NMR experiments on acetals—XXXIX: Conformational insights through NMR studies at 300 MHz of 2‐methyl‐4‐halogenomethyl‐1,3‐dioxolanes

Abstract

Abstract NMR spectra of cis ‐ and trans ‐2‐methyl‐4‐halogeno‐methyl‐1,3‐dioxolanes have been analysed at 300 MHz. Some of the extracted parameters facilitate easy distinction between these 1,3‐dioxolanes and the corresponding structurally isomeric 2‐Methyl‐5‐halogeno‐1,3‐dioxanes. Criteria enabling configurational assignments to be made for the cis ‐ trans isomers of the dioxolane series are tested. The Me‐2 group causes an upfield shift (0·2 to 0·3 ppm) of a trans proton at position 5, but the reversed shift for the corresponding cis proton. This competes with, or even overwhelms the effect of the CH 2 X‐4 substituent, which by virtue of its pronounced preferential rotameric orientation and in comparison with a simple Me‐group, has no large upfield effect on the shift of the syn ‐adjacent proton. Shift criteria and coupling constants J H‐4, H‐5 in cis ‐ and trans derivatives allow further conformational insights into these 1,3‐dioxolanes.