In the field of plant science, strigolactones (SLs) are one of the groups of compounds that have received the most attention in recent years. The first SL, strigol, was introduced to the world of natural product chemistry in 1966. For the next 40 years, SLs have been recognized as seed germination stimulants for root parasitic weeds. However, this situation has changed drastically in the past 15 years, as SLs were reported to be signaling substances in arbuscular mycorrhizal symbiosis in 2005 and a new class of plant hormones regulating shoot branching in 2008. Since then, attention to SL has increased dramatically, and SL-related research has been progressing rapidly. However, while there are many review articles written from the perspective of plant science, there are very few written from the perspective of synthetic organic chemistry. Therefore, in this paper, I would like to review the history of SL research and introduce SL research from the viewpoint of synthetic organic chemistry, focusing on my own research. In addition to the synthetic studies on natural SLs, the application of artificial SL analogs for the control of root parasitic weeds and attempts to help elucidate the biosynthetic pathway of SLs are also presented.

α-Methylene-γ-butyrolactone is a pharmaceutically important motif which is often found in bioactive molecules. Allylation of ketones or aldehydes with β-ester-functionalized allylating agents is known to be one of the reliable synthetic methods for the methylene lactones, while reactions using β-amido-functionalized ones, which provide the precursors for methylene lactones and analogous methylene lactams, have not been fully explored. Herein, we summarized our research on the development of selective syntheses of α-methylene-γ-butyrolactones and α-methylene-γ-butyrolactams. Addition of β-amido-functionalized allylstannanes to N-carbonyl imides was catalyzed by indium chloride and zinc chloride, providing methylene-lactam-fused spirolactones through unique ring opening-reclosure. These allylstannanes were applied to the highly enantioselective syntheses of methylene lactones and methylene lactams, in which indium-catalyzed asymmetric allylation of α-ketocarbonyl compounds was established. We also developed β-amido-functionalized allylboronates as an alternative allylating agent. These reagents showed sufficient nucleophilicity toward N-carbonyl imides in the presence of catalytic zinc bromide and basic additives, and the resulting adducts were employed as a common intermediate for the syntheses of methylene-lactone- and methylene-lactam-based spiro compounds. Furthermore, the remarkable water-stability of the allylboronates led to the development of asymmetric allylation of acetophenones in water. The reaction was catalyzed with zinc hydroxide and a chiral aminophenol reagent, affording a variety of homoallylic alcohols with up to 98% ee.

The automated electrochemical assembly is one of our arrial points for development of electrochemical methods for oligosaccharide synthesis. The one-pot sequential process of activation of thioglycoside building block by electrochemical oxidation and coupling with thioglycoside building block can be carried out in automated manner, and muliple cycles of the process enables us to synthesis oligosaccharides with desired chain length. We have synthesized oligoglucosamines with β-1,6- and β-1,4-glycosidic linkages. Total synthesis of TMG-chitotriomycin and Myc-IV (C16 : 0, S), which are biologically active oligoglucosamines derivatives, has also achieved. Ionic liquid tags were introduced to glucosamine building blocks to facilitate separation of products; however, they worked not only separation tag but also electrolyte and stereo-controlling group in electrochemical glycosylation. We also found that electrochemical glycosylation was also useful to convert linear oligosaccharides of glucosamines into the corresponding cyclic oligosaccharides.

Transition-metal-catalyzed C-C bond activation is one of the most fascinating reactions to construct new C-C bonds because it can convert a simple carbon skeleton directly to a highly complicated carbon framework. In particular, transformations of biphenylene via C-C bond activation are well-known for the synthesis of polycyclic hydrocarbons such as phenanthrene and tetraphenylene. Since the products derived from biphenylene were still limited, there was room for investigation of biphenylene chemistry. Herein we summarize our recent progress for the new reactions of biphenylene by transition-metal catalysts to synthesize various polycyclic hydrocarbons. We achieved an iridium-catalyzed intermolecular [4＋1] cycloaddition of biphenylenes with alkenes to give 9,9-disubstituted fluorene derivatives and we conducted a mechanism study by DFT calculations. In addition, we developed a new strategy to cleave the sterically hindered C-C bond of biphenylene by using an alkene and an alkyne moiety, respectively, which were used as both a directing group and a reaction site. As a result, dihydrobenzofluoranthene and axially chiral benzofluoranthene derivatives were obtained, which have unique photophysical properties. Furthermore, we achieved skeletal rearrangement via the cleavage of two different C-C bonds by a gold catalyst at ambient temperature and we elucidated reactivity and regioselectivity by experimental and computational studies.

Star-shaped π-conjugated molecules, which are branched molecules with a general structure consisting of several (more than three) linear π-conjugated chains connected to a central core, are an attractive target. This is because their molecular properties originating from the star-shaped geometric and electronic structures can be different to their linear-shaped π-conjugated analogues. Donor (D)-π-acceptor (A) molecules, which are composed of donor (electron rich) and acceptor (electron deficient) segments connected by a π-spacer, are also an attractive target due to their inherent photophysical properties based on the intramolecular charge transfer (ICT) process from a donor to an acceptor through a π-spacer. Examples of such photophysical properties include a solvent-polarity responsive fluorescence from the highly-polarized ICT excited state (fluorosolvatochromism) and a cation responsive fluorescence of the D-π-A molecule with an amino-donor-type cation receptor based on on-off switching of the ICT process triggered by a cation binding event. Our current efforts have focused on the synthesis and elucidation of the ICT-based photophysical functions of the star-shaped (D-π)₃-A molecules which have a complexed structure of the star-shaped and D-π-A structures. Herein, we report the synthesis and photophysical properties, including fluorosolvatochromic and proton-sensing properties of 2,4,6-tris(5-aryl-2-thienyl)-1,3,5-triazine, -pyrimidine, and -pyridine derivatives, which are designed as the star-shaped (D-π)₃-A molecules consisting of an electron deficient N-heteroaromatic core (A) and three donor aryl terminals (D) linked by a thiophene spacer (π).

Foldamer is a synthetic oligomer that folds into the specific conformation in the solution state. The functional group in the main (or side) chains assembles in the confined-fold structure, which works as efficient catalysts in several chemical reactions. This review introduces such foldamer catalysts.