Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers.

CAN 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

Stereolabile and Configurationally Stable Atropisomers of Hindered Aryl Carbinols.

CAN 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.-..

Locked Chromophores as CD and NMR Probes for the Helical Conformation of Tetraamidic Macrocycles

A 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 \u201cunstable\u201d 3,3'-biphenyl spacer, the preferred conformation of a \u201cwrapped\u201d 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\u2013aryl 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

A 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

Structure, Conformation and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers.

Low temperature and NOE NMR spectra of four of the title compounds indicate that they adopt a synclinal (sc) conformation, in agreement with the prediction of ab-initio computations. In the case of the most hindered derivative (compound 4) the conformation is syn-periplanar (sp), as also shown by X-ray diffraction. Such stereolabile sp- or sc-atropisomers exist as two conformational enantiomers: the corresponding enantiomerisation barriers, covering the range 6.6 to 9.7 kcal mol-1, could be measured for all the examined compounds. In two cases (compounds 3 and 5) the minor antiperiplanar (ap) atropisomer has been also observed, and the sc to a

Ring Inversion Dynamics of Derivatives of Thianthrene Di- and Tetraoxide.

CAN 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

Kinematic characterization of functional reach to grasp in normal and motor disabled children

The upper limb kinematics were assessed during the execution of a functional task in healthy adults, children and in children with motor disabilities (i.e. hemiplegic cerebral palsy (HCP) and movement disorders (MD)). The quantitative assessment was performed considering the time durations, the amplitude of movements at different joints and the periodicity of the acceleration patterns. Compared to adults, healthy children showed increased motion amplitudes both at the head and at the trunk; this is suggestive of a reduced ability to stabilize the head during reaching. Furthermore, healthy children showed a reduced periodicity of the acceleration patterns which is interpreted as an indication of the on-going maturation process of the central nervous system. Subjects with HCP and MD showed increased movement duration; however this general finding does not account for specific differences. Indeed, children with HCP showed reduced range of motion (ROM) of the shoulder on the frontal plane which is counterbalanced by the introduction of compensatory movements of the trunk. Conversely, in children with MD, the ROM is well-preserved whereas the movements of the head are increased especially at higher speed. Finally, the periodicity of the end-effect is dramatically reduced both in HCP and MD. This suggests the existence of out-of-phase corrective strokes that may indicate an increased variability of the motor control commands. The results of this study reinforce the evidence that kinematic analysis may add valuable information to understand the developmental process in healthy children and to differentiate distinct levels of impairment in children with neurological disorders

Thiocyanate-free ruthenium(II) 2,2′-bipyridyl complexes for dye-sensitized solar cells

Thiocyanate-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