Enantiospecific syntheses of pseudopterosin aglycones.: Part 2.: Synthesis of pseudopterosin K-L aglycone and pseudopterosin A-F aglycone via a B→BA→BAC annulation strategy

The enantiomeric aglycones of pseudopterosins K-L and A-F are synthesised from (-)- and (+)-isopulegol respectively. Key features are (a) the construction of the C3 stereogenic centre by a directed ep oxidation-reduction sequence (K-L); (b) the creation of the C3 stereogenic centre by a Pfaltz asymmetric conjugate reduction (A-F); (c) benzannulation of a cyclic ketone starting with an alpha -oxoketene-S,S-acetal to give a tetrahydronaphthol ether; and (d) a diastereoselective intramolecular electrophilic aromatic substitution using an allylic sulfone as the electrophilic trigger to complete the hexahydro-1H-phenalene core. An X-ray structure of compound 50 was determined.