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Tandem Pummerer-Diels-Alder Reaction Sequence. A Novel Cascade Process for the Preparation of 1-Arylnaphthalene Lignans

Tandem Pummerer-Diels-Alder Reaction Sequence. A Novel Cascade Process for the Preparation of 1-Arylnaphthalene Lignans

Albert Padwa, John E. Cochran, C. Oliver Kappe

J Org Chem. 1996 May 31;61(11):3706-3714.

PMID: 11667219

Abstract:

The alpha-thiocarbocation generated from the Pummerer reaction of an o-benzoyl-substituted sulfoxide is intercepted by the adjacent keto group to produce an alpha-thio isobenzofuran as a transient intermediate which undergoes a subsequent Diels-Alder cycloaddition with added dienophiles. Acid-catalyzed ring-opening of the cycloadduct followed by aromatization gave an arylnaphthalene derivative. With acetylenic dienophiles, the tandem cyclization-cycloaddition sequence provided tetralones which result from a pinacol-type rearrangement of the primary cycloadducts. The versatility of the approach is highlighted through the synthesis of taiwanin C and E and justicidin E. The alpha-thiocarbocation generated from the Pummerer reaction of benzo[1,3]dioxol-5-yl-[6-[(ethylsulfinyl)methyl]benzo[1,3]dioxol-5-yl)methanone is intercepted by the adjacent keto group to produce an alpha-thioisobenzofuran as a transient intermediate which undergoes a subsequent Diels-Alder cycloaddition with dimethyl maleate. The initially formed Diels-Alder cycloadduct was readily converted to 5-benzo[1,3]dioxol-5-yl-8-(ethylthio)naphtho[2,3-d][1,3]dioxole-6,7-dicarboxylic acid dimethyl ester by loss of water on treatment with p-toluenesulfonic acid. Desulfurization of the thionaphthalene with Ra/Ni followed by hydrolysis of the less hindered methyl ester afforded 5-benzo[1,3]dioxol-5-ylnaphtho[2,3-d][1,3]dioxole-6,7-dicarboxylic acid 6-methyl ester which was further transformed into taiwanin C and justicidin E in good yield. Oxidation of the initial Diels-Alder cycloadduct with NaIO(4) in the presence of RuCl(3) followed by extrusion of ethyl sulfinate gave a naphthol derivative which can be converted into taiwanin E.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP1073355142	Benzo[c][1,2,5]oxadiazole-5-boronic acid pinacol ester		1073355-14-2	Price
AP1168135032	Benzo[c][1,2,5]thiadiazol-5-ylboronic acid pinacol ester		1168135-03-2	Price

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