Aldehydes crossed

This involves hydride transfer from an aldehyde molecule lacking an a-H atom, e.g. HCHO, R3CCHO, ArCHO, to a second molecule of either the same aldehyde (disproportionation) or sometimes to a molecule of a different aldehyde ( crossed Cannizzaro). The reaction requires the presence of strong bases, and with, for example, PhCHO the rate law is found to be,... 

Silylation using the silylacetate (3.1.14) [49] involves the initial formation of the acetate carbanion, which abstracts a proton from the carbonyl compound or alcohol (Scheme 3.2, Table 3.5). When the reaction with a ketone is conducted in the presence of an aldehyde, crossed aldol products are obtained (see Chapter 6). 

Removal of the prepeptide Hydroxylation of Pro and Lys residues Clycosylation of 5Hyl and Asn (4) Oxidation of Cys in propeptides Assemblage to form triple helix Removal of the propeptide Staggered deposition to form fibrils Oxidation of Lys and 5Hyl to aldehydes Cross-linking to form supramolecules... 

As demonstrated by MacMillan and coworkers, a-oxygenated aldehydes are very good reaction partners in the aldehyde-aldehyde crossed-aldol reaction. The products are tetroses, and one further aldol step affords a range of hexoses, i.e. differentially protected monosaccharides, in a two-step synthesis (Scheme 20) [203],... 

Keywords aldehyde, cross-Cannizzaro reaction, microwave irradiation, alcohol... 

Human serum albumin Desolvation, glutar aldehyde cross-linkingoi heat denatura-tion None Ethanol used for processing [93]... 

The functionalization of polymers is another useful feature of the hydrosilation reaction. The introduction of highly fluorinated alkyl chains by the hydrosilation of Si-H groups of the polymer with fluorinated aUcenes is a typical example. Poly(phenylsilanes) obtained by the dehydrocouphng polymerization of phenylsilane undergo AIBN initiated free radical hydrosilation with aUcenes, ketones and aldehydes. Cross-linking and branching of polymers can also be easily accomplished using hydrosilation. 

McMillan and co-workers [146] have reported the first example of direct enantioselective aldehyde-aldehyde cross-aldol reaction using small molecules as catalysts. Subsequently, they have described the enantioselective dimerization and cross-coupling of a-oxygenated aldehydes to provide eiythrose architecture. A second L-proline-catalyzed aldol reaction generates hexoses (O Scheme 22) [147]. 

One normally expects antibodies to have a low tolerance to substrate modifications, however an ongoing feature of these aldolase antibodies is their wide scope. They accept a remarkable range of aldol donors and acceptors and perform crossed-, intramolecular- and retro-variants of this reaction, with high yields, rates, and stereospecificities [81,82,83]. Substrate modification experiments have revealed that when acetone is the aldol donor in a ketone-aldehyde crossed aldol reaction, stereoinduction is linked to attack of the sz-face of a prochiral aldehyde with typically >95% ee and when hydroxyacetone is the donor substrate, attack occurs preferentially at the re-face of the aldehyde leading to a diastereomeric a,P-dihydroxy ketones with the two stereogenic centers having an a-syn configuration. This reaction leads to stereospecificities of typically 70 to >99% ee. 

Few examples of what might be described as an intermolecular coupling reaction on inactivated alkenes has appeared [62], Thus ketyl radicals generated from aromatic aldehydes and ketones underwent intermolecular addition to the para position of another aldehyde. Cross-coupling reactions are not feasible in these systems and typically yields are quite low. 

Scheme 5.8 Proline-catalysed direct aldehyde cross-aldol reaction.

Glucose aldehyde cross-linked alkylatedchitosan Biomedical Field - - Below 0.1 pm [103]... 

The trimethylsilyl (TMS) group is a widely used protecting group and Lewis acid, as well as an important functional group for numerous substrates. For these reasons, we deemed the TMS group to be generation one. As described earlier (Scheme 20), we have demonstrated that the use of triflimide as a catalyst initiator is very effective for the aldehyde cross-aldol reaction [34, 35]. The success of this reaction proved to be maximal with the use of triflimide as the catalyst as well as the use of tris(trimethylsilyl)silyl (TTMSS) enol ethers (Scheme 20) [36, 37]. The use... 

Boxer MB, Yamamoto H (2006) Tris(trimethylsilyl)silyl-govemed aldehyde cross-aldol cascade reaction. J Am Chem Soc 128 48 9... 

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