An experimental and theoretical study of the asymmetric lithiation of 1,2,3,5,6,7-hexahydro-3a,4a-diazacyclopenta[def]phenanthren-4-one |
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Authors: | Metallinos Costa Dudding Travis Zaifman Josh Chaytor Jennifer L Taylor Nicholas J |
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Institution: | Department of Chemistry, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada. metallic@brocku.ca |
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Abstract: | The inability of bis-N-Boc-protected octahydrophenanthroline to undergo asymmetric lithiation with (-)-sparteine is circumvented by use of a urea functionality as the directing group. Asymmetric lithiation followed by electrophile quench gives products substituted alpha to nitrogen in better yield (17-30%) but slightly lower enantiomeric ratio (er 84:16) than analogous lithiation of N-Boc-piperidine (er 87:13). Computational studies at the MP2/6-316(d)//B3LYP/6-316(d) level indicate that the prochiral equatorial S-hydrogen is removed preferentially over the pro-R hydrogen, with a difference in transition state activation energies of 1.26 kcal/mol, corresponding to a predicted er of 89:11. The predicted stereochemistry of the reaction was confirmed by single-crystal X-ray analysis of an aryl dibromide prepared from the enantiomerically enriched alpha-methyl-substituted product. |
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