Dieckmann solvent-free

Toda, F., Suzuki, T., Higa, S. Solvent-free Dieckmann condensation reactions of diethyl adipate and pimelate. J. Chem. Soc., Perkin Trans. 11998, 3521-3522. 

Dieckmann condensation reactions of diesters normally should be carried out in dried solvent under reflux under an inert atmosphere [21, 22]. Furthermore, Dieckmann reactions are often carried out under high dilution conditions in order to avoid intermolecular reaction [21, 22], However, it was found that the reaction of diethyl adipate (58a) or pimelate (58b) proceeds efficiently in the absence of solvent under air [23]. It is also possible to isolate the reaction products from the reaction mixture directly by distillation [23], These results establish a completely solvent-free procedure throughout the reaction and work-up of the reaction mixture. This is a very clean, green, simple and economical procedure. 

For comparison with the solvent-free reaction, the Dieckmann condensation of 58a and 58b was studied using the same base in toluene under reflux. As shown in Table 15-15, there is no marked difference between the yields of the solvent-free and solution reactions. This clearly shows that the solvent-free reaction is much better than the other in terms of simplicity, cleanliness and economy. 

The Dieckmann condensation was reported once in the literature to proceed in solvent-free conditions. Toda, Higa and coworkers showed the condensation of adipate (24) and pimelate di-esters (34) to give the corresponding cyclized products using powdered base such as (BuONa, EtOK, and EtONa). The reaction proceeds with the solid di-esters that have been mixed by using a mortar and a pestle with the powdered base for 10 min, and the mixture was then kept in a desiccator for 60 min and then neutralized with p-TSOH-H20 to give the condensed product in 60-82% yields. ... 

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