Chiral homoenolate

Stereoselectivity of chiral homoenolate equivalents in reactions of heterocycles 99S365. 

Allyltitanium complexes derived from a chiral acetal have been reacted with carbonyl compounds and imines [63], While the chiral induction proved to be low with carbonyl compounds, high induction was observed with imines. This complex represents the first chiral homoenolate equivalent that reacts efficiently with imines. Finally, the reactions with electrophiles other than carbonyl compounds and imines, namely a proton source, NCS, and I2, furnished the corresponding alkene, chloro, and iodo derivatives in good yields [64]. 

The optically active cyclopropane 10 gave the chiral homoenolate of isobutyrate Eq. (14), an ethereal solution of which is both chemically and configurationally stable for a week at room temperature, and consequently can serve as a useful chiral building block [23],... 

Metalated SMP allylamines or enamines have been used as the first chiral homoenolate equivalents (d synthons eq 5). ... 

Generation of Enantioenriched, Configurationally Stable Organolithium Reagents. (1 S,2E)-1 -(N.Af-Diisopropyl-carbamoyloxy)-l-methyl-2-butenyllithium-(—)-sparteine is configurationally stable in solution and is obtained by kinetic resolution of the racemic 2-alkenyl carbamate by n-butyllithium-(—)-sparteine with >80% de (eq 4). The enantioenriched allylstan-nane, obtained on y-stannylation, was used as chiral homoenolate reagent. The methoxycarbonylation (a, inversion) yields enantioenriched 3-alkenoates. ... 

Aldol-iype Condensation. Dimetalation of (R)-(+)-3-(p-tolylsulfinyl)propionic acid with Lithium Diisopropylamide produces a chiral homoenolate dianion equivalent which reacts with carbonyl compounds to afford p-sulfinyl-y-hydroxy acids these spontaneously cyclize to give the corresponding p-sulfinyl 7-lactones (eq 2) ... 

Several other chiral homoenolate anion equivalents have been successfully exploited for asymmetric homoaldoi reactions, e.g. (48) and (49), ° but their preparation seems more laborious than that of the carbamates (47). 

Solid-state photoreactions of heterocycles in two-component crystals 98S1. Stereoselectivity of chiral homoenolate equivalents in reactions of heterocycles ... 

Beak et al. reported that chiral homoenolate equivalents can be formed by di-lithiation of an amide and highly diastereoselective reaction with electrophiles to provide the benzylically substituted products [76]. Treatment of (S)-N-(T-phenylethyl)-3-phenylpropionamide 95 with 2.2 equivalents of sec-BuLi and TMEDA followed by addition of an electrophile affords the alkylated products 96 in 46-55% yield with 90 10-94 6 drs (Scheme 27). 

The chiral homoenolate (18) can be alkylated with high diastereoselectivity to provide an asymmetric synthesis of jS-alkyl-aldehydes, with enantiomeric excesses of up to 67% [equation (8)]. ... 

The species (24) is a chiral homoenolate anion equivalent, and reacts with alkyl halides to give chiral j8-substituted aldehydes after hydrolysis. The anion (25) can also act as a homoenolate anion equivalent or, alternatively, as an acyl anion equivalent. For instance, reaction at -78 C results in electrophilic attack at the a-position (acylanion equivalent), whereas at 0°C y-attack is observed (homoenolate anion equivalent). 

As part of a review on the use of metallated 2-alkenyl carbamates as chiral homoenolate reagents for asymmetric synthesis, the preparation of the a-D-allo cofigurated methyl-branched 3,6-dideoxy compound (56) as well as the fi-D-talo derivative (57) have been reported starting from R-benzyl lactaldehyde. For the preparation of the a-L-allo and the P-L-w/o compounds see Vol. 21, p.l44. 

An efficient asymmetric synthesis of 3-phenylalkanals has been developed, based on a chiral homoenolate equivalent, using a recoverable chiral auxiliary derived from atrolactic acid (Scheme 10). ... 

A-heterocyclic carbenes (NHC) are also efficient organocatalytic tools for generating homoenolate equivalents from a,P-unsaturated aldehydes. These reactive intermediates display a versatile reactivity in a number of catalytic transformations attesting to an important synthetic potential [38]. Recently, Scheldt et al. [39a] accomplished the first enantioselective protonation of a homoenolate species generated by a chiral NHC precursor 93 in the presence of DIE A and an excess of ethanol as the achiral proton source (Scheme 3.46). The suggested mechanism involves an initial addition of NHC 93 to the enal 89 followed by a formal 1,2-proton shift resulting in the formation of the chiral homoenolate equivalent 91. A diastereose-lective P-protonation/tautomerization sequence leads to the acyl triazolinium inter-... 

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