Absolute Configurations of Melanoxadin, MR‐93A, Melanoxazal, and MR‐93B

Fungal metabolites melanoxadin, MR-93A, melanoxazal, and MR-93B were synthesized with the key stereogenic centers derived from commercially available chiral building blocks. The optically active synthetic products with well-defined absolute configurations provided authentic samples for the stereoisomers of these oxazole-containing natural products and thus allowed for unambiguous assignments of their relative and absolute configurations. The large discrepancies in the optical rotations between the natural and the pure synthetic samples are discussed. Some errors in the previously reported NMR signal assignments are also corrected.