"Solvent-less" mechanochemical approach to the synthesis of allobetulin and some of its esters
Various significant biological activities have been recently found for allobetulin and its derivatives which in combination with their low toxicities lead to an increased research effort. In the present work allobetulin and some its acyl derivatives have been synthesized by different reactions using a grindstone method. All reactions were carried out at room temperature. Allobetulin (1a) and allobetulin 3-O-formate (1b) were prepared by reacting betulin with trifluoroacetic acid (TFA) and HCOOH, consecutively. The reactions time was 30-40 min, and the yield of the products was 82 and 98 %, respectively. Allobetulin (1a) under the action of TFA for 2 hours affords allobetulin 3-O-trifluoroacetate (3) in 95 % yield. Whereas, the treatment of betulin diacetate with TFA for 30 min gives allobetulin 3-O- acetate (2a) in 92 % yield. The formation of products was detected by TLC using C<sub>6</sub>H<sub>6</sub>:CH<sub>2</sub>Cl<sub>2</sub>:CH<sub>3</sub>OH (5:5:1) as eluent and the spots were revealed after spraying the TLC plates with reagent (1 % phosphomolybdic acid-water) followed by heating at 110 ºC for 5 minutes to show a characteristic blue colour. The present procedure is simple, efficient, and environmentally benign. The structures of all products were confirmed by <sup>1</sup>H NMR, <sup>13</sup>C NMR, and FT-IR spectroscopy.