Abstract
| - The effect of blocking groups and stereochemistry of the substituents on the regio- and stereoselectivityin intramolecular nitrone−alkene cycloaddition (INAC) of hept-6-enoses are reported. l-ribo-Hept-6-enose 12 and d-lyxo-hept-6-enose 15, both containing a 2,3-O-isopropylidene blocking group, and l-xylo-hept-6-enose 23 and d-arabino-hept-6-enose 30, both with a 2,3-O-trans-diacetal blocking group, wereprepared from d-ribose and d-arabinose, respectively. With N-alkyl hydroxylamine, lactols 12 and 15underwent an INAC reaction to give cis-fused isoxazolidines exclusively whereas lactols 23 and 30 gavea mixture of cis-, trans-fused isoxazolidines (cyclohexanols) and bridged isoxazolidines (cycloheptanols).With the 2,3-O-trans-diacetal protecting group, the bridged bicyclo[4.2.1]isoxazolidines (cycloheptanols),via the endo mode of INAC cyclization, were synthesized for the first time from unbranched sugarderivatives 23 and 30. The stereochemical outcomes of these reactions were rationalized on the basis oftransition state energies obtained by computation. The present INAC showed trivial temperature, butsignificant solvent dependence. For lactols 23 and 30, performing the INAC in 2-propanol gave the bestyields of fused isoxazolidines (cyclohexanols) whereas in dichloromethane afforded the best yields ofbridged isoxazolidines (cycloheptanols).
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