152 Non-Aromatic Heterocycles
Substituted derivatives of all three systems are able to undergo concerted thermal ring opening, generating ylides that
can be trapped in 32 cycloaddition reactions, for example providing a route to pyrrolidines (from aziridines).
The heteroatom in aziridines and thiiranes can be eliminated via various cycloreversion reactions, for example via
N-nitrosation of aziridines leading to loss of nitrous oxide (N 2 O), or by the reaction of thiiranes with trivalent phosphorus
compounds, when a phosphorus sulfi de is eliminated.
A related elimination of sulfur dioxide occurs during the Ramberg–Bäcklund synthesis of alkenes, which generates an
episulfone as a transient intermediate.
Diaziridines and dioxiranes are relatively stable isolable systems, although dioxiranes can be explosive and should be
handled in dilute solution. Dimethyldioxirane is a relatively strong oxidant but can show good selectivity: its reactivity
is similar to that of a peracid but it has the advantage of producing a neutral, volatile by-product (acetone). Dioxiranes
are obtained by reaction of ketones with OXONE ® (2KHSO 5 .KHSO 4 .K 2 SO 4 ) the active ingredient being potassium
peroxymonosulfate.
Oxaziridines, prepared by OXONE ® oxidation of imines, are selective oxygen-transfer reagents; in particular, the
camphor-derived reagent is widely used for enantioselective oxygenation of enolates and other nucleophiles.