Controlled helical orientation of carbazole in amino acid derived poly(N‐propargylamide)s

Novel L ‐alanine and L ‐glutamic acid derivatized, carbazole‐containing N‐propargylamides [N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐alanine N′‐propargylamide and N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐glutamic acid‐γ‐benzyl ester N′‐propargylamide] were synthesized and polymerized with (nbd)Rh⁺[η⁶‐C₆H₅B^?(C₆H₅)₃] (nbd = norbornadiene) as a catalyst to obtain the corresponding polymers with moderate molecular weights in high yields. Polarimetry, circular dichroism, and ultraviolet–visible spectroscopy studies revealed that both poly[N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐alanine N′‐propargylamide] and poly[N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐glutamic acid‐γ‐benzyl ester N′‐propargylamide] took a helical structure with a predominantly one‐handed screw sense in tetrahydrofuran, CHCl₃, and CH₂Cl₂. The helix content of poly[N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐alanine N′‐propargylamide] could be tuned by heat or the addition of a protic solvent, and the helical sense of poly[N‐(9‐carbazolyl) ethyloxycarbonyl‐L ‐glutamic acid‐γ‐benzyl ester N′‐propargylamide] was inverted by heat in CHCl₃ or in mixtures of tetrahydrofuran and CH₂Cl₂. Poly[N‐(9‐carbazolyl) ethyloxycarbonyl‐L ‐alanine N′‐propargylamide] and poly[N‐(9‐carbazolyl)ethyloxycarbonyl‐L ‐glutamic acid‐γ‐benzyl ester N′‐propargylamide] also took a helical structure in film states. They showed small fluorescence in comparison with the monomers and redox activity based on carbazole. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 253–261, 2007     

Synthesis and chiral recognition properties of poly(N‐propargylamide) gels derived from ornithine and lysine

Copolymerization of ornithine‐ and lysine‐derived N‐propargylamides, N‐α‐tert‐butoxycarbonyl‐N‐δ‐fluorenylmethoxycarbonyl‐L ‐ornithine N′‐propargylamide ( 1 ), N‐α‐tert‐butoxycarbonyl‐N‐ε‐fluorenylmethoxycarbonyl‐L ‐lysine N′‐propargylamide ( 2 ), N‐α‐fluorenylmethoxycarbonyl‐N‐δ‐tert‐butoxycarbonyl‐L ‐ornithine N′‐propargylamide ( 3 ), and N‐α‐fluorenylmethoxycarbonyl‐N‐ε‐tert‐butoxycarbonyl‐L ‐lysine N′‐propargylamide (4) with dipropargyl adipate was carried out using (nbd)Rh⁺[η⁶‐C₆H₅B^?(C₆H₅)₃] as a catalyst in THF to obtain polymer gels in 80–93% yields. The gels adsorbed N‐benzyloxycarbonyl L ‐alanine, N‐benzyloxycarbonyl L ‐alanine methyl ester, and (S)‐(+)‐1‐phenyl‐1,2‐ethanediol preferably than the corresponding optical isomers. The order of chiral discrimination was poly( 1 ) > poly( 4 ) > poly( 2 ), poly( 3 ) gels. The fluorenylmethoxycarbonyl groups of the gels could be partly removed by piperidine treatment, leading to increase of adsorptivity but decrease of chiral recognition ability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4175–4182, 2008     

Ring‐opening metathesis polymerization of amino acid‐functionalized norbornene derivatives

Amino acid‐derived novel norbornene derivatives, N,N′‐(endo‐bicyclo[2.2.1] hept‐5‐en‐2,3‐diyldicarbonyl) bis‐L ‐alanine methyl ester (NBA), N,N′‐(endo‐bicyclo[2.2.1]hept‐5‐en‐2,3‐diyldicarbonyl) bis‐L ‐leucine methyl ester (NBL), N,N′‐(endo‐bicyclo[2.2.1]hept‐5‐en‐2,3‐diyldicarbonyl) bis‐L ‐phenylalanine methyl ester (NBF) were synthesized and polymerized using the Grubbs 2nd generation ruthenium (Ru) catalyst. Although NBA, NBL, and NBF did not undergo homopolymerization, they underwent copolymerization with norbornene (NB) to give the copolymers with M_n ranging from 5200 to 38,100. The maximum incorporation ratio of the amino acid‐based unit was 9%, and the cis contents of the main chain were 54–66%. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5337–5343, 2006     

Microwave‐assisted synthesis and characterization of heterocyclic,and optically active poly(amide‐imide)s incorporating L‐amino acids

N,N′‐Pyromelliticdiimido‐di‐L ‐alanine ( 1 ), N,N′‐pyromelliticdiimido‐di‐L ‐phenylalanine ( 2 ), and N,N′‐pyromelliticdiimido‐di‐L ‐leucine ( 3 ) were prepared from the reaction of pyromellitic dianhydride with corresponding L ‐amino acids in a mixture of glacial acetic acid and pyridine solution (3/2 ratio) under refluxing conditions. The microwave‐assisted polycondensation of the corresponding diimide‐diacyl chloride monomers ( 5–7 ) with 4‐phenyl‐2,6‐bis(4‐aminophenyl) pyridine ( 10 ) or 4‐(p‐methylthiophenyl)‐2,6‐bis(4‐aminophenyl) pyridine ( 12 ) were carried out in a laboratory microwave oven. The resulting poly(amide‐imide)s were obtained in quantitative yields, and they showed admirable inherent viscosities (0.12–0.55 dlg^?1), were soluble in polar aprotic solvents, showed good thermal stability and high optical purity. The synthetic compounds were characterized by IR, MS, ¹H NMR, and ¹³C NMR spectroscopy, elemental analysis, and specific rotation. Copyright © 2008 John Wiley & Sons, Ltd.     

Determination of kynurenic acid levels in rat brain microdialysis samples and changes in kynurenic acid levels induced by N‐acetylaspartic acid

The levels of kynurenic acid, an endogenous antagonist of α₇ nicotinic acetylcholine and N‐methyl‐D ‐aspartate receptors, were measured in microdialysis samples obtained from the prefrontal cortices of rats using column‐switching high‐performance liquid chromatography with fluorescence detection. When the perfusate was constantly infused at a rate of 1.0 μ/min, the in vitro recovery of kynurenic acid through the dialysis membrane was approximately 20.4%, and the precision was within 1.31%. Endogenous kynurenic acid in the microdialysis sample was clearly detected using column‐switching high‐performance liquid chromatography. As an application study, N‐acetyl‐L ‐aspartic acid, an endogenous metabolite and precursor of N‐acetyl‐L ‐aspartyl‐L ‐glutamic acid, which is an agonist of metabotropic glutamate receptors, was infused for 120 min through the microdialysis probe. The kynurenic acid level significantly increased during the infusion of N‐acetyl‐L ‐aspartic acid, suggesting that kynurenic acid might have some association with N‐acetyl‐L ‐aspartic acid in vivo. Copyright © 2009 John Wiley & Sons, Ltd.     

A silaproline‐containing dipeptide

The silaproline‐containing dipeptide N‐(3,3‐di­methyl‐1‐pivaloyl‐1‐aza‐3‐sila‐5‐cyclo­pentyl­carbonyl)‐l ‐alanine iso­propyl­amide, C₁₇H₃₃N₃O₃Si, has two independent molecules in the asymmetric unit and each adopts a β‐II folded conformation, where the amide on the terminal C interacts intramolecularly with the pivaloyl O atom. The five‐membered silaproline ring is C^β‐puckered, an infrequent conformation for the homol­ogous proline ring.     

Synthesis and properties of optically active amino acid based polyacetylenes bearing eugenol and fluorene moieties

Novel optically active amino acid based polyacetylenes bearing eugenol and fluorene moieties were synthesized, and their properties, including chiroptical ones, were analyzed. N‐[1‐(3,4‐Dimethoxyphenyl)‐2‐propyloxycarbonyl]‐L ‐alanine N′‐propargylamide ( 1 ), N‐[1‐(3,4‐dimethoxyphenyl)‐2‐propyloxycarbonyl]‐L ‐alanine propargyl ester ( 2 ), N‐(9‐fluorenylmethoxycarbonyl)‐L ‐alanine N′‐propargylamide ( 3 ), and N‐(9‐fluorenylmethoxycarbonyl)‐L ‐alanine propargyl ester ( 4 ) were polymerized with a rhodium‐zwitterion catalyst in tetrahydrofuran to afford the corresponding polymers with moderate molecular weights ranging from 10,800 to 17,300 in good yields. Because of the large specific rotation and circular dichroism (CD) signal, it was concluded that the poly(N‐propargylamide)s [poly( 1 ) and poly( 3 )] took a helical structure with a predominantly one‐handed screw sense. The solvent and temperature could tune the helical structure of poly( 1 ). On the other hand, the poly(propargyl ester)s [poly( 2 ) and poly( 4 )] exhibited only small specific rotations and CD signals. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 810–819, 2006     

Ab Initio MP2 and Density Functional Theory Computational Study of AcAlaNH2 Peptide Hydration: A Bottom‐Up Approach

AcAlaNH₂?n H₂O (n=1–13) complexes have been proposed as models to account for water solvent effects on the molecular properties of N‐acetyl‐L ‐alanine amide. Ab initio computations are planned to evaluate peptide–water interactions and to provide a means for approximating relative effects of the short‐range many‐body interactions arising in real solution without introducing any external parameters intended to quantify empirical or semiempirical potential‐energy functions. The present bottom‐up approach reveals the formation of compact ring clusters of water molecules strongly bonded to peptidic polar groups throughout hydrogen bonds. The explicit coordination of water molecules around the peptide renders the fully extended (FE) and polyproline II (PPII) conformers more stable with respect to the 3₁₀ helix or γ turn. The alternance of donor and acceptor groups on both sides of the FE and PPII conformers allows for synergy and extensive H‐bonding.     

Glycosidic linkage,N‐acetyl side‐chain,and other structural properties of methyl 2‐acetamido‐2‐deoxy‐β‐d‐glucopyranosyl‐(1→4)‐β‐d‐mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis–trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.     

Total Synthesis,Absolute Configuration,and Biological Activity of Xyloallenoide A

The novel natural product xyloallenoide A, isolated from the marine mangrove endophytic fungus from the South China Sea, and its diastereoisomer xyloallenoide A1, which contain N‐methyl‐substituted amino acids, were synthesized. The absolute configurations of the amino acid units of xyloallenoide A were finally confirmed to be L ‐Lys, Me‐D ‐Val, and Me‐L ‐Ala. This report represents a practical and attractive alternative for the synthesis of N‐methyl‐substituted cyclotripeptides. In the preliminary bioassay, synthetic xyloallenoide A showed marginal activities against KB (IC₅₀=9.6 μM ) and KBv200 cells (IC₅₀=10.3 μM ), and xyloallenoide A1 was inactive against KB and KBv200 cells.     

cyclo(‐Cha–Oxz–d‐Val–Thz–Ile–Oxz–d‐Val–Thz‐) N,N‐di­methyl­acetamide dihydrate: a square form of cyclo­hexyl­alanine‐incorporated ascidiacycl­amide having the strongest cytotoxicity

The title compound, 1‐cyclo­hexyl­methyl‐1‐de(1‐methyl­propyl)­asci­dia­cycl­amide N,N‐di­methyl­acet­amide di­hy­drate, C₃₉H₅₆N₈O₆S₂·C₄H₉NO·2H₂O, a cyclo­hexyl­alanine‐incorporated ascidiacycl­amide analogue ([Cha]ASC), shows a square form similar to natural ASC. On the other hand, CD (circular dichroism) spectra showed [Cha]ASC to have a folded structure in solution, making it the second known analogue to show a discrepancy between its crystal and solution structures. Moreover, the cytotoxicity of [Cha]ASC (ED₅₀ = 5.6 µg ml⁻¹) was approximately two times stronger than that of natural ASC or a related phenyl­alanine‐incorporated analogue, viz. cyclo(‐Phe–Oxz–d ‐Val–Thz–Ile–Oxz–d ‐Val–Thz‐) ([Phe]ASC), and was confirmed to be associated with the square form. However, [Phe]ASC was previously shown to be folded in the crystal structure, which suggests that the difference between the aromatic and aliphatic rings affects the molecular folding of the ASC mol­ecule.     

N‐Acetyl‐l‐tyrosine methyl ester monohydrate at 293 and 123 K

The crystal structure of a protected l ‐tyrosine, namely N‐acetyl‐l ‐tyrosine methyl ester monohydrate, C₁₂H₁₅NO₄·H₂O, was determined at both 293 (2) and 123 (2) K. The structure exhibits a network of O—H...O and N—H...O hydrogen bonds, in which the water molecule plays a crucial role as an acceptor of one and a donor of two hydrogen bonds. Molecules of water and of the protected l ‐tyrosine form hydrogen‐bonded layers perpendicular to [001]. C—H...π interactions are observed in the hydrophobic regions of the structure. The structure is similar to that of N‐acetyl‐l ‐tyrosine ethyl ester monohydrate [Soriano‐García (1993). Acta Cryst. C 49 , 96–97].     

Functionalized 2‐Hydrazinobenzothiazole with Isatin and Some Carbohydrates under Conventional and Ultrasound Methods and Their Biological Activities

Several chemical reactions were carried out on 3‐(benzothiazol‐2‐yl‐hydrazono)‐1,3‐dihydro‐indol‐2‐one ( 2 ). 3‐(Benzothiazol‐2‐yl‐hydrazono)‐1‐alkyl‐1,3‐dihydro‐indol‐2‐one 3a , 3b , 3c have been achieved. Reaction of compound 2 with ethyl bromoacetate in the presence of K₂CO₃ resulted the uncyclized product 4 . Reaction of compound 2 with benzoyl chloride afforded dibenzoyl derivative 5 . Compound 2 was smoothly acetylated by acetic anhydride in pyridine to give diacetyl derivative 6b . Moreover, when compound 4 reacted with methyl hydrazine, it yielded dihydrazide derivative 7 , whereas the hydrazinolysis of this compound with hydrazine hydrate gave the monohydrazide derivative 8 . {N‐(Benzothiazol‐2‐yl‐N′‐(3‐oxo‐3,4‐dihydro‐2H‐1,2,4‐triaza‐fluoren‐9‐ylidene)hydrazino]‐acetic acid ethyl ester ( 9 ) was prepared by ring closure of compound 8 by the action of glacial acetic acid. In addition, the reaction of 2‐hydrazinobenzothiazole ( 1 ) with d ‐glucose and d ‐arabinose in the presence of acetic acid yielded the hydrazones 10a , 10b , respectively. Acetylation of compound 10b gave compound 11b . On the other hand, compound 13 was obtained by the reaction of compound 1 with gama‐d ‐galactolactone ( 12 ). Acetylation of compound 13 with acetic anhydride in pyridin gave the corresponding N¹‐acetyl‐N²‐(benzothiazolyl)‐2‐yl)‐2,3,4,5,6‐penta‐O‐acetyl‐d ‐galacto‐hydrazide ( 14 ). Better yields and shorter reaction times were achieved using ultrasound irradiation. The structural investigation of the new compounds is based on chemical and spectroscopic evidence. Some selected derivatives were studied for their antimicrobial and antiviral activities.     

Remarkable Shape‐Sustaining,Load‐Bearing,and Self‐Healing Properties Displayed by a Supramolecular Gel Derived from a Bis‐pyridyl‐bis‐amide of L‐Phenyl Alanine

A series of bis‐amides decorated with pyridyl and phenyl moieties derived from L ‐amino acids having an innocent side chain (L ‐alanine and L ‐phenyl alanine) were synthesized as potential low‐molecular‐weight gelators (LMWGs). Both protic and aprotic solvents were found to be gelled by most of the bis‐amides with moderate to excellent gelation efficiency (minimum gelator concentration=0.32–4.0 wt. % and gel–sol dissociation temperature T_gel=52–110 °C). The gels were characterized by rheology, DSC, SEM, TEM, and temperature‐variable ¹H NMR measurements. pH‐dependent gelation studies revealed that the pyridyl moieties took part in gelation. Structure–property correlation was attempted using single‐crystal X‐ray and powder X‐ray diffraction data. Remarkably, one of the bis‐pyridyl bis‐amide gelators, namely 3,3‐Phe (3‐pyridyl bis‐amide of L ‐phenylalanine) displayed outstanding shape‐sustaining, load‐bearing, and self‐healing properties.     

Insulin Secretagogues from Moringa oleifera with Cyclooxygenase Enzyme and Lipid Peroxidation Inhibitory Activities

Bioassay‐directed isolation and purification of the methanol extract of Moringa oleifera fruits yielded bioactive N‐benzyl thiocarbamates, N‐benzyl carbamates, benzyl nitriles, and a benzyl ester. Among these, methyl 2‐[4‐(α‐L ‐rhamnopyranosyl)phenyl]acetate ( 2 ), N‐[4‐(β‐L ‐rhamnopyranosyl)benzyl]‐1‐O‐α‐D ‐glucopyranosylthiocarboxamide ( 3 ), 1‐O‐phenyl‐α‐L ‐rhamnopyranoside ( 5 ), and 4‐[(β‐D ‐glucopyranosyl)‐(1→3)‐(α‐L ‐rhamnopyranosyl)]phenylacetonitrile ( 6 ) are novel, and their structures were determined by spectroscopic methods. The known compounds isolated and characterized from the MeOH extract were niazirin (=4‐(α‐L ‐rhamnopyranosyl)phenylacetonitrile; 1 ), niazicin A (=methyl N‐{4‐[(4′‐O‐acetyl‐α‐L ‐rhamnopyranosyl)benzyl]}thiocarbamate; 4 ), methyl N‐{4‐[(α‐L ‐rhamnopyranosyl)benzyl]}carbamate ( 7 ), and methyl N‐{4‐[(4′‐O‐acetyl‐α‐L ‐rhamnopyranosyl)benzyl]}carbamate ( 8 ). The combined yield of these compounds from dried M. oleifera fruits was 1.63%. In rodent pancreatic β‐cells (INS‐1), compounds 4, 5, 6, 7 , and 8 at 100 ppm significantly stimulated insulin release. Cyclooxygenase‐1 (COX‐1) and cyclooxygenase‐2 (COX‐2) enzyme inhibition assays revealed that 5 and 6 were most active at 83 ppm. Compound 6 , however, demonstrated greater specificity for inhibition of COX‐2 enzyme (46%) than COX‐1 enzyme. Lipid peroxidation assays revealed that 4 and 6 at 50 ppm inhibited peroxidation reactions by 80 and 95%, respectively, while 3 and 8 inhibited lipid peroxidation by 35%. These compounds did not inhibit the cell growth when tested with human breast (MCF‐7), central nervous system (CNS, SF‐268), lung (NCI‐H460), or colon (HCT‐116) cancer cell lines. Moreover, these compounds were not cytotoxic at the concentrations tested.     

Conformational and electronic study of N‐acetyl‐L‐isoleucine‐N‐methylamide using DFT and IPCM calculations

A conformational and electronic study on N‐acetyl‐L ‐isoleucine‐N‐methylamide was carried out. All side‐chain as well as backbone conformations were explored for this compound. Multidimensional conformational analysis predicts 81 structures in the case of N‐acetyl‐L ‐isoleucine‐N‐methylamide, 53 relaxed structures were determined at the DFT (B3LYP/6‐31G(d)) level of theory. An exhaustive electronic study employing the atoms‐in‐molecules (AIM) method was carried out. In addition, the effects of three solvents (water, acetonitrile, and chloroform) were included in the calculations using the isodensity polarizable continuum model (IPCM) method. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Bioorthogonal Labeling,Bioimaging, and Photocytotoxicity Studies of Phosphorescent Ruthenium(II) Polypyridine Dibenzocyclooctyne Complexes

The synthesis, characterization, photophysics, lipophilicity, and cellular properties of new phosphorescent ruthenium(II) polypyridine complexes functionalized with a dibenzocyclooctyne (DIBO) or amine moiety [Ru(N^N)₂(L)](PF₆)₂ are reported (L=4‐(13‐N‐(3,4:7,8‐dibenzocyclooctyne‐5‐oxycarbonyl) amino‐4,7,10‐trioxa‐tridecanyl‐aminocarbonyl‐oxy‐methyl)‐4′‐methyl‐2,2′‐bipyridine bpy‐DIBO, N^N=2,2′‐bipyridine bpy ( 1 a ), 1,10‐phenanthroline phen ( 2 a ); L=4‐(13‐amino‐4,7,10‐trioxa‐tridecanylaminocarbonyl‐oxy‐methyl)‐4′‐methyl‐2,2′‐bipyridine bpy‐NH₂, N^N=bpy ( 1 b ), phen ( 2 b )). The strain‐promoted alkyne–azide cycloaddition (SPAAC) reaction of the DIBO complexes 1 a and 2 a with benzyl azide were studied. Also, the DIBO complexes 1 a and 2 a can selectively label N‐azidoglycans located on the surface of CHO‐K1 and A549 cells that were pretreated with 1,3,4,6‐tetra‐O‐acetyl‐N‐azidoacetyl‐D ‐mannosamine (Ac₄ManNAz). Additionally, the intracellular trafficking and localization of these biomolecules were monitored using laser‐scanning confocal microscopy. Interestingly, the biolabeling and cellular uptake efficiency of the DIBO complexes 1 a and 2 a were cell‐line dependent, as revealed by flow cytometry and ICP‐MS. Furthermore, the complexes showed good biocompatibility toward the Ac₄ManNAz‐pretreated cells in the dark, but exhibited photoinduced cytotoxicity due to the generation of singlet oxygen.     

Structural properties of D‐mannopyranosyl rings containing O‐acetyl side‐chains

The crystal structures of 1,2,3,4,6‐penta‐O‐acetyl‐α‐d ‐mannopyranose, C₁₆H₂₂O₁₁, and 2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranosyl‐(1→2)‐3,4,6‐tri‐O‐acetyl‐α‐d ‐mannopyranosyl‐(1→3)‐1,2,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranose, C₄₀H₅₄O₂₇, were determined and compared to those of methyl 2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranoside, methyl α‐d ‐mannopyranoside and methyl α‐d ‐mannopyranosyl‐(1→2)‐α‐d ‐mannopyranoside to evaluate the effects of O‐acetylation on bond lengths, bond angles and torsion angles. In general, O‐acetylation exerts little effect on the exo‐ and endocyclic C—C and endocyclic C—O bond lengths, but the exocyclic C—O bonds involved in O‐acetylation are lengthened by ～0.02 Å. The conformation of the O‐acetyl side‐chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°‐alcoholic C atom or bisecting the H—C—H bond angle of a 1°‐alcoholic C atom. Of the two C—O bonds that determine O‐acetyl side‐chain conformation, that involving the alcoholic C atom exhibits greater rotational variability than that involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of O‐acetyl side‐chain conformations in saccharides. Experimental evidence was also obtained to confirm density functional theory (DFT) predictions of C—O and O—H bond‐length behavior in a C—O—H fragment involved in hydrogen bonding.     

Probing Effect of Solvation on Photoexcited Quadrupolar Donor-Acceptor-Donor Molecule via Ultrafast Raman Spectroscopy

The symmetric and quadrupolar donor-acceptordonor (D-A-D) molecules usually exhibit excitedstate charge redistribution process from delocalized intramolecular charge transfer (ICT) state to localized ICT state. Direct observation of such charge redistribution process in real-time has been intensively studied via various ultrafast time-resolved spectroscopies. Femtosecond stimulated Raman spectroscopy (FSRS) is one of the powerful methods which can be used to determine the excited state dynamics by tracking vibrational mode evolution of the specific chemical bonds within molecules. Herein, a molecule, 4, 4′-(buta-1, 3-diyne-1, 4-diyl)bis(N, N-bis(4-methoxyphenyl)aniline), that consists of two central adjacent alkyne (-C≡C-) groups as electron-acceptors and two separated, symmetric N, N-bis(4-methoxyphenyl)aniline at both branches as electron-donors, is chosen to investigate the excited-state photophysical properties. It is shown that the solvation induced excited-state charge redistribution in polar solvents can be probed by using femtosecond stimulated Raman spectroscopy. The results provide a fundamental understanding of photoexcitation induced charge delocalization/localization properties of the symmetric quadrupolar molecules with adjacent vibrational markers located at central position.

     

Syntheses and structures of four new mixed‐amide phosphoric triamides

Phosphoric triamides have extensive applications in biochemistry and are also used as O‐donor ligands. Four new mixed‐amide phosphoric triamide structures, namely rac‐N‐tert‐butyl‐N′,N′′‐dicyclohexyl‐N′′‐methylphosphoric triamide, C₁₇H₃₆N₃OP, (I), rac‐N,N′‐dicyclohexyl‐N′‐methyl‐N′′‐(p‐tolyl)phosphoric triamide, C₂₀H₃₄N₃OP, (II), N,N′,N′′‐tricyclohexyl‐N′′‐methylphosphoric triamide, C₁₉H₃₈N₃OP, (III), and 2‐[cyclohexyl(methyl)amino]‐5,5‐dimethyl‐1,3,2λ⁵‐diazaphosphinan‐2‐one, C₁₂H₂₆N₃OP, (IV), have been synthesized and studied by X‐ray diffraction and spectroscopic methods. Structures (I) and (II) are the first diffraction studies of acyclic racemic mixed‐amide phosphoric triamides. The P—N bonds resulting from the different substituent –N(CH₃)(C₆H₁₁), (C₆H₁₁)NH–, 4‐CH₃‐C₆H₄NH–, (tert‐C₄H₉)NH– and –NHCH₂C(CH₃)₂CH₂NH– groups are compared, along with the different molecular volumes and electron‐donor strengths. In all four structures, the molecules form extended chains through N—H…O hydrogen bonds.