Shovan Mondal

One-pot Crabbé homologation-radical cascade cyclisation with memory of chirality

The tandem Crabbé homologation-radical rearrangement of terminal enediynes leads, in a one-pot procedure, to the enantioselective synthesis of six- and seven-membered ring α-aminoesters bearing a quaternary stereocenter based on the phenomenon of memory of chirality.

Mechanistic Investigation of Enediyne-Connected Amino Ester Rearrangement. Theoretical Rationale for the Exclusive Preference for 1,6- or 1,5-Hydrogen Atom Transfer Depending on the Substrate. A Potential Route to Chiral Naphthoazepines

Memory of chirality (MOC) and deuterium-labeling studies were used to demonstrate that the cascade rearrangement of enediyne-connected amino esters 1a and 1b evolved through exclusive 1,5- or 1,6-hydrogen atom transfer, subsequent to 1,3-proton shift and Saito-Myers cyclization, depending on the structure of the starting material. These results were independently confirmed by DFT theoretical calculations performed on model monoradicals. These calculations clearly demonstrate that in the alanine series, 1,5-hydrogen shift is kinetically favored over 1,6-hydrogen shift because of its greater exergonicity. In the valine series, the bulk of the substituent at the nitrogen atom has a major influence on the fate of the reaction. N-Tosylation increases the barrier to 1,5-hydrogen shift to the benefit of 1,6-hydrogen shift. The ready availability of 1,6-hydrogen atom transfer was explored as a potential route for the enantioselective synthesis of naphthoazepines.

One-pot Sonogashira coupling-cyclization toward regioselective synthesis of benzosultams.

A one-pot method for the Sonogashira coupling and cyclization of 2-bromobenzenesulfonamides and terminal alkynes is presented. This method allows access to a variety of substituted benzosultams regioselectively in excellent yields. The reasons for regioselectivity are interpreted through density functional theory (DFT) studies.

Photoactivated cyclization of aryl-containing enediynes coated gold nanoparticles: enhancement of the DNA cleavage ability of enediynes.

Two novel enediynes containing an aromatic ring and substituted by two thiol functions as end-groups were designed and studied as functionalizing agent of gold nanoparticles. Phototriggered cyclization of the capping agent under UV-visible irradiation was investigated. Interestingly, the length of the thiol-substituted chain was shown to influence significantly the cyclization rate. Depending on the length of the spacer, either polymerization or simple cyclization of the coating agent was evidenced. The present study underscores the possibility of finely controlling the fate of the coating agent (polymerization/cyclization). Nanocomposites were characterized by UV-visible absorption spectroscopy, dynamic light scattering (DLS) technique and transmission electron microscopy (TEM) measurements. Finally, the ability of the colloidal solutions to induce photoinitiated damages to PcDNA3 supercoiled DNA was evaluated. Interestingly, an increase as high as 50% of the DNA cleavage could be registered when adding enediynes-capped gold nanoparticles to solutions of enediynes. In particular, the enhancement of DNA scission was observed in both thermal and photochemical activation modes.

Enantioselective Synthesis of Carbocycles and Heterocycles by Radical/Polar and Polar/Radical Cascades**

nly twenty years ago, enantioselective synthesis, involvingradical intermediates, was still regarded as rather limited.However, the knowledge of the parameters essential forstereocontrol and the use of chiral auxiliaries in radicalreactions had already reached a very high level of develop-ment. Another breakthrough came from the work of Porter,Sibi, and co-workers with the introduction of chiral Lewisacids.

Synthesis and characterization of novel cholesterol based mesogenic compounds using ‘click’ chemistry

An efficient route for the synthesis of hitherto unreported non-symmetric dimesogens consisting of a cholesteryl moiety and an aromatic mesogenic unit interconnected through a pentamethylene or decamethylene spacer by Cu(I)-catalyzed azide–alkyne cycloaddition ‘click’ chemistry has been developed. The aromatic units consist of two phenyl rings linked to a triazole moiety. A new trimesogen consisting two cholesteryl esters and an aromatic segment joined by pentamethylene spacer has also been synthesized. The newly synthesized mesogens show N*, TGB and partially bilayered SmC* (SmCd*) phases. The liquid crystalline phases were investigated with the help of polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction studies. DFT study was also used to locate the orientation of the dipole at the centre position i.e. at ester linkage. All the compounds were characterized from their elemental analyses and spectral data.

Theoretical Study To Explain How Chirality Is Stored and Evolves throughout the Radical Cascade Rearrangement of Enyne-allenes

This article reports a theoretical study to explain how the intrinsic property of chirality is retained throughout the radical cascade rearrangement of an enantiopure chiral enyne-allene (bearing one stereogenic center) selected as a model for this family of reactions. Calculations at the MRPT2/6-31G(d)//CASSCF(10,10)/6-31G(d) level of theory were used to determine the entire reaction pathway which includes singlet state diradicals and closed-shell species. The cascade process involves three elementary steps, i.e., by chronological order: Myers-Saito cycloaromatization (M-S), intramolecular hydrogen atom transfer (HAT), and recombination of the resulting biradical. The enantiospecificity of the reaction results from a double transmission of the stereochemical information, from the original center to an axis and eventually from this axis to the final center. The first two steps lead to a transient diradical intermediate which retains the chirality via the conversion of the original static chirogenic element into a dynamic one, i.e., a center into an axis. The only available routes to the final closed-shell tetracyclic product imply rotations around two σ bonds (σ(C-C) and σ(C-N), bonds β and α respectively). The theoretical calculations confirmed that the formation of the enantiomerically pure product proceeds via the nonracemizing rotation around the σ(C-C) pivot. They ruled out any rotation around the second σ(C-N) pivot. The high level of configurational memory in this rearrangement relies on the steric impediment to the rotation around the C-N bond in the chiral native conformation of the diradical intermediate produced from tandem M-S/1,5-HAT.

Synthesis and Mesomorphic Behavior of Novel Liquid-Crystalline Thiophene Derivatives

A new class of thiophene-based cholesteryl and ferrocenyl derivatives was synthesized and characterized by common analytical techniques. Liquid-crystalline properties were investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Thiophene-based cholesteryl derivatives show homeotropic liquid-crystalline phase sequence crystal → smectic A → cholesteric → isotropic, whereas the ferrocenyl derivative is nonmesomorphic. The effects of thiophene ring, terminal group, and length of the polymethylene spacers on the mesomorphic behavior are discussed.

Cooperative use of VCD and XRD for the determination of tetrahydrobenzoisoquinolines absolute configuration: a reliable proof of memory of chirality and retention of configuration in enediyne rearrangements.

The absolute configurations (AC) of azaheterocylic compounds resulting from the cascade rearrangement of enediynes involving only light atoms were unambiguously assigned by the joint use of vibrational circular dichroism (VCD) and copper radiation single crystal X-ray diffraction (XRD). These AC determinations proved that the rearrangements of enediynes proceeded with memory of chirality and retention of configuration.

Stabilization of TGB Phase in the Binary Mixture of Rod-Shaped Compounds

A miscibility study of two cholesterol based rod-shaped compounds (R1 and R2) is presented. Both the compounds possess in common enantiotropic liquid crystalline phase sequence Cr → SmA → N* → Isotropic, which is generally exhibited by cholesterol-based compounds. Additionally, one of the rod-shaped compounds (R1) possesses a twist-grain boundary (TGB) phase over a short temperature range. In the binary mixture of these two rod-shaped compounds the TGB phase was stabilized over a long temperature range compared to that in the individual molecule R1. In the binary mixture up to 20% (w/w) of compound R1 possessing the TGB phase exhibited enhancement of the temperature range of the TGB phase compared to that of individual compound R1. Further gradual increase of percentage of R1 in the binary mixture did not show any enhancement of the TGB phase compared to that of individual R1.