Benzylic chiral compounds

It is known that electrochemical reduction of oximes in protic media occurs in two steps the N—O bond is first reduced to form an imine and the latter is then reduced to afford a primary amine1,29. Tallec has shown that the amine from oxime 33 can be trapped intramolecularly (equation 16)35. Interestingly, the SS diastereomer predominates the chiral pyrrolidine ring derivative serves to control the stereochemistry of formation of the new benzylic chiral center. Electrochemical reductive cross-coupling of O-methyl oximes with carbonyl compounds in isopropanol at a tin cathode affords adducts (equation 17) which can be reduced further to 2-amino alcohols36. In this fashion, menthone could... 

It is presumed that the initial step for the intermolecular as well as intramolecular carbolithiation is an energetically favorable coordination of the lithium atom with the n-system, which serves to establish the geometry of the system prior to addition. The chiral benzylic organolithium compound 45, obtained after the carbolithiation step, reacted diastereoselectively with a number of electrophiles, yielding a formal inversion of the configuration. 

The presence of a free alcohol is not essential as a similar enantiomeric excess is obtained with a rerr-biityl ether or dialkylamines [128], The chiral, thermodynamically favored benzylic organolithium compound 92Li, obtained after the carbometallation step, may react with a number of electrophiles in a highly diastereoselective manner with a fornial inversion of configuration [84, 85] as already described in Scheme 7-71. In all cases examined, the product is obtained in a diastereomeric ratio of >98 2 prior to purification and in 83% ee. Thus, two chiral centers in an acyclic system have been created in a singlepot operation. 

The research group of Muzart and Henin studied extensively the palladium-catalyzed EDP of allyl- or benzyl-carboxylated compounds. Mainly two types of substrates, prochiral enol carbonates A and racemic (3-keto esters B, were used to afford enols C as transient species [25]. In the presence of a chiral proton source, asymmetric protonation/tautomerization of enols led to enantioenriched ketones D... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

Alcohol benzylation. The derivatization is catalyzed by TMSOTf, which does not cause racemization of sensitive chiral compounds. 

Since the Betti reaction results in a product with a benzylic chiral centre, several studies have been reported whereby the product enantiomers are resolved. Betti reported in 1906 that his base 10 could be classically resolved following separation of the corresponding tartaric acid salts. More recently Hu and co-workers have reported a high yielding and scaleable modification to this procedure by using tartaric acid in acetone. In acetone, two different compounds were formed, the (/ )-enantiomer giving the aminal 19 and the (iS)-enantiomer giving the tartaric acid salt 20. 

Small rotations are also observed for another interesting group of chiral compounds those which are chiral due to the presence of isotopes, llie chiral benzyl alcohol (24) and the diphenylmethanol (25) have both been obtained as pure enantiomers and show rotations of +0.7 and -1.0 respectively. Even the acetic acid (26) chiral due... 

After extraction of the fresh samples of cherries, 35 volatiles were identified by the GC-MS analysis of the flavor extract. The main components of the flavor extracts were hexanal, 3-methyl butanol, limonene, /> nn5-2-hexenal, 1-hexanol, cw-3-hexen-l-ol, tram-3-hexen-l-ol, /ranj -2-hexen-l-ol, linalool, benzaldehyde, 1-octanol, benzyl acetate, benzyl alcohol, and a-terpineol. Due to the nature of the static vacuum extraction technique and the use of a high boiling solvent (iso-octane), compounds more volatile than hexanal were lost during concentration of the solvent extract in a rotary evaporator. This is one of the disadvantages of the static vacuum SDE technique and alternative methods for the analysis of highly volatile chiral compounds need to be used ( e.g., dynamic headspace followed by MDGC). 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Diaziridines also show slow nitrogen inversion, and carbon-substituted compounds can be resolved into enantiomers, which typically racemize slowly at room temperature (when Af-substituted with alkyl and/or hydrogen). For example, l-methyl-3-benzyl-3-methyl-diaziridine in tetrachloroethylene showed a half-life at 70 °C of 431 min (69AG(E)212). Preparative resolution has been done both by classical methods, using chiral partners in salts (77DOK(232)108l), and by chromatography on triacetyl cellulose (Section 5.08.2.3.1). 

In addition reactions to chiral carbonyl compounds, the stereochemical course taken by resonance-stabilized alkali metals or magnesium benzyl anions resembles that taken by localized carbanions of similar bulk. Thus, conditions can be delineated which lead to either the steric approach or chelation control the following serve as examples. 

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

Durst and coworkers were the first to report the condensation of chiral a-sulphinyl carbanions with carbonyl compounds . They found that metallation of (+ )-(S)-benzyl methyl sulphoxide 397 followed by quenching with acetone gives a mixture of dia-stereoisomeric S-hydroxy sulphoxides 398 in a 15 1 ratio (equation 233). The synthesis of optically active oxiranes was based on this reaction (equation 234). In this context, it is interesting to point out that condensation of benzyl phenyl sulphoxide with benzaldehyde gave a mixture of four j8-sulphinyl alcohols (40% overall yield), the ratio of which after immediate work-up was 41 19 8 32 . 

In a-methyl-benzylcalcium and a-Me3Si-benzylcalcium complexes, energy barriers for inversion of the chiral benzylic carbon (17-19 kcal mol-1) are concentration independent, suggesting that a dissociative mechanism is involved that involves Ca-Ca bond breakage. The a-methyl-benzylcalcium compounds are less stable and show a... 

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