Lewis silylenol ethers

For example, using (/ )-5-trimethylsilyl-2-cyclohexenone as the chiral Michael acceptor, optically active m // .v-3.5-disubstituied cyclohexanones 1 are obtained via a Lewis acid catalyzed addition of silylenol ethers or ketene acetals. 

The third cycloaddition substrate explored the feasibility of a vinyl nitro functionality as an activated dipolarophile (98, Scheme 1.9c). Preparation of nitroalkene oxidopyridinium betaine 98 began with silylenol ether 92, which was treated with methoxydioxolane in the presence of Lewis acid catalyst, TrC104, to afford keto dioxolane 93 in 58 % yield [47]. Ketone 93 then underwent a-nitration by treatment with /-BuONCL and KOt-Bu to provide nitro ketone 84 (91 %), which was then converted to the nitroalkene functionality via reduction under Luche conditions to... 

Aldol reactions of aldehydes with cycloakanones were performed in ionic liquids and catalyzed by FeCl3-6H20 [32]. Mukaiyama aldol reactions of silylenol ethers with aldehydes can be carried out in aqueous media however, among several Lewis acidic catalysts investigated, iron compounds were not the optimal ones [33], If silyl ketene acetals are applied as carbon nucleophiles in Mukaiyama aldol reactions, cationic Fe(II) complexes give good results. As catalysts, CpFe(CO)2Cl [34] and [CpFe(dppe) (acetone)] BF4 [35] [dppe = l,2-bis(diphenylphosphano)ethane] were applied (Scheme 8.8). No diastereomeric ratio was reported for product 26a. 

With the ability to make use of fluoroglycosides as C-glycosidation substrates, chloroglycosides have been comparably useful. As shown in Scheme 2.5.10, Allevi, et al.,89 condensed silylenol ethers with 2,3,4,6-tetra-Obenzyl-a-D-glucopyranosyl chloride. Silver triflate was utilized as the Lewis acid catalyst. In the examples studied, the yields exhibited were consistently greater than 80%. Furthermore, complete a selectivity was observed. 

Lewis-acid-promoted alkylations of silylenol ethers and silyl ketene acetals [195] with Co-complexed acetylenic acetals [196] and acetylenic aldehydes [197,198] (Scheme 4-56) also proceed with fair to excellent syn diastereoselectivity, in contrast to the low selectivity reactions of the free acetylenic derivatives [199, 200]. Reactions of the complexed aldehydes with lithium enolates are stereospecific, with (Z)-enolates giving syn selectivity and ( )-enolates anti selectivity [201]. The complementary stereoselectivity of the crossed aldol reactions of free and cobalt-complexed propynals with silyl ketene 0,S-acetals has been elaborated by Hanoaka exclusive syn selectivity is exhibited by the complexes and high anti selectivity is found with pro-... 

The Mukaiyama-Michael addition of diazo-silylenol ethers CH2=C(OTBS)-C(N2)C02R to enones R CH=CHCOR, catalysed by a chiral Cu(II) complex (10mol%), acting as a Lewis acid, affords the corresponding y-functionalized diazoacetoacetates with <96% ee ... 

Full reports and further improvements on the Lewis acid-catalysed reactions of olefins with methyl propiolate have been published. Monosubstituted and 1,2-disubstituted olefins give mainly cyclobutene esters whereas other olefins tend to undergo ene reactions leading to 2,5-dienoates. With add-sensitive molecules, ethylaluminium dichloride is the preferred catalyst. Silylenol ethers react to give derivatives of hydroxycyclobutene esters. 

The BINOL-Ti-catalyzed aldol reaction of chiral P-benzyloxy aldehyde and silyl enol ether provides a facile route to the stereoselective synthesis of both syn and anti-diastereomers of p,S-dihydroxy thioesters, which can be used as the key intermediates for P-lactone synthesis [111] (Scheme 14.39). The high diastereoselectivity of this reaction was found to be dictated by the chirality of the BINOL-Ti catalyst rather than that of the chiral aldehyde substrate. A catalytic enantioselective resolution of secondary alcohol containing silylenol ether moiety has been realized by an aldol reaction catalyzed by BINOL-Ti Lewis acid [112]. 

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