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Lionel Moisan

Publications and source records attributed to Lionel Moisan.

5 recordsLinked to original sources
PubMed2004

Catharanthinol and dihydrocatharanthinol: two Iboga-class alkaloids.

The title compounds are indole alkaloids of the Iboga class. In both compounds, viz. catharanthinol methanol solvate, C(20)H(24)N(2)O.CH(4)O, (I), and dihydrocatharanthinol monohydrate, C(20)H(26)N(2)O.H(2)O, (II), a nitrogen-containing seven-membered ring is fused to the indole system and shares two sides with a tricyclic isoquinuclidine group. The main difference between (I) and (II) is the presence of a C=C bond in the isoquinuclidine ring in (I). The presence of amine and hydroxy groups in these molecules and of methanol [in (I)] or water [in (II)] solvent molecules results in intra- and/or intermolecular hydrogen bonding.

Alkaloids↗
PubMed2004

In the golden age of organocatalysis.

The term "organocatalysis" describes the acceleration of chemical reactions through the addition of a substoichiometric quantity of an organic compound. The interest in this field has increased spectacularly in the last few years as result of both the novelty of the concept and, more importantly, the fact that the efficiency and selectivity of many organocatalytic reactions meet the standards of established organic reactions. Organocatalytic reactions are becoming powerful tools in the construction of complex molecular skeletons. The diverse examples show that in recent years organocatalysis has developed within organic chemistry into its own subdiscipline, whose "Golden Age" has already dawned.

Journal Article↗
PubMed2002

An ion pair formed by protonated Fe(cp*py)2 and the octanuclear cluster U8Cl24O4(cp*py)2 [cp*py is tetramethyl-5-(2-pyridyl)cyclopentadiene].

In bis[1,1',2,2',3,3',4,4'-octamethyl-5-(2-pyridinio)-5'-(2-pyridyl)ferrocene] di-mu(3)-chloro-hexadeca-mu(2)-chloro-hexachlorodi-mu(4)-oxo-di-mu(3)-oxo-bis[(eta(5),kappaN)-1,2,3,4-tetramethyl-5-(2-pyridyl)cyclopentadienyl]octauranium(IV) dichloromethane tetrasolvate, [Fe(C(14)H(17)N)(C(14)H(16)N)](2)[U(8)Cl(24)O(4)(C(14)H(16)N)(2)].4CH(2)Cl(2), (I), two protonated Fe(cp*py)(2) units [cp*py is tetramethyl-5-(2-pyridyl)cyclopentadiene] form an ion pair with the dianionic centrosymmetric cluster U(8)Cl(24)O(4)(cp*py)(2). The latter is the highest nuclearity assemblage in the chemistry of uranium (non-uranyl) compounds reported to date.

Journal Article↗
PubMed2001

Enantioselective Organocatalysis.

The last few years have witnessed a spectacular advancement in new catalytic methods based on metal-free organic molecules. In many cases, these small compounds give rise to extremely high enantioselectivities. Preparative advantages are notable: usually the reactions can be performed under an aerobic atmosphere with wet solvents. The catalysts are inexpensive and they are often more stable than enzymes or other bioorganic catalysts. Also, these small organic molecules can be anchored to a solid support and reused more conveniently than organometallic/bioorganic analogues, and show promising adaptability to high-throughput screening and process chemistry. Herein we focus on four different domains in which organocatalysis has made major advances: 1) The activation of the reaction based on the nucleophilic/electrophilic properties of the catalysts. This type of catalysis has much in common with conventional Lewis acid/base activation by metal complexes. 2) Transformations in which the organic catalyst forms a reactive intermediate: the chiral catalyst is consumed in the reaction and requires regeneration in a parallel catalytic cycle. 3) Phase-transfer reactions: The chiral catalyst forms a host-guest complex with the substrate and shuttles between the standard organic solvent and the second phase (i.e. a solid, aqueous, or fluorous phase in which the organic transformation takes place). 4) Molecular-cavity-accelerated asymmetric transformations: the catalyst can select between competing substrates, depending on size and structure criteria. The rate acceleration of a given reaction is similar to the Lewis acid/base activation and is the consequence of the simultaneous action of different polar functions. Herein it is shown that organocatalysis complements rather than competes with current methods. It offers something conceptually novel and opens new horizons in synthesis.

Journal Article↗
PubMedDate not supplied

Computational gestalts and perception thresholds.

In 1923, Max Wertheimer proposed a research programme and method in visual perception. He conjectured the existence of a small set of geometric grouping laws governing the perceptual synthesis of phenomenal objects, or "gestalt" from the atomic retina input. In this paper, we review this set of geometric grouping laws, using the works of Metzger, Kanizsa and their schools. In continuation, we explain why the Gestalt theory research programme can be translated into a Computer Vision programme. This translation is not straightforward, since Gestalt theory never addressed two fundamental matters: image sampling and image information measurements. Using these advances, we shall show that gestalt grouping laws can be translated into quantitative laws allowing the automatic computation of gestalts in digital images. From the psychophysical viewpoint, a main issue is raised: the computer vision gestalt detection methods deliver predictable perception thresholds. Thus, we are set in a position where we can build artificial images and check whether some kind of agreement can be found between the computationally predicted thresholds and the psychophysical ones. We describe and discuss two preliminary sets of experiments, where we compared the gestalt detection performance of several subjects with the predictable detection curve. In our opinion, the results of this experimental comparison support the idea of a much more systematic interaction between computational predictions in Computer Vision and psychophysical experiments.

Animals↗