Efficient and practical aerobic radical addition of thiophosphites to alkenes

Thiophosphites, but not phosphites, add onto alkenes under very mild conditions when treated with triethylborane and oxygen, and deliver the expected adducts in high isolated yields. This method, featuring a high atom economy, complements those described in literature and can be used to initiate tandem cascade processes. © 2001 Published by Elsevier Science Ltd

Microstructural Characterization of Hydrogen Peroxide Initiated Hydroxytelechelic Polyisoprene

International audienc

NMR Characterization of a Hydrogenated Hydroxytelechelic Polyisoprene

International audienc

Determination of substituents distribution in carboxymethylpullulans by NMR spectroscopy

The distribution of carboxymethyl substituents in the alpha-(1 --> 6)-linked maltotriosyl repeating units of a carboxymethylpullulan (CMP) series was investigated by high resolution NMR spectroscopy on very short oligomers (DPn = 1.2-1.5) obtained by acid hydrolysis. A series of 2D NMR experiments on parent pullulan, hydrolysed pullulan and CMP was used to assign the proton and carbon chemical shifts of CMP acid hydrolysates. The degree of substitution (DS) and the relative distribution of -CH2COONa groups at OH-2, OH-3, OH-4 and OH-6 of glucose residues (DSi) were determined from H-1 NMR measurements. From a set of CMP samples, widely different in degree of substitution, it was observed that the substitution at C-2 is predominant and decreases according to the order C-2 > C-3 > C-6 > C-4. Taking into account the availability of each OH group in the parent pullulan, an order of relative reactivity of hydroxyl groups is defined according to the relation: R-i = DSi/n(i), where n(i) is the number of free OH groups in a maltotriose unit (MTU) for a given site C-i, the reactivity order was found to be OH-2 > OH-4 > OH-6 > OH-3. (C) 2000 Elsevier Science Ltd. All rights reserved

Efficient and practical aerobic radical addition of thiophosphites to alkenes

Thiophosphites, but not phosphites, add onto alkenes under very mild conditions when treated with triethylborane and oxygen, and deliver the expected adducts in high isolated yields. This method, featuring a high atom economy, complements those described in literature and can be used to initiate tandem cascade processes. © 2001 Published by Elsevier Science Ltd

Efficient and practical aerobic radical addition of thiophosphites to alkenes

Thiophosphites, but not phosphites, add onto alkenes under very mild conditions when treated with triethylborane and oxygen, and deliver the expected adducts in high isolated yields. This method, featuring a high atom economy, complements those described in literature and can be used to initiate tandem cascade processes. © 2001 Published by Elsevier Science Ltd

Efficient and practical aerobic radical addition of thiophosphites to alkenes

Thiophosphites, but not phosphites, add onto alkenes under very mild conditions when treated with triethylborane and oxygen, and deliver the expected adducts in high isolated yields. This method, featuring a high atom economy, complements those described in literature and can be used to initiate tandem cascade processes. © 2001 Published by Elsevier Science Ltd

Aryldithioethyloxycarbonyl (Ardec): A new family of amine protecting groups removable under mild reducing conditions and their applications to peptide synthesis

The development of phenyl-dithioethyloxycarbonyl (Phdec) and 2-pyridyldithioethyloxycarbonyl (Pydec) protecting groups, which are thiollabile urethanes, is described. These new disulfide-based protecting groups were introduced onto the epsilon-amino group of L-lysine; the resulting amino acid derivatives were easily converted into Nalpha-Fmoc building blocks suitable for both solid- and solution-phase peptide synthesis. Model dipeptide-(Ardec)s were prepared by using classical peptide couplings followed by standard deprotection protocols. They were used to optimize the conditions for complete thiolytic removal of the Ardec groups both in aqueous and organic media. Phdec and Pydec were found to be cleaved within 15 to 30 min under mild reducing conditions: i) by treatment with dithiothreitol or beta-mercaptoethanol in Tris-HCl buffer (pH 8.5-9.0) for deprotection in water and ii) by treatment with beta-mercapto-ethanol and 1,8-diazobicyclo-[5.4.0]undec-7-ene (DBU) in N-methyl-pyrrolidinone for deprotection in an or ganic medium. Successful solid-phase synthesis of hexapeptides Ac-Lys-Asp-Glu-Val-Asp-Lys(Ardec)-NH 2 has clearly demonstrated the full orthogonality of these new amino protecting groups with Fmoc and Boc protections. The utility of the Ardec orthogonal deprotection strategy for site-specific chemical modification of peptides bearing several amino groups was illustrated firstly by the preparation of a fluorogenic substrate for caspase-3 protease containing the cyanine dyes Cy 3.0 and Cy 5.0 as FRET donor/acceptor pair, and by solid-phase synthesis of an hexapeptide bearing a single biotin reporter group. ©Wiley-VCH Verlag GmbH & Co. KGaA

NMR studies of the R2 repeat and related peptide fragments of the DNA binding domain of c-Myb. New light on the structure and folding of R2.

The solution structure of the R2 repeat of the DNA binding domain of the protooncogene c-Myb contains a N-terminal structural motif comprising two antiparallel helices. The motif is stabilized by interactions involving conserved residues. The recognition region in C-terminal position is flexible. This structure differs from that of R2 of another c-Myb protein

First Direct Evidence of an ortho-Lithiated Aryloxetane: Solid and Solution Structure, and Dynamics

Oxetanes are key synthons for asymmetric synthesis and also effective in directing ortho-lithiation. This work first reports the solution and the solid-state structure of an ortho-lithiated aryloxetane (1-Li) in the presence/absence of a bidentate ligand such as N,N,N′,N′-tetramethylethylenediamine (TMEDA). Single crystal X-ray diffraction analysis of 1-Li revealed a singular crystallographic structure in which the asymmetric unit comprises a core where the lithium atom is coordinated to the nitrogen atom of half a molecule of TMEDA and intramolecularly stabilised by the oxetane ring oxygen. This aggregation state is unprecedented in ortho-lithiated arenes. Variable temperatures multinuclear magnetic resonance (1H, 7Li, 13C) mono- and two-dimensional NMR studies and DFT computations supported the coexistence in solution of three chelated bridged dimeric aggregates, in slow equilibration at 180 K. The major isomer is an heterochiral aggregate on the basis of 1H,7Li-HOESY and 1H,1H-NOESY experiments. Conclusions were supported by the preparation of enantiomerically enriched (S)-1-Li. The privileged formation of homochiral aggregates from racemic mixtures may also have implications for the development of chiral resolution processes