C-13 NMR CHEMICAL-SHIFTS OF HETEROCYCLES - EMPIRICAL SUBSTITUENT EFFECTS IN 5-HALOMETHYLISOXAZOLES

Evaluation by empirically derived equations for the substituent effect (alpha, beta, gamma, delta) on the C-13 NMR chemical shifts for C-3, C-4, C-5 and halomethyl-substituent carbon (C-6) in isoxazoles 1-5 [where C-3 substituent (R1) = H, alkyl or phenyl, C-4 substituent (R2) = H, alkyl, and C-5 substituent (R3) = di- or trihalomethyl, methyl and H], taking as reference the compound 1a, is reported. From the calculated values for the alpha, beta, gamma, delta effects for each substituent it was possible to estimate the chemical shift of each carbon of the compounds 1-5. The C-13 chemical shifts of the C-3, C-4, C-5, C-6 of these compounds, can be estimated with good precision: 94% of the calculated chemical shifts are found to be within +/-1.0ppm, and 100% are found to be within +/-1.5ppm.