Sesqui- and triterpenoids of the liverwort Conocephalum japonicum

Toyota, Masao; Asakawa, Yoshinori

doi:10.1016/s0031-9422(00)95098-2

Cited by 22 publications

(9 citation statements)

References 16 publications

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“…The resulting enantiomerically enriched vinylsilanes can be protodesilylated, converted to the corresponding vinyl halides, or oxidized to β-acyl-substituted enones in >66% yield. The utility of the NHC–Cu-catalyzed reaction is illustrated through a concise total synthesis8 of natural product riccardiphenol B 9…”

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Formation of Vinyl-, Vinylhalide- or Acyl-Substituted Quaternary Carbon Stereogenic Centers through NHC−Cu-Catalyzed Enantioselective Conjugate Additions of Si-Containing Vinylaluminums to β-Substituted Cyclic Enones

2010

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A catalytic method for enantioselective conjugate addition (ECA) of Si-containing vinylaluminum reagents to β-substituted cyclopentenones and cyclohexenones is described. Reactions are promoted by 1.0-5.0 mol % of a bidentate NHC-Cu complex, which is prepared from air stable CuCl 2 •2H 2 O and used in situ, and typically proceed to completion within 15-20 minutes. The requisite vinylmetals are generated efficiently by a site-selective hydroalumination of an alkyne with dibal-H. The desired products, containing a quaternary carbon stereogenic center, are obtained in 48-95% yield after purification and in 89:11 to >98:2 enantiomer ratio (er). The vinylsilane moiety within the products can be functionalized to afford acyl, vinyliodide or desilylated alkenes in 67% to >98% yield and with >90% retention of the alkene's stereochemical identity. The utility of the catalytic process is illustrated in the context of a concise enantioselective synthesis of riccardiphenol B.Catalytic enantioselective conjugate additions of easily accessible C-based nucleophiles to unsaturated carbonyls faciliates the synthesis of a range of enantiomerically enriched biologically active molecules and are therefore of significant value. 1 Nevertheless, in spite of recent advances, 2 notable shortcomings persist in this area, particularly in the context of reactions furnishing quaternary carbon stereogenic centers. 3 One deficiency relates to the paucity of protocols for catalytic conjugate additions of vinyl units. 4 A report regarding Cuphosphine-catalyzed enantioselective conjugate addition (ECA) reactions of vinylaluminums with β-substituted cyclic enones has appeared; however, substrate and reagent scope is narrow and the preparation of the requisite vinylmetals via vinylhalides can be limiting (details below). 5 We set out to examine whether Cu complexes of N-heterocyclic carbenes (NHCs) can effect ECA of vinylmetals to generate quaternary carbon stereogenic centers. 6 In this context, use of Si-substituted vinylmetals would be attractive (Scheme 1) for several reasons. First, efficient and stereoselective hydroaluminations of silylacetylenes with inexpensive diisobutyaluminum hydride (dibal-H) afford the requisite vinylaluminums. 6 Thus, the need for stereoselective synthesis of vinylhalides, only a few of which can be purchased and whose preparation often requires strongly acidic conditions, 7 would not be necessary. Second, in instances where the vinylmetal is a small molecule, the impracticality of isolation or purification of volatile alkynes or vinylhalides can be avoided. Third, and perhaps most importantly, as illustrated in Scheme 1, efficient synthesis of a variety of * amir.hoveyda@bc.edu .Supporting Information Available: Experimental procedures and spectral, analytical data for all products (PDF). This material is available on the web: http://www.pubs.acs.org. enantiomerically enriched products that cannot be directly prepared or, at least, easily synthesized by a catalytic process would become feasible due to the ...

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Formation of Vinyl-, Vinylhalide- or Acyl-Substituted Quaternary Carbon Stereogenic Centers through NHC−Cu-Catalyzed Enantioselective Conjugate Additions of Si-Containing Vinylaluminums to β-Substituted Cyclic Enones

2010

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“…A piscicidal diterpene dialdehyde was isolated from R. lobata var. yakushimensis (6) which contain a sesquiterpene portion attached to a quinol ring, from a Japanese collection of R. crassa, but did not mention any biological activities (7). Sesquiterpene/quinones and sesquiterpene/ quinols are rare in terrestrial plants, but quite common from marine sources (8).…”

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Sesquiterpene/Quinol from a New Zealand Liverwort, Riccardia crassa

Perry

Foster²

1995

J. Nat. Prod.

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“…Zeorin (Tsuda et al 1967) has been isolated from liverworts (Bard on et al 1999, 2002, Toyota and Asakawa 1993 and other vascular plants (Wong et al 1986;Inayama et al 1989;Wu et al 1990;Shiojima and Ageta 1994). There are a few reports of zeorin isolation or detection from lichen extracts (Shibata et al 1965, Yoshioka et al 1966, although it is not clear whether the mycobiont is the producer.…”

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confidence: 99%