%0 Generic
%A Bach, Thorsten
%A Grosch, Benjamin
%A Strassner, Thomas
%A Herdtweck, Eberhardt
%D 2003
%T Enantioselective [6π]-Photocyclization Reaction of an Acrylanilide
Mediated by a Chiral Host. Interplay between Enantioselective
Ring Closure and Enantioselective Protonation
%U https://acs.figshare.com/articles/dataset/Enantioselective_6_-Photocyclization_Reaction_of_an_Acrylanilide_Mediated_by_a_Chiral_Host_Interplay_between_Enantioselective_Ring_Closure_and_Enantioselective_Protonation/3697149
%R 10.1021/jo026602d.s004
%2 https://acs.figshare.com/ndownloader/files/5787579
%K trans product ent
%K intermediates 8
%K chiral lactam 4
%K Enantioselective Ring Closure
%K trans compound ent
%K cis compound ent
%K amide 4 favor
%K chiral amide 4
%K DFT
%K enantioselective protonation
%K NMR
%K enantioselective ring closure
%K chiral complexing agent
%K conrotatory ring closure
%X The [6π]-photocyclization of the anilides 1a and 5 was studied in the absence and in the presence
of the enantiomerically pure chiral lactam 4. The relative configuration of the products was
unambiguously established by single-crystal X-ray crystallography and by NMR spectroscopy. A
significant enantiomeric excess was observed upon reaction of compound 1a to its photocyclization
products at −55 °C employing lactam 4 as a chiral complexing agent in toluene as the solvent
(66% yield). The trans product ent-3a was obtained in 57% ee, and the minor diastereoisomer (trans/cis = 73/27), cis product ent-2a, was obtained in 30% ee. DFT calculations were conducted modeling
the complexation of intermediates 8 and ent-8 to host 4. In agreement with steric arguments
concerning the conrotatory ring closure of 1a, the formation of ent-8 is favored leading to the more
stable complex 4·ent-8 as compared to 4·8. Whereas the enantioselectivity in the photocyclization
to trans compound ent-3a increased upon reduction in the reaction temperature, the enantiomeric
excess in the formation of cis compound ent-2a went through a maximum at −15 °C (45% ee) and
decreased at lower temperatures. Deuteration experiments conducted with the pentadeuterated
analogue of 1a, d5-1a, revealed that the protonation of the intermediates 8 and ent-8 is influenced
by chiral amide 4. In the formation of ent-3a/3a, both the enantioselective ring closure and the
enantioselective protonation by amide 4 favor the observed (6aS,10aS)-configuration of the major
enantiomer ent-3a. In the formation of ent-2a/2a, the enantioselective ring closure (and the
subsequent diastereoselective protonation) favors the (6aR,10aS)-configuration that is found in
compound 2a. Contrary to that, the enantioselective protonation by amide 4 shows a preference
for ent-2a with the (6aS,10aR)-configuration.
%I ACS Publications