Synthesis of Cycloprodigiosin Identifies the Natural Isolate as a Scalemic Mixture

The
enantiomers of the natural product cycloprodigiosin were prepared
using an expedient five-step synthetic sequence that takes advantage
of a Schöllkopf–Barton–Zard (SBZ) pyrrole annulation
with a chiral isocyanoacetate and a nitrocyclohexene derivative. Using
chiral HPLC and X-ray crystallographic analyses of the synthetically
prepared material and natural isolate (isolated from the marine bacterium Pseudoalteromonas rubra), naturally occurring cycloprodigiosin
was determined to be a scalemic mixture occurring in an enantiomeric
ratio of 83:17 (R)/(S) at C4′.