Heteroaromatic diols

Heteroaromatic diols, 40 257 Heterobimetallics, 43 179-254, see also Metals cyano-bridged, 43 244-249 molecular, 43 249... 

The site of dihydroxylation in heterocycles depends on the nature of the heteroaromatic system (Scheme 9.31) usually, electron-rich heterocycles like thiophene are readily biooxidized but give conformationally labile products, vhich may undergo concomitant sulfoxidation [241]. Electron deficient systems are not accepted only pyridone derivatives give corresponding cis-diols [242]. Such a differentiated behavior is also observed for benzo-fused compounds biotransformation of benzo[b] thiophene gives dihydroxylation at the heterocyclic core as major product, while quinoline and other electron-poor systems are oxidized at the homoaromatic core, predominantly [243,244]. 

It should be noted that several sensitive groups (amino, heteroaromatic, etc.) were unaffected [24], The same reagent effected the highly selective oxidation of a number of 1,4-diols to cyclic hemiacetals (y-lactols), a conversion not previously possible in one step, for example [25] ... 

Figure 2 The structures of ceramide 1, a schematic structure of ceramide that illustrates the sites at which structural modifications may be introduced by chemical synthesis, and the structure of a representative ceramide found in human skin (2). Structural changes in the ceramide molecule have been introduced at many sites. Alterations include the configuration at C-2 and C-3 the lengths of the fatty amide chain and of the aliphatic chain attached to C-5 the positions of the unsaturation and the secondary hydroxy group the replacement of the hydroxyl groups with other atoms (hydrogen or fluorine) or functionalities (methoxy, methylthio, and keto) the incorporation of aromatic, heteroaromatic, and other rings in place of the alkenyl side chain of the sphingoid base and the replacement of the carbonyl group of the carboxamide group. In addition, the 2-amino-1,3-diol functionalities have been incorporated into cyclic structures.

The indicator variables are D2 and D3 for simple X-catechols,D2 = 1 andforX-naphtha-lene diols, D3 = 1. The negative coefficients with both terms (D2 and D3) underscore the detrimental effects of these structural features in these inhibitors. Thus, discontinuities in the structural features of the molecules of this data set are accounted for by the use of indicator variables. An indicator variable may be visualized graphically as a constant that adjusts two parallel lines so that they are su-perimposable. The use of indicator variables in QSAR analysis is also described in the following example. An analysis of a comprehen-, sive set of nitroaromatic and heteroaromatic compounds that induced mutagenesis in TA98 cells was conducted by Debnath et al., and QSAR 1.57 was formulated (172). 

On the other hand, another cooperative catalysis approach was developed by Oh and Kim with a highly diastereo- and enantioselective domino aldol-cyclisation reaction occurring between aldehydes and methyl a-isocyanoacetate. The process employed a combination of a chiral cobalt complex derived from brucine amino diol and an achiral thiourea. The reaction was applicable to a range of aliphatic, aromatic and heteroaromatic aldehydes, providing the corresponding chiral oxazolines in good yields and diastereoselectivities of up to >90% de combined with good to excellent enantioselectivities of up to 98% ee, as shown in Scheme 7.12. 

Clerici and Porta have extensively investigated the reaction promoted by aqueous TiCb in acidic media [80]. Under these conditions, electron-withdrawing substituted aliphatic, aromatic and heteroaromatic carbonyl compounds give 1,2 diols by a homo-coupling reaction ... 

A three-component synthesis of pyrroles, which unites enolisable ketones, primary amines, and vicinal diols, was reported by Beller. Remarkably, these reactions generate two C-N bonds, one C-C bond, liberate one equivalent of H2, and two equivalents of water in a single synthetic operation. A broad range of substituents is tolerated, including heteroaromatics, esters, amides, and aryl halides. Primary drawbacks of these methods include high reaction temperatures and the inability to access certain regio-isomers, such as 2-ff-3-allqrl pyrroles. 

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