1 H‐NMR.‐spektroskopische Bestimmung der Enantiomerenreinheit von Allencarbonsäureestern mit optisch aktiven Europium‐Verschiebungsreagenzien

1 H‐NMR. Spectroscopic Determination of Enantiomeric Purities of Allenic Esters Using Optically Active Europium Shift Reagents The racemic allenic methyl esters 3‐‐7 and the racemic allenic diesters 8‐‐10 ( cf. Scheme 2 ) in 1,1,2‐trichloro‐1,2,2‐trifluoroethane (TCFE) and CCl 4 in the presence of optically active tris[3‐(heptafluorobutyryl)‐(+)‐camphorato]europium(III) (Eu(hfc) 3 ) have induced unlike 1 H‐NMR. shift differences (ΔΔδ) for the protons of the methoxycarbonyl groups of their enantiomers. In some allenic esters the shift reagent causes additional separation of resonance signals; thus, further substituents on the allenic framework may be differentiated in the racemic mixture. This finding provides a widely applicable method for the determination of absolute enantiomeric purities of allenic esters and their corresponding acids. Accordingly we found for optically pure (+)‐( S )‐2‐methyl‐2,3‐pentadienoic acid ((+)‐( S )‐ 13 ; cf. Fig. 2 ) a calculated [α]   589 20value of + 73.3 ± 1.8°. Finally, the substituent effects on ΔΔδ‐values ( cf. Table 1‐‐3 ) are discussed.