Synthesis of Divergent Benzo[b]fluorenones through Cycloaromatization Reactions of 1,5-Enynols and 1,5-Diynols

Abstract

A facile and efficient synthesis of divergent benzo­[b]­fluorenones is described through the use of dichlorobenzoquinone-promoted oxidative cycloaromatization reactions of acyclic 1,5-enynols and 1,5-diynols. The success of these cascade reactions depends on the chemoselectivity of the initial Meyer–Schuster rearrangement to produce allenol intermediate, which is followed by regioselective Schmittel cyclization and the subsequent Friedal–Crafts alkylation or radical attack at the terminal Ar moiety. Only an oxidant and a solvent were required in the reaction, thus delivering a small library of the expected polycarbocyclic products with excellent functional group tolerance under metal-free conditions. The absorption and photoluminescence properties of the selected benzo­[b] fluorenones were also investigated. The results indicated that the compound (2h) which contained an electron-donating 4-OMe group at the phenyl moiety displayed deep green color emission (491 nm).

Recent Developments in the Meyer‐Schuster Rearrangement

“…A little later, the same group established that a similar process was possible starting from alkene derivatives 131 . In that case, two equivalents of DDQ were used [78b] …”

Regiospecific Palladium-Catalyzed Cross-Coupling Reactions Using the Operational Equivalent of 1,3-Dilithiopropyne

“…47,48 To explore functional group compatibility, we further used 4-iodoacetophenone (19) as a substrate. We were pleased to observe that the coupling product, 4-(propyn-1yl)acetophenone (20), was isolated in 82% yield ( Table 2, entry 5), and no reaction at the carbonyl group was observed. Under similar conditions, methyl 2-iodobenzoate (21) gave 85% yield of methyl 2-(propyn-1-yl)benzoate (22) (entry 6).…”

“…12 It has been suggested that this cross-coupling reaction is difficult to achieve with lithium acetylides using catalytic amounts of palladium, because the highly nucleophilic alkynyllithiums may displace the ligands (e.g., PPh 3 , Cl -) from the palladium complexes, forming lithium palladates that do not exhibit any catalytic activity. 13 O-Substituted arylalkynes are very useful intermediates in the synthesis of heterocyclic compounds 14 such as phthalides and isocoumarins, 15 furocoumarins, 16 2-substituted benzofurans, 17 indoles, 18 1,2-benzothiazine 1,1-dioxides, 19 benzoisothiazoles, benzofluorenones, 20 dihydroisobenzofuranes, 21 and isoquinolones. 14 Specifically, 1-propynylarenes that can be obtained by the above alkynylation protocols using prop-1-yne, are not only very valuable synthetic intermediates, but are also present as key motifs in a wide number of natural products, 22 many of which have important biological activity.…”

Regioselectivity Switch Towards the Development of Innovative Diels‐Alder Cycloaddition and Productive Applications in Organic Synthesis

“…In 2004, Carlos Saa and colleagues reported an intramolecular dehydro DA cycloaddition of diarylacetylenes where thermal cyclization of 1‐[2‐(arylethynyl)phenyl]‐3‐trimethylsilylpropynones 75 underwent rearrangement in notable switching between benzo[b]fluorenones and benzo[c]fluorenones yielded a mixture of products, including benzo[ b ]fluorenones 78 and benzo[ c ]fluorenones 79 that have immense bioactivity and necessity in organic synthesis (see Scheme 12). [122–124] The formation of benzo[b]fluorenones 78 via 80 arises from direct aromatization‐isomerization of the initial [4+2] cyclized product whereas, benzo[c]fluorenones 79 via 81 governed by six electron electrocyclic ring opening followed by aromatization‐isomerization with promising yields up to 93% as depicted in Scheme 12. The remarkable switch between benzo[b]‐ and benzo[c]fluorenones expedient during [4+2] cycloaddition pathway was one of the prominent accomplishments in the history of organic synthesis pursuits.…”