We report the determination
of the absolute configuration of a
diterpenoid, namely, ballonigrin lactone A (BLA), by comparison of
the computed optical rotations, [α]D, of its two
diastereomers using density functional theory (DFT) calculations to
the experimental [α]D value of +22.4. One of the
diastereomers having configurations 4S, 5R, 6S, 10S, 15S was named
“α-BLA,” and the other one with configuration
4S, 5R, 6S, 10S, 15R was called “β-BLA”. Six conformers
for each diastereomer (α-BLA and β-BLA) of BLA were identified
through their conformational analysis. [α]D values
of these six conformations for each diastereomer were calculated using
DFT at the mPW1PW91/6-311G(d,p)/SMDChloroform level of
theory, leading to the conformationally averaged [α]D values of −96.8 for α-BLA and +65.1 for β-BLA.
Thus, it was found that the experimental [α]D value
of +22.4 was of 4S, 5R, 6S, 10S, 15R, i.e., β-BLA. Experimental
and computed nuclear magnetic resonance (NMR) data were also compared,
and this comparison was in accordance with the conclusion drawn from
the comparison of [α]D values. Finally, the results
were augmented with the calculation of the DP4 analysis, and the probability
obtained also endorsed our earlier calculations.
