ABSTRACT
Saturated and partially unsaturated heterocycles are widely distributed as natural products. N-Methylpyrrolidone and sulfolane are useful dipolar aprotic solvents, with characteristics like those of dimethylformamide and dimethyl sulfoxide. Aziridines are also found in anti-tumour agents, such as the mitomycins. Thiiranes also occur naturally, as plant products such as thiirane-2-carboxylic acid, isolated from asparagus. The main chemical aspect in which compounds with a nitrogen in a five- or six-membered ring differ from their acyclic counterparts is in the possibility open to them to be dehydrogenated to the corresponding aromatic system. The presence of some enamine, at equilibrium, is demonstrated by the conversion of piperideine into a dimer; indeed, the ability of these two systems to serve as both imines and enamines in such aldol-like condensations is at the basis of their roles in alkaloid biosynthesis. The regiochemistry of ring opening is determined mainly by steric and to a lesser extent by inductive and electronic effects.
