Enantiospecific total synthesis of ent-10, 11-thapsan-10-ol

First enantiospecific total synthesis of optical antipode of the sesquiterpene 10,11-epoxythapsan-10-ol has been described. (R)-Carvone has been employed as the chiral starting material and a combination of intramolecular alkylation and Criegee fragmentation are employed for intramolecular stereospecific transfer of the chirality. An intramolecular diazoketone cyclopropanation and regioselective cyclopropane ring cleavage reactions have been employed for the creation of the three requisite contiguous quaternary carbon atoms.

Curing Reactions in Elastomers. 11. Carboxy- Terminated Polybutadiene

Attempts have been made to understand the curing reactions in carboxy-terminated polybutadiene (CTPB), which happens to be the most practical binder in advanced solid composite propellants. The curing of CTPB has been studied for different ratios of curing agents (MAPO and Epoxide) by gel content, molecular weight, crosslink density, and penetration temperature measurements, and the optimum composition of curators for effective curing of CTPB has been determined. Activation energy calculations on the curing of CTPB with 9.5% epoxide and 0.5% MAPO in the temperature range 75100°C gave 14.1 kcal/mol for which a diffusion-controlled or acid-catalyzed epoxide ring opening mechanism has been suggested for the curing process in CTPB.

The Structure of 9b-Methyl- 3afl,3b~,5 a~t,9a0h9bfl, 11 afl-perhydrophenanthro-[2,l-b]furan-7-one

C17H2602, M, = 262, triclinic, PI, a = 8.513(2), b = 8.970(2), c = 11.741(3)A, a = 120.51 (5), fl = 93.30(4), y = 68.43(4) ° , V = 708.9,/k 3, Z = 2, D O = 1.213, D e = 1.227 Mg m -a, g(Mo Ka, 2 = 0.7107 ,&) = 0.084 mm -1, F(000) = 288. The structure, solved by direct methods, has been refined to an R value of 5.9% using 1361 intensity measurements. The ring junctions, in sequence from either end of the polycycle, are cis, trans and cis.

Microsomal 11-Alpha-hydroxylation of progesterone in Aspergillius ochraceus: Part I: Characterization of the hydroxylase system

Microsomes (105,000xg sediment) prepared from induced cells of A.ochraceus was found to hydroxylate progesterone to 11-Alpha-hydroxyprogesterone (11a-OHP) in high yields (85-�90% in 30 min.) in the presence of NADPH and O2. The pH optimum for the hydroxylase was found to be 7.7. However, for the isolation of active microsomes grinding of the mycelium should be carried out at pH 8.3. Metyrapone, carbon monoxide, SKF-525A, p-CMB and N-methyl maleimide inhibited the hydroxylase activity indicating the involvement of cytochrome P-450 system. The inhibition of the hydroxylase by cytochrome Image and the presence of high levels of NADPH-cytochrome Image reductase in induced microsomes suggest that the reductase could be one of the components in the hydroxylase system.

Modeling Finite Buffer Effects on TCP Traffic over an IEEE 802.11 Infrastructure WLAN

The network scenario is that of an infrastructure IEEE 802.11 WLAN with a single AP with which several stations (STAs) are associated. The AP has a finite size buffer for storing packets. In this scenario, we consider TCP controlled upload and download file transfers between the STAs and a server on the wireline LAN (e.g., 100 Mbps Ethernet) to which the AP is connected. In such a situation, it is known (see, for example, (3), [9]) that because of packet loss due to finite buffers at the Ap, upload file transfers obtain larger throughputs than download transfers. We provide an analytical model for estimating the upload and download throughputs as a function of the buffer size at the AP. We provide models for the undelayed and delayed ACK cases for a TCP that performs loss recovery only by timeout, and also for TCP Reno.

State Dependent Attempt Rate Modeling of Single Cell IEEE 802.11 WLANs with Homogeneous Nodes and Poisson Arrivals

Analytical models of IEEE 802.11-based WLANs are invariably based on approximations, such as the well-known mean-field approximations proposed by Bianchi for saturated nodes. In this paper, we provide a new approach for modeling the situation when the nodes are not saturated. We study a State Dependent Attempt Rate (SDAR) approximation to model M queues (one queue per node) served by the CSMA/CA protocol as standardized in the IEEE 802.11 DCF. The approximation is that, when n of the M queues are non-empty, the attempt probability of the n non-empty nodes is given by the long-term attempt probability of n saturated nodes as provided by Bianchi's model. This yields a coupled queue system. When packets arrive to the M queues according to independent Poisson processes, we provide an exact model for the coupled queue system with SDAR service. The main contribution of this paper is to provide an analysis of the coupled queue process by studying a lower dimensional process and by introducing a certain conditional independence approximation. We show that the numerical results obtained from our finite buffer analysis are in excellent agreement with the corresponding results obtained from ns-2 simulations. We replace the CSMA/CA protocol as implemented in the ns-2 simulator with the SDAR service model to show that the SDAR approximation provides an accurate model for the CSMA/CA protocol. We also report the simulation speed-ups thus obtained by our model-based simulation.

Experiences With WM: A Centralised Scheduling Approach for Performance Management of IEEE 802.11 Wireless LANs

In our earlier work ([1]) we proposed WLAN Manager (or WM) a centralised controller for QoS management of infrastructure WLANs based on the IEEE 802.11 DCF standards. The WM approach is based on queueing and scheduling packets in a device that sits between all traffic flowing between the APs and the wireline LAN, requires no changes to the AP or the STAs, and can be viewed as implementing a "Split-MAC" architecture. The objectives of WM were to manage various TCP performance related issues (such as the throughput "anomaly" when STAs associate with an AP with mixed PHY rates, and upload-download unfairness induced by finite AP buffers), and also to serve as the controller for VoIP admission control and handovers, and for other QoS management measures. In this paper we report our experiences in implementing the proposals in [1]: the insights gained, new control techniques developed, and the effectiveness of the WM approach in managing TCP performance in an infrastructure WLAN. We report results from a hybrid experiment where a physical WM manages actual TCP controlled packet flows between a server and clients, with the WLAN being simulated, and also from a small physical testbed with an actual AP.

X-Ray Studies On Crystalline Complexes Involving Amino-Acids And Peptides .11. Peptide Aggregation And Water-Structure In The Crystals Of A Hydrated 1-1 Complex Between L-Histidyl-L-Serine And Glycyl-L-Glutamic Acid

The hexahydrate of a 1:1 complex between L-histidyl-L-serine and glycyl-L-glutamic acid crystallizes in space group P1 with a = 4.706(1), b= 8.578(2), c= 16.521(3) ÅA; α= 85.9(1), β= 89.7(1)°, = 77.4(1). The crystal structure, solved by direct methods, has been refined to an R value of 0.046 for 2150 observed reflections. The two peptide molecules in the structure have somewhat extended conformations. The unlike molecules aggregate into separate alternating layers. Each layer is stabilized by hydrogen bonded head-to-tail sequences as well as sequences of hydrogen bonds involving peptide groups. The arrangement of molecules in each layer is similar to one of the plausible idealized arrangements of L-alanyl-L-alanine worked out from simple geometrical considerations. Adjacent layers in the structure are held together by interactions involving side chains as well as water molecules. The water structure observed in the complex provides a good model, at atomic resolution, for that in protein crystals. An interesting feature of the crystal structure is the existence of two water channels in the interfaces between adjacent peptide layers.

Gas-crystal photoreaction: the structures of 4,4'-dimethoxy(thiobenzophenone) (I), C15H14O2S, and 4,4'-bis(dimethylamino)thiobenzophenone (II), C17H20N2S

(I): M r = 258.34, triclinic, Pi, a = 9.810 (3), b=9.635(3), e=15.015(4)A, a=79.11(2), #= 102.38 (3), y = 107.76 (3) o, V= 1308.5 A 3, Z = 4, Din= 1.318 (3) (by flotation in KI solution), D x = 1.311 g cm -3, Cu Ka, 2 = 1.5418/~, g = 20-05 cm -1, F(000) = 544, T---- 293 K, R = 0.074 for 2663 reflections. (II): M r = 284.43, monoclinic, P2~/c, a= 17.029 (5), b=6.706 (5), c= 14.629 (4), t= 113.55 (2) ° , V=1531.4A 3, Z=4, Dm=1.230(5) (by flotation in KI solution), Dx= 1.234gem -3, Mo Ka, 2 = 0.7107 A, g = 1.63 cm-1; F(000) = 608, T= 293 K, R = 0.062 for 855 reflections. The orientation of the C=S chromophores in the crystal lattice and their reactivity in the crystalline state are discussed. The C--S bonds are much shorter than the normal bond length [1.605 (4) (I), 1.665 (8) A (II) cf. 1.71 A].

Ni(II)chelates of 4,9-dimethyl-5,8-dizadodeca-4,8-diene-2,11-dione-3,10-diphenyl hydrazone and related molecules

Syntheses and structural characterization of Ni(II) chelates of a new series of symmetric and unsymmetric tetradentate linear ligands are described. Preparative routes involve either the direct reaction between a metal complex and arene diazonium diazonium salts or a simple metal incorporation into the independently synthesized ligands. Recent X-ray structure determination of 4,9-dimethyl-5,8-diazadodeca-4,8-diene-2,11-dione-3,10-di(4′-methyl phenyl) hydrazonatonickel(II) complex reveals the geometry around the Ni(II) to be very close to square planar. The expected distortion because of the disposition of bulky aromatic groups on the neighbouring nitrogens is minimized by their projection in the opposite directions from the plane. PMP, IR and electronic spectral data for the complexes are quite in agreement with this structure.

Bromination of Intermolecular Chelate Linkage Isomers of 3-Oximino-4,9-Dimethyl (or 4,7,9-Trimethyl)-5,8-Diazadodeca-4,8-Diene-2,11-Dionato Nickel(II)

The reaction of the title complexes (FIG. 1) with N-bromosuccinimide or bromine in chloroform yields isomeric bromo complexes on substitution of the γ-CH carbon proton by bromine. The brominated products have been characterised by ir, pmr, electronic absorption spectra, conductivity and magnetic susceptibility measurements. The linkage isomerisation of the brominated products in chloroform has been shown to depend on the diamine residue.

11-Epi-azadirachtin H from Azadirachta indica

A new tetranortriterpenoid 11-epi-azadirachtin H has been isolated from the methanolic extracts of Azadirachta indica seeds. Its structure is proposed on the basis of various spectral analyses.

Modeling finite buffer effects on TCP traffic over an IEEE 802.11 infrastructure WLAN

The network scenario is that of an infrastructure IEEE 802.11 WLAN with a single AP with which several stations (STAs) are associated. The AP has a finite size buffer for storing packets. In this scenario, we consider TCP-controlled upload and download file transfers between the STAs and a server on the wireline LAN (e.g., 100 Mbps Ethernet) to which the AP is connected. In such a situation, it is well known that because of packet losses due to finite buffers at the AP, upload file transfers obtain larger throughputs than download transfers. We provide an analytical model for estimating the upload and download throughputs as a function of the buffer size at the AP. We provide models for the undelayed and delayed ACK cases for a TCP that performs loss recovery only by timeout, and also for TCP Reno. The models are validated incomparison with NS2 simulations.