Mismatched case

Aliphatic, aromatic and vinylic aldehydes can be employed in this reaction with similar yields and enantioselectivities. When chiral aldehydes are utilized, excellent diastereoselectivity is obtained for matched cases, whereas mismatched cases yield products with moderate to good diastereoselectivity (Scheme 9.13a) [67]. The limitation of the methodology is that only terminal vinylepoxides can be obtained. 

For example the aldol addition of (S)-2-cyclohexylpropanal is more stereoselective with the enolate (S)-5 than with the enantiomer (R)-5. The stereoselectivity of these cases derives from relative steric interactions in the matched and mismatched cases. 

Figure 33. Dimensionless spoutdiametersasafunctionof dimensionless height for small columns. Case A test case Case B all dimensionless parameters matched, bed diameter halved Case C particle Reynolds number mismatched Case D Froude number mismatched Case E density ratio, Reynolds number mismatched Case F bed Reynolds number mismatched Case G internal friction angle, loose packed voidage mismatched. (From He et al., 1995.)...

Scheme 13 Matched and mismatched cases of regioselectivity in the opening of 32 by 22...

In studies directed toward the total synthesis of tedanolide [38], the addition of y-substituted ketene acetal 60 to aldehyde 84 generated unsaturated ester 85 in 62% yield (Scheme 30). The resulting double bond could be further functionalized by applying Sharpless asymmetric dihydroxylation conditions. Hence diol 86, which represents the mismatched case due to unfavorable... 

The combination of the geometrical preference of the tether and the stereochemical preference of the dipolarophile substituent can be seen in the intramolecular cycloadditions of alkyl nitronates, (Scheme 2.6) (99). When the tether is restricted to two atoms, only the endo approach of the tether is observed in up to a 100 1 ratio, independent of the configuration of the disubstituted dipolarophile. However, in the case of a three-atom linker, there exists a matched and mismatched case with respect to the observed stereoselectivities. With a (Z)-configured dipolarophile, only the exo isomer was observed since the ester moiety also approaches on the exo to the nitronate. However, with an ( )-configured dipolarophile, the ester group is forced to approach in an endo manner to accommodate an exo approach of the tether, thus leading to lower selectivity. 

Experiments with the enantiomerically pure enolate of 6 and bromide 7 revealed that (R)-6 and (S)-7 constitute a matched pair of reactants, while (S)-6 and (S)-l are a mismatched pair66. The total lack of diastereoselection in the mismatched case was attributed to competitive racemization of 7 by liberated bromide ion which had time to occur because of a slower reaction rate between the mismatched reactants. 

In the mismatched case the phenyl substituent hinders the approach of the electrophile from the face anti to the C4-substituent, thus a lower selectivity is obtained. 

Consequently, matched/mismatched cases [25] can result, and indeed our investigations on cooperative effects of stereogenic elements in such systems revealed 9 be the matched case and 23 (which is also easily prepared by following a directed deprotonation-silylation-deprotonation-trapping-desilylation sequence [11]) to be the mismatched case in diethylzinc additions to aldehydes [26]. Later, these investigations were extended to more complex systems such as 24 [27], but ferrocene 9 still remains superior to all other compounds. 

Fig. 7. The evolution of the harmonic power with propagation distance (arbitrary units) for ADM,WBM and MD phase-matching (solid line). The Cerenkov power changes from quadratic (solid line) to linear long (dashed line) growth. For QPM for alternate sections with no X1-2 1 (short dashed line). The phase-mismatched case is also shown (dotted line)...

The mismatched case of a Still-Gennari olefination, which corresponds to the matched case of Figure 9.18, can be found in Figure 9.19. There, reagent control and substrate control act against each other. The substrate in Figure 9.19 is the enantiomer... 

The reactions of titanated 133 and 134 with (R) and (5) TBDMS-protected lactaldehyde [MeCH(OTBDMS)CHO] are also highly diastereoselective. Very high levels of diastereoselectivity (> 98%) were observed when the facial selectivity of the allylic sulfoximine anion matched that of the chiral aldehyde (the matched case).86,87 In the mismatched cases the diastereoselectivities were less but still... 

The reagent s moderate facial preference makes it an ideal choice for illustrating the concept of double asymmetric induction. The chiral lithium (Z)-enolate, which also exhibits a moderate enantiofacial preference in reaction with achiral aldehydes, reacts with the reagent to afford a 61 28 ratio of products (eq 6). This mismatched case of asymmetric induction indicates that the facial preferences of the two compounds are working at crosspurposes. With the reagent s enantiomer, (45)-2 -dimethyl-l,3-dioxolane-4-carboxaldehyde, a greater than 97 3 ratio of products is obtained, indicating matched facial preferences. 

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