Synthetic Applications of Carbohydrate-derived Donor-Acceptor Cyclopropanes

In this account, we elaborate our group's contribution towards understanding the chemistry of carbohydrate-derived donor-acceptor (DA) cyclopropanes. Our work was mainly focused on the ring opening of these versatile chiral synthons under the influence of Lewis acid promoters like electrophilic halogen species, TMSOTf, BF₃.OEt₂, etc. We studied various modes of ring opening on these DA cyclopropanes, envisaging the access to intriguing molecular architectures. These modes of reaction of the DA cyclopropanes can be controlled by strategically introducing an electron-withdrawing group (EWG) onto the cyclopropane ring, which could direct the ring cleavage by polarizing the cyclopropane C−C bond. Our studies also revealed that the ring opening is sluggish in the absence of an EWG. Using this concept, we demonstrated the synthesis of various biologically interesting molecular skeletons, viz., glycoamino acids (GAA), GAA nucleotides, α-levoglucosan amino acid, and septano-oligosaccharides, with high selectivity. We also applied our understanding to the first stereoselective synthesis of (S)-(−)-longianone and confirmed its absolute configuration. Apart from the inherently activated DA cyclopropanes, we introduced the in situ generation of DA cyclopropanes, starting from vinylcyclopropanes (VCPs). The ring-opening and ring-expansion chemistry of these easily accessible synthons was studied. The chemistry developed for carbohydrate-fused cyclopropanes was also applied for carbohydrate-derived spiro-cyclopropanes. The Lewis-acid-mediated ring opening of spiro-DA-cyclopropanes enabled easy access to fused furopyrans and spirolactones.     

Characterization of ZrC reinforced AA6061 alloy composites produced using stir casting process

In the present study, effect of ZrC vol.% on mechanical properties of AA6061 metal matrix composites (MMCs) produced via stir casting technique was investigated. The vol.% of ZrC particles was varied as 5,10 and 15. The composites were characterized for its microstructure and mechanical properties and their relationships were obtained. The scanning electron microscope (SEM) images revealed uniform distribution and good bonding between the AA6061 alloy and the ZrC particles. The mechanical properties of the AA6061 alloy was found to significantly improve with the addition of ZrC particles from 5 to 15 vol.%, the hardness increased from 32 to 68 HV, yield strength increased from 50 to 86 MPa and the ultimate tensile strength increased from 118 to 165 MPa. However, the % of elongation of the composite samples decreased with 15 vol.% addition of ZrC particles. Sliding wear behaviour of the composites was investigated using a pin-on-disc wear tester at a load of 9.8 N and addition of ZrC particles was significantly found to reduce the wear rate of AA6061 alloy.

     

On the Intrinsic Thermal Stability in HVDC Cables

The paper deals with some important aspects of steady state thermal stability in the insulation of HVDC cables. The resistively graded electric field distribution and the temperature distribution, under steady state, in dc cables, are interrelated. The dc conductivity is known to be a function of stress as well as of temperature. Reasonably convincing theories of electric stress and temperature distribution are now available in the literature. Until recently, the thermal runaway was believed, primarily, to be a consequence of an inordinate run away in the sheath temperature due to dynamic interaction with the ambient. While this premise is partly true, there are other overriding factors controlling the stability. This paper presents a detailed investigation on this aspect and it is shown that the multi-factor dependent dc conductivity alone causes the thermal instability, boundary conditions at sheath having a marginal role.     

Two-stage intonation modeling using feedforward neural networks for syllable based text-to-speech synthesis

This paper proposes a two-stage feedforward neural network (FFNN) based approach for modeling fundamental frequency (F₀) values of a sequence of syllables. In this study, (i) linguistic constraints represented by positional, contextual and phonological features, (ii) production constraints represented by articulatory features and (iii) linguistic relevance tilt parameters are proposed for predicting intonation patterns. In the first stage, tilt parameters are predicted using linguistic and production constraints. In the second stage, F₀ values of the syllables are predicted using the tilt parameters predicted from the first stage, and basic linguistic and production constraints. The prediction performance of the neural network models is evaluated using objective measures such as average prediction error (μ), standard deviation (σ) and linear correlation coefficient (γ_X,Y). The prediction accuracy of the proposed two-stage FFNN model is compared with other statistical models such as Classification and Regression Tree (CART) and Linear Regression (LR) models. The prediction accuracy of the intonation models is also analyzed by conducting listening tests to evaluate the quality of synthesized speech obtained after incorporation of intonation models into the baseline system. From the evaluation, it is observed that prediction accuracy is better for two-stage FFNN models, compared to the other models.     

Quasi-three-dimensional elastic stresses in rotating disks

A method is presented to obtain stresses and displacements in rotating disks by taking into account the effect of out-of-plane restraint conditions at the hub. The stresses and displacements are obtained in a non-dimensional form, presented in the form of graphs and compared with the generalized plane stress solution.     

Converging spherical and cylindrical shock waves

The self-similar solutions for converging spherical and cylindrical strong shock waves in a non-ideal gas satisfying the equation of state of the Mie-Gruneisen type are investigated. The equations governing the flow, which are highly non-linear hyperbolic partial differential equations, are first reduced to a Poincaré-type ordinary differential equation with suitable approximation. Such an approximation helps in obtaining the self-similar solutions and the similarity exponent numerically by phase-plane analysis.