Enolate aluminium

Prior to the advent of triphenylphosphine-stabilized CuH [6a, b, 13], Tsuda and Saegusa described use of five mole percent MeCu/DIBAL in THE/HMPA to effect hydroalumination of conjugated ketones and esters [26]. The likely aluminium enolate intermediate could be quenched with water or TMSCl, or alkylated/acylated with various electrophiles (such as Mel, allyl bromide, etc. Scheme 5.5). More... 

Fig. 19. The polymerization flask designed for the synthesis of high-molecular-weighi (Mn>500,000) poly(methyl methacrylates) by the high-speed living polymerization with the (porphyrinato)aluminium enolate (2)-Lewis acid (3e) systems...

The stereoselective synthesis of /(-branched a-halocarboxylic acids containing two newly formed chiral centres (155) has been accomplished by a reaction consisting of 1,4-addition of dialkylaluminium chlorides to a,/(-unsaturated A -acyloxazolidinones (154) followed by quenching the intermediate aluminium enolate with /V-halosuccini-mides. The most efficient stereo-control was achieved with oxazolidines derived from glucosamine (154). Although /(-branched aliphatic a-halo carboxylic acids were synthesized stereo selectively, the highest stereoselectivity was observed for (3-aryl substrates.112... 

The regioselectivity of the double nucleophilic addition of ketene silyl acetals to a,/3-unsaturated imines has been found to be highly dependent on the subtle difference in the reactivities of the ketene silyl acetals the factors are mainly derived from the ability of the ketene silyl acetals to undergo the silicon-aluminium exchange reaction, where the aluminium enolate preferentially undergoes 1,4-addition.209... 

Aldol reactions of boron enolates are frequently more diastereoselective than aldol reactions of, for example, lithium or aluminium enolates. This is partly ascribed to the relatively short boron-oxygen bond length (B-O = 1.36-1.47 A, Li-0 = 1.92-2.00 A, Al-0 = 1.92 A) which exacerbates the unfavourable 1,3-diaxial interactions that occur between the boron substituent... 

The next step must be the restoration of the aromaticity of the ring by the removal of the proton at the site of attack. This gives the aluminium enolate of the ketone. There is a proton now available to convert the aluminium enolate to the ketone and this is the final product. This is a useful reaction because it has added a benzene ring to a quaternary carbon atom—conjugate addition has overcome steric hindrance. 

The synthesis was planned around the reaction of a specific enolate of ester 136 with the epoxide 137. This reaction was expected to give mainly trans 138 and is chemoselective both because of the usual enolate problem and because 137 contains a terminal alkyne. The lithium enolate was too basic and the aluminium enolate was used instead. The reaction gave an 85 15 mixture of trans and cis 138 and also an 85 15 mixture of trans and cis 139 after cyclisation. Dihydroxylation by osmylation gave a mixture of diols 140 this was deliberate so that they could determine the stereochemistry at C-2 . To the surprise of the chemists, natural rubrynolide was identical to one of the minor (i.e. cis) diols in the 15% part of the mixture. Careful NMR analysis showed that it was 135a. 

Yields of the cyclized product are in the range 49—69%. Cyclization of a-bromo-carboxylates of o>-hydroxy-aldehydes to macrolides by an intramolecular aldol addition has been carried out by formation of the aluminium enolate (Zn, Et2Al-Cl) yields vary between 35 and 68%. 

Diisobutylaluminium hydride lieri-butyllithium I cuprous iodide Stereospecific reductive generation of aluminium enolates from a,p[Pg.136]

Et2AlCl-Assisted Carbonyl Addition. A new method [111] for the preparation of jS-hydroxy-ketones involves the coupled attack of dialkylaluminium chloride and zinc on an a-bromoketone. An aluminium enolate is generated which gives a facile addition to another carbonyl compound, present in the system (illustrated below) ... 

The formation of Z in the cyclane acylation can be tentatively explained by the obtention of the aluminium enolate from the first intermediate of the reaction followed by an additional acylation. A possible alternative process is the acylation of the 8, Y-ethylenic ketone. But the elimination during the acylation reaction is a known step. 

Furthermore, three-component coupling tandem Michael-aldol reactions were achieved by trapping the aluminium enolate intermediate with an aldehyde (Scheme 19.16a). Initiated by the asymmetric Michael addition of malonic esters to cyclic enones, several natural products were synthesised... 

Scheme 6 In situ trapping of aluminium enolates by Alexakis [36]...

The aluminium enolates generated after ECA do not react directly with electrophiles, probably due to their high stability. However, they can be trapped in situ by silylation, carbonation and 0-acylation in good yields (Scheme 6). These intermediates 7-9 can eventually be used in Tsuji reactions or ozonolysis, for example, to generate more elaborated adducts [36]. 

The 1,4-addition of dimethylphenylthio-aluminium to the enone (120) affords the corresponding aluminium enolate, which undergoes intramolecular aldol condensation leading to fused-ring cyclohexenones, and the inherent... 

Maruoka et have utilized an intramolecular aldol coupling in their synthesis of pentadecanolide (465), which arose from an extension of work on the use of aluminium enolates derived from a-halocarbonyl compounds (Scheme 68). 

Aldol Reactions.—A new highly effective aldol synthesis employs an aluminium enolate derived regiospecifically from the appropriate a-halogeno-ketone using zinc and dialkylaluminium chloride (Scheme 18). ... 

Trapping of the initially formed aluminium enolate from a,/8-unsaturated ketones and diethylaluminium cyanide, with chlorotrimethylsilane-pyridine yields the corresponding silylenol ethers (35), which can be alkylated or phenyl-sulphenylated (Scheme 22). ... 

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