers of a 6-coordinate complex. It resulted in controlled polymerization and some stereoselectivity (max. P, = 0.63 and max. Ps = 0.64). MALDI-ToF mass spectrometry confirmed that the chiral alkoxide ligand was an initiating group. 

Fujiwara has reported a unique chiral lanthanoid(II) alkoxide-promoted asymmetric Mukaiyama aldol reaction.38 Stoichiometric amounts of the chiral alkoxide, however, were required for good enantioselectivity. 

Fig. 13.42. Helmchen synthesis of enantiomerically pure a-alkylated carboxylic acids. The deprotonation of the propionic acid ester results in the "f "-enolate in the solvent THF and in the "Z"-enoLate in the solvent mixture THF/HHPA. In these projections, both enolates react preferentially from the front. The "f" -enolate results in a 97 3 mixture of 5- and fi-configured a-benzyl-propionic acid esters (X marks the chiral alkoxide group), while the "Z"-enolate results in a 5 95 mixture. Chromatographic separation and reduction of the C(=0)—Xc groups afford alcohol B with 100% ee from the " "-enolate and alcohol ent-B with 100% ee from the "Z"-enolate.

Chiral alkoxides are also obtained from the analogous aldehydes albeit with lower selectivity. Methyl ketones are selectively attacked on the face, unblocked by the bulky PPhs. The obtained ee values match or exceed the ones found with other classic enantioselective reductants. 

Chiral alkoxides are reduced with retention of configuration reduction of chiral halogermanes results in inversion of configuration. ... 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

The simple chiral lithium alkoxide 5 was applied to the Michael reaction of methyl phenylacetate with methyl acrylate to give the corresponding adduct in 84% ee (Eq, (12.6)) [19]. The catalytic enantioselective Michael reaction was also effected by the chiral alkoxide 5. 

Tab. 5.1 Representative examples of highly tactic polymers prepared from chiral alkoxides and phenoxides, initiators, and monomers.

Table 2. Chiral Alkoxides, Initiators, Monomers, and Properties of the Resulting Polymers "...

Apparently, the unreactive species formed after 50% conversion contains one equivalent of the product alcoholate of 27, one equivalent of the remaining lithium acetylide 25 and two equivalents of the chiral alkoxide 26, forming a mixed 3 1 alkoxide-acetylide tetramer, which shows a similar reactivity as 29. The product alcoholate of 27 was suggested to exhibit chelating abilities similar to the amino alcoholate 26. Then 26 and lithiated 27 could block via chelation the lithiums close to the acetylide moiety and cause the observed low reactivity, which is also apparent in 29. However, lithiated [ ketoaniline 27 did not show... 

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