(4-Pentenyl)azaborole I was prepd. from the lithium salt of 2,5-dehydro-1H-1,2-azaborole and Br(CH2)3CH:CH2 and underwent reversible intramol. cycloaddn. to give tricyclic azaborecine II. The transition of I to II was monitored by 11B NMR. The crystal structure of II and of an intermediate (between I and II) was reported.