Syntheses and Preliminary Biological Evaluations of the Dibromopyrrole‐Containing Marine Natural Products Agesasine A, Agesasine B, Longamide E and Various Congeners

Abstract

Total syntheses of the title marine natural products have been achieved and so confirming the structures originally assigned to them. Upon subjecting agesasine A and its corresponding ethyl ester to Mitsunobu conditions, a 1,5-cyclodehydration reaction takes place to give 2-oxazolines. In contrast, on subjecting agesasine B to the same Mitsunobu conditions, a simple dehydration reaction occurs to give the corresponding acrylate. A total synthesis of longamide E was achieved by engaging a 1,2-disubstituted pyrrole in a lactam-forming reaction and this was followed by a two-fold and fully regio-controlled bromination reaction. A distinctly different and possibly biomimetic route was used to synthesize, via the open-chain natural product nakamurine B, longamide B and its methyl ester. Preliminary biological evaluations of the title alkaloids and various analogues against a small human cancer cell line panel reveals cytotoxic properties that vary significantly with structure.

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

“…As a result of our ongoing interest in establishing abbreviated total syntheses of pyrrole-containing marine alkaloids, an ever-expanding class of fascinating natural product, we undertook, as a prelude to the present study, total syntheses of compounds 4 and 5 . The symmetrical nature of the latter target and the possibility that this could serve as a precursor to the former prompted our pursuing denigrin E ( 5 ) as the initial objective of our studies.…”