Synthesis of 5-methylidenehexahydropyrrolo[l,2-a]imidazoles and 6-methylideneoctahydropyrrolo[l,2-a]pyrimidines by the reaction of 1-alkynyl-1-chlorocyclopropanes with lithium derivatives of 1,2- and 1,3-diaminoalkanes

A reaction of 1-alkynyl-1-chlorocyclopropanes with excess of lithium derivative of 1,2-diaminoethane leads to 5-methylidenehexahydropyrrolol,2-a]imidazoles in 35–72% yields, whereas analogous reaction with lithium derivative of 1,3-diaminopropane gives 6-methyl-ideneoctahydropyrrolol,2-a]pyrimidine in up to 50% yield. A mechanism of these unusual multi-step processes includes dehydrochlorination of 1-alkynyl-1-chlorocyclopropanes to form conjugated alkynylcyclopropenes capable of addition of monoalkylamide ions at the double bond, leading to the corresponding secondary cyclopropylamines; the latter under the reaction conditions isomerize to the linear imines, which further undergo a cascade cyclization with sequential involvement of the C=N and C≡C bonds.