Ring-Opening Reactions of Aminocyclopropanes and Aminocyclobutanes

Nitrogen-containing compounds are an important class of molecules in medicinal chemistry, chemical biology, biochemistry, material sciences or environmental sciences. Organic nitrogen occurs in many forms, ranging from small building blocks such as urea, amino acids, nucleotides, to complex natural products, drug molecules, proteins, nucleic acids, among others. The transformation of easily-accessed nitrogen-containing building blocks into more complex nitrogen-containing structures is therefore important for the synthesis of bioactive compounds or functional materials. Interested in donor-acceptor aminocyclopropanes and aminocyclobutanes, we first developed a Lewis-acid catalyzed formal [4+1] cycloaddition of donor-acceptor aminocyclobutanes with isocyanides, which resulted in the formation of cyclopentene-1,2-diamines. We then attempted to develop a formal [3+2] cycloaddition of donor-acceptor aminocyclopropanes with electron-deficient alkenes, which was mechanistically inspired by the Morita-Baylis-Hillman reaction. Apart from the donor-acceptor system, we were also attracted by simple aminocyclopropanes without electron-withdrawing group at vicinal position. We anticipated that the incorporation of cyclopropylamine into carboxylic-containing compounds followed by ring opening functionalization reactions could be used for the synthesis of more complex products. Therefore, we first disclosed an oxidative ring-opening strategy to transform acyl, sulfonyl or carbamate protected aminocyclopropanes into 1,3-dielectrophilic carbon intermediates bearing a halide atom (Br, I) and a N,O-acetal. Replacing the alkoxy group of the N,O-acetal was achieved under acidic conditions via an elimination-addition pathway, while substitution of the halides by nucleophiles was done under basic conditions via a SN2 pathway, generating a wide range of 1,3-difunctionalized propylamines. Based on the transformation described above, we further discovered an efficient synthesis of 4-amino thiochromans starting from simple aminocyclopropanes and thiophenols through a formal [3+3] annulation reaction. Next, we developed an oxidative ring-opening strategy to transform cyclopropylamides and cyclobutylamides into fluorinated imines. The imines can be isolated in their more stable hemiaminal form, with the fluorine atom installed selectively at the terminal position. Both cheap benzophenone with UV A light or organic and inorganic dyes with blue light could be used as photoredox catalysts to promote this process. Various fluorinated amines were then obtained by nucleophilic attack on the hemiaminals in one pot, giving access to a broad range of useful building blocks for medicinal chemistry. Finally, we turned our attention to the synthesis of diamines, which are essential building blocks for the synthesis of agrochemicals, drugs and organic materials, yet their synthesis remains challenging, as both nitrogen moieties need to be differentiated and diverse substitution patterns (1,2-, 1,3 or 1,4) are required. We described a new strategy giving access to 1,2-, 1,3- and 1,4- amido azides as orthogonally protected diamines based on the nitrogen-directed diazidation of alkenes, cyclopropanes and cyclobutanes.Other unsuccessful efforts over the last four years have also been described, together with some preliminary results of ring-opening cyanation and arylation reactions.