DPFGSE-MDEC-J-resolved COSY: An efficient method for selectively measuring proton-proton spin coupling constants of the multiplet signals.

ABSTRACT

Natural products such as polyketides often possess various spin systems, consisting of a methine group directly bonded to a methyl group (e.g., ─CHA ─CHB (CH3 )─CHC ─). The methine proton HB splits into a broadened multiplet by coupling with several vicinal protons, rendering analysis difficult of n JH-H with respect to HB in double-quantum filtered COSY or exclusive COSY. For the purpose of measuring n JH-H in the aforesaid spin system, we have developed new techniques, named multifrequency homo-decoupling (MDEC)-J-resolved COSY and DPFGSE-MDEC-J-resolved COSY. This method incorporates MDEC pulse scheme into J-resolved COSY for the selective decouple of individual methyl groups, avoiding decoupling of the target protons resonating in methyl region. Determinations of n JH-H of the multiplet signals can easily be performed using the proposed pulse sequence.