Complex polycyclic diazo ketone

The total synthesis of the diterpenoid tropone, harringtonolide was accomplished in the laboratory of L.N. Mander. The key step to form the seven-membered ring was the Buchner reaction of a complex polycyclic diazo ketone intermediate. Upon treatment with rhodium mandelate, an unstable adduct was formed and was immediately treated with DBU to afford the less labile cycloheptatriene. 

While most of the initial studies have involved the transition metal-catalyzed decomposition of a-carbonyl diazo compounds and have been reviewed [3-51], it appears appropriate to highlight again some milestones of these transformations, since polycyclic structures could be nicely assembled from acyclic precursors in a single step. Two main reactivities of metalo carbenoids derived from a-carbonyl diazo precursors, namely addition to a C - C insaturation (olefin or alkyne) and formation of a ylid (carbonyl or onium), have been the source of fruitful cascades. Both of these are illustrated in Scheme 27 [52]. The two diazo ketone functions present in the same substrate 57 and under the action of the same catalyst react in two distinct ways. The initially formed carbenoid adds to a pending olefin to form a bi-cyclop. 1.0] intermediate 58 that subsequently cyclizes to produce a carbonyl ylide 59, that is further trapped intramolecularly in a [3 + 2] cycloaddition. The overall process gives birth to a highly complex pentacyclic structure 60. 

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