Morgan Donnard

Quaternary centres bearing nitrogen (α-tertiary amines) as products of molecular rearrangements

Quaternary centres bearing a nitrogen substituent (α-tertiary amines and their derivatives) are found in a variety of bioactive molecules but pose a major challenge in synthesis, particularly when enantiomeric purity is required. Approaches comparable to those used for tertiary alcohols are typically hampered by the poor electrophilicity of imines, requiring powerful nucleophiles that may also act as bases. A set of powerful alternative approaches make use of the rearrangement of readily available precursors, often (but not always) with formation of a new tertiary carbon to nitrogen bond. In this Feature Article we review the scope, limitations and specificities of some of these rearrangements in order to illuminate their synthetic potential.

Cyclocarbopalladation/Cross-Coupling Cascade Reactions in Sulfide Series: Access to Sulfur Heterocycles

Cyclocarbopalladation/cross-coupling cascade reactions were applied for the first time in a sulfur series and represent an efficient route to access sulfur heterocycles. Stille or Suzuki-Miyaura cross-coupling was successfully used as the final reaction. The products are original benzothiolane and isothiochromane scaffolds with a stereodefined tetrasubstituted exocyclic double bond. To illustrate the application of this method to the synthesis of bioactive molecules, a sulfur analogue of the anticancer agent tamoxifen was prepared as a potential selective estrogen-receptor modulator.

Geometry-Selective Synthesis of E or Z N-Vinyl Ureas (N-Carbamoyl Enamines)

N-Vinyl ureas are emerging as a valuable class of compounds with both nucleophilic and electrophilic reactivity. They may be made by capturing the enamine tautomer of an imine with an isocyanate, a reaction which in general leads to the E isomer of the vinyl urea. Deprotonation of such a vinyl urea, or of an allyl urea, generates a dipole stabilized Z-allyl anion which may be protonated to return the Z-vinyl urea. Isomerization of an allyl urea with a Ru complex provides an alternative route to E-vinyl ureas.

Synthesis of sulfur heterocycles via domino metal-mediated reactions

GRAPHICAL ABSTRACT ABSTRACT Two methodologies to access S-heterocycles and mixed N,S-heterocycles via metal-mediated domino reactions are described. One involves a cyclocarbopalladation/cross-coupling domino process and leads to benzene-fused five- or six-membered sulfur heterocycles with a stereo defined tetrasubstituted exocyclic double bond. The other consists in a three-component domino reaction between 2-aminophenyl disulfide, copper cyanide, and an electrophile to access N-substituted 2-amino benzothiazoles. Preliminary results in the use of the second method to access N-substituted 2-imino benzothiazoles are also reported.

Synthesis of Benzo[c]silole Derivatives Bearing a Tetrasubstituted Exocyclic C=C Double Bond by Palladium-Catalyzed Domino Reactions

The synthesis of diversely substituted 2,3-dihydro-benzo[c]siloles through an unprecedented palladium-catalyzed domino sequence is reported, involving a cyclocarbopalladation of an internal silylalkyne. This reaction proceeds with complete stereoselectivity to lead to a fully substituted exocyclic C=C double bond. Notably, the overall domino sequence appears to be crucial to obtain the desired cyclic vinylsilanes.

Synthesis of Sulfur-Containing Exo-Bicyclic Dienes and Their Diels–Alder Reactions To Access Thiacycle-Fused Polycyclic Systems

The stereocontrolled synthesis of unprecedented sulfur-containing exo-bicyclic 1,3-dienes is reported through a palladium-catalyzed reductive cyclization of sulfur-linked 2-bromoenynes. The fused bicyclic structure provides a better stability to the thiacyclic diene compared to the simple 3,4-dimethylenetetrahydrothiophene. Their reactivity toward several dienophiles has been investigated, and various original thiacycle-fused polycyclic systems have been obtained with high or total diastereoselectivity. Moreover, they are the first exo-bicyclic dienes used in Diels-Alder reactions. The relative configurations of four cycloadducts have been unambiguously assigned by X-ray crystallographic analysis. Mechanistic details of the cycloadditions have been examined by computational means.