Chirality pseudo-chiral centers

Johnson rearrangement of allylic alccdiols (274) and (277) led to the enantiomeric 7,8-unsaturated esters (276) and (279), respectively (Scheme 48). Both transition states (275) and (278) favor a pseudo-equatorial position of the benzyloxymethylene substituent the newly formed chiral center is obtained in very hi optical purity throu the chirality transfer process. As a consequence of this geometrical preference, secondary allylic alcohols invariably provide predominantly ( )-configured double bonds upon thermal Gaisen rearrangement The ( -selectivity usually increases with the steric bulk of the C-2 substituent, an effect which was rationalized by a pseudo-1,3-diaxial interaction in the transition state leading to the (Z)-alkene (280 Figure S). ... 

In addition to the R and S designations, compounds with two chiral centers may also be identified by stereochemical nomenclature that describes the entire system. For example, the erythro and threo nomenclature derived from carbohydrate chemistry may be employed to describe the relative positions of similar groups on each chiral carbon. Thus, the ephedrines are designated as erythro forms since the similar groups (OH and NHCH3) are on the same side of the vertical axis of the Fischer projection, and the pseudo-ephedrines are designated as threo forms since like groups are on opposite sites of the vertical axis of the projection (Fig. 10). 

If every tertiary carbon atom in the chain is asymmetric, one might expect the polymer to exhibit optical activity. Normally, homoatomic carbon chains show no optical activity because two long chains constitute part of the group variatious as these become longer (and more alike) in relation to the chiral center, the optical activity decreases to a vanishingly small value. Vinyl polymers derived from (CH2=C XY) monomers fall into this category as they are centrosymmetric relative to the main chain, and the tertiary carbons arc thrai only pseudo-asymmetric. 

When the crystal of [/ -l-(methoxycarbonyl)ethyl](cyclopentylamine)-cobaloxime, (i -l-mce)(cpa)cobaloxime, 8, was exposed to visible light, another mode of racemization was observed [42], The crystal belongs to the monoclinic system and the space group is P2i. There are two crystaUographically independent molecules, A and B, and one solvent benzene molecule in the asymmetric unit as shown in Fig. 3.32. There is a pseudo inversion center between A and B. A disordered benzene molecule is situated on the pseudo inversion center between the chiral 1-mce groups of A and B. The molecular stractures of A and B are shown in Fig. 3.33. The 1-mce groups of A and B take syn and anti conformations, respectively. This structure is very similar to that of the previous cha complex except that the two 1-mce groups have different conformations. 

Assign R ov S configuration to each chirality center in pseudo-ephedrine, an over-the-counter decongestant found in cold remedies. 

QN and QD consist of a planar quinoline and a rigid quinuclidine ring and are different in the configuration of Cs and C9 chiral centers, thus diastereomers to each other (Fig. 9a). Interestingly, QN and QD CSPs display pseudo-enantiomeric property in most cases. It means that an opposite elution order can be obtained when the same enantiomeric pair is separated on both CSPs. This property is likely due to... 

Clearly, upon using the enantiomeric catalyst [(S,S) instead of (R,R)] the opposite enantioselectivity of the overall process results. However, this effect is also seen with catalysts that are of analogous configuration, but not derived from trans-1,2-diaminocyclohexane (DACH). For example, the pseudo-ephedrine derived catalyst shown in Scheme 5, having (5)-configuration at the centers of chirality, shows some preference for the (5)-azlactone kinetically favors the (5)-azlactone in alcoholytic ring opening [37]. 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

---