Metalation and halogen‐metal exchange in the imidazo[1,2‐a]quinoxaline series

The n‐butyllithium and lithium 2,2,6,6‐tetramethylpiperidide metalation and the halogen‐metal exchange of imidazo[1,2‐a]quinoxaline derivatives followed by quenching with various electrophiles were studied. The reaction conditions have been optimized and various C₁ substituted imidazo[1,2‐a]quinoxalines were obtained in high yields.     

A method to analyze performance of digital connections

A comprehensive performance analysis method that models, at bit level, the error performance of individual links in an end-to-end connection is presented. The link model accounts for the burst-error behaviour of each individual link. A method to concatenate several individual links and extract a model for the end-to-end connection is given. This resulting end-to-end model can be used to calculate performance measures such as bit error rate and block error rate for any given block size. A procedure to compute the probability distribution of errors within a specific block is also developed. Finally, a method to compute the probability distribution of blocks having a certain error rate over a given period of time is presented. The utility and power of the model are illustrated with the help of an example connection     

Analysis of Organic Matter in Water of Low TO C (Total Organic Carbon) Content by Chromatographic Techniques

Abstract

The problems encountered when analyzing organic pollutants in drinking water stem from the large number of unknown compounds at very low concentration.

The concentration of the organics is carried out on n-alkyl silica, after a rigorous cleaning, or by liquid-liquid extraction. The complexity of these extracts renders a direct analysis by gas chromatography coupled with mass spectrometry impossible; hence, a pre-separation step is required.

Reversed and normal phase chromatography have been investigated with these extracts. The former leads to informative fingerprints but subsequent identification is difficult.

Normal phase liquid chromatography is more suitable and a separation by chemical classes of increasing polarity is applied to water extracts. The eluent is fractionated, each fraction is then gently evaporated and subsequently analyzed by GC. Identification is then possible by coupling with mass spectrometry.

These procedures are used to follow the change in organic matter during the two last steps of the drinking water treatment: ozonation and filtration on active charcoal. Another application is the analysis of humic extracts.     

A new class of arylpiperazine derivatives: the library synthesis on SynPhase lanterns and biological evaluation on serotonin 5-HT(1A) and 5-HT(2A) receptors

An efficient solid-supported method for the synthesis of a new class of arylpiperazine derivatives containing amino acid residues has been developed. A 72-membered library was synthesized on SynPhase Lanterns functionalized by a BAL linker. A one-pot cleavage/cyclization step of aspartic and glutamic acid derivatives yielded succinimide- and pyroglutamyl-containing ligands (chemsets 9 and 10). The library representatives under study showed different levels of affinity for 5-HT(1A) and 5-HT(2A) receptors (estimated K(i) = 24-4000 and 1-2130 nM, respectively). Several dual 5-HT(1A)/5-HT(2A) ligands were found, of which two (9(3,3) and 9(3,5)) displayed high 5-HT(2A) affinity comparable to that of the reference drug ritanserin. A set of individual fragment contributions for the prediction of 5-HT(1A) and 5-HT(2A) affinity of all the library members were defined on the basis of the Free-Wilson analysis of 26 compounds. An alkylarylpiperazine fragment had essentially the same impact on the affinity for both receptors, whereas different terminal amide fragments were preferred by 5-HT(1A) (chemset 17, R(2) = adamantyl) and 5-HT(2A) (chemset 9, R(2) = norborn-2-ylmethyl) binding sites.     

Link layer retransmission schemes for circuit-mode data over the CDMA physical channel

In the last few years, wide-area data services over North American digital (TDMA and CDMA) cellular networks have been standardized. The standards were developed under three primary constraints: (i) compatibility with existing land-line standards and systems, (ii) compatibility with existing cellular physical layer standards that are optimized for voice, and (iii) market demands for quick solutions. In particular, the IS-95 CDMA air interface standard permits multiplexing of primary traffic (e.g., voice or circuit data) and secondary traffic (e.g., packet data) or in-band signaling within the same physical layer burst. In this paper, we describe two radio link protocols for circuit-mode data over IS-95. The first protocol, Protocol S, relies on a single level of recovery and uses a flexible segmentation and recovery (FSAR) sublayer to efficiently pack data frames into multiplexed physical layer bursts. We next describe Protocol T, that consists of two levels of recovery. Protocol T has been standardized for CDMA circuit-mode data as IS-99 (Telecommunications Industry Association, 1994). We provide performance comparisons of the two protocols in terms of throughput, delay and recovery from fades. We find that the complexity of the two level recovery mechanism can buy higher throughput through the reduced retransmission data unit size. However, the choice of TCP (and its associated congestion control mechanism) as the upper layer of recovery on the link layer, leads to long fade recovery times for Protocol T. The two approaches also have significant differences with respect to procedures and performance at handoff and connection establishment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ab initio mechanistic study of radical reactions: The regioselectivity of the addition of atomic hydrogen to fluoroacetylene and 1,1-difluoroethylene

Ab initio molecular orbital calculations of the transition states and barrier heights for the addition of atomic hydrogen to fluoroactylene and 1,1-difluoroethylene are presented. The relative reactivity of these compounds and the regioselectivity of the addition reactions are analyzed in terms of chemically interpretable contributions by means of the energy decomposition scheme proposed by Morokuma. The usual PMO approach of the problem is discussed in this context. A rationalization of some of the experimental rules of radical reactivity is given.     

Tandem mass spectrometry of amidated peptides

The behavior of C-terminal amidated and carboxylated peptides upon low-energy collision-induced dissociation (CID) was investigated. Two sets of 76 sequences of variable amino acid compositions and lengths were synthesized as model compounds. In most cases, C-terminal amidated peptides were found to produce, upon CID, an abundant loss of ammonia from the protonated molecules. To validate such MS/MS signatures, the studied peptides contained amino acids that can potentially release ammonia from their side chains, such as asparagine, glutamine, tryptophan, lysine and arginine. Arginine, and to a lesser extent lysine, was shown to induce a competitive fragmentation leading to the loss of ammonia from their side chains, thus interfering with the targeted backbone neutral release. However, when arginine or lysine was located at the C-terminal position mimicking a tryptic digest, losses of ammonia from the arginine side chain and from the peptide backbone were completely suppressed. Such results were discussed in the frame of peptidomic or proteomic studies in an attempt to reveal the presence of C-terminal amidated peptides or proteins.