Asymmetric Induction in the Amine-Induced Ring-Opening of 3-Bromo-5-ethyl-2-isopropylthiophene 1,1-Dioxide using L-Prolinol.
作者:Anders Tsirk、Salo Gronowitz、Anna-Britta Hörnfeldt、Olle Matsson、Jadwiga Trocha-Grimshaw、Ole Hammerich、Inger Søtofte、Bengt Långström
DOI:10.3891/acta.chem.scand.52-0533
日期:——
The diastereomers (1-(2S)-[(2R) (3E,5Z)-5-bromo-7-methyl-3,5-octadien-2-yl]-tetrahydro-1H-pyrrol-2yl) methanol (6) and (1-(2S)-[(2S)(3E,5Z)-5-bromo-7-methyl-3, 5-octadien-2-yl] tetrahydro-1H-pyrrol-2-yl) methanol 7) were formed in a 65:35 ratio from 3-bromo-5-ethyl-2-isopropylthiophene, 1,1-dioxide (5) and L-prolinol. (5R,5aS,8R,9R,9aS,9bS)-Ethyl 7-bromo-8-isopropyl-5-methyl-2,3 ,5,5a,8,9,9a,9b-octahydro-1H-pyrrolo[2,1-a] isoindole-9-carboxylate (10) and (5S,5aS,8R,9R,9aS,9bS)-ethyl 7-bromo-8-isopropyl-5-methyl-2,3,5,5a,8, octahydro-1H-pyrrolo[2,1-a]isoindole-9-carboxylate (12) were synthesized from 6 and 7 in order to establish the absolute configuration of the ring-opened products. The asymmetric induction at C(2) in the ring-opened products can be explained by the preferential formation of E tautomers over Z tautomers and the preferential formation of the E(2S) (15) and Z(2S) (17) enantiomers over the E(2R) (14) and Z(2R) (16) enantiomers when 5 tautomerizes. The tautomers form complexes with the L-prolinol dimers, and the enantiotopic face that will preferentially be attacked by another L-prolinol equivalent in a Michael addition will be the one anti to the dimer and syn to the isopropyl group.