Acceptor reaction applications

F. Paul, E. Oriol, D. Auriol, and P. Monsan, Acceptor reaction of a highly purified dextransucrase with maltose and oligosaccharides. Application to the synthesis of controlled molecular weight dextrans, Carbohydr. Res., 149 (1986) 433-441. 

In the case of propionaldehyde, a range of aldehyde acceptors are applicable with good yields and selectivities. The scope includes allq l-derived aldehydes (80-88% yield, 97-99% ee) and one aryl-derived aldehyde (81% yield, 99% ee). The relevance of this discovery was proven soon after by the same group, by applying the intermolecular, proline-catalysed aldol reaction as part of a two-step synthesis of carbohydrates (Scheme 5.9). The... 

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote ... 

Van Leusen and co-workers also demonstrated the utility of dilithio-tosylmethyl isocyanide (dilithio-TosMIC) to extend the scope of the application. Dilithio-TosMIC is readily formed from TosMIC and two equivalents of n-butyllithium (BuLi) in THF at -70"C. Dilithio-TosMIC converts ethyl benzoate to oxazole 14 in 70% yield whereas TosMIC monoanion does not react. In addition, unsaturated, conjugated esters (15) react with dilithio-TosMIC exclusively through the ester carbonyl to provide oxazoles (16). On the other hand, use of the softer TosMIC-monoanion provides pyrroles through reaction of the carbon-carbon double bond in the Michael acceptor. 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

Besides a polymerization of the Michael acceptor, a double alkylation of the starting ketone, by reaction with a second Michael acceptor molecule, may take place as a side reaction, and thus further reduce the yield. The polymerization of the enone 2 as well as the double alkylation of the starting ketone can be avoided by application of a modern procedure for the Robinson annulation that uses an organotin triflate as catalyst." ... 

Whereas SHMT in vivo has a biosynthetic function, threonine aldolase catalyzes the degradation of threonine both l- and D-spedfic ThrA enzymes are known [16,192]. Typically, ThrA enzymes show complete enantiopreference for their natural a-D- or a-t-amino configuration but, with few exceptions, have only low specificity for the relative threo/erythro-configuration (e.g. (122)/(123)) [193]. Likewise, SHMT is highly selective for the L-configuration, but has poor threo/erythro selectivity [194]. For biocatalytic applications, the knovm SHMT, d- and t-ThrA show broad substrate tolerance for various acceptor aldehydes, notably induding aromatic aldehydes [193-196] however, a,P-unsaturated aldehydes are not accepted [197]. For preparative reactions, excess of (120) must compensate for the unfavorable equilibrium constant [34] to achieve economical yields. 

The y-keto nitriles shown in Table I were prepared by the cyanide-catalyzed procedure described here. This procedure is generally applicable to the synthesis of y-diketones, y-keto esters, and other y-keto nitriles. However, the addition of 2-furancarboxaldehyde is more difficult, and a somewhat modified procedure should be employed. Although the cyanide-catalyzed reaction is generally limited to aromatic and heterocyclic aldehydes, the addition of aliphatic aldehydes to various Michael acceptors may be accomplished in the presence of thioazolium ions, which are also effective catalysts for the additions. 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

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