Stereoselective Reactions of Phthalimido‐Substituted Radicals Derived from (±)‐Threonine: A comparison with reactions of N ‐phthaloyliminium ions

Stereoselective reactions of phthalimido‐substituted radicals derived from (±)‐threonine with different radical traps are reported ( Scheme 3, Table 1 ). A strong influence of the nature of the radical trap on the stereoselectivity was noticed. Small nucleophilic radical traps gave preferentially the syn products. The observed selectivities are explained with the A 1,3 strain model and depend on steric and electronic effects ( Fig. 2 ). Reactions with electrophilic radical traps such as diphenyl diselenide gave the anti diastereoisomers with moderate stereocontrol, presumably due to stereoelectronic effects. The same stereochemical outcome, i.e. , preferential formation of the anti products, was observed for the reactions of the related N ‐phthaloyliminium ion ( Scheme 5, Table 2 ). The stereochemistry of the ionic reaction is rationalized by a Felkin‐Anh model ( Fig. 3 ).