Tetrahydropyridine (THP) ring expansion under the action of activated terminal alkynes. The first synthesis and X-ray crystal structure of tetrahydropyrimido[4,5-d]azocines

Tetrahydropyridopyrimidines (THPPm) 1–3 underwent tandem cleavage–cyclization piperidine ring enlargement under the action of terminal activated alkynes to produce tetrahydropyrimido[4,5-d]azocines 4–7 in good preparative yields. The latter compounds are representatives of a new heterocyclic system.