Manuele Musolino

The reactions of dimethyl carbonate and its derivatives

The worldwide urge to embrace a sustainable and bio-compatible chemistry has led industry and academia to develop of chlorine-free methodologies focused on the use of CO2 and CO2-based compounds as feedstocks, promoters and reaction media. In this scenario, dialkyl carbonates (DACs) and in particular dimethyl carbonate (DMC) occupy a privileged position due to their low toxicity, high biodegradability and peculiar reactivity. Nowadays, the large-scale production of DACs is carried out through clean processes (i.e., phosgene-free processes), which include the direct insertion of CO2 into epoxides, allowing – in principle – recycling of the carbon dioxide emitted during carbonate degradation. This groundbreaking achievement has definitely drawn attention toward the conception of procedures to activate the rather stable DACs with the aim of employing these compounds as green alternatives to their reactive chlorinated analogues. DACs are ambident electrophiles, which under appropriate conditions can undergo BAc2- or BAl2-nucleophilic substitution to give, respectively, alkoxycarbonylation and alkylation reactions. The many efforts devoted to improving the industrial suitability of organic carbonates have unveiled an intriguing and innovative chemistry as demonstrated by the numerous publications and patents published on these compounds over the last thirty years. This review reports on DACs as alkoxycarbonylating agents and their applications in industry and fine synthesis, as well as alkylating agents including allylic alkylation using palladium catalysts and the Pd/Ti bimetallic system and anchimerically driven alkylations via mustard carbonates. Moreover, the reactivity of organic carbonates toward several substrates and under different reaction conditions is described along with some distinctive DAC-mediated cyclization and transposition reactions. The synthesis of olefins and ethers under both liquid and gas phase conditions via thermal decarboxylation of organic carbonates is also reported.

The diketopiperazine-fused tetrahydro-β-carboline scaffold as a model peptidomimetic with an unusual α-turn secondary structure

Summary Aiming at restricting the conformational freedom of tryptophan-containing peptide ligands, we designed a THBC (tetrahydro-β-carboline)-DKP (diketopiperazine)-based peptidomimetic scaffold capable of arranging in an unusual α-turn conformation. The synthesis is based on a diastereoselective Pictet–Spengler condensation to give the THBC core, followed by an intramolecular lactamization to complete the tetracyclic THBC-DKP fused ring system. The presence of conformers bearing the intramolecular thirteen-membered hydrogen bond that characterizes the α-turn structure is confirmed by 1H NMR conformational studies. To the best of our knowledge, this scaffold represents one of the rare examples of a designed constrained α-turn mimic.

5-Membered cyclic ethers via phenonium ion mediated cyclization through carbonate chemistry

Abstract Cyclization of 2-(2-hydroxyethyl)phenol via DMC chemistry in acidic conditions is herein discussed for the first time. Reaction conditions have been investigated and optimized. This substrate is quite appealing as it incorporates a 2-hydroxyethyl moiety in ortho to the aromatic hydroxyl group capable of stabilizing the related phenonium ion. When the reaction mechanism was investigated via theoretical calculations, the results suggest that the most favorable pathway encompasses a DMC-mediated formation of the phenonium ion that is converted into the 2-(2-methoxyethyl)phenol. The related cyclic ether is then formed via intramolecular cyclization of this intermediate. This peculiar cyclization reaction is another example of the versatility of DMC herein used as solvent, methoxycarbonylation agent and leaving group in the intramolecular cyclization leading to the phenonium ion.

β-Aminocarbonates in Regioselective and Ring Expansion Reactions

The reactivity of β-aminocarbonates as anisotropic electrophiles has been investigated with several phenols. Products distribution shows that the regioselectivity of the anchimerically driven alkylation reaction depends on the nucleophiles. The results suggest that in the presence of nucleophiles that are also good leaving groups, the reaction takes place under thermodynamic control favoring the attack on the most sterically hindered carbon of the cyclic aziridinium intermediate. Furthermore, when an enantiomerically pure pyrrolidine-based carbonate was used, the reaction with phenols proceeds via a bicyclic aziridinium intermediate leading to the stereoselective synthesis of optically active 3-substituted piperidines via ring expansion reaction. These results were confirmed both by NMR spectroscopy and X-ray diffraction analysis.