Synthesis of Oxazolidinones and 1,2-Diamines fromN-Alkyl Aziridines

Abstract

Reactions of N-alkyl-substituted aziridines with LiI followed by an electrophile are discussed. In the first series of reactions, the electrophile is carbon dioxide and the product is an oxazolidinone. In all cases, either no reaction occurred or a high yield of product was obtained. HMPA had to be added to some reactions to dramatically improve the regiochemistry. Net retention of stereochemistry was observed. In the second series of reactions, the electrophile is an iminium salt and the product is a 1,2-diamine. Here the reaction is highly regioselective in THF without the addition of HMPA. Unlike the oxazolidinone chemistry, the diamine formation works equally well with or without the addition of LiI. With respect to the regiochemistry, the results are the same with and without added LiI. However, with respect to the stereochemistry, in the presence of added LiI, the reaction with the iminium salt goes with net retention of stereochemistry. In contrast, with no added LiI, in some cases the reaction goes with net retention and in some cases with net inversion of stereochemistry.Because N-alkyl-substituted aziridines are readily available and highly reactive three-membered heterocyclic ring systems, 1 we have been interested in their conversion to other organic compounds, especially to oxazolidinones 2 and 1,2-diamines 3 (Scheme 1). These conversions involve reacting the aziridine with LiI followed by an electrophile. It was found that when no electrophile is added to the reaction mixture, after the aqueous work-up, a six membered ring dimer of the aziridine (a piperazine) is obtained. Unlike many reactions of aziridines, 4 for our reactions, neither an electron-withdrawing group nor pre-formation of an aziridinium salt is required.In this paper, we summarize our previously reported results. In addition, we discuss the synthesis of the aziridines, give yields and isomeric ratios for all reactions, and give full experimental and spectroscopic details. Most importantly, for the first time, we show that the diamine formation reaction can be done with or without the addition of LiI. Although the regiochemistry is the same with and without added LiI, the stereochemistry is not. Synthesis of Aziridines and Iminium SaltsAs shown in Scheme 2, almost all the aziridines were synthesized from the corresponding epoxide and primary amine. The epoxide was treated with the primary amine in the presence of lithium perchlorate to afford the 1,2-aminoalcohol. Then, the aminoalcohol was cyclized using triphenylphosphine and bromine. 5 The exception to this was cis-2,3-diphenylaziridine, which was prepared by the treatment of tribenzylamine N-oxide with n-butyllithium. 6 Also as shown in Scheme 2, the iminium salts were synthesized from the corresponding secondary amine. The secondary amine was treated with MeOH and paraformaldehyde to generate a gem-aminoether, which, without any purification, was added to methyltrichlorosilane to yield the iminium salt. 7