Enolates, aluminum boron

The lithium enolate generated using lithium diisopropylamide [4111-54-0], lithium 2,2,6,6-tetramethylpiperidide [58227-87-1], or lithium hexamethyldisilazide [4039-32-17 is a chemical reagent that reacts with other reactants to give a variety of products (37). In the quest for improved stereospecificity, enolates with different cations such as silicon, aluminum, boron, and zinc have also been used (38). In group transfer polymerization, ketene silyl acetals, eg, (CH3)2C=C [OSi(CH3)3] (OCH3) are employed as initiators (39). 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

BicycIo[4.2.0]octan-2-ones were cleaved by Lewis acids such as aluminum trichloride, boron trifluoride, tin(IV) chloride and zinc(II) acetate in the presence of acetic anhydride to afford enol acetates of cycloocta-l,5-dienols.155... 

Reviews on stoichiometric asymmetric syntheses M. M. Midland, Reductions with Chiral Boron Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 2, Academic Press, New York, 1983 E. R. Grandbois, S. I. Howard, and J. D. Morrison, Reductions with Chiral Modifications of Lithium Aluminum Hydride, in J. D. Morrison, ed.. Asymmetric Synthesis, Vol. 2, Chap. 3, Academic Press, New York, 1983 Y. Inouye, J. Oda, and N. Baba, Reductions with Chiral Dihydropyridine Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 4, Academic Press, New York, 1983 T. Oishi and T. Nakata, Acc. Chem. Res., 17, 338 (1984) G. Solladie, Addition of Chiral Nucleophiles to Aldehydes and Ketones, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 6, Academic Press, New York, 1983 D. A. Evans, Stereoselective Alkylation Reactions of Chiral Metal Enolates, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 1, Academic Press, New York, 1984. C. H. Heathcock, The Aldol Addition Reaction, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 2, Academic Press, New York, 1984 K. A. Lutomski and A. I. Meyers, Asymmetric Synthesis via Chiral Oxazolines, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 

Ethyl aluminum dichloride mediates a formal [5 + 2] cycloaddition of complex (164) and (166) with enol silyl ethers to produce the highly strained seven-membered rings (165) and (167) respectively (Schemes 239 -240). Excellent stereoselectivity is observed in both cases. A related double alkylation affords complexed seven-membered rings via a formal [4 - - 3] cycloaddition. Incorporation of fluorine is observed using boron trifluoride etherate (Scheme 241). 

In sharp contrast to the utility of chiral boron Lewis acids, chiral aluminum Lewis acids have been little used for asymmetric aldol reactions of silyl enolates since the first example reported by Reetz et al. [115]. Fujisawa et al. have reported that an equimolar amount of a chiral Lewis acid prepared from Et2AlCl and a bor-nane-2,3-diol promotes the aldol reaction of 48 in moderate yields with good enantioselectivity [127]. 

The only main Group III metal, other than boron, that has been utilized in the aldol reaction is aluminum, the enolates of which behave rather capriciously in terms of stereochemistry. The A1—C bond is relatively weak. However, aldol reactions with aluminum enolates derived from chiral acyl-iron complexes proceed with high asymmetric induction. 

In 1974, Jeffery et al. reported the first use of well-defined dimeric or trimeric aluminum enolates in the aldol reaction. - Reaction of dimethyl-Z(0)-4,4-dimethylpent-2-en-2-oxyalane (123) with acetaldehyde or benzaldehyde provides the anti aldol products (124) as chelated dimers involving five-coordinate aluminum atoms, as shown in Scheme 51. However, the corresponding (0)-enolate (125) gives dimeric products of syn configuration which undergo isomerization, if not hydrolyzed immediately, to provide the same anti products. This syn-anti isomerization is much more facile with benzaldehyde, and is in sharp contrast to aldol reactions mediated by boron. 

Complementary to the acylation of enolate anions is the acid-catalyzed acylation of the corresponding enols, where the regiochemistry of acylation can vary from that observed in base-catalyzed reactions. Although the reaction has been studied extensively in simple systems, it has not been widely used in the synthesis of complex molecules. The catalysts most frequently employed are boron trifluoride, aluminum chloride and some proton acids, and acid anhydrides are the most frequently used acylating agents. Reaction is thought to involve electrophilic attack on the enol of the ketone by a Lewis acid complex of the anhydride (Scheme 58). In the presence of a proton acid, the enol ester is probably the reactive nucleophile. In either case, the first formed 1,3-dicarbonyl compound is converted into its borofluoride complex, which may be decomposed to give the 3-d>ketone, sometimes isolated as its copper complex. 

Catalytic hydrogenation of 1 in the presence of rhodium on aluminum oxide proceeds smoothly to afford (5)-hexahydromandelic acid (3) [2]. Subsequent treatment of 3 with ethyllithium provides in 75% yield the ketone 4, which is 0-silylated to afford 5. Generated in situ with the appropriate dialkylboron triflate and 5, the boron enolates 6a—c react with a variety of aldehydes to provide exclusively a mixture of syn-d o products 7 and 8 in 70-80% yields, often with excellent stereoselectivities. 

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