Sébastien Comesse

New and Expeditious Tandem Sequence Aza-Michael/Intramolecular Nucleophilic Substitution Route to Substituted γ-Lactams: Synthesis of the Tricyclic Core of (±)-Martinellines

A new and highly diastereoselective tandem reaction aza-Michael/intramolecular nucleophilic substitution is presented. This unprecedented tandem reaction between N-substituted alpha-bromoacetamides and Michael acceptors proceeds with good yields and excellent diastereoselectivity to provide the corresponding trisubstituted gamma-lactam systems. An application to the concise synthesis of the tricyclic core of (+/-)-martinelline alkaloids is also described.

A Domino N-Amidoacylation/Aldol-Type Condensation Approach to the Synthesis of the Topo-I Inhibitor Rosettacin and Derivatives†

The pot, atom, and step-economic synthesis of Rosettacin topo-I poison and its derivatives has been achieved using a novel domino N-amidoacylation/aldol-type condensation, followed by decarboxylation of the ester function. The key domino procedure simply involves mixing HOBt ester as new reagent with lactam and NaH together in THF or THF/ DMF. The reaction seems to be general and led to suitable N-heterocyclic products in moderate to good yields.

Hydrophobically modified xanthan: an amphiphilic but not associative polymer.

Hydrophobic octyl moieties have been grafted in various densities onto the carboxylic acid functions of xanthan under its ordered conformation. The outcoming amphiphilic and associative properties were studied by fluorescence spectroscopy and rheology. Results showed that the conformation of xanthan is not affected by the chemical modification and remains the same as the native one. Additionally, xanthan derivatives do not show any viscoelastic enhancement; nevertheless, their dynamics is strongly slowed down: the higher the grafting density, the slower the relaxation. We proved that hydrophobically modified xanthan, even being amphiphilic, does not exhibit any additional associating properties compared to the unmodified xanthan. The high stiffness of xanthan helices does not allow the derivatives to adopt the organization usually observed for flexible amphiphilic polymers. On the basis of these observations, a model depicting such a singular behavior is proposed.

Synthesis of highly functionalized pyrrolidines as tunable templates for the direct access to (±)-coerulescine and the tricyclic core of martinellines

An aza-Michael induced ring closure (aza-MIRC) tandem reaction of benzyl (2-bromoethyl)carbamate with various Michael acceptors is described. The N-Cbz-β-gem-disubstituted pyrrolidines thus obtained were proved to be versatile intermediates for the rapid access to both martinelline and spirooxindole backbones. An application of this strategy towards an expedient 4 step total synthesis of (±)-coerulescine is also presented.

Domino process optimized via ab initio study for an alternative access to bicyclic lactams.

A totally new acid-free domino process to access highly functionalized bicyclic γ- and δ-lactams starting from commercially available and inexpensive ethoxymethylene derivatives is reported. Mechanisms elucidated by computational calculations led to new reaction conditions that boosted the yields up to 3.5 times higher.

Synthesis of Oxazolidin-4-ones: Domino O-Alkylation/Aza-Michael/Intramolecular Retro-Claisen Condensation

An original and rapid domino reaction for access to oxazolidin-4-ones is presented. Simply by heating α-bromoamido alcohol in the presence of KNaCO3 and water with readily prepared Michael acceptors, an unprecedented molecular rearrangement is generated. This new methodology enables the hitherto unreported synthesis of functionalized oxazolidin-4-ones. The process was proved to be compatible with a wide variety of substrates, and high regioselectivities were achieved.

Simple Access to Highly Functional Bicyclic γ- and δ-Lactams: Origins of Chirality Transfer to Contiguous Tertiary/Quaternary Stereocenters Assessed by DFT

This paper describes the synthesis of both polysubstituted oxazolo-pyrrolidinones and -piperidinones by a domino process. The methodology is based on the reaction between hydroxyl halogenoamides and Michael acceptors, which leads efficiently to bicyclic lactams. The process is compatible with unsymmetrical electron-withdrawing groups on the Michael acceptor, which allows the formation of two contiguous and fully controlled tertiary and quaternary stereocenters. In the case of tetrasubstituted Michael acceptors, two adjacent quaternary stereocenters are formed in good yield. Starting from (R)-phenylglycinol derived amides results in the formation of enantioenriched bicyclic lactams in low to good yields and with high levels of stereoselectivity, thus greatly increasing the scope and interest of this strategy. The origins of chirality transfer and diastereoselectivity were studied by DFT calculations and have been attributed to a kinetic control in one of the last two steps of the reaction sequence. This selectivity is dependent upon both the substituents on the Michael acceptor and the sodium cation chelation.

Chemo-, Regio-, and Stereoselective Synthesis of Polysusbtituted Oxazolo[3,2-d][1,4]oxazepin-5(3H)ones via a Domino oxa-Michael/aza-Michael/Williamson Cycloetherification Sequence

The access to new oxazolo[3,2-d][1,4]oxazepin-5(3H)-ones starting from α-bromoamido alcohols and Michael acceptors under mild conditions is presented. This domino process proved to be chemo-, regio-, and stereoselective and allows the formation of a large diversity of highly functional 7-membered rings in good yields up to 95%. The complete shift of the regioselectivity of the intermediate enolate from a C-C to a C-O bond formation, contrary to the already known alkylations of such ambident nucleophiles, is mostly triggered by steric effects. The last step of the sequence was modeled by DFT giving some important insights for this C-C vs C-O bond shift.

AN ENANTIOSELECTIVE APPROACH TO SYNTHESIS OF ADVANCED CHIRAL TEMPLATES FOR THE CONSTRUCTION OF INDOLIC AND INDOLIZIDINIC ALKALOIDS

A new approach for the syntheses of chiral indolic and indolizidinic templates is presented. They were obtained in three steps starting from a chiral primary amino-vinylsilane. The key step was the N-acyliminium π-cyclization, leading diastereoselectively to the formation of the six membered ring templates.

Chemical modification of xanthan in the ordered and disordered states: an open route for tuning the physico-chemical properties

The impact of the chain stiffness on physicochemical properties has been studied by chemical modification of xanthan under both ordered and disordered conformations. Corresponding rheological properties were studied and results showed that amphiphilic xanthan exhibited completely different behaviors depending on its conformation during modification. Xanthan, when modified under ordered conformation, exhibits similar behavior to non-modified one, only the chain relaxation being strongly slowed down. Therefore, the high stiffness of xanthan helices does not allow hydrophobic moieties to associate. Oppositely, xanthan modified under its disordered conformation displayed a chemical gel-like behavior without any relaxation of the chain within the studied frequency range nor with temperature, which is unexpected for this length of alkyl chains. These different viscoelastic properties can be correlated to the regioselectivity of the grafting; the latter can be controlled by the conformation of xanthan during modification, thus by the synthesis conditions.