Stereo- And regioselectivity in an intramolecular nitrone-alkene cycloaddition of hept-6-enoses with a trans-acetonide blocking group

From sugar to cycloadduct: The effect of the trans-acetonide blocking group and the stereochemistry of the substituents on the regio- and stereoselectivity in the intramolecular nitrone–alkene cycloaddition (INAC) reaction of hept-6-enoses (see scheme) is reported and studied by using theoretical analysis. The positional effect of the trans-acetonide blocking group and the effect of the stereochemistry of the substituents on the regio- and stereoselectivity in intramolecular nitrone–alkene cycloaddition (INAC) reactions of hept-6-enoses are reported. Hept-6-enoses with a 2,3-trans-acetonide group were reacted with N-alkyl hydroxylamine to give a mixture of exo and endo cycloadducts (cyclohexanols and cycloheptanols, respectively). Complete formation of endo cycloadducts (cycloheptanols) was realized for the INAC reaction of hept-6-enoses containing a 3,4-trans-O-isopropylidene ring. Similarly, reaction of a hept-6-enose possessing a 4,5-trans-acetonide group surprisingly afforded exo cycloadducts (cyclohexanols) exclusively. The regio- and stereochemical outcomes of these reactions were rationalized on the basis of transition-state energies obtained by computation.