Asymmetric Synthesis by James Morrison (Eds.)

By James Morrison (Eds.)

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Ratios determined by N M R s p e c t r o s c o p y . a b >97 : 3 - 1 3 1. 49 Alkylation of Chiral Enolates ester enolates (entries A-C). As expected, syn-substituents on the 7position of this ring system effectively screen the exo-enolate face (Table 16, entry J). In these two reported examples endo-directed, electrophile attack exceeded 97 : 3 (133). Before leaving the discussion of endocyclic and exocyclic enolates, attention should be drawn to the unprecedented observations reported on the alkylation of the enolates derived from acetonides 37 and 38 [Eq.

One classical enolate system is shown in Eq. (28) (83, 84). In the conformational^ cox 28A locked enolate 27, alkylation can occur either via electrophile approach from the equatorial (path E) or axial (path A) face of the cyclohexylidine ring system. In both diastereomeric transition states leading to products 28E and 28A, the imposed geometric constraints imparted by stereoelectronic factors are essentially equivalent, assuming a reactantlike transition-state geometry. 7 depending on both enolate and electrophile structure (83, 84).

Again assuming reactantlike transition-state geometries, equatorial 7r-facial selection to give diastereomer 36 should be strongly favored due to the added steric hindrance imposed by the 3-axial substituent (R = Me, OMe) on the axial πface. In fact, conformational^ locked cyclohexylidine-derived enolates possessing 3-axial substituents exhibit exceptional levels (^30:1) of 1. Alkylation of Chiral Enolates 35 TABLE 12 Diastereoselective Alkylation of Substituted Cyclohexylidine Enolates: 1,3- and 1,4-Asymmetric Induction Entry Reaction C 0 2R A Reference Me C 0 2R Me^ f^N LDA I M e ^ - ^ M e CO2R e ^ - ^ M e 84 ^"NVIE -75 C R = H R = Me C 0 2R Β Me C 0 2R Me ί1 [Ί ^ " ^ O M e _ 7 5.

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