Total Syntheses of the Structures Assigned to Denigrins A, B, C, F, and G, 3,4-Diaryl-Pyrrole and -Pyrrolidinone Alkaloids, and the Conversion of Congener B into the Co-metabolite Spirodactylone

The title marine natural products have been prepared
by total synthesis
and in the case of congeners 3, 6, and 7 for the first time. Each of these was obtained by manipulation
of readily prepared denigrin B (2). The structure, 3, assigned to denigrin C is shown to be incorrect. Reaction
of compound 2 with DDQ has led, in high yield, to the
related natural product spirodactylone (16), while treating
the corresponding permethyl ether 15 with PIFA/BF3·Et2O provides compound 20, embodying
an isomeric framework.