| Popis: |
The deprotonation of β-stereogenic alkyl carbamates 8 with sec-butyllithium/TMEDA proceeds kinetically controlled with differentiation between the diastereotopic protons at the α-methylene group. The lithium intermediates 9A and 9B are trapped by electrophiles to give carboxylic acid esters 10A/B and stannanes 11A/B, 13A/B. The diastereomeric ratio of A to B is independent of the electrophile. The highest efficiency is achieved with β-aryl-β-alkyl-substituted starting compounds 8d–f. The trends in the experimentally observed diastereoselectivities are reflected by the results of semiempirical PM3 calculations on the transition states of the deprotonation reaction. — As a result of double stereodifferentiation, application of the chiral base sec-butyllithium/(-)-sparteine gives rise to efficient kinetic resolution of racemic alkyl carbamates. |
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