Corrected Structure of Natural Hyacinthacine C1 via Total Synthesis

Cross-Coupling as a Key Step in the Synthesis and Structure Revision of the Natural Products Selagibenzophenones A and B

Selagibenzophenone A (1) and its isomer selagibenzophenone B (2) were recently described as natural products from Selaginella genus plants with PDE4 inhibitory activity. Herein, we report the first total syntheses of both compounds. By comparing spectroscopic data of the synthetic compounds with reported data for the isolated material, we demonstrate that the structure of one of the two natural products was incorrectly assigned, and that in fact isolated selagibenzophenone A and selagibenzophenone B are identical compounds. The synthetic strategy for both 1 and 2 is based on a cross-coupling reaction and on the addition of organometallic species to assemble the framework of the molecules. Identifying a suitable starting material with the correct substitution pattern is crucial because its pattern is reflected in that of the targeted compounds. These syntheses are finalized via global deprotection. Protecting the phenols as methoxy groups provides the possibility for partial control over the selectivity in the demethylation thanks to differences in the reactivity of the various methoxy groups. Our findings may help in future syntheses of derivatives of the biologically active natural product and in understanding the structure–activity relationship.

New total synthesis and structure confirmation of putative (+)-hyacinthacine C3 and (+)-5-epi-hyacinthacine C3

A unique synthesis of polyhydroxylated pyrrolizidine alkaloids, namely (+)-hyacinthacine C₃ and (+)-5-epi-hyacinthacine C₃ is presented. The strategy relies on a 1,3-dipolar cycloaddition of an l-mannose derived nitrone, which owing to its great syn-stereoselectivity builds up the majority of the required stereocenters. The following key steps include Wittig olefination and iodine-mediated aminocyclisation, that provide two epimeric pyrrolizidines with the appropriate configuration. As a result, structure and steric arrangement of the first synthetically prepared (+)-hyacinthacine C₃ are proved to be correct, clearly confirming the inconsistency with the stereochemistry assigned to the natural sample. With respect to the previously proven glycosidase inhibitory activities, the antiproliferative effect of (+)-hyacinthacine C₃ and (+)-5-epi-hyacinthacine C₃ was evaluated using several cell line models.

A second total synthesis of (+)-hyacinthacine C₃ is reported. As a result, structure of the first synthetically prepared alkaloid is proved to be correct, clearly confirming the inconsistency with the stereochemistry assigned to the natural sample.