- Studies on the synthesis of landomycin A. Synthesis of the originally assigned structure of the aglycone, landomycinone, and revision of structure
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The originally proposed structure (2) of landomycinone, the aglycone of landomycin A, has been synthesized and shown to be nonidentical to the naturally derived landomycin A aglycone. The synthesis of 2 features the Doetz benzannulation reaction of chromium carbene 5 and alkyne 6, and the intramolecular Michael-type cyclization reaction of the phenolic naphthoquinone 20. It is proposed that natural landomycinone possesses the alternative structure 3, but attempts to access this structure via the Michael-type cyclization of the isomeric phenolic naphthoquinone 38 have been unsuccessful.
- Roush, William R.,Neitz, R. Jeffrey
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p. 4906 - 4912
(2007/10/03)
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- POLYBROMINATED AROMATIC COMPOUNDS V. REACTION OF PENTABROMOTOLUENE WITH SODIUM METHOXIDE IN PYRIDINE
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During the reaction of pentabromotoluene with sodium methoxide in pyridine, together with the products from methoxydebromination and protodebromination, the products from substitution of both the bromine atom in the aromatic ring and hydrogen atom in the methyl group by methoxy groups are formed.The directing effect of the methyl and methoxymethyl groups during substitution of the bromine in the pentabromophenyl ring by the methoxide ion was studied, and it was shown that the directing selectivity of these substituents is low.The methyl group is predominantly a meta-orientant, whereas the methoxymethyl group is predominantly a para-orientant.
- Shishkin, V. N.,Lapin, K. K.,Tanaseichuk, B. S.,Butin, K. P.
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p. 1882 - 1887
(2007/10/02)
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- Naturally Occurring Dibenzofurans. Part 5. Synthesis of Melacarpic Acid
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The total synthesis of the lichen dibenzofuran melacarpic acid (5) was achieved by intramolecular Ullman coupling of methyl 5-bromo-4-(2-bromo-5-methoxy-3-methylphenoxy)-6-heptyl-2-methoxybenzoate (48) or methyl 6-heptyl-5-iodo-4-(2-iodo-5-methoxy-3-methyl-4-nitrophenoxy)-2-methoxybenzoate (46) followed by appropriate transformations.
- Carvalho, Christopher F.,Sargent, Melvyn V.
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p. 1613 - 1620
(2007/10/02)
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- Electrophilic Substitution with Rearrangement. Part 9. Dienones derived from Brominations of o-, m-, and p-Cresol
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Regiospecific protodebromination of ring-substituted bromophenols derived from 2-, 3-, or 4-methylphenol can be effected by heating them with aqueous hydrogen iodide; the synthetic scope of this reaction has been explored.These di- and poly-bromophenols can generally be converted by further bromination in aqueous acetic acid into dienones , which have now been shown to have the 4-bromo-2,5-dienone rather than the 2-bromo-3,5-dienone structure.The rearrangemens of these dienones to ring-substituted polybromophenols by treatment with sulfuric acid have been investigated; where more than one product is formed, the regioselectivity differs from that prevailing in the corresponding direct bromination of the phenol with liquid bromine.The alternative rearrangements of these dienones in aprotic solvents with and without illumination have been compared with results obtained by reaction of methylphenols with bromine under the same conditions.Characteristic differences between the behaviours of 2-, 3-, and 4-methyl-substituted compounds reflect the specific reactions available to the particular dienones.
- Brittain, Judith M.,Mare, Peter B. D. de la,Newman, Paul A.
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