A new entry for the synthesis of N-acyl-N′-substituted guanidines

Efficient construction of the hexacyclic ring core of palau'amine: the pK a concept for proceeding with unfavorable equilibrium reactions

Palau'amine has received a great deal of attention as an attractive synthetic target due to its intriguing molecular architecture and significant immunosuppressive activity, and we achieved its total synthesis in 2015. However, the synthesized palau'amine has not been readily applicable to the mechanistic study of immunosuppressive activity, because it requires 45 longest linear steps from a commercially available compound. Here, we report the short-step construction of the ABCDEF hexacyclic ring core of palau'amine. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with unfavorable equilibrium reactions, and a palau'amine analog without the aminomethyl and chloride groups is synthesized in 20 longest linear steps from the same starting material. The palau'amine analog is confirmed to retain the immunosuppressive activity. The present synthetic approach for a palau'amine analog has the potential for use in the development of palau'amine probes for mechanistic elucidation.

A palau'amine analog (2) was synthesized from 2-cyclopentenone in 20 steps. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with the unfavorable equilibrium reactions.

Benzoylguanidines as Anion-Responsive Systems

A series of benzoylguanidinium salts was prepared and the changes in UV/Vis spectra, triggered by the presence of anions, were investigated. All compounds undergo deprotonation with basic anions like dihydrogenphosphate and acetate in acetonitrile. The most pronounced spectral changes were obtained by deprotonation of N¹ -benzoyl-N³ -(p-nitrophenyl) guanidinium chloride which shows the naked-eye visible color change from colorless to yellow. Measured pK_a (BH⁺ ) in acetonitrile ranges from 12-16, which is comparable to the pyridinium cations. The proton transfer equilibria were also tested in acetonitrile/water mixture where all but the most acidic derivatives showed pK_a (BH⁺ ) of 4-6 units which corresponds to apparent association constants of 10⁴ -10⁶  dm³  mol^-1 . UV/Vis spectra of neutral and protonated forms were modelled by the TD-DFT approach using CAM-B3LYP and PBE0 functionals and compared to CC2 results. In the case of CAM-B3LYP, a parameter ω, defining amount of long-range exchange correction, was varied to achieve the best agreement with the experimental spectra. The optimized ω parameters are 0.10 a₀ ^-1 for neutral benzoylguanidines and 0.20 a₀ ^-1 for neutral nitrobenzoyl and protonated systems. The larger ω parameter in the latter is ascribed to more pronounced charge transfer character of the HOMO-LUMO transition - the one responsible for the lowest energy absorption band.

Investigation of Novel Pesticides with Insecticidal and Antifungal Activities: Design, Synthesis and SAR Studies of Benzoylpyrimidinylurea Derivatives

In order to find pesticides with insecticidal and antifungal activities, a series of novel benzoyl pyrimidinylurea derivatives were designed and synthesized. All target compounds were identified by ¹H-NMR spectroscopy and HRMS. Insecticidal and antifungal activity of these compounds were evaluated and the structure-activity relationships (SAR) were clearly and comprehensively illustrated. Compound 7, with low toxicity to zebrafish (LC50 = 378.387 µg mL-1) showed 100% inhibition against mosquito (Culex pipiens pallens) at 0.25 µg mL-1. Both compounds 19 and 25 exhibited broad-spectrum fungicidal activity (>50% inhibitory activities against 13 phytopathogenic fungi), which were better than those of the commercial pesticide pyrimethanil (>50% inhibitory activities against eight phytopathogenic fungi). Furthermore, compounds 19 and 25 exhibited protective activity against Sclerotinia sclerotiorum on leaves of Brassica oleracea L. during in vivo experiments.

Insights on the Auxochromic Properties of the Guanidinium Group

UV/vis spectra of phenylguanidine (PHGU) in the gas phase and in acetonitrile have been simulated by TD-DFT calculations. Several DFT hybrid and long-range corrected functionals were tested with respect to CASPT2 gas phase calculations. Solvent effects were considered using polarizable continuum model (PCM) and compared with the measured data in acetonitrile. Comparison with isoelectronic phenylurea and related phenyltiourea was done as well. The PBE0 and long-range corrected CAM-B3LYP functionals were selected to investigate the effect of protonation on the excitation energies and absorption intensities of PHGU and several guanidine derivatives with different aromatic chromophoric groups (naphthyl, anthracenyl, quinolinyl, anthraquinonyl, and coumarinyl). Also, the effect of complexation and specific interactions through hydrogen bonds with different anions was examined. It was shown that the protonation of the guanidine subunit shifts the low energy absorption bands toward higher energies (hypsochromic shift). The shift is reduced upon complexation with anions. In phenylguanidine salts, λmax values are correlated to the anion basicity and strength of H-bonding. The observed changes diminish upon increase of chromophoric size (naphthyl, anthracenyl). Theoretical predictions of UV/vis spectra correlate well with experimentally measured spectra of selected guanidine derivatives and their salts.

Total synthesis of palau'amine

Palau'amine has received a great deal of attention in the past two decades as an attractive synthetic target by virtue of its intriguing molecular architecture and significant immunosuppressive activity. Here we report the total synthesis of palau'amine characterized by the construction of an ABDE tetracyclic ring core including a trans-bicylo[3.3.0]octane skeleton at a middle stage of total synthesis. The ABDE tetracyclic ring core is constructed by a cascade reaction of a cleavage of the N-N bond, including simultaneous formation of imine, the addition of amide anion to the resulting imine (D-ring formation) and the condensation of pyrrole with methyl ester (B-ring formation) in a single step. The synthetic palau'amine is confirmed to exhibit excellent immunosuppressive activity. The present synthetic route has the potential to help elucidate a pharmacophore as well as the mechanistic details of immunosuppressive activity.

A new and environmentally benign synthesis of aroylguanidines using iron trichloride

A new synthetic approach for the guanylation of aroylthioureas using iron trichloride is presented.

A mild method for the synthesis of carbamate-protected guanidines using the Burgess reagent

A simple method for the synthesis of carbamate-protected guanidines from primary amines is described. A variety of thioureas derived from primary amines and isothiocyanates react with the Burgess reagent to give the corresponding guanidines via either a stepwise or one-pot procedure. By tuning the carbamoyl units of isothiocyanates and the Burgess reagent, differentially N,N'-diprotected guanidines can be obtained. Selective deprotection of the products affords N-monoprotected guanidines.

Chlorotrimethylsilane activation of acylcyanamides for the synthesis of mono-N-acylguanidines

A simple and efficient one-pot method for the synthesis of monoprotected guanidines is presented. Treatment of an acylcyanamide with chlorotrimethylsilane generates a reactive N-silylcarbodiimide capable of guanylating a variety of amines. Typically the reaction is complete in 15 min for primary and secondary aliphatic amines at rt. Hindered amines and anilines are also competent nucleophiles but require extended reaction times.

On the structure of palau'amine: evidence for the revised relative configuration from chemical synthesis

Hexacyclic congeners 3 and 4 of palau'amine, which incorporate both guanidine functional groups and have the cis configuration of the azabicyclo[3.3.0]octane core, are prepared in 14 steps from cycloadduct 6. Synthetic access to these analogues allows the first direct comparison of NMR data for hexacyclic diguanidine structures having the originally proposed cis-azabicyclo[3.3.0]octane fragment with data for natural alkaloids of the palau'amine family. This comparison provides convincing evidence in favor of the recently proposed structural revision of these marine alkaloids, fully supporting the trans configuration of the central azabicyclo[3.3.0]octane ring system of palau'amine and congeners.