Radical reactions regiocontrol

The stereochemical requirements for the synthesis were fulfilled starting from enantiomerically pure alkynes 16, whereas the regiocontrol was achieved, when necessary (the [4-1-1] annulation can afford mixtures of quinoline derivatives, see Scheme 6), using ort/ o-(trimethylsilyl)-substituted arylisonitriles 15 (Scheme 7). In conclusion, the authors set up an asymmetric, regioselective, and widely applicable protocol that allowed the synthesis of more than fifty different compounds, proving once more the broad scope and functional-group tolerance of radical reactions. 

Another interesting reaction type detected from the photoreaction of tertiary amine-ketone system which have found use in alkaloids chemistry [172] involves sequential electron transfer and proton transfer followed by back electron transfer to yield iminium cations (Eq. 35) leading to dealkylation products upon aqueous workup. The regiocontrol in these dealkylations is dictated by preferential deprotonation at the less branched amine cation radical a-carbon. 

The reactions show a high degree of stereo- and regiocontrol, and apparently proceed through a radical derived by hydrogen abstraction from the allylic position of a 2- or 3-alkyl group. Attack occurs selectively at a methyl rather than an ethyl substituent, e.g. in 5, but when reaction at ethyl does occur, e.g. 5 - 6 or 8 9, the exocyclic alkene is produced with low... 

Scheme 3. Reactions top) and transition structures bottom) of 4-penten-l-oxyl radieal cyclizations. Quoted yields refer to c o(Bu3SnH) = 0.44 M. Shaded balls denote atoms and positions which are important in regiocontrol (6-endo-trig-reaction of 5) or in 5-exo-trig stereocontrol in substituted 4-penten-l-oxy 1-radicals (for example 12, Scheme 4, or 19 and 22, Scheme 5) [16, 45, 46]...

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