Enantiospecific Photochemical Transformations under Elevated Pressure

Enantiospecific axial‐to‐point chiral transfer in light‐induced transformations was efficient under elevated pressure at high temperatures. Model photoreactions with atropisomeric compounds showed higher enantioselectivity in the photoproducts under elevated pressure. The ee values in the photoproducts were rationalized based on the increased stability of optically pure atropisomeric compounds at elevated pressure, even at high temperatures.     

Automatic analyzer for highly polar carboxylic acids based on fluorescence derivatization–liquid chromatography

A sensitive, versatile, and reproducible automatic analyzer for highly polar carboxylic acids based on a fluorescence derivatization–liquid chromatography (LC) method was developed. In this method, carboxylic acids were automatically and fluorescently derivatized with 4‐(N,N‐dimethylaminosulfonyl)‐7‐piperazino‐2,1,3‐benzoxadiazole (DBD‐PZ) in the presence of 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium chloride by adopting a pretreatment program installed in an LC autosampler. All of the DBD‐PZ‐carboxylic acid derivatives were separated on the ODS column within 30 min by gradient elution. The peak of DBD‐PZ did not interfere with the separation and the quantification of all the acids with the exception of lactic acid. From the LC‐MS/MS analysis, we confirmed that lactic acid was converted to an oxytriazinyl derivative, which was further modified with a dimethoxy triazine group of 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium chloride (DMT‐MM). We detected this oxytriazinyl derivative to quantify lactic acid. The detection limits (signal‐to‐noise ratio = 3) for the examined acids ranged from 0.19 to 1.1 µm , which correspond to 95–550 fmol per injection. The intra‐ and inter‐day precisions of typical, highly polar carboxylic acids were all <9.0%. The developed method was successfully applied to the comprehensive analysis of carboxylic acids in various samples, which included fruit juices, red wine and media from cultured tumor cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Construction of a pentacyclic ring system of isoryanodane diterpenoids by SmI2-mediated transannular cyclization

Isoryanodane diterpenoids possess a densely oxygenated and intricately fused pentacycle (ABCDE-ring), although its unique ring system has not yet been assembled chemically. The 6/5/7-membered BCD-rings of isoryanodanes differ from the 5/6/6-membered rings of the structurally-related ryanodanes. We previously achieved the total synthesis of ryanodine and five other ryanodane diterpenoids by employing the common ABDE-tetracyclic intermediate 1. Herein, we report a new strategy for constructing isoryanodane ABCDE-ring structure 3 from 1 via a combination of the oxy-Cope rearrangement and SmI₂-mediated transannular cyclization.     

Symmetry‐Driven Strategy for the Assembly of the Core Tetracycle of (+)‐Ryanodine: Synthetic Utility of a Cobalt‐Catalyzed Olefin Oxidation and α‐Alkoxy Bridgehead Radical Reaction

Ryanodine ( 1 ) is a potent modulator of intracellular calcium release channels, designated as ryanodine receptors. The exceptionally complex molecular architecture of 1 comprises a highly oxygenated pentacyclic system with eleven contiguous stereogenic centers, which makes it a formidable target for organic synthesis. We identified the embedded C₂‐symmetric tricyclic substructure within 1 . This specific recognition permitted us to design a concise synthetic route to enantiopure tricycle 9 by utilizing a series of pairwise functionalizations. The four tetrasubstituted carbon centers of 9 were effectively constructed by three key reactions, a dearomatizing Diels–Alder reaction, the kinetic resolution of the obtained racemic 14 through asymmetric methanolysis, and the transannular aldol reaction of the eight‐membered diketone 10 . A new combination of cobalt‐catalyzed hydroperoxidation and NfF‐promoted elimination enabled conversion of the hindered olefin of 9 into the corresponding ketone, thus realizing the desymmetrization. Finally, the tetrasubstituted carbon was stereospecifically installed by utilizing the α‐alkoxy bridgehead radical to deliver the core tetracycle 7 with the six contiguous tetrasubstituted carbon centers. Consequently, the present work not only accomplishes efficient assembly of four out of the five fused rings of 1 , but also develops two new powerful methodologies: two‐step ketone formation and bridgehead radical reaction.     

Entropy‐Driven Diastereoselectivity Improvement in the Paternò–Büchi Reaction of 1‐Naphthyl Aryl Ethenes with a Chiral Cyanobenzoate through Remote Alkylation

The precise stereocontrol of photocycloaddition reactions is still a significant challenge owing to their mechanistic complexity and the involvement of highly reactive and short‐lived intermediates. Attempts have hitherto been made through structural modifications, mostly by introducing steric conflicts, to increase the difference between the enthalpic barriers. Herein, we show that entropy plays a crucial role in influencing the diastereoselectivity of a Paternò–Büchi reaction. Remote meta alkylation of the donor caused nominal changes in its photophysical properties as well as those of the exciplexes derived thereof. Nevertheless, the diastereomeric excess of the oxetane product was greatly improved by about 40 %. This enhancement, which is not accompanied by any significant changes in the photophysical properties, is difficult to rationalize by conventional enthalpic control concepts based on repulsive steric and/or attractive intermolecular interactions as well as electronic perturbations. Differential activation parameters and compensatory enthalpy–entropy relationships revealed that the diastereoselectivity enhancement is not simply enthalpic but also entropic in origin.     

An application of a laser technique to the velocity measurement of the liquid-fuel film within intake pipe of internal combustion engines

A new experimental technique is developed by combining a flash photolysis method with a laser-photodiode system to measure the velocity of the flow of thin liquid films. The technique is applied to the measurement of the velocity of the liquid-fuel film within the intake pipe of an internal combustion engine operated under firing condition. The results show that the velocity of the fuel film is of the order of 1/100 of the mean air velocity inside the intake pipe.     

Forced oscillations of a vertical continuous rotor with geometric nonlinearity

Nonlinear forced oscillations of a vertical continuous rotor with distributed mass are discussed. The restoring force of the rotor has geometric stiffening nonlinearity due to the extension of the rotor center line. The possibility of the occurrence of nonlinear forced oscillations at various subcritical speeds and the shapes of resonance curves at the major critical speeds and at some subcritical speeds are investigated theoretically. Consequently, the following is clarified: (a) the shape of resonance curves at the major critical speed becomes a hard spring type, and (b) among various kinds of nonlinear forced oscillations, only some special kinds of combination resonances have possibility of occurrence.     

Interchangeability of vortex-breakdown types

In order to investigate the connection between the bubble and the spiral form of vortex breakdown, experiments were conducted: an external disturbance in the form of an azimuthally spinning waveform was imposed in a pipe. The azimuthal wave number was varied by adjusting the phase difference among four oscillating pistons mounted circumferentially on the pipe. By imposing a disturbance of zero azimuthal wave number, a spiral was transformed into a bubble, and this occurred only for selective piston frequencies; the vortex breakdown which altered from the spiral to the bubble moved upstream, where it remained as a bubble as long as the external disturbance remained. Once the disturbance was removed, the bubble returned to a spiral. By imposing a disturbance of azimuthal wave number +1 (the first circumferential mode rotating in the same direction as the mean swirl), a bubble was transformed into a spiral for selective piston frequencies, and the spiral moved downstream. These preferred frequencies were found to be the same as the unexcited frequencies observed in the spiral in its natural state. As long as the external disturbance was imposed, the breakdown altered from the bubble to the spiral remained as a spiral; once the disturbance was removed, the spiral reverted to a bubble. By imposing a disturbance with azimuthal wave number -1 (the first circumferential mode rotating in the opposite direction to the mean swirl), no change was detected in either a bubble or a spiral. By imposing a disturbance with azimuthal wave number 2 (the second circumferential mode), for selective piston frequencies a bubble was transformed into what appears to be the so-called two-tailed type. Thus, it appears that hydrodynamic instability plays a role in interchanging vortex breakdown types, and a comparison with available stability theories is discussed.