A novel circuit for independent control of electron energy and emission current of a hot cathode electron source

The implementation of a non-linear combination of two reference voltages to control the anode voltage in the previously described biasing system of an electron source with a hot cathode allows elimination of the correlation between the emission current and the accelerating voltage. The presented system is highly suitable for applications in electron-impact mass spectrometers, ionization gauges and other instruments (for example, electron microscopes).     

Studies toward the synthesis of alpha-fluorinated phosphonates via tin-mediated cleavage of alpha-fluoro-alpha-(pyrimidin-2-ylsulfonyl)alkylphosphonates. Intramolecular cyclization of the alpha-phosphonyl radicals

Treatment of the alpha carbanions generated from several alpha-(pyrimidin-2-ylsulfonyl)alkylphosphonates with Selectfluor gave high yields of the alpha-fluoro-alpha-(pyrimidin-2-ylsulfonyl)alkylphoshonates, which were desulfonylated [Bu(3)SnH/2,2'-azobisisobutyronitrile (AIBN)/benzene or toluene/Delta] to give alpha-fluoroalkylphosphonates. "Catalytic" tin hydride, generated from tributyltin chloride and excess polymethylhydrosiloxane in the presence of potassium fluoride, also effected removal of the pi-deficient alpha-(pyrimidin-2-ylsulfonyl) group from the phosphonate esters. Substitution of Bu(3)SnD for Bu(3)SnH gave access to alpha-deuterium-labeled phosphonates. Prolonged treatment of alpha-(pyridin-2-ylsulfonyl)alkylphosphonate with excess Bu(3)SnH/AIBN or catalytic tin hydride also effected desulfonylation but in moderate yields. This represents a mild new methodology for removal of the synthetically useful pi-deficient heterocyclic sulfone moiety and an alternative route for the preparation of alpha-fluorinated phosphonates. Desulfonylation is suggested to proceed via attack of tin radical at an oxygen (or sulfur) atom of the sulfonyl group to give a stabilized alpha-phosphonyl radical intermediate. The latter was found to undergo 5-exo-trig ring closure to give the corresponding 2-methylcyclopentylphosphonates. Treatment of diethyl 1-bromohex-6-enylphosphonate with Bu(3)SnH/AIBN produced an analogous mixture of ring-closure products. Treatment of [(2-bromo-5- methoxyphenyl)(fluoro)(pyrimidin-2-ylsulfonyl)]methylphosphonate with Bu(3)SnH resulted in an intramolecular radical [1,5]-ipso substitution reaction and migration of the pyrimidinyl ring to give fluoro[5-methoxy-2-(pyrimidin-2-yl)phenyl]methylphosphonate.     

Convenient Synthesis of Julolidines Using Benzotriazole Methodology

Reaction of N,N-bis[(benzotriazol-1-yl)methyl]aniline (2) with 1-vinylpyrrolidin-2-one gives a mixture of diastereomeric 1,7-bis(2-oxopyrrolidin-1-yl)julolidines 3. After reduction of 3 with LAH, the predominant trans diastereomer of 1,7-di(pyrrolidin-1-yl)julolidine (4) is separated. Reaction of 2 with ethyl vinyl ether yields predominantly trans-1,7-di(benzotriazol-1-yl)julolidine (11). Stepwise synthesis from tetrahydroquinoline 15 gives access to julolidines with two different substituents on C-1 and C-7. Reaction of 1-[(benzotriazol-1-yl)methyl]-1,2,3,4-tetrahydroquinoline (25) with enolizable aldehydes gives a mixture of tetrahydroquinolines 26-29 which are converted into single julolidine products upon treatment with sodium hydride, LAH, or phenylmagnesium bromide. Reactions of 1,2,3,4-tetrahydroquinolines with benzotriazole and 2 molar equiv of enolizable aldehydes gives 1,2,3-trisubstituted julolidines 38-41, which with lithium aluminum hydride, sodium hydride, or a Grignard reagent produce single diastereomers of products 42, 43, and 45, respectively.     

The modified `phosphine imide' reaction: a safe and soft alternative ureas synthesis

In this work we present a new way for the direct and general synthesis of urea compounds from primary amines by the modified `phosphine imide' reaction. A large panel of amine structures are compatible with the smooth reaction conditions. Particularly in the case of sensitive l-aminoesters, it is interesting to note that the stereochemistry at the asymmetric centre was unmodified in the reaction conditions.     

Living polymerization with reversible chain transfer and reversible deactivation: The case of cyclic esters

Polymerization of cyclic esters at the properly chosen conditions can be treated as living polymerization, in agreement with the tentative definition of the Nomenclature Commission of IUPAC (Macromolecular Division) requiring that no irreversible transfer or irreversible termination take place. For these processes the most kinetic or structural (end group) studies do not reveal any deviation. However, since in these polymerizations reversible transfer to backbones of macromolecules and/or reversible deactivation take place, the molar mass distribution can be Poissonian only at certain conditions. These processes have been studied quantitatively and the corresponding rate constants were determined. Thus, the importance of these processes could be established by comparing the rate constants of transfer and/or deactivation with rate constants of propagation. In this way, polymerizations of cyclic esters were used to illustrate the meaning and scope of the definition of “living polymerization”, a process from which irreversible transfer and deactivation are absent and in which living polymers are formed, i. e. composed of macromolecules that do not irreversibly loose their ability to grow.     

Controlled polymerization of cyclic esters. structure of initiators and of active species related to the selectivity of initiation and propagation

This paper reviews mostly the already published work of our own, complemented with some new data and related to the control (i.e. selectivity) in the anionic and covalent (pseudoanionic) polymerization of cyclic esters. Ionic and covalent growing species are briefly compared in terms of their susceptibility to undergo side reactions. Then, structures of covalent initiators (mostly multivalent metal alkoxides) are discussed from the view point of their behavior in initiation and formation of active species. R₂AlOR', (RO)₃Al, and other metal alkoxides are discussed.     

Biodegradable Polyesters for Tissue Engineering

Summary: Paper describes basic characteristics of synthesis and properties of aliphatic polyesters used for tissue engineering. Described is also synthesis of polyester containing block copolymers suitable for surface modification. Described are methods used for scaffold fabrication with required porosity. In particular, presented are methods according to which scaffolds are made from prefabricated polyester micro- and nanoparticles.