Ring opening of reactions of aziridines by different π-nucleophiles can be applied in the synthesis of biologically active analogs of amaryllidaceae alkaloids. π-Nucleophiles linked to aziridine ring nitrogen or linked to oxygen on the C-2 of aziridine could be used to synthesize hexahydrophenanthridin-6-one. This type of structure can be found in many amaryllidaceae alkaloids like pancratistatin and narciclasine. These plants have been in use for thousands of years as herbal remedies. These alkaloids have been receiving an attention because of their potentially useful medicinal properties including anticancer and antiviral properties. Attempts to synthesize hexahydrophenanthridin-6-one by intramolecular cyclization of aziridines with π-nucleophiles(linked to aziridine nitrogen) under highly unfavorable 5-endo cyclization failed. A new ring opening reaction of an unactivated aziridine by a π-nucleophile (linked to oxygen onthe C-2 of aziridine) was developed for the synthesis of hexahydro-1H-benzo[c]chromen-1-amine. These types of molecules are present in amaryllidaceae alkaloids like homolycorane, neruscine and mesanane. A similar kind of substrate was used in the synthesis of 7-deoxypancratistain that has a hexahydro- phenanthridin 6-one skeleton.
The second mode of intramolecular cyclization of aziridines with π-nucleophiles can be used for the synthesis of azabicyclo[3.2.1]octanes. These molecules are potential ligands for the nicotinic acetylcholine receptor. The nature of the aziridine substituent plays a major role in the initial ring opening and subsequent cyclization to provide azabicyclo[3.2.1]octanes. Synthesis of 5-phenyl 6-tosyl 6-azabicyclo[3.2.1]octane was reported earlier in our research group. Attempts to deprotect the tosyl group failed under a variety of conditions to provide sufficient quantities of the deprotected product. Hence, a new synthetic route was developed for the synthesis of the aziridine required for the
aziridine-π-nucelophile cyclization. While N-diphenylphosphinyl and N-H aziridines participate in the initial ring opening reactions, they failed to undergo subsequent cyclization to provide the desired bicyclic product. In contrast, a p-nitrobenzenesulfonyl group on the aziridine nitrogen leads efficiently to the bicyclic ring system and can be readily deprotected.