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 [wh ere 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 calcula ted values for the alpha, beta, gamma, delta effects for each substitu ent it was possible to estimate the chemical shift of each carbon of t he compounds 1-5. The C-13 chemical shifts of the C-3, C-4, C-5, C-6 o f these compounds, can be estimated with good precision: 94% of the ca lculated chemical shifts are found to be within +/-1.0ppm, and 100% ar e found to be within +/-1.5ppm.