Racemic 2-alkylbranched acids are transformed to diastereomeric derivatives with (S)-2-hydroxy-3-phenylpropionic acid-N-methylamide or (S)-(-)-l-phenylethylamine and separated by liquid chromatography to pure diastereoisomers, which are subsequently hydrolyzed to yield optically pure acids. Enantiomeric alcohols are generated by LiAlH 4 -reduction of the corresponding acids, esters are synthesized by different methods. The odour impression of the enantiomeric compounds is investigated.2-Alkylalkanoic acids, esters and alcohols are well appreciated aroma compounds of fruits and other foodstuffs 1). In particuliar 2-methylbutanoic acid and its esters are known to be important compounds of fruits like apples 2) and pineapples 3). Until now odour quality only was investigated with racemic substances of this type, although it is generally known that enantiomers may exhibit rather different odour sensations 4 ). 2-Alkylbranched acids and alcohols are important for pheromone synthesis and liquid crystal investigations, too. Recently, investigations on lipase-catalyzed enantioselective esterification of 2-methylalkanoic acids have been reported by EngeI 5 ).This paper now describes the generation of optically pure 2-methylbutanoic acid (1), 2-methylpentanoic acid (2) 2-methylhexanoic acid (3), 2-ethylhexanoic acid (4), the corresponding alcohols 2-methyl-butan-l-01 (5), 2-methyl-pentan-I-ol (6), 2-methyl-hexan-I-ol (7), 2-ethyl-hexan-l-ol (8) and the esters etlJyl-2-methylbutanoate (9) and prenyl-2-methylpentanoate (10).In order to determine their absolute configurations racemic acids 1-4 were converted to diastereomeric amides with (S)-(-)-l-phenylethylamine (SPEA) and chromatographically (LC) separated into optically pure diastereoisomers. Diastereomeric amides were prepared either by a way earlier described by Kaneda 6 ) with equimolar ratio of free acid and free amide by catalysis of N,N'-Dicylclohexylcarbodiimide (DCC) in dry tetrahydrofurane (THF) or by an other way versus acid chloride and free amine under catalysis of 4-Dimethylaminopyridine (DMAP). Absolute configurations of the 2-alkylalkanoic acids moieties were eludicated by comparison with easily available (S)-2-methylbutanoic acid la and by IH-NMR data of tlJeir diastereomeric (S)-1-phenylethylamides 7 ) (Helmchen's model), by chromatographic behaviour of tlJese 965
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