Inclusivef 2 (1270) meson production in νp and\bar vp charged current interactions

Using data obtained with the bubble chamber BEBC at CERN, the inclusivef ₂ (1270) meson production invp and \(\bar vp\) charged current reactions is studied. It is found thatf ₂ production occurs mainly in events with a hadronic invariant massW?7 GeV. In these events, the averagef ₂ multiplicity is about half the average ρ^O multiplicity, and thex _F andp _T ² distributions of thef ₂ agree in shape with those of the ρ^O. The predictions of a semi-empirical model (Wells model) are in accord with the measured multiplicities atW>7 GeV, whereas at lowerW the model predicts too largef ₂ multiplicities.     

Approaches to unsaturated analogues of nucleosides comprising four- and six-membered rings

Unsaturated nucleoside analogues 21, 22, 46, and 54, comprising four- and six-membered rings, were synthesized using two different approaches. The 2-benzyloxycycloalkanones 23a and 23b served as starting materials for both methods. Conversion to methylenecyclobutanes 29a and 29b was followed by addition of bromine via pyridinium perbromide to give vicinal dibromides 30a and 30b. Reaction of 29a with Br2 gave a ring-contracted cyclopropane derivative 31. Alkylation-elimination of adenine with 30a gave bromoalkene 32 as the major product and adenine-containing unsaturated derivatives 33, 34, and 35 as minor components. Vicinal dibromide 30b gave the Zaitsev cyclohexene 45 as the only product. Epoxidation of 29a and 29b afforded oxiranes 36a and 36b which were used in alkylation of adenine to furnish hydroxy derivatives 37a, 37b, 38a, and 38b. Beta-elimination via mesylates 39a and 40a using tBuOK/DMF gave Z- and E-methylenecyclobutanes 34 and 35. With an excess of base the E-bis-methylenecyclobutane 41 was obtained. Mesylation of cyclohexane derivatives 37b and 38b gave the Z- and E-N6-mesylated product 48. By contrast, the N6-benzoyl derivatives 49 and 50 afforded O-mesyl intermediates 51 and 52. Beta-elimination gave both Hofmann and Zaitsev products 53 and 45. O-Debenzylation of 34 and 35, 45, and 53 afforded analogues 21, 22, 46, and 54. The E-isomer 22 was also obtained by hydroboration procedure from E-bis-methylenecyclobutane 41.     

Practical aspects of splitless injection of semivolatile compounds in fast gas chromatography

Possibilities and practical aspects of implementation of splitless injection of larger volumes for fast GC purposes utilizing narrow-bore column, hydrogen as carrier gas, fast temperature programming under programmed flow conditions and commercial instrumentation were searched. As a model sample semivolatile compounds of a broad range of volatility and polarity (7 n-alkanes and 19 pesticides) were chosen. Peak shapes, peak broadening and peak areas and its repeatability were evaluated under various experimental set-ups (liner/injection technique combinations). Various factors, such as liner design, injection technique, retention gap length, compound volatility and polarity, the solvent used, initial oven temperature influenced compound focusation and/or maximal injection volume. Combination of analytical column (CP-Sil 13 CB 25 m long, 0.15 mm i.d., film thickness 0.4 microm) with normal-bore retention gap (1 m long, 0.32 mm i.d.) allowed maximal injection volume 8 microl for 4 mm i.d. liner used without any peak distortion when solvent recondensation in the retention gap was employed.     

Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)

We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems.     

Strategy for the synthesis of polymeric supports with hydrazone linkers for solid-phase alkylation of ketones and aldehydes

A new approach to polymeric supports useful for the immobilization of aldehydes and ketones via hydrazone linkers is reported. The new strategy gives supports with better properties and is effective for the synthesis of all supports previously used for the alkylation of ketones anchored as hydrazones. In contrast to other approaches, the new strategy also provided a polymer with an economical C2 spacer linker. The supports were used for immobilization of ketones 3-pentanone, acetone, N-benzylpiperidone, and aldehydes hexanal and 3-phenylpropanal in the form of their hydrazones. The polymer-supported hydrazones were subjected to alpha-alkylation (LDA/RX) followed by acidic, reductive, or oxidative cleavage/workup procedures to provide alpha-alkylated aldehydes or ketones as well as corresponding primary amines, alcohols, nitriles or acids.     

Highly Chemoselective Synthesis of Indolizidine Lactams by SmI2‐Induced Umpolung of the Amide Bond via Aminoketyl Radicals: Efficient Entry to Alkaloid Scaffolds

Samarium(II) iodide enables a wide range of highly chemoselective umpolung radical transformations proceeding by electron transfer to carbonyl groups; however, cyclizations of important nitrogen‐containing precursors have proven limited due to their prohibitive redox potential. Herein, we report the first reductive cyclizations of unactivated cyclic imides onto N‐tethered olefins using SmI₂/H₂O. This new umpolung protocol leads to the rapid synthesis of nitrogen‐containing heterocycles that are of particular significance as precursors to pharmaceutical pharmacophores and numerous classes of alkaloids. The reaction conditions tolerate a wide range of functional groups. Excellent chemoselectivity is observed in the cyclization over amide and ester functional groups. Such unconventional reactivity has important implications for the design and optimization of new bond‐forming reactions by umpolung radical processes. The reaction advances the SmI₂ cyclization platform to the challenging unactivated N‐tethered acyl‐type radical precursors to access nitrogen‐containing architectures.