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

Natural products such as polyketides often possess various spin systems, consisting of a methine group directly bonded to a methyl group (e.g., ─CH A ─CH B (CH 3 )─CH C ─). The methine proton H B splits into a broadened multiplet by coupling with several vicinal protons, rendering analysis difficult of n J H–H with respect to H B in double‐quantum filtered COSY or exclusive COSY. For the purpose of measuring n J H–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 J H–H of the multiplet signals can easily be performed using the proposed pulse sequence.