The aryl C-glycoside structure is, among the plenty of biologically active natural products, one of the distinct motifs embedded. Because of the potential bioactivity as well as the synthetic challenges, these structures have attracted considerable interest, and extensive research toward the total synthesis has been performed. This Review focuses on the synthetic strategies and tactics employed in the total synthesis of this class of natural products. The Introduction describes the historical background, structural features, and synthetic problems associated with aryl C-glycoside natural products. Next the Review summarizes the methods for constructing the aryl C-glycoside bonds. Completed total syntheses—and, in some cases, selected examples of incomplete syntheses—of natural aryl C-glycosides are also summarized. Finally described are the strategies for constructing polycyclic structures, which were utilized in the total syntheses.

Total Synthesis of Aryl C-Glycoside Natural Products: Strategies and Tactics

Reactions of ketene acetals-14 The use of simple mixed vinylketene acetals in the annulation of quinones

silyl], asymmetric variants became extensively studied and utilized in the synthesis of enantioenriched six-membered oxygen-containing heterocycles. Indeed, several efficient metal-based chiral Lewis acids have been developed for this reaction. Recently, Rawal and co-workers introduced a metal-free, hydrogen-bonding catalyst for asymmetric HDA reactions of 1-dimethylamino-3-tert-butyldimethylsilyloxy-1,3-butadiene (Rawal s diene) with aldehydes [Eq. (1), TBS = tert-butyldimethylsilyl]. Despite all this progress however, the present catalytic systems for asymmetric HDA reactions are largely limited to the originally introduced dienes, and substituted and functionalized dienes have proven to be highly challenging substrates. As a consequence, only few catalytic asymmetric syntheses of 2,6-disubstituted dihydropyrones have been reported and the catalytic enantioselective synthesis of 2,5,6-trisubstituted dihydropyrones is entirely unknown. 7] Herein, we report a highly enantioselective HDA reaction of aldehydes with substituted 1,3bis(silyloxy)-1,3-dienes catalyzed by a novel, highly fluorinated, chiral disulfonimide [Eq. (2)]. Recently our group has introduced chiral disulfonimides (DSI) as effective catalysts of asymmetric Mukaiyama and vinylogous Mukaiyama aldol reactions with aldehydes. We hypothesized that such a catalyst could also promote the mechanistically related HDA reactions of 1,3-bis(silyloxy)1,3-dienes, which surprisingly have previously been utilized only in non-asymmetric catalysis. Our interest in this particular transformation was further stimulated by speculating that the lack of established catalytic asymmetric HDA reactions might stem from a competing silylium-catalyzed background reaction. We had previously shown that disulfonimide-based catalysis offers an efficient solution to this problem in Mukaiyama aldolizations. To explore the principal utility of our DSI catalysts in HDA reactions, we initially studied our previously used chiral disulfonimide catalyst 1 in the reaction of 2-naphthaldehyde with the commercially available diene 3. These experiments showed that the product 4 could indeed be obtained in the presence of 5 mol% of catalyst 1 at 78 8C in Et2O in good yield and reasonable enantioselectivity (Table 1, entry 1). We reasoned that improving the steric and electronic properties of the catalyst by incorporating large perfluorinated substitu-

Highly Enantioselective Hetero‐Diels–Alder Reaction of 1,3‐Bis(silyloxy)‐1,3‐dienes with Aldehydes Catalyzed by Chiral Disulfonimide

Despite their simplicity and synthetic usefulness, cyclisation reactions of 1,3-dicarbonyl dianions with 1,2-dielectrophiles are problematic, since both dianions and 1,2-dielectrophiles are highly reactive compounds (low reactivity matching). In addition, 1,2-dielectrophiles are often rather labile, and reactions with nucleophiles can result in polymerisation, decomposition, formation of open-chained products, elimination or SET-reactions. These intrinsic limitations can be overcome by a proper reactivity tuning and by the use of electroneutral dianion equivalents (masked dianions) in Lewis acid catalysed reactions. The cyclisations reported herein allow for an efficient, regio- and stereoselective one-pot synthesis of biologically relevant ring systems.

Regio‐ and Diastereoselective Cyclization Reactions of Free and Masked 1,3‐Dicarbonyl Dianions with 1,2‐Dielectrophiles

This review has been written in order to update the literature on anthraquinones occurring in the Rubiaceae. Since appearance of the excellent book on naturally-occurring quinones by R.H. Thomson (120) in 1971 about 50 new anthraquinones have been isolated from members of the Rubiaceae. Also several new methods have been used for structure analysis and separation of the anthraquinones which have not been subject to review before. For this review we have confined ourselves to the naturally occurring anthraquinones and therefore the synthesis of anthraquinones is not included. The article covers the literature from 1969 to 1984.

Anthraquinones in the Rubiaceae

Regiospecific reactions of some vinylogous ketene acetals with haloquinones and their regioselective formation by dienolization

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Regiospecific reactions of some vinylogous ketene acetals with haloquinones and their regioselective formation by dienolization