Enolates configurational stability

T(2-Alkylcycloalk-l-enyl)methyl carbamates of type 56 are useful 1,2-dianion synthons that can be combined with two aldehydes in adjacent positions to provide a versatile synthesis of [fjannulated tetrahydrofurans (Scheme 81). At first, a carbanion of carbamate 56, which exhibits considerable configurational stability, is generated by (—)-sparteine-mediated deprotonation this is then converted to an optically active homoaldol product 57 with up to %% ee. An (it)-oxonium ion, which is subsequently formed under the influence of BF3, undergoes an intramolecular Mukaiyama-type addition of the enolic moiety onto the carbonyl group of a second aldehyde in the least-hindered conformation. Finally, the carhamoyl group is extmded, and after aqueous workup, diastereomerically pure tetrahydrofurans can be isolated <2005ASC1621>. 

The research group of Trost has shown that stabilized enolates derived from -diesters and a-phe-nylsulfonyl esters are excellent nucleophiles for palladium(0)-catalyzed reactions with allylic acetates. Since these reactions involve ir-allylpdladium intermediates, they proceed with net retention of configuration and, therefore, differ from traditional displacement reactions where inversion is the rule. Moreover, the transition metal imposes a template effect on intramolecular versions of these reactions and... 

Those decarboxylases involving net retention of configuration generally require catalytically essential divalent metal ions that may be directly involved in stabilizing enol intermediates generated from the decarboxylation step. In the case of malic enzyme, an active site divalent metal ion (Mn ) has been implicated to be near bound substrate, on the basis of NMR paramagnetic relaxation... 

There are two other reactive species whose cis-trans isomerism can be important to the distribution of products, organometallics derived from vinyl halides, and enolate anions. The configurational stability of these species will be discussed in detail in Sections 8.4.F, 8.5.D and in Sections 9.2.C, 9.5, respectively. The... 

In the key step of this convergent synthesis, chiral enamine reacts with immo-nium salt, affording a chiral Mannich base with 100 % e.e. Due to the tendency to enolization, this intermediate has limited configurational stability. Reduction under mild conditions by a two-step protocol afforded S)-fenpropimorph with 95.1 % e.e. 

The preferential -configuration of the enol esters, derived from p-dicarbonyl compounds under phase-transfer conditions, contrasts with the formation of the Z-enol esters when the reaction is carried out by classical procedures using alkali metal alkoxides. In the latter case, the U form of the intermediate enolate anion is stabilized by chelation with the alkali metal cation, thereby promoting the exclusive formation of the Z-enol ester (9) (Scheme 3.5), whereas the formation of the ion-pair with the quaternary ammonium cation allows the carbanion to adopt the thermodynamically more stable sickle or W forms, (7) and (8), which lead to the E-enol esters (10) [54],... 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

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