A Route to the C,D,E Ring System of the Aspidosperma Alkaloids.

A short synthetic sequence leading to the formation of the C,D,E-ring subunit of the Aspidosperma alkaloids is reported. This route is based on a ring fragmentation/intramolecular azomethine ylide 1,3-dipolar cycloaddition reaction sequence that gives the desired tricyclic product as a single diastereomer. A γ-amino-ß-hydroxy-α-diazo carbonyl compound is shown to fragment in the presence of a Lewis acid to give an iminium product that can be directly reduced to the corresponding amine.

Synthesis of demissidine by a ring fragmentation 1,3-dipolar cycloaddition approach.

A synthesis of the steroidal alkaloid demissidine from epiandrosterone is reported. A ring fragmentation reaction that efficiently ruptured the D-ring of a diazo ester derivative of epiandrosterone to provide an aldehyde tethered ynoate product was key to this sequence. Incorporation of the indolizidine framework was achieved by an azomethine ylide 1,3-dipolar cycloaddition.