SYNTHESIS OF THE BICYCLO[3.3.1]NONANE CORE OF HUPERZINE A AND NOVEL PYRIDINE-FUSED TRICYCLES BY CYCLISATION OF PYRIDINE-BASED RADICALS

The cyclisation of 3-pyridyl radicals and (2-pyridyl)methyl radicals, generated from (pyridyl)cyclohexenols 5 to 8, has been examined as part of a model study directed towards the synthesis of huperzine A. The 3-pyridyl radical, generated from 3-bromopyridine 5, undergoes 5-exo-trig cyclisation to give hexahydroindenopyridine 10. Related pyridine-fused tricycle 12 is formed by 5-exo-trig cyclization of the (2-pyridyl)methyl radical derived from selenide 7b, while the radicals generated from selenides 8b and 19 undergo 6-exo-trig cyclisation to give the bicyclo[3.3.1]nonane core of huperzine A.