Thermal [2+2] Intramolecular Cycloadditions of Fuller-1,6-enynes

摘要

The transition-metal-catalyzed cyclization of enynes (mostly 1,6-enynes) 1 has been extensively studied as it provides a powerful method for the construction of carbo- and heterocyclic systems (2-5) in a simple manner (Scheme 1). Cycloisomerization of enynes to form novel diene structures is currently a major challenge both synthetically and mechanistically. Among the different cyclic products obtained, metathesis-type product 5 is a useful building block whose formation involves an interesting mechanism in which the carbon-carbon double bond of the enyne is completely cleaved. In this chemical transformation, cyclobutene species 6 were suggested as precursors of dienes 5. Cyclobutenes 6 have been considered to be highly strained species as they are the equivalent of a trans-cycloheptene, a species not known to exist at ambient temperature. However, the cyclobutene isomer 7 is an olefin that is stable at 118 ℃. Furthermore, cyclobutene 8 has recently been isolated as a stable and well-characterized silane derivative.