Non Lewis acids

A catalytic system consisting of cat. Sml2, Zn/Hg, Lil, and Me3SiOTf induces spirolactonization (Scheme 20) [56]. Me3SiOTf plays a similar role in converting the intermediary alkoxides to the silyl ethers. The efficacy of Lil depends on the formation of Sml3 from Sm OTf, which facilitates reduction by Zn/Hg. The Lewis acidity of Zn(II) is reduced by conversion to a non-Lewis-acidic species such as Li2Znl2(OTf)2. 

Non-Lewis acid catalyzed Diels-Alder reactions. 355... 

Non-Lewis acid catalyzed Dieis-Aider reactions... 

In recent years, supramolecular chemistry has produced a number of systems which have been shown to be able to effectively catalyze a Diels-Alder reaction. Most systems selectively afforded only one diastereomer because of a pre-organized orientation of the reactants. These systems include cyclodextrines, of which applications in Diels-Alder chemistry have recently been reviewed89. Some other kinds of non-Lewis acid catalyzed Diels-Alder reactions, including catalysis by proteins and ultrasound, have been discussed by Pindur and colleagues90. 

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

The bottom part of Figure 10.32 shows how monomeric organolithium compounds as a contact ion pair (A), a solvent-separated, non-Lewis-acidic ion pair (B) or as a solvent-separated, Lewis-acidic ion pair (C) react with cyclohexenone as a prototypical a,fi-unsaturated ketone. 

The addition of the solvent-separated, non-Lewis-acidic ion pair B to cyclohexenone also proceeds in a single step. The coordinatively saturated Li (solv)4 cation is just a casual... 

Reviewed was the use of our newly developed Pd-catalyzed glycosylation reaction for the incorporation of rare and unnatural sugars on unique biologically relevant structures. This de novo approach for the introduction of carbohydrate structures, under non-Lewis acidic conditions, into preexisting structural motifs is unique. To date, there simply does not exist an equivalent chemical or biochemical method for the stereoselective glycosylation of under-fimctionalized D- and L-sugars, which can be elaborated into a myriad of carbohydrate analogues. Not to mention this method also allows for complete a/(3-stereocontrol. The power and uniqueness of this approach is probably best... 

Non-Lewis acid cataiyzed Dieis-Alder reactions... 

A major advantage of the non-Lewis acid catalyzed cycloaddition is the possibility of carrying out the domino [4 + 2]/[3 + 2] cycloaddition in a one-pot fashion, since electron-poor alkenes react much faster with the nitronate formed in situ than electron-rich alkenes [14c, 20, 21[. This multicomponent reaction then provides the nitroso acetals in a single transformation, without the need to isolate the nitronate which was formed first, prior to the 1,3-dipolar cycloaddition. 

Lewis acidic nucleophiles (AlMea, ZnEt2, i-Bu2AlH) and a few non-Lewis acidic sources (NaBH(OAc)3, NaBHsCN)-... 

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