Naturally occurring 5-methyoxy-3(2H)-furanones. Re-assignment of structures to the aspertetronin group of natural products

Synthetic and spectroscopic investigations establish that the aspertetronin (also known as gregatin and graminin) group of natural products isolated from $\text{Aspergillus}$ and $\text{Cephalosporium sp}$., has the 5-methoxy-3(2H)-furanone structure (4) rather than the previously assigned O-methyl tetronic acid structure (3).