Total synthesis tetronasin (1), a tetronic acid polyether ionophore antibiotic, is reported in which the γ-unsubstituted α-acyltetronic acid structure is constructed by cyclization of a β-keto ester derivative 18 derivedfrom tricyclic aldehyde intermediate 17 by reaction with methyl (diazoacetoxy)acetate in the presence of zirconium tetrachloride.
An efficient entry to the tetrahydropyran/cyclohexane moiety of tetronasin has been developed. An aldol reaction between a cyclohexanecarboxaldehyde, 8, and a (tetrahydropyranyl)acetate, 9, under controlled conditions followed by dehydration of the adduct 10 afforded predominantly (E)-ester 11, which on photoisomerization and subsequent reduction with iso-Bu2AlH provided the B/C ring system 13.