Organic chemistry is constantly concerned with effecting reactions at a particular centre in a complex molecule, and if possible with a high and predictable level of stereoselectivity. In the light of much accumulated ex perience within organic chemistry it is usually possible to assess the likeli hood of alternative reaction pathways at least qualitatively. However, well based expectations can be falsified, and the experiments directed to the synthesis of vitamin B12 which led to Woodward's recognition of orbital symmetry control in organic chemistry are an instructive example. Our limi...