Synthesis of Bridged Azabicycles from Pyridines and Pyrrole by a Diallylboration – Ring Closing Metathesis Sequence

Abstract

AbstractA straightforward method for the preparation of bridged azabicycles by a diallylboration–ring closing metathesis (RCM) sequence was elaborated. The precursors for the metathesis reaction were prepared by reductive trans‐diallylation of aromatic heterocycles such as pyridines, isoquinoline and pyrrole with triallylborane and an alcohol. Heating of the trans isomers with one equivalent of triallylborane allowed the corresponding cis isomers – or (in the case of pyrrole) a mixture of cis and trans isomers – to be obtained. Boc protection of the amine function gave the precursor for the RCM reaction with axial orientations of allylic groups. The chemical yields of this stage are very good and the amounts of Grubbs’ catalyst required for the reaction do not exceed 2.5 mol‐% (first generation cat.) or 3.7 mol‐% (second generation cat.). Azabicyclo[4.2.1]nonene, obtained from pyrrole, is a key structural motif of the toxin pinnamine. The structures of cis‐1,3‐diallyl‐1,2,3,4‐tetrahydroisoquinoline and azabicyclo[4.3.1]decadiene were studied by single‐crystal X‐ray analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)