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Exploring Intra- and Intermolecular Alkyne Annulations From Biologically Active Terpenoids to Polycyclic Aromatic Hydrocarbons
AdvisorChalifoux, Wesley A.
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Various trialkylsilyl-substituted alkynes were utilized to prepare highly functionalized and reactive conjugated 2,4-diynones via Friedel−Crafts acylation reaction. We then developed a highly regioselective Lewis acid promoted (Me2AlCl) Diels–Alder reaction of these conjugated 2,4-diynones to produce substituted 2-alkynyl-1,4-cyclohexadiene (‘skipped’ cyclohexadiene) products. The scope of this synthetic methodology was studied utilizing a variety of cyclic and acyclic dienes as well as with a diverse array of 2,4-diynones. The synthetic utility of these 2-alkynyl-1,4cyclohexadienes was demonstrated making use of functionalities (alkene and enone moiety) present in these molecules.A highly regioselective synthesis of 2,3-disubstituted chromen-4-one derivatives was accomplished from readily available internal alkyne and 2-methoxybenzoyl chlorides. This Lewis acid promoted method features relatively mild reaction conditions to synthesize a variety of 2,3-disubstituted chromen-4-one derivatives in one pot with up to 93% yield. The reaction proceeds via a domino intermolecular Friedel-Crafts acylation/intermolecular vinyl carbocation trapping (or oxa-Michael addition)/demethylation reaction sequence. We are able to establish a broad scope of this method and showed a brief synthetic utility of these disubstituted 2,3-didubstiuted chromen-4-one derivatives. The synthesis of pyreno[a]pyrene based helicene derivatives has been achieved via a four-fold benzannulation reaction of alkynes with 30-49% yield. We have demonstrated an easy and efficient protocol featuring a Suzuki cross-coupling reaction between bis(trifluoromethanesulfonyloxy)naphthalene and various 2,6-diynylphenyl borates followed by triflic acid (TfOH) promoted alkyne benzannulation. The two enantiomers of each of these pyreno[a]pyrene based helicene derivatives were separated using chiral HPLC to study their photophysical and chiroptical properties. Migration of one phenyl substituent was also observed under the reaction conditions (TFA, CH2Cl2, rt), producing rearranged side product. The very first chiral teropyrene derivatives have been synthesized utilizing an InCl3-promoted four-fold benzannulation reaction of alkynes. The X-ray crystallographic analysis of chiral teropyrene showed that the substituents in the bay-regions exhibit considerable repulsion and results in considerable twisting of the polycyclic aromatic hydrocarbon (PAH) backbone to arrive at an end-to-end twist angle 52°. The inversion barrier of the enantiomers was found to be 30 kcal/mol, high enough to allow separation of the enantiomers to study them spectroscopically.