A short synthesis of (-)-dendrobine. Some observations on the nickel mediated radical cyclisation and on the Pauson-Khand reaction

The total synthesis of (-)-dendrobine was accomplished in 13 steps starting from (+)-verbenol. The key step is a radical cascade starting with a carbamyl radical generated by reaction of the N-benzoyloxy-carbamyl derivative of trans-verbenol with tributylstannane. This allows the one step establishment of three contiguous stereogenic centres, including the difficult cis-vicinal amino alcohol motif, and ultimately the control of the configuration of the remaining four asymmetric carbons. The synthesis also features the use of the Pauson-Khand reaction where an interesting solvent effect was observed. Thus, the use of a coordinating solvent such as acetonitrile was found necessary to suppress the formation of a ring-opened by-product resulting from scission of a C-N bond. In an ancillary model study, we observed an unusual but interesting 1,4-hydrogen atom abstraction which, however, thwarted our initial synthetic plan.