Catalyst- and excess reagent recycling in aza-Michael additions

16 alpha-Azolyl-pregnenolone derivatives were prepared via 2-butyl-1,1,3,3-tetramethylguanidine (n-Bu-TMG) catalysed aza-Michael addition of 16-dehydropregnenolone (16-DHP) carried out in [bmim][BF4]. The application of the guanidine base and the imidazolium ionic liquid made it possible to recycle not only the catalyst/solvent mixture but also the excess of the N-heterocyclic reagent. By the introduction of CO2 at the end of the reaction, both the guanidine base and the unreacted (excess) reagent could be converted into ionic species that remained dissolved in the ionic liquid phase, while the steroid components were extracted with an apolar solvent. After the removal of CO2, the experiment could be repeated by the addition of the steroid substrate and only an equimolar amount of the N-heterocycle. The methodology was successfully applied to a number of N-heterocycles, such as imidazole, pyrazole, 1,2,3- and 1,2,4-triazoles, and benzimidazole. Indazole and indole could also be converted into the corresponding products, but a stronger base had to be used to obtain a recyclable system. The use of a [bmim][BF4]/nBu-TMG solvent/catalyst mixture was shown to facilitate the recycling of solvents, catalysts and unreacted N-heterocycles (used in excess) in the aza-Michael reaction of various N-heterocycles with a steroidal substrate.