Pd/C–Cu mediated direct and one-pot synthesis of γ-ylidene butenolides
D. RambabuS. BhavaniKumara Swamy NalivelaSoumita MukherjeeMandava Venkata Basaveswara RaoManojit Pal
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The strategies for the synthesis of naturally occurring ten-membered-ring lactones, which are the most abundant of the medium-ring lactones, are discussed. According to their structures and/or biosynthesis, the molecules are classified in monocyclic polyketides, monocyclic oxylipins, aliphatic bicyclic and aromatic bicyclic lactones. In each subsection, the lactones are presented in chronological order of their isolation. Representative complex systems bearing multiple functionalities in addition to the lactone moiety, as well as a selection of recent synthetic approaches to non-natural medium-ring lactones, are briefly presented.
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Diazoethane partly alkylates 3-acetylcoumarin giving 3-acetyl-4-ethylcoumarin, and partly expands the lactone ring giving, after addition of a second molecule of diazoethane, the benzoxepinopyrazoline (IX) as a racemate of assigned stereochemistry. This compound readily loses nitrogen and undergoes a second ring expansion to form the benzoxocin derivative (XII) as a single (racemic) stereoisomer. Treatment of (XII) with diazoethane induces immediate ring expansion giving the benzoxonin derivative (XIII), again as a single (racemic) stereoisomer. There is no further reaction with diazoalkanes, notwithstanding the presence of a highly activated double bond; the resistance is attributed to the internal compression that would result within a highly convoluted molecule if addition occurred.The structures and conformations of the foregoing products and of some by-products have been established by spectroscopic methods. Additionally, the benzoxocin derivative (XII) has been shown to yield meso-dimethyl-succinic acid upon exhaustive ozonolysis, and a convenient method is described for the preparation of small amounts of dimethylmaleic anhydride by the reaction of diazomethane with methylmaleic anhydride.A parallel is drawn between these reactions and certain of those found with quinone–diazoalkane adducts, and it is suggested that ring expansions are observed because the methyl group introduced with the diazoethane molecule forces the pyrazoline into the necessary conformation.
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A concise and diastereoselective construction of the ABCD ring system of spirochensilide A is described. The key steps of this synthesis are a semipinacol rearrangement reaction to stereoselectively construct the AB ring system bearing two vicinal quaternary chiral centers and a Co-mediated Pauson–Khand reaction to form the spiro-based bicyclic CD ring system. This chemistry leads to the stereoselective synthesis of 13(R)-demethyl spirochensilide A, paving the way for the first asymmetric total synthesis of (−)-spirochensilide A.
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Abstract Stereoselective β‐mannosylation has been recognized as one of the greatest challenges of carbohydrate chemistry. Herein, we described a practical method for stereoselective construction of β‐mannosides by using a 2,6‐lactone‐bridged thiomannosyl donor through the remote acyl‐group participation as well as the steric effect of O ‐4 substituent. The two effects are enabled through the conversion of a regular mannopyranosyl 4 C 1 conformation into a 2,6‐lactone bridged conformation. The lactone donor could be readily prepared in three steps on a gram scale and the β‐mannosylation proceeded smoothly with high stereoselectivity for primary, secondary and tertiary alcohol acceptors. In addition, this strategy was successfully applied to the synthesis of a naturally occurring trisaccharide.
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The skeletal structure of azalomycin F4a (1) was determined on the basis of available data about the physicochemical properties of F4a and about the structures of the degradation products of this compound and of its derivatives. The structure was found to consist of a 36-membered lactone ring bearing multiple hydroxy functions, a diene and a dienoic ester group, as well as a side chain with an N-methylguanidine moiety as its terminal. One of the hydroxyl groups on the lactone ring forms a hemiketal ring with the keto group on a ring carbon, and another hydroxyl group forms a hemiester with a malonic acid moiety.
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Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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The title compound, C(33)H(46)O(7), is an unusual oxydation product of the therapeutic agent glycyrrhetinic acid that has, in comparison to the latter, a distinctly altered triterpene structure with one five- and four six-membered carbocycles complemented by a γ-lactone ring with a spiro-junction and a ring double bond. The junction between the five-membered ring C, a cyclo-penta-none ring, and the six-membered ring D, previously in question, was found to be cis, confirming earlier structure assignments based solely on chemical transformations. In the solid state, the compound exhibits five intra- and four inter-molecular C-H⋯O inter-actions with H⋯O distances less than or equal to 2.70 Å and C-H⋯O greater than 100°.
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Abstract A highly stereoselective first total synthesis of amphidinin B is described. The key steps involved in this synthesis are the generation of the exo ‐double bond in the C 1 –C 9 segment, the Barbier allylation, enzymatic kinetic resolution, and the construction of the C 10 –C 21 segment by Sharpless asymmetric epoxidation, base‐induced epoxide ring‐opening, radical cyclization, diastereoselective reduction of the exo ‐cyclic double bond, one‐pot allylation followed by debenzylation, Evans alkylation, and Yamaguchi esterification.
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