Aldehydes chain elongation

The reagent ( )-l-(diisopropylaminocarbonyloxy)-2-butenyltitanium was also successfully applied for the chain elongation of the chiral aldehyde 9 in the total synthesis of 22,23-dihydroaver-mectin B]b. Starting from ent-9, the products ent-11 and ent-10 are formed in the same ratio110. 

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

A diene system with unsymmetrical 1,4-disubstitution is converted to the iron carbonyl complex 1 which is resolved into its enantiomers. The aldehyde function is conformationally locked in the transoid position and is diastereofacially shielded from the bottom face. Nucleophiles attack from the top face with high selectivity. Alternatively, chain elongation leads to the triene 2 which is reacted with diazomethane. Cerium(IV) oxidation removes the metal and furnishes the substituted cyclopropane 3. 

In principle, bifunctional aldehydes should be able to engage in twofold enzymatic aldol additions to both of their acceptor carbonyls in a fashion to be classified as a tandem reaction, that is, without the need for isolation of intermediates. Depending on the specificity of the enzyme used and on the functionalization in the starting material, the isomeric constitution as well as the absolute and relative stereochemistry should be deliberately addressable. Therefore, we engaged in a program to evaluate the scope and the Hmitations of such two-directional chain elongation processes for the construction of extended poly functional molecules [36]. 

Fig. 14.32. Aldehyde alkyne chain elongation via [l,2]-rearrangement of a vinyl carbenoid (Corey—Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig olefina-tion and form the 1,1-dibro-moalkene (B). In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid D is formed stereoselectively. The carbenoid undergoes H migration to form the alkyne C. The alkyne C reacts immediately with the second equivalent of n-BuLi to give the lithium acetylide and is reconstituted by reprotonation during aqueous workup.

Fig. 11.28. Aldehyde alkyne chain elongation via [1 -rearrangement of a vinyl carbene (Seyferth procedure). First, a Horner-Wadsworth-Emmons olefination of the aldehyde is carried out to prepare alkene A. Upon warming to room temperature, alkene A decomposes and gives the vinyl carbene B. From that, the alkyne is formed by way of a [1,2]-rearrangement.

Zero-length cross-linkers are reactive molecules that activate functional groups on surfaces without any chain elongation or incorporation in molecules that have to be attached. In this approach, all reactive surface modifications, for example aldehydes, epoxy-groups, and halogenated surfaces are also termed zero-length cross-linkers. 

The authors had changed the starting material to the substituted tetralone 135 174), which was converted into the enol acetate 136 when treated with acetic anhydride and acid (Scheme 13). Subsequent ozonization afforded aldehyde 137. Chain elongation was achieved by Wittig condensation using Corey-Chaykovsky... 

Eukaryotic peptide chain elongation factor-2 Triosephosphate isomerase Myo-inositol-phosphate synthase Mannose-1-phosphate guanylj transferase Alcohol dehydrogenase Aldehyde reductase Ade5... 

A handle based on a C-terminal semicarbazone 37 that is compatible with Boc/Bzl chemistry has also been developed. At the end of the chain elongation, peptide aldehydes are cleaved by treatment with dilute aqueous acid and formaldehyde.f l... 

Chain Elongation of Aldehydes through Nucleophilic Additions... 

CHAIN ELONGATION OF ALDEHYDES THROUGH NUCLEOPHILIC ADDITIONS... 

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