In Silico Reassignment of (+)-Diplopyrone by NMR Calculations: Use of a DP4/J-DP4/DP4+/DIP Tandem to Revise Both Relative and Absolute Configuration.

The structural revision of natural (+)-diplopyrone (ND) was achieved by quantum NMR calculations. A DP4/J-DP4/DP4+ tandem suggested 3 as the most likely structure, but ECD calculations did not match the experimental values. The second more probable structure (6epi-1) showed the right ECD spectrum and high DP4/DP4+ probabilities obtained after fitting. However, further analysis of the MTPA-ND derivatives by DP4+/DIP calculations demonstrated that the absolute configuration at C-9 had been incorrectly assigned. Then, the structure of ND was proposed as ent-3.