No abstract

Although the potential advantages of forward linear prediction for the processing of multi-dimensional NMR f 1 spectra are well established, this method is surprisingly little used for 2D spectra used for organic structure determination. A detailed investigation of the advantages and limitations of forward linear prediction for this purpose f 1 is reported. This is a reliable technique which is particularly useful for 1H-detected 13C-1H shift correlation spectra, allowing up to 16-fold linear prediction of the 13C axis of HSQC spectra. In general, the use of linear prediction allows one to obtain comparable 2D spectra in one quarter of the time or double the sensitivity in a comparable time relative to similar spectra without linear prediction. The one exception is the absolute value COSY spectrum, where linear prediction beyond a factor of two gives poor results. Linear prediction is generally superior to zero Ðlling as a time-saving technique, although the di †erence between the two approaches disappears as the data point resolution approaches the natural linewidth. By contrast, forward linear prediction is not f 1 f 2 recommended.

A mixture of glycosidic acids, each of which consisted of a tetrasaccharide unit linked by an anomeric carbon to I lhydroxypal~nitic acid and containing short-chain fatty acids ester-linked to the sugar units, was isolated from Ipotnoeu stalls. Basic hydrolysis allowed isolation of a single compound whose structure was determined by two-dimensional NMR methods to be (S)-1 I-hydroxypalmitic acid 11-0-~-quinovopyranosy1-(1-4)-a-rhamnopyranosyl-(1,2)-~-glycopyranosyl-( I-2)-P-quinovopyranoside. Further HPLC separation of the original mixture allowed separation of a compound that was shown, by 2D NMR, to have 2-methylbutanoic acid molecules linked to C-4 of the terminal quinovose and C-2 of rhamnose, 3-hydroxy-2-methyl butanoic acid linked to C-6 of glucose, and with the palmitic acid forming a lactone at C-3 of rhamnose. Closer inspection revealed that this compound was actually a mixture of diastereomers involving different ((2R.3R) and (2S,3S)) enantiomers of 3-hydroxy-2-methylbutanoic acid bonded to the chiral tetrasaccharide core. Key ~~r r l s :tetrasaccharide, two-dimensional NMR. A partir de l'lpomoea stons, on a isole un melange d'acides glycosidiques qui contiennent chacun une unite tetrasaccharide like par un carbone anomerique a I'acide 1 1-hydroxypalmitique et qui contiennent des esters d'acides gras a chaines plus courtes lies a des unites de sucre. De leur hydrolyse basique on a pu isoler qu'un seul compose dont on a determine la structure par des mtthodes de RMN bidimensionnelles; il s'agit du 11-0-P-quinovopyranosyl-(1,4)-arhamnopyranosyl-(l,2)-~-glucopyranosyl-(1,2)-~-quinovopyranoside de l'acide (S)-1 1 -hydroxypalmitique. Une separation plus poussee du melange original par CHLP a permis de separer un compose qui, selon la RMN en 2D, contiendrait des molCcules d'acide 2-m6thylbutanoique likes au C-4 du quinovose terminal et au C-2 du rhamnose, de I'acide 3-hydroxy-2-mCthylbutanoique lie au C-6 du glucose et de I'acide palmitique formant une lactone en C-3 du rhamnose. Un examen plus minutieux a rCvClC que ce compose est en realite un melange de diastCrCoisombres impliquant differents ((2R,3R) et (2S,3S)) Cnantiomeres de l'acide 3-hydroxy-2-methylbutanoique lies a un noyau tetrasaccharide chiral.

No abstract

No abstract