Formal [3 + 2] and [3 + 3] additions of acceptor-substituted cyclopropylmethylsilanes to allenylsilanes

1,3-Dipolar synthons formed from vicinal TBDPS-substituted cyclopropyl alkyl/phenyl ketones on treatment with Lewis acids such as TiCl(4) and Et(2)AlCl reacted with allenylsilanes to furnish [3 + 2] and [3 + 3] adducts with high regio- and stereocontrol. [reaction: see text]     

Theoretical studies on the stereochemical course of 1,3‐dipolar cycloaddition of azomethine ylides derived from indol‐2,3‐dione and thiazolidine‐4‐carboxylic acid

1,3‐Dipolar cycloadditon of azomethine ylides with different dipolarophiles leads to the formation of novel heterocyclic spiro compounds having two or more chiral centers. The theoretical studies (HF/3–21G) on the 1,3‐dipolar cycloaddition reaction between ethene and azomethine ylide A4 derived from isatin and thaizolidine‐4‐carboxylic acid, indicates that the energy barrier for this addition is about ～ 8 kcal/mol higher than that in simplest azomethine ylide A1 . HF/3–21G studies on a series of azomethine ylides A2 and A3 suggested that the increased barrier is mainly due to stabilization of azomethine ylides arising from aromatic indol nucleus. Semi‐empirical studies indicate that the cycloaddition is streocontrolled as the transition states corresponding to only the stericlly allowed paths could be located on the potential energy surface.     

Microdetermination of fluoride by spectrophotometry of arsenazo(III)-complex of Zr back-extracted from htta in benzene

A spectrophotometric method for the determination of trace amounts of fluoride in aqueous solution is developed. It is based on the back extraction of Zr(IV) from its thenoyltrifluoroacetonate complex in benzene by aqueous F^– followed by the development of Zr-arsenazo(III) coloured complex and the measurement of the absorbance at 665 nm. Optimum conditions and effect of interfering ions are studied. The coefficients of variation obtained are 1.5% and 2.2% in 11 determination at the F^– concentration level of 10.0 g/ml and 1.6 g/ml. An excess of Al(III) is used for decomplexing Zr-fluoride prior to the colour formation if the fluoride amount is high. The method is useful for the determination of trace impurities of fluoride in nuclear fuel after separating fluoride from fuel matrix by pyrohydrolysis.     

Studies towards the total synthesis of Phostriecin

A synthetic approach toward the phostriecin, an antitumor natural product is described. The key features of the present synthesis are Wittig reaction, synthesis of homoallylic alcohol using Brown’s protocol (alkoxyallylboration) and RCM for the creation of unsaturated lactone moiety of phostriecin.     

Impact of silica nanoparticles dispersion on the dielectric and electro-optical properties and absorption spectra of host ferroelectric liquid crystal

The present work concerns with the investigation of the effect of dispersion of Silica (SiO₂) nanoparticles (NPs) in host ferroelectric liquid crystal (FLC) KCFLC10S on the dielectric and electro-optical properties and ultraviolet-visible (UV-VIS) absorption spectra of the pristine and dispersed systems. We have found that the dispersion of SiO₂ NPs in the host FLC strongly influences the various properties of dispersed systems. No evidence of aggregates and clumps in the dispersed system has been observed. Due to SiO₂ NPs dispersion, a rapid decrease in dielectric permittivity ε’, increase in conductivity σ with frequency, increase in spontaneous polarisation P_s and decrease in switching time with bias voltage have been observed. Based on the absorption spectra, we have also made an attempt to link the electro-optical and dielectric response with the mechanism of FLC–NPs interactions.     

Direct synthesis of 6-sulfonylated phenanthridines via silver-catalyzed radical sulfonylation-cyclization of 2-isocyanobiphenyls

A convenient and straightforward synthesis of 6-sulfonylated phenanthridines via silver-catalyzed sequential radical insertion, cyclization and aromatization of 2-isocyanobiphenyls is reported. The protocol does not require a phenanthridine scaffold as a substrate and presents a highly regioselective synthesis of 6-alkyl/arylsulfonyl phenanthridines. The protocol utilizes readily available and easy to handle sodium sulfinates as sulfonating agents and potassium persulfate as an oxidant to afford good to excellent yields of the desired products in a one-pot operation at room temperature.     

Synthesis of 9-CD3-9-cis-retinal cofactor of isorhodopsin

We report the synthesis of 9-CD₃-9-cis-retinal via a six-step procedure from β-ionone. The steps involve an initial deuteration of the methyl ketone of β-ionone followed by two consecutive Horner-Wadsworth-Emmons (HWE) coupling reactions and their corresponding DIBAL reductions. A final oxidation of the allylic alcohol of the retinol leads to the target compound. This deuterium labeled retinoid is an important cofactor for studying protein-retinoid interactions in isorhodopsin.     

Chemoselective one-pot access to benzo[e]indole-4,5-diones and naphtho[2,1-b]thiophene-4,5-diones via copper-catalyzed oxidative [3 + 2] annulation of α-oxoketene N,S-acetals/β-ketothioamides with α-/β-naphthols

An operationally simple and efficient one-pot method for the synthesis of 1-aroyl (or alkanoyl)-2-thioalkyl-3-aryl (or alkyl)-3H-benzo[e]indole-4,5-diones and naphtho[2,1-b]thiophene-4,5-diones has been devised by copper-catalyzed cross-coupling of α-oxoketene N,S-acetals/β-ketothioamides with α-/β-naphthols in open air for the first time. The key to the success of this transformation is the room temperature oxidation of α-/β-naphthol to 1,2-naphthoquinone as a reactive species, which undergoes formal [3 + 2] annulation with α-oxoketene N,S-acetals/β-ketothioamides via cascade sequence of Michael addition/tautomerization/oxidation/cyclization/aromatization reactions, enabling addition of a pyrrole/thiophene ring onto naphthoquinone moiety. Further, benzo[e]indole-4,5-diones were transformed to pentacyclic fused phenazine derivatives under solvent-free conditions. Based on our experimental outcomes, a tentative mechanistic rationale for this chemoselective protocol is proposed, which is well validated and supported by the control experiments.     

Synthetic studies toward pondaplin: total synthesis of pondaplin analogues

Syntheses of synthetic analogues of pondaplin 1 have been achieved. Final macrolide construction was accomplished using a Keck macrolactonization reaction.     

Studies in sesquiterpenes—LV: Isolongifolene(part 6): Mechanism of rearrangement of longifolene to isolongifolene-I

The gross mechanism of rearrangement of longifolene to isolongifolene has been elucidated by using site-specifically labelled longifolene-4, 4, 5, 5-d₄ and shown to follow the pathway proposed by Berson et al., which involves an exo, exo Me shift, in preference to the endo, endo Me migration route proposed earlier. An efficient synthesis of longifolene-4, 4, 5, 5-d₄, the key compound in the present investigation, is described.