Fluorescence imaging is a powerful technique for visualizing and quantifying dynamically changing biological events with high spatial resolution and sensitivity, and it has been widely applied in both fundamental biological research and clinical practice. However, conventional “always-on” fluorescent probes emit signals irrespective of their localization, often resulting in high background fluorescence and low imaging contrast in biological environments. To address these limitations, activatable fluorescent probes, which switch from a non-emissive “off” state to an emissive “on” state only in response to specific biological stimuli, have attracted increasing attention owing to their high signal-to-noise ratios and improved detection sensitivity. A variety of activation mechanisms have been developed, including the removal of quenching groups, changes in molecular planarity, and modulation of π-conjugation length through elimination reactions. Among these strategies, probes activated by recovery of the π-conjugated system provide unique opportunities for rational molecular design. In particular, cyanine dyes incorporating nucleophilic functional groups represent a promising yet relatively underexplored platform for activatable probe development. In this account, we summarize our recent efforts on the design and synthesis of nucleophile-containing cyanine-based activatable fluorescent probes. We focus on enzyme-responsive systems, including probes activated by esterases and aldehyde dehydrogenase, and discuss their photophysical properties, activation mechanisms, and applications in biological imaging.

In contrast to the considerable progress in the development of methodologies for amide bond formation in amines (i.e., primary amines, secondary amines, anilines), the development of direct N-acylation of less nucleophilic N-heterocycles and amides with carboxylic acids is still challenging. In this article, we describe the direct N-acylation of less nucleophilic heterocycles and amides with carboxylic acids promoted by the 4-(N,N-dimethylamino)pyridine N-oxide (DMAPO)/di-tert-butyl dicarbonate (Boc₂O) system. The new one-pot method, which does not involve pre-activation of substrates, enables the direct N-acylation of a wide variety of nitrogen nucleophiles such as indole, carbazole, pyrrole, pyrazole, lactam, and anilide with carboxylic acids in high yield. This method also enables one-pot direct synthesis of bulky N-acyl heterocycles starting from a wide variety of less nucleophilic N-heterocycles and sterically hindered α-fully substituted carboxylic acids. Recently, we have also successfully developed a direct and selective N1-acylation of indazole by utilizing this method. In addition, a new synthesis of N1-functionalized alkyl indazoles utilizing N1-acyl indazoles as starting materials was achieved. The new protocol is useful for the selective synthesis of structurally diverse N1-functionalized alkyl indazoles, which are difficult to synthesize by other methods such as the Mitsunobu reaction and classical S_N2 alkylation of indazole. Our protocol is also amenable to one-pot direct N-acylation of sulfoximines and can be carried out under mild reaction conditions. Furthermore, we demonstrated a simple and practical synthesis of N-acyl oxazolidinones which can then be used in various asymmetric transformations. The new method exhibits excellent functional group tolerance and broad substrate scope. As the present method is practical, operationally simple, and scalable, it should find wide applications in both academic and industrial laboratories.

Nucleophilic addition to carbonyl compounds is one of the most fundamental transformations in organic synthesis. Carbonyl carbon atoms serve as electrophilic carbinol cation synthons in reactions with nucleophiles. Umpolung of the carbonyl reactivity should permit carbonyl compounds to react as nucleophilic carbinol anions with electrophiles. We have developed photocatalytic umpolung reactions of carbonyl compounds to generate anionic carbinol synthons through multielectron reduction. Here two approaches are presented. The first approach exploits carbon dioxide-promoted electron transfer (CO₂ET) process. Under photocatalytic conditions in the presence of CO₂, aromatic aldehydes and ketones undergo two-electron reduction to generate carbinol anion equivalents. The resulting nucleophilic species participate in a range of C-C bond forming reactions, including carboxylation, cross-pinacol coupling with second carbonyl compounds, and 1,4-addition to electron-deficient olefins. This concept is further extended to α,β-unsaturated carbonyl compounds, where homoenolate anion equivalents are generated through CO₂ET process. The second approach relies on a newly developed diazabenzacenaphthenium photocatalyst (N-BAP) with high photoredox abilities and visible-light absorption. In combination with ammonium oxalate as a traceless reductant, N-BAP promotes unprecedented four-electron reduction of esters to generate carbinol anion equivalents. The resulting carbinol anions undergo protonation to afford alcohols and react nucleophilically with carbonyl electrophiles to form unsymmetric 1,2-diols.

Amycolamicin, which exhibits potent antibacterial activity against drug-resistant bacteria including MRSA, VRE, PRSP, and BLNAR, was discovered from the soil actinomycete Amycolatopsis sp. MK575-fF5 and Kibdelosporangium sp. MA7385 by the Igarashi and Singh groups, respectively. The unique hybrid structure of amycolamicin combined with its promising biological activity attracted significant attention from the synthetic community. Herein, we describe the details of our convergent total synthesis of amycolamicin, which features: (1) a protecting group-free intramolecular Diels-Alder reaction of a hydroxy tetraenal to construct a trans-decalin system in a highly diastereoselective manner; (2) a diastereoselective nucleophilic addition of a p-methoxybenzyloxy-substituted vinyllithium reagent to an α,β-bisalkoxy ketone intermediate to provide the corresponding tertiary alcohol as a single diastereomer; (3) a β-selective glycosylation of a trans-decalinol intermediate using a bicyclic N,O-acetal as a glycosyl donor; (4) a completely stereoconvergent N-acylation of an anomeric N-glycoside mixture with a β-keto thioester; and (5) a nucleophilic ring-opening of a cyclic carbonate protecting group with 2,4-dimethoxybenzyl amine to install a β-hydroxy carbamate structure.

New natural products are the starting point for related research areas such as drug development, biosynthesis, mode of action, and organic synthesis. However, intensive screening activity over the past decades has led to frequent reisolation of known compounds. With the first priority to obtain new chemical structures, we have employed HPLC/UV-guided chemical screening instead of activity-guided screening. In this article, new compounds discovered in our laboratory are presented, focusing on structural features, structure determination, biosynthesis, and synthetic studies.