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).