Efficient Construction of Pyrano[3,2‐a]carbazoles: Application to a Biomimetic Total Synthesis of Cyclized Monoterpenoid Pyrano[3,2‐a]carbazole Alkaloids

We have developed a highly efficient route to 2‐hydroxy‐3‐methylcarbazole ( 1 ) via a palladium‐catalyzed construction of the carbazole skeleton. Using 1 as relay compound, different methods for annulations of pyran rings by reaction with terpenoid building blocks have been tested. The Lewis acid promoted reaction of 1 with prenal ( 21 ) opened up an efficient route to girinimbine ( 3 ) and the corresponding reaction with citral ( 25 ) afforded mahanimbine ( 5 ). Oxidation of compounds 3 and 5 provided murrayacine ( 4 ) and murrayacinine ( 6 ). Following the biogenetic proposal, mahanimbine ( 5 ) has been exploited for efficient biomimetic syntheses of the cyclized monoterpenoid pyrano[3,2‐a]carbazole alkaloids cyclomahanimbine ( 7 ), mahanimbidine ( 8 ) and bicyclomahanimbine ( 9 ). The interconversions of 5 , 7 , 8 and 9 are described and mechanistic implications are discussed. Structural assignments are unambiguously verified by X‐ray crystal structure determinations. Moreover, cyclomahanimbine ( 7 ) was transformed into murrayazolinine ( 10 ) and exozoline ( 11 ).     

The interaction of disodium salt of 4,6-dinitro-1-oxobenzo-[6,5-c]-2,1,3-oxadiazolediol-5,7 with NaOH and HCl

Disodium salt of 4,6-dinitro-1-oxobenzo-[6,5-c]-2,1,3-oxadiazolediol-5,7 (I) was synthesized. The experimental dependences of the formation function on the solution pH were obtained by the potentiometric titration method for the reaction of NaOH and HCl with I, and the equilibrium constants for the above reactions were calculated. The reaction of I with HCl gives the acid sodium salt of 4,6-dinitro-1-oxobenzo-[6,5-c]-2,1,3-oxadiazolediol-5,7, which is the coordination compound with the Na⁺ cation. The title compound was studied by X-ray diffraction.     

Self-assembling systems based on amphiphilic alkyltriphenylphosphonium bromides: elucidation of the role of head group

A systematic study of the aggregation behavior of alkyltriphenylphosphonium bromides (TPPB-n; n=8, 10, 12, 14, 16, 18; here n is the number of carbon atoms in alkyl groups) in aqueous solutions has been carried out and compared with trimethyl ammonium bromides (TMAB-n). Critical micelle concentrations (cmcs) of TPPB-n and TMAB-n decrease with the number of carbon atoms with the slope parameter of ca.0.3. The low cmcs and effective solubilization power toward Orange OT indicate high micellization capacity of phosphonium surfactants. The low counterion binding parameter β is revealed for TPPB-10 and TPPB-12, while high counterion binding of ≥80% is observed for high TPPB-n homologs. Values of the surface potential ψ calculated on the basis of pK(a) shifts of p-nitrophenols is similar for both series and monotonously increase with alkyl chain length. Several points indicate non-monotonic changes within TPPB-n series. There are peculiarities of the tensiometry and solubilization plots for high homologs and above mentioned increases in counterion binding on transiting from low to high molecular weight surfactants. Differences in aggregation behavior between TPPB and TMAB series and between low and high homologs can be due to the specific structural character of the TPP(+) cation, which is supported by X-ray data.     

Electrochemical nickel-induced fluoroalkylation: synthetic, structural and mechanistic study

Electrocatalytic generation of nickel catalysts in low oxidation states by reduction of nickel complexes with various ligands (2,2'-bipyridine, 2,2':6',2'-terpyridine, (S,S)-2,6-bis(4-phenyl-2-oxazolin-2-yl)-pyridine) in the presence of olefinic substrates and fluoroalkyl halides leads to new organic products derived from addition-dimerization processes. Due to the presence of two stereocenters in the dimerization products two diastereomers were characterized by a variety of analytical techniques including multi-dimensional NMR methods and X-ray single crystal diffraction. The formation of dimers was prevented by the inclusion of the hydrogen atom donor tributyltin hydride. The cyclic voltammetry study of selected nickel complexes along with fluoroalkyl halides demonstrated that Ni(I)L is the active form of the catalyst.