A new route to 1-phenylnaphthalenes by cycloaddition: a simple and selective synthesis of some naphthalene lignan lactones

Total synthesis of justicidin B, justicidin E, and taiwanin C: A general and flexible approach toward the synthesis of natural arylnaphthalene lactone lignans

Lignans are widely present in traditional medicinal plants. Many natural arylnaphthalene lactone lignans (NALLs) isolated from the genera Justicia, Haplophyllum, and Phyllanthus possess interesting biological activities. Herein, we report a general strategy for the total synthesis of this kind of lignans. Features of this new approach are an aryl-alkyl Suzuki cross-coupling to introduce the dioxinone unit, a cation-induced cyclization to construct the aryl dihydronaphthalene, and base-mediated oxidative aromatization to furnish the arylnaphthalene core. By incorporating these key transformations, the total syntheses of justicidins B and E and taiwanin C covered type I and type II NALLs were accomplished.

Ultrasound-assisted synthesis of RO- and RS-substituted (hetero)acenes via oxo- and thio-Friedel-Crafts/Bradsher reactions

Two heteroatom-variants of the Friedel-Crafts/Bradsher cyclization of o-acetalaryl(aryl)methyl ethers and o-dithioacetalaryl(aryl)methyl thioethers, have been realized with the ultrasound assistance. The environmentally friendly "oxo-variant" (Oxo-F-C/B), proceeding in a medium containing mineral acid and a large amount of water (HCl_aq/CH₃CN) led to a very efficient formation of RO-substituted (hetero)acenes in less than 5 min. In the "thio-variant" (Thio-F-C/B), o-dithioacetalaryl(aryl)methyl thioethers underwent ultrasound-assisted cyclization in nonaqueous medium (FeCl₃/KI/EtOH) in less than 25 min., in lower yields than in the "oxygen variant" to give RS-substituted (hetero)acenes. The RO-(hetero)acenes cyclized at 25-60 °C in aqueous media but did not cyclize in organic solvents while the RS-(hetero)acenes required higher temperatures 55-60 °C and cyclized in organic solvents but did not react in aqueous media. The acceleration of the ultrasound-assisted reactions compared to the reactions carried out under silent conditions exceeded 7500 times in the most effective example of the oxo-variant and on average 2 times for the thio-variant. The plausible reaction mechanisms under ultrasound and silent conditions have been proposed. The ultrasonic mechanism involves disturbing of solvation layers and formation of the reactive ("naked") carbocations upon operation of the shock wave produced by the bubble collapse. The o-acetalaryl(aryl)methyl ethers underwent a selective ultrasound-assisted deacetalization to give o-formylaryl(aryl)methyl ethers, without subsequent cyclization under the acidic reaction conditions.

Regiocontrolled Benzannulation of Diaryl(gem-dichlorocyclopropyl)methanols for the Synthesis of Unsymmetrically Substituted α-Arylnaphthalenes: Application to Total Synthesis of Natural Lignan Lactones

[reaction: see text] An efficient synthesis of highly substituted alpha-arylnaphthalene analogues has been developed utilizing Lewis acid-promoted regiocontrolled benzannulation of aryl(aryl')-2,2-dichlorocyclopropylmethanols (aryl not equal aryl'; abbreviated as AACMs). Both AACM diastereomers were easily prepared via highly stereoselective addition (>95/5) of ArLi to gem-dichlorocyclopropropyl aryl' ketones. The choice of Lewis acids determined the cyclization regioselectivity of the present benzannulation. TiCl4 and SnCl4 used the chelation pathway, whereas silyl triflates used a nonchelation pathway to give unsymmetrically substituted regioisomeric alpha-arylnaphthalenes in 40-91% yields with moderate to excellent regioselectivity (TiCl4 or SnCl4; >99/1-3/1, TBDMSOTf; >1/99-1/4). Thus, the alpha-aryl or alpha-aryl' moiety (accessory aryl group) was alternatively introduced to alpha-arylnaphthalenes by choosing either the order of the reaction sequences or the appropriate catalyst. Application of the present method to the total synthesis for unsymmetrically substituted natural lignan lactones, justicidin B, retrojusticidin B, dehydrodesoxypodophyllotoxin, and a related analogue, 5'-methoxyretrochinensin, was demonstrated. Lignan retrolactones (retrojusticidin B and 5'-methoxyretrochinensin) were synthesized by the conventional lactonization of the diol precursor, whereas a novel Bu2SnO-mediated monoacylation method was applied to the synthesis of normal lignan lactones (justicidin B and dehydrodesoxypodophyllotoxin).

Antiviral activity of arylnaphthalene and aryldihydronaphthalene lignans

A practical method for large scale synthesis of 1-arylnaphthalene and 1-aryl-1,2-dihydronaphthalene lignans is described. The method makes use of the classic Stobbe condensation followed by regioselective reactions that provide access to both the common and retrolactone lignans, e.g., 2 and 3. A total of 25 compounds, many of which are known natural products, were prepared and their antiviral activity against human cytomegalovirus measured.Key words: lignan, Stobbe, arylnaphthalene, antiviral.