Page 31 - Tyrosine-Based Bioconjugations - Jorick Bruins

Another quinone methide bioconjugation method was developed by Lei et al., whom reported that a (4+2) cycloaddition reaction of a vinyl thioether with ortho-quinolinone could be employed for cellular organelle imaging (Scheme 10).125-127 After incubating quinolinone (38) at 37 ˚C, quinone methide derivatives (38) were formed that were found to undergo cycloaddition with a vinyl thioether. Michael addition of free thiols to the formed quinone methide was also observed, but due to reversibility of this reaction, the (4+2) cycloaddition product (40) was obtained as the main product. When biotin or fluorescein (FITC) were attached to the quinone methide precursor (41 and 42, respectively), labeling and imaging of vinyl thioether-bearing BSA and vinyl thioether-bearing Taxol inside live cells was achieved.125 A 2nd generation ortho- quinolinone (43) was developed with approximately 20-fold higher reaction rate (2.8 x 10-2 M-1 s-1 versus 1.5 x 10-3 M-1 s-1) and its bioorthogonality to SPAAC was demonstrated.127 While this method is chemoselective and bioorthogonal to the broadly applied SPAAC conjugation approach, the ortho-quinolinone quinone methide requires introduction of the vinyl thioether via a non-selective lysine-NHS conjugation strategy. Furthermore, with a reported rate of 2.8 x 10-2 M-1 s-1 for the second generation, the reaction is considerably slower (approximately 105- fold) the iminoquinone conjugation methods.110 Similar to acrylamides reaction with iminoquinones, cycloaddition takes place via normal electron-demand Diels–Alder cycloaddition on the oxygen and methylene group.
Scheme 10. (4+2) cycloaddition with vinyl thioethers and ortho-quinolinones, for the 2nd generation precursor: R1 = biotin or fluorescein piperazine.
General Introduction
29
1