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As a result of recent insights in the molecular structures and medicinal uses of the tannins, this class of natural products has enjoyed renewed scientific interest. This is reflected on the one hand in the enormously increased number of publications reporting on the isolation, characterization and biological activity of the tannins, and on the other hand in the steadily growing number of papers reporting tannin syntheses. In this review the most recent advances in the total synthesis of ellagitannins are discussed. In nature the ellagitannins occur usually only as the R-or only as the S-configured atropisomer, while the corresponding opposite atropisomers are seldom found. The configuration of the hexahydroxydiphenoyl units (HHDP's) found in ellagitannins is usually determined by the linkage position of the HHDP's with the polyol residues of the ellagitannins. Those ellagitannins of which the HHDP's are linked to the 2,3-or 4,6-positions of D-glucopyranose, have the S configuration; the corresponding 3,6-(HHDP)-ellagitannins are R-configured. Very few exceptions exist where both atropisomers are known. With one atropisomer showing the "normal" configuration, the other atropisomer is usually indicated in the literature as "unusual ellagitannin". This observed strict atropodiastereoselectivity in the biosynthesis of normally configured and unusual ellagitannins has led to the formulation of several hypotheses, which will be examined, in addition to the overview of effective concepts and strategies for the synthesis of ellagitannins.
The synthesis of racemic tetrangomycin (11) is reported. In the key step the hexahydrobenz[α]anthraquinone 8 is generated in a regioselective Diels‐Alder reaction of diene 5 with bromonaphthoquinone 7. The conversion of 8 into 11 involves only three operations: treatment with diethyl ether–boron trifluoride, hydrogen peroxide and photooxygenation.
The Diels-Alder reactions of the naphthoquinones 1-4 with 1 he dienes Sa-? were investigated. The regioisomeric outcome of the adducts lOa-lOg, 16, and 23 is in agreement with theoretical predictions from frontier orbital calculations. The adduct 10a was further converted to the non-natural 1e.g. 15) and the adduct 16 to the natural angucyclinones MM 47755 (19a) and X-14881 E (22) by silicon-oxygen exchange and photooxygenation. The addition of naphtoquinones 2 and 4 to dienes 6 and 7 afford the Diels-Alder adducts 16 and 23, respectively, which are valuable intermediates for further transformation into non-aromatic angucyclinones.The angucyclinones are an emerging type of quinone antibiotic that have recently attracted much attention"]. They can be divided into two large groups depending on 1 he presence or absence of a C-glycosidic moiety [']. Among {he non-C-glycosidic compounds the tetrangomycin type with aromatic ring B represents the more simple antibiotics. We have recently described the synthesis of racemic rabelo1nycin[~1 and tetrang~rnycin[~] using a Diels-Alder reaction lor the construction of the angular skeleton (for related work using a Diels-Alder approach see r e f~. [~-~l .The problem of the introduction of the labile tertiary hydroxy group at C-3 was solved by the use of silylated diene building blocks and the angular carbonyl group was introduced by a terminal photooxidation step. We now describe a further extension of our method with particular emphasis on the investigation of the regiochemistry of the Diels-Alder step, the possibility of extension to non-aromatic angucyclinones Thus, the Wittig-Horner reaction of the ketone 8L31 with tert-butyl (diethoxyphosphory1)acetate afforded the unsaturated ester 9a (E/Z mixture, 69%) that was converted to the vinyl ketene acetal5b in the usual manner by deprotonation with LDA and treatment with trimethylsilyl chloride (92%) (Scheme 1). The silyloxydiene 6 was prepared by starting from the ethyl ester 9b, which was reduced with LAH in toluene to the allylic alcohol 9d31. PCC oxidation to the unsaturated aldehyde 9d proceeded in good yield (85%). The crude reaction product could be converted to the ( H Z ) mixture of the silylvinyl ether by treatment with zinc chloride/triethylamine in toluene in the presence of trimethylsilyl chloride.The Diels-Alder reactions of the naphthoquinones 1-3 with the vinyl ketene acetal 5a were studied first (Scheme 2). Not surprisingly, the unsubstituted 1 ,4-naphthoquinone 1 was much less reactive than juglone (2) or the halogenated derivative 3, and the reaction mixture had to be refluxed in toluene for several hours to afford the adducts 10a and 10b (8-deoxyangucyclinones). The reactivity and the regiochemical outcome of the reaction of the quinones 2 and 3 are in perfect agreement with previous experimental observations [20,21] and the frontier orbital considerations of Houk et a1.[221. Overlap of the frontier orbitals with the larger coefficients of diene and dienophile is energetically favored and dominates the regio...
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