An Approach to the Total Synthesis of Diterpenoid Plant Growth Substances Isolated From Gametophytes of the Fern Genus Anemia

Reduction of the oxoindan acid (9) by potassium in liquid ammonia followed by in situ alkylation with 3-methylbut-2-enyl bromide furnished enol ether (14) which was transformed into the acetal (17) and thence aldehydes (18; R = COCH3 and COCHCl2). Acid- catalysed aldol reactions then afforded the ethanoindene derivatives (19; R=COCH3 and COCHCl2). Further elaboration gave carbinol (28) which is envisaged as an intermediate for the total synthesis of the diterpenoid antheridiogens obtained from gametophytes of the fern genus Anemia, including acid (2). The relative stereochemistry of acetal (17) was established by X-ray crystallographic analysis.