Benzaldehydes, diastereomeric, asymmetric

Recently, much attention has been devoted to the use of car-banions derived from optically active a-substituted sulfoxides in asymmetric synthesis. Thus, the condensation of the lithium salt of the optically active dithioacetal mono-5-oxide 51 with benzaldehyde yields a mixture of diastereomeric adducts 307, which were converted in high chemical yield into the corresponding optically active a-methoxyaldehyde 308, having 70% optical purity (316). 

Chiral enamine derivatives have also been used as electron-rich alkenes. The oxazoline derivative 17 reacted with benzaldehyde to yield the two stereoiso-meric oxetanes 18a and 18b with a diastereomeric excess of 67% (Scheme 5) [12]. A significantly higher diastereoselectivity was observed in the case of the reaction of the pyrrolidine derivatives 19 where the enamine function is localized inside the five membered ring [13]. Then the oxetane 20a (R — n-Cgil g) was used in an asymmetric synthesis of the antifungal alkaloid (+ )-preussin. The approach of the 3n,7T excited ketone preferentially occurred syn with respect to... 

Synthesis of optically active alcohols.1 For asymmetric synthesis of alcohols, the carbanion salt (2) is generated from optically active (R)-methyl p-tolyl sulfoxide (I).2 The optically active carbanion reacts with benzaldehyde (3) to give a 1 1 diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxides (4a) and (4b) in 84% yield-These can be separated by silica gel chromatography and fractional crystallizations to give (4a, oq, + 91.7°, 17% yield) and (4b, aD + 202.8°, 15.5% yield). Raney nickel desulfurization of (4a) and (4b) gives (SH-)-l-phenylethanol (5a, aD — 42.6°) and R-(+)-l-phenylethanol (5b, aD +42.1°), respectively, in about 60% yield. Since the specific rotation of optically pure (5b) is aD +43.5°, alcohols of high optical purity can be obtained in this way. 

Various ferrocene-based organosilanols 165 have been synthesized in two steps fi om chiral 2-ferrocenyl oxazolines 163. Diastereoselective ortho-lithiation with sec-BuLi followed by electrophilic attack with chlorosilanes gave diastereomerically enriched 164, which were oxidized in air with [IrCl(C8Hi2)]2 as catalyst to give, after purification, stereochemically homogeneous samples of 165. Their application in asymmetric phenyl transfer reactions to substituted benzaldehydes afforded products with high ee (up to 91%) <050L1407>. 

A soln. of benzaldehyde and phenyl isoeyanide allowed to stand with (-)-menthoxyacetic acid at —5 for 4 days, and another 2 days after dilution with ether—> (-)-menthoxyacetylmandelic acid anilide. Crude Y 80% pure Y 56%.—The reaction is subject to asymmetric induction and yields one diastereomeric form in excess. It may be used, in the Marckwald sense, for the synthesis of optically active a-hvdroxy acids. (F. e. and limitations s. R. H. Baker and L. E. Linn, Am. Soc. 70, 3721 (1948).)... 

Additionally, the Chinese group realized a lOOmmol-scale example of this asymmetric synthesis, starting from p-NOj-benzaldehyde achieving comparable ee (97% ee) and yield (90%), despite a decrease in the diastereomeric ratio (dr 11 1) [29],... 

Diastereomeric phosphoramides have been employed to catalyse the asymmetric aldol addition of trichlorosilyl enolates to benzaldehyde. Good anti/syn product ratios were achieved, but these were reversed on employing a more hindered catalyst, and the ratios were also affected by the catalyst concentration. A mechanistic switchover is proposed one transition-state geometry involves a 1 1 complex (cat-alyst enolate) favoured by a hindered catalyst in low concentration, while the other route involves a 2 1 stoichiometry. 

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochi-ral and therefore the corresponding chromium tricarbonyl compounds are chiral. (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25). Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26). The optically active chromium complexes can be prepared by di-astereoselective ortho lithiation of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation of the chiral ort/io-substituted benzaldehyde am-inals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes. The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions. ... 

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