Alkoxide derivatives, preparation from

Disubstituted dihydrofurans and dihydropyrans were prepared via allylic etherification [68] in a similar manner to dihydropyrroles (cf Section 9.4.6). Thus, diaste-reoisomeric ethers were generated by the reaction of cinnamyl tert-butyl carbonate with the copper alkoxide prepared from (Rj-l-octen-3-ol, depending on which enantiomer of the phosphoramidite ligand was used (Scheme 9.39). Good yields and excellent selectivities were obtained. RCM in a standard manner gave cis- and trans-dihydrofuran derivatives in good yield, and the same method was used for the preparation of dihydropyrans. 

Aryl fluoroalkyl ethers have been prepared from the reaction, at room temperature in HMPA, of fluo-ro-substituted alkoxides with activated fluoro-,149 nitro-,149 and, at 150 °C, also chloro-arenes150,151 and some chloro-substituted pyrazines (equation 15), pyrimidines, quinolines,150,152 and pyridines.152 Disubstitution was observed in die presence of comparably activated leaving groups such as in 2,4- and 2,6-di-chloronitro- or cyano-benzenes, whereas regiospecific substitution took place at position 4 in 3,4-dichloronitro- or cyano-benzene and at position 2 in 2-fluoro-6-chlorocyanobenzene.151 Steric hindrance and the number of fluorine substituents in the alkoxide pose limits to the reactivity. Thus, tertiary alkoxides, or alkoxides containing more than four fluorine substituents, displace activated nitro and fluoro, but not chloro substituents.149,150 The secondary hexafluoro-2-propoxide anion does not react even with the more reactive nitro and fluoro derivatives.149... 

Kobayashi and colleagues developed a catalytic enantioselective method for the allylation of imines 24 by substituted allylstannanes 25 with chiral zirconium catalysts 26 and 27 prepared from zirconium alkoxides and l,l -bi-2-naph-thol derivatives (Scheme 10) [19]. The allylation of aromatic imines 24 with 25 afforded the corresponding homoallylic amines 28 in good yields (71-85%) with high stereoselectivities (87-99% ee). 

Metals. Lanthanide metals are also considered as valuable precursors. For example, alkoxides derived from cheap and low-boiling-point alcohols have been alternatively synthesized from metals in the presence of HgQ2 as catalyst [133]. Representative and specific methods of preparation include transmetalla-tion reactions (Eq. 7-9) [134], using ammonia solutions of ytterbium and europium as synthetic reagents (Eq. 10) [135] and the generation of thiolate complexes from disulfides (Eq. 11) [136],... 

Olefinic aldehydes have been prepared by bromination of the diethyl-acetal derivatives followed by dehydrobromination (cf. Acetals and Ketals) the unsaturated aldehydes are readily liberated by mild acid treatment of their acetals. Alkoxy aldehydes have also been synthesized through acetal intermediates, which in turn are prepared from sodium alkoxides and bromoacetals. ... 

A comprehensive group of polyesters contains hindered piperidine or piperazine (HALS) moieties. Most of these stabilizers were prepared under transesterification conditions, using tetraalkyl titanates, lithium amide or sodium alkoxide as catalysts. Terminal HALS group was built in under these conditions into a polyether-polyester. Polyester 145 was prepared from a reactive diester derived from piperazinedione and fljco-alkylidenediol (n = 2-15) [188], A similar system contains 2,2,6,6-tetramethylpiperidine moiety [189]. 

Enantioselective Conjugate Addition to Prochiral Enones of Organometallic Reagents Modified with Ephedrine. Enantioselective conjugate addition to 2-cyclohexenone with chiral organo(alkoxo)cuprates [MCu(OR )R] has been studied. When the cuprate is prepared from the lithium alkoxide of ephedrine, Phenyllithium, and Cul, 3-phenylcyclo-hexanone with 50% ee is obtained. The enantioselectivity reaches 92% ee in enantioselective ethylation when a chiral diamino alcohol derived from ephedrine is employed (eq 13). ... 

Preparation of Derivatives. Enoate derivatives are prepared from the corresponding chiral alcohol by treatment with acry-loyl chloride in the presence of Triethylamine and catalytic 4-Dimethylaminopyridine or the appropriate carboxylic acid chloride and Silveril) Cyanide. Alkynyl ethers are readily available from the potassium alkoxide by treating with Trichloroethylene, in situ dechlorination with n-Butyllithium, and electrophilic trapping. Trapping the intermediate anion with a proton source or lodomethane followed by Lindlar reduction of the alkynyl ether affords the corresponding vinyl and l-(Z)-propenyl ether, respectively, while reduction of the alkynyl ether with Lithium Aluminum Hydride affords the l-( )-propenyl ether. 

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

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