Synthesis of Citrifurans A and D through Assembly of the Natural Products Citrinin and Gregatin A.

The synthesis of citrifuran A, a structurally unique natural heterodimer, was accomplished in a bioinspired manner by a thermal [4 + 2] cycloaddition between citrinin and gregatin A, the former of which was synthesized via a Diels-Alder-initiated aromatization reaction and the latter via a Cu-mediated 5- cyclization-bromination as the key steps, respectively. On the other hand, an acid/base-promoted cycloaddition of citrinin with gregatin A followed by hydrolysis of the resulting intermediate furnished citrifuran D.