Reactivity of N-Phenyl-1-Aza-2-Cyano-1,3-Butadienes in the Diels-Alder Reaction

It is found that N-phenyl-2-cyano-1-azadiene 4, prepared via a two-step, one-pot, sequence from acrylanilide, undergoes efficient [4 + 2] cycloaddition with a complete range of electron rich, electron poor, and neutral dienophiles under remarkably mild thermal conditions (90-120 degrees C for 20-48 h). Regiospecific formation of the alpha-cycloadduct wherein the dienophile substituent is alpha to nitrogen is observed for vinyl ethers and styrene, whereas the Diels-Alder reactions with methyl acrylate and methyl vinyl ketone (MVK) produce alpha/beta mixtures in which the alpha-cycloadduct is the major regioisomer (approximately 4-5:1). An essentially identical reaction pattern was observed in the Diels-Alder reaction of N-(p-methoxyphenyl)-2-cyano-1-azadiene 18 and the 4-methyl-substituted azadiene 27. For compound 19 derived from cycloaddition of 18 with ethyl vinyl ether, facile conversion to the dihydropyridine 21 through loss of EtOH on brief acid treatment was also noted. The 2,4-cis-disubstitution pattern confirmed by X-ray diffraction for the major cycloadduct 29 isolated from the reaction of 27 with styrene provides evidence for the endo mode of cycloaddition in the Diels-Alder reaction of N-phenyl(aryl)-2-cyano-1-azadienes. Calculation of the frontier orbital energies and coefficients, as well as the transition state geometries for the [4 + 2] cycloaddition of N-phenyl-2-cyano-1-azadiene 4 with methyl vinyl ether, styrene, and MVK were carried out at the RHF AM1 level (MOPAC, Version 5.0). The FMO treatment indicates that the reaction of 4 with methyl vinyl ether occurs under LUMO(diene) control, whereas in contrast, the corresponding cycloaddition with MVK occurs preferably under HOMO(diene) control. A high degree of asynchronicity is observed in the calculated transition states for reaction of 4 with the three representative dienophiles. In all cases the transition states leading to the alpha-cycloadducts are lower in energy than those giving the beta-products. However, at the AM1 level the exo cycloaddition mode is found to be the preferred, this result contrasting with experimental results for azadiene 27.     

Perception of syllable timing by prebabbling infants

Adults hear alternating syllables with isochronous syllable onset-onset times as having a long-short, alternating rhythm when the syllables differ in initial consonant. This occurs because adults attend to syllable-internal events, called the "P centers" or "stress beats", rather than to syllable onsets. Thus they report that stress-beat aligned speech is isochronous and stress-beat aligned clicks are synchronized with the speech. The question asked here is whether, like adults, infants attend to the timing of syllable stress beats. In experiment 1, infants showed differences in time to habituate to sequences of alternating monosyllables, [bad] and [strad], having two different onset-onset times (onset- and stress-beat-timed) and two different placements of clicks on the syllables (on syllable onsets and on stress beats). Infants habituated more slowly to sequences with clicks on the stress beats than to sequences with clicks on syllable onsets and most slowly of all to stress-beat-timed speech with clicks on the stress beats. To interpret these findings, a second experiment was run using sequences only of the syllable [strad] so that speech timing measured according to onsets and stress beats was the same. Syllables had isochronous timing or a long-short alternating rhythm, corresponding to two possible ways of hearing the stress-beat-timed speech of experiment 1. In addition, two patterns of click placement were compared, uniform and syncopated, corresponding to two ways of hearing the stress-beat aligned clicks of experiment 1. The patterns of sucking times in the two experiments match exactly if stress-beat aligned speech in experiment 1 is identified with the isochronous speech of experiment 2 and the stress-beat aligned clicks of experiment 1 match with the uniformly timed clicks of experiment 2. It is inferred from this correspondence that infants perceive stress beats and stress-beat timing of syllables as adults do.     

Ring currents in the porphyrins: pi shielding, delocalisation pathways and the central cation

It is shown that the ipsocentric orbital-based model explains how the charge of the central cation drives the delocalisation pathway in metalloporphyrins. A positive charge +Ze at the centre of the porphin ring gives rise to a two-way radial transfer of charge within the pi structure of the porphin macrocycle. This manifests itself in a change of pathway of the global pi current, as Z increases from Z = 0, from an inner- through a bifurcated- to an outer-pathway. Changes of pathway can be interpreted in terms of a specific pi shielding effect whereby electrons in high-lying pi orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in pi structure are essentially independent of accompanying changes in the sigma structure.     

Synthesis of C-linked glycopyranosyl serines via a chiral glycine enolate equivalent

[formula: see text] The stereoselective preparation of C-linked D-gluco- and D-galactopyranosyl L-serines in their alpha and beta forms is herein reported. The syntheses require the conversion of the allyl C-glycopyranosides into their iodoethyl derivatives, which then undergo substitution with the Williams' chiral glycine enolate equivalent. Deprotection and acetylation affords Boc-protected amino acids for peptide synthesis.