13C NMR studies of the friedelane series of triterpenoids and the conformation of the D and E rings in friedelan‐7‐one

The 13C chemical shifts in the spectra of friedelan‐7‐one (1), methyl 3‐oxofriedelan‐25‐oate (2), methyl friedelan‐25‐oate (3), 7‐oxofriedelan‐3‐α‐yl acetate (4), friedelan‐3‐one (5), 2‐hydroxy‐3, 4‐seco‐friedelan‐3‐oic acid (6), methyl 2‐oxo‐3,4‐seco‐friedelan‐3‐oate (7), methyl 2‐hydroxy‐3,4‐seco‐friedelan‐3‐oate (8) and 3,4‐seco‐friedelan‐2,3‐diol (9) have been fully assigned using two‐dimensional heteronuclear correlation NMR spectroscopy, BB proton decoupled, DEPT and selective heteronuclear spin decoupling methods. These assignments and the observation of substantial NOEs between 26‐methyl and 28‐methyl and between 28‐methyl and 30‐methyl support the boat to boat conformations for the D and E rings in 1. Corrections and correlations are made of some earlier literature assignments of friedelanones.