Methodologies for the synthesis of functionalized naphthols and progress toward the total synthesis of vineomycinone B₂ methyl ester and actinophyllic acid
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A general and efficient methodology for the synthesis of cis-2-substituted-1,2-dihydro-1-naphthols and 2-substituted-1-naphthols from oxabenzonorbornadienes was developed. The procedure involved the sequential palladium-catalyzed ring opening of oxabenzonorbornadienes with aryl or vinyl halides followed by oxidation of the intermediate dihydronaphthols with IBX. The scope of the palladium-catalyzed coupling was extended to a variety of halides such as aryl iodides and bromides bearing both electron-withdrawing and -donating groups, vinyl bromides, and glycal iodides. Oxidation of cis-2-substituted-1,2-dihydro-1-naphthols using IBX led to 2-substituted-1-naphthols in good to excellent yields. Application of such methodologies successfully led to a Group II C-aryl glycoside model. The double intramolecular benzyne-furan cycloadditions and naphthyne-furan cycloadditions were developed. In the model studies, furans and the reacting benzynes or naphthynes were linked with silicon tethers and, thus, the regiochemistry of Diels-Alder cycloadditions could be controlled. Two different tactics for converting the resultant oxabenzonorbornadienes to substituted anthrarufins were demonstrated. The first method entails the initial cleavage of the silicon tethers followed by regioselective ring opening of the oxabenzonorbornadienes and oxidation of the central ring giving the target anthrarufin, whereas the second features the regioselective ring opening of the oxabenzonorbornadienes followed by protiodesilylation and oxidation. Application of the chemistry demonstrated in the model double benzyne-furan cycloadditions successfully led to a total synthesis of vineomycinone B2 methyl ester. This strategy enables the rapid assembly of the glycosyl-substituted aromatic frameworks of complex C-aryl glycoside antibiotics from simple starting materials. We propose an oxidative Mannich reaction for the synthesis of an indole natural product, actinophyllic acid. The synthesis features the selective oxidation of a C3-alkyl indole followed by intramolecular nucleophilic addition of enol ether to build a bicylcic framework. This approach may provide a convergent access to the skeleton of actinophyllic acid. Currently, we have prepared indole 5.77, and continuing efforts will be focused on the formation of azepino[4,3-b]indole 5.78 and the key oxidative Mannich reaction.