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Seven new diterpenes, helioporins A-G (1–7), have been isolated from the blue coral Heliopora coerulea . The structures determined by spectroscopic data and chemical conversions are related to the aglycons of pseudopterosins and seco-pseudopterosins, which are antiinflammatory glycosides from Caribbean gorgonians. Treatment of helioporin A ( a ) with TMSI/NaI followed by Na 2 S 2 O 3 gave helioporin E ( 5 ) and an unusual product ( 11 ) resulting from CC bond cleavage. Reaction of helioporin B ( 2 ) with Tf 2 O/C 5 H 2 NMe t -Bu 2 followed by catalytic reduction gave a cyclization product ( 14 ) which was identical with a compound derived from 5 . All helioporins were chemically correlated and their relative stereochemistry was related to that of the aglycon of pseudopterosin G ( 10 ). Helioporins A and B showed antiviral activity against HSV1 and C-G cytotoxicity against P388.

A revised stereostructure for helioporin D was unequivocally established by its synthesis from the seco-pseudopterosin aglycone and by careful analysis of NMR data. As the corresponding benzodioxole derived from the pseudopterosin A aglycone was not identical with helioporin E, it was proven that helioporin D and E do not belong to the same stereochemical series.