304 research outputs found
A selective and operationally simple approach for removal of methoxy-, allyloxy-, and benzyloxycarbonyl groups from carbinols
No full textRegioselective Acetolysis of Highly O-Benzylated Carbohydrates Promoted by Iodine or an Iodine/Silane Combined Reagent: Use of Isopropenyl Acetate as an Alternative to Acetic Anhydride
No full textRegioselective acetolytic de-O-benzylation of poly-O-benzylated
sugars can be triggered by the activation of isopropenyl
acetate (IPA) with either an iodine/silane combined
reagent or iodine alone. Unlike other known acetolysis procedures,
the protocols presented here do not suffer from the
useofharshacidicreagentsandexcessamountsofhigh-boiling
acetic anhydride. The activation of IPA with iodine and
triethylsilane (or the cheaper polymethylhydrosiloxane,
PMHS)usuallyresultsinshorterreactiontimes,withtheacetolysis
occurringpreferentially (with some exceptions)at primary
benzyloxy groups. With anomerically armed sugars,
anomeric iodination can be a concomitant process to give
highly reactive intermediates which can be converted in situ
into workable and useful building blocks upon suitable
quenchingconditions.Ontheotherhand,IPAactivationwith
iodine alone allows the reactions to occur under milder conditions,albeitoverlongertimes.Inalmostallreported examples, IPA can be used in moderate excess (5equiv.), but its
employment as the solvent is crucial with an amino sugar
model compound otherwise recalcitrant to any acetolytic
modification. An additional advantage of these conditions
lies in the unprecedented possibility of incorporating the
acetolysis step into one-pot synthetic sequences leading to
multiple functional modifications of saccharidic substrates.
As to the regioselectivity, most reactions seem to be controlled
bysteric factors, asthe most accessibleprimary benzyloxy
groups are commonly acetolyzed. However, a couple
of disclosed examples display that under suitable structural
conditions,strainreliefeffectsmightbearulingfactorforthe
regiocontrol of the processes. Overall, the reported protocols
offer complementary options for experimentally easy access
to a wide range of useful saccharide building blocks featuring
a varied profile of protecting groups
The Youth Experience Gap: Explaining Differences across EU Countries
No full textThis note aims to provide a theoretical framework to think of the youthunemployment problem and a classification of EU countries according to the way they address it.The key factor to explain youth unemployment is what we call the youth experience gap. To helpyoung people fill it in and ease school-to-work transitions, every EU country provides a mix ofpolicy instruments, including different degrees and types of labour market flexibility, of educationaland training systems, of passive income support schemes and fiscal incentives. Five differentcountry groups are detected whose outcomes in terms of youth unemployment are dramaticallydifferent: a) the North-European; b) the Continental European; c) the Anglo-Saxon; d) the South-European; e) New Member States. The Lisbon strategy provides guidelines in line with thetheoretical framework discussed here, but it is costly and hard to implement.Youth Unemployment Problem, Youth Experience gap, Youth Employment Policy, Lisbon Strategy
Good Masters: A Reply to Massimo Fusillo, Daniele Scalise, Filippo La Porta and Gianluigi Rossini
No full textA final comment given by Pierpaolo Antonello, author of Dimenticare Pasolini
Probing the interactions between all components of the catalytic pool for homogeneous olefin polymerisation by diffusion NMR spectroscopy
No full textDiffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp2ZrMe2/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe3 depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C6D6 with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)(2) and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D) MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp2Zr(mu-Me)(2)AlMe2]MeMAO (1) OSIP (outer sphere ion pair) and [Cp2Zr+Me center dot center dot center dot MeMAO(-)] (2) ISIP (inner sphere ion pair) that form upon activation of Cp2ZrMe2. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions
An easy and versatile approach for the regioselective de-O-benzylation of protected sugars based on the I2/Et3SiH combined system.
No full textOne-pot catalytic glycosidation/Fmoc removal - An iterable sequence for straightforward assembly of oligosaccharides related to HIV gp120.
No full textThe iodine/triethylsilane system: a versatile combination with multiple applications in carbohydrate chemistry
No full textA one-pot glycosidation-transient protecting group removal iterable scheme for the rapid assembly of the D1 tetrasaccharide of gp 120
No full textThe I2/Et3SiH system: A versatile combination with multiple applications in carbohydrate chemistry
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