Carbon-skeleton chromatographic

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

When the crude reaction mixture is chromatographed on A1203 the trans-octalone 153 is obtained in 57 % overall yield as a result of epimerization. The potential of this route to prepare bicyclic or polycyclic carbon skeletons under mild conditions and in a stereocontrolled manner is evident. 

The method for determining the carbon skeleton can be realized in several versions. First, the hydrogenolysis, hydrogenation or dehydrogenation of substances can be carried out independently of their chromatographic determination, by using known chemical methods (e.g., refs. 9-11). 

Hydrogenation methods were successfully employed in the analysis not only of hydrocarbons. but also of compounds of other classes. Preliminary hydrogenation before the chromatographic column is a rapid and precise method for establishing the carbon skeleton of the sample compound. 

In the synthesis of 9-bromoretinaldehyde (469), the tetraene side chain was built up from the phosphonium salt (171). The latter was reacted with (466) to give the ester mixture (467), which was reduced with diisobutyl aluminum hydride and oxidized with manganese(IV) oxide to give the isomeric aldehyde (468). This was coupled to the phosphonate (166) to form the carbon skeleton of the aldehye (469). After reduction, oxidation, and chromatographic separation, the aldehyde (469) and its 13-cis isomer were obtained (Motto et al., 1980). 

The identification of terpenoic compounds is very complicated because these substances usually occur in mixtures of closely related compounds, and they are sensitive to various kinds of isomerization and to the rearrangement of the carbon skeleton. Gas chromatography (GC) has proved to be the most useful and fastest method of separating the terpenes in complex mixtures substantial progress has been achieved after making use of capillary columns. The very complex nature of many essential oils does not allow a complete chromatographic separation even on high-resolution columns. 

Brownlee, R. G., and R. M. Silverstein A micro-preparative gas chromatograph and a modified carbon skeleton determinator. Anal. Chem. 40, 2077—2079 (1968). 

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