Preparation of phenalenes and hydronaphthacenes through tandem alkyne Fischer-carbene complex coupling and inter- or intramolecular Diels-Alder reactions

The [5+5] route to the phenanthrene skeleton

This account describes the historical development of the coupling of γ,δ-unsaturated Fischer carbene complexes and o-alkynylbenzaldehydes, which directly affords hydrophenanthrene ring systems in a process where each reactant contributes five carbons to the newly-formed bicyclo[4.4.0]decane ring system. The process has been termed net [5+5] cycloaddition. Use of the reaction to produce various medicinally important natural products and/or their parent ring systems is discussed.

Total synthesis of (+)-antofine and (-)-cryptopleurine

The tylophorine alkaloid anticancer compounds antofine and cryptopleurine have been synthesized in optically active form. Both syntheses employ optically pure α-amino acids as the starting materials, require only seven steps from known 2-ethynylpyrrolidine or 2-ethynylpiperidine derivatives, and are free of protecting groups. Key steps include an alkyne hydration and a chromium carbene complex based net [5+5]-cycloaddition step. Alkyne hydration was accompanied by racemization of the resulting β-aminoketone under most of the conditions examined, and successful minimization of this side reaction was achieved through careful pH control and choice of metal additive. Final ring closure involves a Bischler-Napieralski reaction using a carbamate (antofine) or urea (cryptopleurine) precursor.