Exploring a Unique Reactivity of 6π-Azaelectrocyclization to Enzyme Inhibition, Natural Products Synthesis, and Molecular Imaging: An Approach to Chemical Biology by Synthetic Chemists

While elucidating the inhibitory mechanism of a hydrolytic enzyme by aldehyde-containing natural products, we discovered a reaction involving a rapid 6 pi-azaelectrocyclization of azatrienes generated from aldehyde with lysine residues. The electrocyclic reaction of the 1-azatriene system, a cyclization precursor, exhibited a substituent effect. Structure-reactivity studies showed that azaelectrocyclization, which usually proceeds in low yield at high temperatures, produced a quantitative yield in less than 5 minutes at room temperature. Asymmetric chiral piperidine synthesis and a one-pot library synthesis of pyridines on solid supports were applied to synthesize pyridine/indole alkaloid-type natural products. Additionally, we developed lysine-based labeling and engineering of biomolecules and living cells based on the rapid 6 pi-azaelectrocyclization. Both 1,4,7,10-tetraazacyclododecane1,4,7,10- tetraacetic acid (DOTA) as a metal chelating agent and fluorescent groups, as well as oligosaccharide structures were introduced efficiently and selectively into surface lysines within 10 minutes at concentrations as low as 10-8 M. The DOTA-labeled somatostatin and glycoproteins were then radiometallated with 68Ga; the receptor-mediated accumulation of somatostatin in pancreas and the oligosaccharide-dependent circulatory residence of glycoproteins were visualized by microPET for the first time. Furthermore, we succeeded to image the trafficking of the fluorescence-labeled lymphocytes noninvasively, while the N-glycan-engineered lymphocytes targeted the colon carcinoma in tumor mouse model; the tumor-targeting cells were thus synthesized using our 6 pi-azaelectrocyclization.