1,720,969 research outputs found
Locked Chromophores as CD and NMR Probes for the Helical Conformation of Tetraamidic Macrocycles
No full textA series of tetraamidic homochiral macrocycles have been built convergently upon the introduction in the covalent scaffold of two 1,1'-binaphthyl-2,2'-diol derivatives, joined by aromatic spacers of differing
shapes and rigidity (p-phenyl, 4,4'-biphenyl, 3,3'-biphenyl) through aromatic amide functionalities,
perfectly positioned to intramolecularly hydrogen bond the naphtholic oxygen acceptors of the
binaphthyl units, in order to effectively lock the central chromophores in spatial proximity. The
combination of several techniques, namely NMR and CD spectroscopies, and computational modeling,
allows for a clear depiction of the structure and conformation of the macrocycles in solution. In the
case of the shape “unstable” 3,3'-biphenyl spacer, the preferred conformation of a “wrapped” form in a
relatively polar (EtOH) solvent is clearly signalled by CD spectroscopy by analyzing the changes in the
shape of the induced CD signal deriving from the central, achiral chromophore absorption band. The
rigid, covalent scaffold in which the two central 3,3'-biphenyl spacers are embedded raises the energetic
barrier for the rotation around the aryl–aryl bonds of the spacers to a value (8.0 kcal mol-1) much
higher than the value calculated in the case of unlocked biphenyls
First total synthesis of natural aplyolides B and D, ichthyotoxic macrolides isolated from the skin of the marine mollusk Aplysia depilans
No full textA convergent pathway is described for the synthesis of the marine macrolides aplyolides B (2) and D (3). Stereoselective preparation of a key fragment was achieved by Sharpless asymmetric dihydroxylation of eneyne 10
Stereolabile and Configurationally Stable Atropisomers of Hindered Aryl Carbinols.
Full text linkCAN 143:132926 AN 2005:441860 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R))
ABSTRACT
CARBINOLS OF THE AR-C(OH)R2 TYPE, AR BEING O-ISOPROPYLPHENYL, EXIST AS STEREOLABILE SYN-CLINAL (S.C.) AND ANTI-PERIPLANAR (AP) ATROPISOMERS WHEN R = ME, ET, I-PR. IN THE LATTER COMPD., THE MAJOR ATROPISOMER ALSO COMPRISES TWO ENANTIOMERIC FORMS THAT INTERCHANGE WITH A BARRIER OF 6.4 KCAL MOL-1. X-RAY DIFFRACTION, NOE EXPTS., AND AB INITIO CALCNS. INDICATE THAT THE S.C.-ATROPISOMER IS THE MORE STABLE FORM WHEN R = ME, I-PR, T-BU BUT IS THE LESS STABLE ONE WHEN R = ET. NMR SPECTRA AT VARIABLE TEMP. ALLOWED THE DETN. OF THE BARRIERS FOR THE INTERCONVERSION OF THE S.C.- INTO THE AP-ATROPISOMERS (ΔG.THERMOD. = 7.6, 8.8, AND 13.5 KCAL MOL-1 FOR ME, ET, I-PR, RESP.). WHEN R IS A TERT-BU GROUP, THE TWO ATROPISOMERS ARE CONFIGURATIONALLY STABLE: THE AP-ATROPISOMER IS OBTAINED AS THE KINETIC CONTROLLED COMPD., WHICH CAN BE TRANSFORMED INTO THE THERMODYNAMICALLY MORE STABLE S.C.-..
Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers.
Full text linkCAN 145:145171 AN 2006:476210 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R))
ABSTRACT
LOW-TEMP. AND NOE NMR SPECTRA OF FOUR OF THE TITLE COMPDS. INDICATE THAT THEY ADOPT A SYNCLINAL (S.C.) CONFORMATION, IN AGREEMENT WITH THE PREDICTION OF AB INITIO COMPUTATIONS. IN THE CASE OF THE MOST-HINDERED DERIV. (COMPD. 4), THE CONFORMATION IS SYN-PERIPLANAR (SP), AS IS ALSO SHOWN BY X-RAY DIFFRACTION. SUCH STEREOLABILE SP- OR S.C.-ATROPISOMERS EXIST AS TWO CONFORMATIONAL ENANTIOMERS: THE CORRESPONDING ENANTIOMERIZATION BARRIERS, COVERING THE RANGE 6.6 TO 9.7 KCAL MOL-1, COULD BE MEASURED FOR ALL THE EXAMD. COMPDS. IN TWO CASES (COMPDS. 3 AND 5), THE MINOR ANTIPERIPLANAR (AP) ATROPISOMER HAS BEEN ALSO OBSD., AND THE S.C. TO AP INTERCONVERSION BARRIER MEASURED (11.7 AND 11.9 KCAL MOL-1, RESP.). IN ADDN., RESTRICTED ROTATION OF THE ISO-PR AND TERT-BU SUBSTITUENTS HAS BEEN DETECTED, AND THE CORRESPONDING BARRIERS HAVE BEEN DETD
Ring Inversion Dynamics of Derivatives of Thianthrene Di- and Tetraoxide.
Full text linkCAN 145:271216 AN 2006:646513 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R))
ABSTRACT
THE RING INVERSION BARRIER FOR THIANTHRENE TETRAOXIDE WAS DETD. BY MAKING USE OF THE VARIABLE TEMP. 13C NMR SPECTRA OF THE 2,7-DIISOPROPYL DERIV. (ΔG.THERMOD. = 6.5 KCAL MOL-1). THE BARRIER IS LOWER THAN THAT MEASURED FOR A TRANS THIANTHRENE DIOXIDE DERIV. (ΔG.THERMOD. = 9.35 KCAL MOL-1). THESE RESULTS AGREE WELL WITH AB INITIO THEOR. PREDICTIONS
Thiocyanate-free ruthenium(II) 2,2′-bipyridyl complexes for dye-sensitized solar cells
No full textThiocyanate-free Ru(II) complexes as photosensitizers for dye-sensitized solar cells (DSSCs) are presented. These classes of complexes have gained in recent times increasing interest due to their more favorable long-term stability compared to conventional thiocyanate-based complexes such as benchmark dyes N719 and Z907. Here two classes of thiocyanate-free ruthenium(II) complexes have been reviewed: (a) bis-bipyridyl complexes bearing an ancillary cyclometallated bidentate ligand; (b) bipyridyl complexes bearing non-cyclometallated bidentate ancillary ligands. The article briefly outlines design strategies and optical and photovoltaic properties of these two families of ruthenium dyes. Although their photovoltaic performances is not as high as that of N719, recent increasing efforts by many research groups have allowed to achieve favorable light harvesting abilities, device photocurrents, and top-ranked power conversion efficiencies in combination with superior long-term stabilities, paving the way to efficient solar devices endowed with long-term chemical and thermal stability for viable large-scale commercialization of DSSCs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A BINOL-based chiral polyammonium receptor for highly enantioselective recognition and fluorescence sensing of (S,S)-tartaric acid in aqueous solution
No full textA chiral ditopic polyammonium receptor featuring two [9]aneN3
moieties separated by a (S)-BINOL linker is able to selectively bind
and sense in water (S,S)-tartaric acid over its (R,R)/meso forms
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