Alkyne Cyclizations: From α-Carbonyl Bicyclic Furans to Expanded Chiral Hexa-peri-hexabenzocoronenes
AuthorHamal, Khagendra Bahadur
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The development and utilization of highly functionalized and reactive dienophiles in the Diels–Alder cyclization reaction is of value in producing diversely functionalized, and therefore useful, cyclic products. We have developed a Diels–Alder reaction of conjugated 2,4-diynones, promoted by Lewis acids (Me2AlCl or EtAlCl2), to produce substituted 2-alkynyl-1,4-cyclohexadiene (‘skipped’ cyclohexadiene) products in good to excellent yields. The reaction was successful with a variety of cyclic and acyclic dienes as well as with a diversity of 2,4-diynones. The diversely functionalized 1,4-cyclohexadiene products that are obtained from this reaction are of utility as intermediates on the way to synthesizing pharmaceutically relevant cyclic and polycyclic compounds. Those heavily functionalized skipped dienes were cleanly converted into dihydroisobenzofuran (bicyclic furan) derivatives in the presence of π-Lewis acid catalysts. Ketone derivatives were obtained by using CuCl2 as catalyst whereas AgNO3 provides aldehyde analogues. We have also demonstrated a facile one-pot approach to construct bicyclic furan derivatives directly from diynone and diene starting materials.We also describe the synthesis, characterization, structure and properties of expanded chiral hexa-peri-hexabenzocoronenes. The Suzuki cross-coupling between bromo substituted hexa-peri-hexabenzocoronene and diynylphenyl borate gives the coupled product. The alkyne benzannulation reaction of coupled product promoted by a mixture of TFA and TfOH afforded the desired final product. The twisted structure of chiral compounds is unambiguously confirmed by X-ray crystallographic analysis and the enantiomers are separated in chiral HPLC.