Second-order asymmetric induction

More recently, Szdntay and co-workers (232) succeeded in performing the enantioselective total synthesis of both natural and unnatural antipodes of yohimbine and -yohimbine. Utilizing second-order asymmetric induction, either the levorotatory or dextrorotatory antipode of key tetracyclic intermediate 400 could be obtained by the use of (—)- or (+)-tartaric acid, respectively. 

The first example of an asymmetric induction at tetragonal silicon was reported by Klebe and Finkbeiner42 and is shown in equation 3. The reaction of a prochiral bis-(acetamido)silane with optically active amino acids led to two diastereomeric 2-silaoxa-zolidones in unequal amounts. These diastereomers were shown to undergo a second-order asymmetric transformation crystallization was accompanied by a rearrangement of the less abundant into the more abundant diastereomer. From the silaoxazolidones, alcoholysis reactions yielded an optically active dialkoxysilane. 

If stoichiometric quantities of the chiral auxiliary are used (i.e., if the chiral auxiliary is covalently bonded to the molecule bearing the prochiral centres) there are in principle three possible ways of achieving stereoselection in an aldol adduct i) condensation of a chiral aldehyde with an achiral enolate ii) condensation of an achiral aldehyde with a chiral enolate, and iii) condensation of two chiral components. Whereas Evans [14] adopted the second solution, Masamune studied the "double asymmetric induction" approach [22aj. In this context, the relevant work of Heathcock on "relative stereoselective induction" and the "Cram s rule problem" must be also considered [23]. The use of catalytic amounts of an external chiral auxiliary in order to create a local chiral environment, will not be considered here. 

A conceptually different approach to interligand asymmetric induction uses chiral phase transfer catalysts. Scheme 3.26 illustrates two examples of such a process using an A -benzylcinchonium halide catalyst. The first is an indanone methylation [150] and the second is a glycine alkylation [151]. Hughes et al. reported a detailed kinetic study of the indanone methylation which revealed a mechanism significantly more complicated than a simple phase-transfer process the reaction is 0.55 order in catalyst and 0.7 order in methyl chloride, deprotonation of the indanone occurs at the interface, and methylation of the enolate (not deprotonation) is rate-determining [150]. Nevertheless, the rationale for the... 

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