Acyl group migration affects the synthesis, isolation, manipulation and purification of all acylated organic compounds containing free hydroxyl groups, in particular carbohydrates. While several isolated studies on the migration phenomenon in different buffers have been reported, comprehensive insights into the overall migration process in different monosaccharides under similar conditions have been lacking. Here, we have studied the acyl migration in different monosaccharides using five different acyl groups by a combination of experimental, kinetic and theoretical tools.The results show that the anomeric configuration in the monosaccharide has a major influence on the migration rate, together with the relative configurations of the other hydroxyl groups and the nature of the migrating acyl group. Full mechanistic model, based on computations, demonstrates that the acyl migration proceeds through an anionic stepwise mechanism with linear dependence on the [OH À ] and the pK a of the hydroxyl group toward which the acyl group is migrating.
A total regioselective synthesis of 1,4,5‐trisubstituted‐1,2,3‐triazoles from aryl azides and enaminones is reported. The use of an ionic liquid in the presence of water and trimethylamine is crucial for the progress of the reaction. The process, consisting of a cascade starting with a water‐promoted 1,3‐dipolar cycloaddition (in agreement with DFT calculations) followed by a base‐promoted retro‐aza‐Michael reaction, shows a completely different reactivity to previously reported base‐promoted reactions between enaminones and sulfonyl azides only leading to disubstituted triazoles. Such reactivity, reported herein, allows preparation of trisubstituted triazoles in a complete regioselective way.
The catalyzed desymmetrizative
ring expansion of alkenylcyclobutanols
promoted by halofunctionalization of the alkene moiety with
N
-bromosuccinimide has been experimentally and computationally
studied. The reaction yields highly enantioenriched cyclopentanones
bearing two all-carbon quaternary stereocenters, one of them being
generated in the rearrangement of the cyclobutane ring and the other
by enantioselective desymmetrization. The reaction is competitive
with the formation of a spiroepoxide, but it turns completely selective
toward the cyclopentanone when a chiral bisphosphonium magnesium salt
is employed as a catalyst. Mechanistic studies support the formation
of an ion pair leading to a complex with only a unit of phosphoric
acid, which is the resting state of the catalytic cycle. Calculations
reproduce in an excellent way the observed reactivity and predict
the effect exerted by the substituents of the aromatic ring linked
to the double bond. The computational studies also revealed the reaction
as a highly asynchronous concerted process taking place as one kinetic
step but in two stages: (i) halogenation of the double bond and (ii)
rearrangement of the cyclobutane. No intermediates are present in
the reaction as energy minima. The experimental scope of the reaction
further confirms the predictions for the observed reactivity and selectivity.
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