Hydroxy acids conformation

CsHi.Os Y-Hydroxy acid Conformation Study(15) T211... 

Many of the reactions induced in organic molecules on irradiation in oxygenated aqueous solution follow a common mechanistic pattern. In this, the hydroxy acids conform strictly, and, because of the obvious relevance of such mechanisms to carbohydrate irradiations, the general pattern will be considered here. ... 

Acyl groups are common in bacterial polysaccharides. The parent acids are fatty acids, hydroxy acids, and amino acids. The simplest acid, formic acid, has only been found as the amide. The occurrence of O-formyl groups had been reported, but proved to be incorrect. A-Formyl groups have been found in different polysaccharides for example, in the 0-specific side-chains of the LPS from Yersinia enlerocolitica 0 9, which are composed of 4,6-dideoxy-4-formamido-D-mannopyranosyl residues. The formyl group can assume two main conformations, s-cis (41) and s-trans (42), which are... 

Figure 5 Conformations of a-hydroxy acids around the C(l)—C(2) bond ...

A detailed study of the iodocyclization of a series of unsaturated hydroxy acids has demonstrated that a ground-state conformational analysis can serve as a reliable indicator of the relative reactivities of various conformations. Thus, while 153 undergoes exclusive iodolactonisation, 154 gives the corresponding iodotetrahydrofuran as the sole product, in full agreement with the MM2 prediction224. 

Meyers and coworkers report the synthesis of enantiomerically enriched a-hydroxy acids (enantiomeric excesses of the products generally range from 30 to 87%) from chiral ketooxazolines such as (86 Scheme 8). In most cases, only moderate selectivities are observed. The highest ratios (62-87% ee) result when aryllithiums are employed as the nucleophile (Table 21). Since the course of the reaction seems to depend on a number of subtle conformational and coordinating effects, predictions of the stereochemical outcome of these processes are difficult. 

TsOH -HjO in 600 ml. of benzene at reflux with stirring under nitrogen for 10 min. and collecting the distillate in a water separator. Workup and crystallization afforded the lactone (2) in nearly quantitative yield. This procedure proved to be inapplicable to the tra/u-diaxial hydroxy acid (3), which can lactonize only if one ring assumes the unstable boat conformation (4). The same catalyst was ineffective in boiling toluene however, when xylene was used as the solvent water was eliminated, and lactone (4) was obtained in 73% yield after chromatography. 

Johnson et found the y-hydroxy acid (3) to resist lactonization by usual methods since the transformation requires a flip of the ring into the boat conformation (4). Lactonization with DCC in pyridine succeeded in some trials but the... 

A group of compounds that protect cartilage from enzymatic degradation Asymmetric alkylation of Evans s chiral enolates Asymmetric synthesis from an enantiomerically enriched hydroxy-acid Part V - Conformational Control and Resolution Kinetic or Not ... 

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