Allylic alcohols and aziridines from alkenes : Studies in asymmetric catalysis
Abstract: The development of methodology for asymmetric catalytic transformation of alkenederivatives is summarized. The studies concern the [a] Cu(I)-catalyzed allylic oxidation,[b] isomerization of epoxides to allyl alcohols, and [c] Cu(I)-catalyzed alkeneaziridination, respectively.[a] The preparation of exo-2-azabicyclo[2.2.1]heptane-3-carboxylic acid 7a is described, along with its application as ligand for the allylic oxidation of cyclopentene and cyclohexene with peresters. The use of 7a resulted in higher yields and levels of asymmetric induction compared to those obtained with (L)-proline, and gave the corresponding 2-cycloalkenyl benzoates of 65% ee, in up to 63% yield.[b] Diamines 15 and 39 were prepared and evaluated as catalysts for the base-mediated rearrangement of meso- and racemic epoxides. Treatment of epoxides with stoichiometric amounts of lithium diisopropylamide and 5 mol% of (1S, 3R, 4R)-3-(1-pyrrolidinyl)methyl-2-azabicyclo[2.2.1]heptane 15a or (2R, 5R)-methylpyrrolidinyl analogue 34b gave the corresponding allylic alcohols in good yields and with excellent enantioselectivity. An average of 95% enantiomeric excess (ee) was achieved in the synthesis of 12 allylic alcohols, several of which are valuable precursors to pharmaceuticals and natural products. Studies of non-linear effects indicate that the reactive species is a monomeric Li-15a, and that the formation of less reactive di- or oligomers is effectively suppressed by the presence of Lewis basic co-solvents.[c] A set of [N-(4-R-benzenesulfonyl)imino]phenyliodinanes 41a-h were evaluated as nitrene precursors for the alkene aziridination. For six out of seven alkenes studied, 41b or c (R=OMe, NO2) gave higher yields and enantioselectivity than those obtained with the commonly used 41a (R=CH3).
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