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Nature produces single enantiomers and diastereomers of complex organic molecules with relative ease compared with the synthetic chemist. The question arises, however, as to whether two species can evolve to select different possible transition states in key biosynthetic steps. A team of researchers have now shown that the selectivity of an intramolecular Diels–Alder reaction differs in two closely related fungi.

Robert Williams and co-workers1 from Colorado State University have isolated a new natural product, (+)-versicolamide B, from a fungi — Aspergillus versicolor — collected in a Hawaiian forest. It is the first member of the stephacidin family of natural products found to exhibit a particular stereochemical relationship (anti) in the bicyclic part of the molecule. The structure of this compound was assigned by 2D-NMR spectroscopy and confirmed by performing a biomimetic total synthesis. The team also isolated single enantiomers of two other compounds (stephacidin A and notoamide B) that are known to be produced by other members of the Aspergillus genus, but with opposite stereochemistry.

The combination of these three metabolites from A. versicolor is noteworthy. It suggests that this species exhibits opposite facial selectivity to A. ochraceus in the Diels–Alder reaction — leading to the unusual conclusion that these two closely related fungi have evolved to produce opposite enantiomers of a number of natural products. However, the production of (+)-versicolamide B indicates that the reaction is less than completely diastereoselective. The corollary is that several other, as yet unisolated, metabolites are likely to be produced by various members of the Aspergillus genus, but an interesting question remains — why have two closely related species evolved this opposite preference, and how?