New synthetic pathways from gibberellins to antheridiogens isolated from the fern genus anemia

Gibbereliin A7 (4) was converted into the antheridiogens 2 and 3 isolated from cultured gametophytes of ferns belonging to the genus Anemia by two independent routes. The first of these established the 9,15-cyclogibberellane
skeleton of 3 by means of an intramolecular alkylation on the 9-ene-l-iodo-16-one 8 which was obtained from the triene acid 7; the 10-hydroxyl of 3 was introduced by syn-SN2' substitution of the 30-mesylate derived from
10. In a second, more efficient approach, lactone 17 was treated with an excess of jV-bromosuccinimide to form dibromide 19, which was also readily converted to a cyclogibberellane derivative, 20. This could be converted
into both antheridiogen 2 from Anemia phyllitidis and antheridiogen 3 from Anemia mexicana. Important features of these syntheses are the solvent-controlled modulation of the effect of the amidine base DBU in the preparation
of 32 and the use of diphenyl- and dimethylboron bromide to effect the contrathermodynamic 1,3-allylic isomerization of lactones 9, 20, and ll,12-dihydro-32.