Unusual Facial Selectivity in the Cycloaddition of Singlet Oxygen to a Simple Cyclic Diene1
摘要:
Syntheses of 5-isopropyl-1,3-cyclohexadiene and syn-5-isopropyl-2,3-dioxabicyclo[2.2.2]octane, by routes that would allow completely diastereoselective introduction of deuterium labels, are described. The reaction of the isopropyl cyclohexadiene with singlet oxygen is shown to give an endoperoxide that is derived by preferential attack on the more sterically hindered face of the diene, A possible mechanistic explanation of this result is that the attack from the less hindered face leads to ''ene'' reaction rather than endoperoxide formation, However, this mechanism would require that the ''ene'' reaction and cycloaddition proceed via a common intermediate-presumably a perepoxide.
Synthesis of carbocyclic core of cortistatin A (1), a novel anti-angiogenic steroidal alkaloid from Indonesian marine sponge, was investigated. Intramolecular Heck cyclization using substrate 18 achieved construction of 7-membered B-ring structure. The presence of steric hindrance around the reaction center was found to favor endo cyclization pathway in this substrate.