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Annulation reactions natural products synthesis

We commenced our own approach toward ( )-phomactin A with an intent to feature an oxa-[3 + 3] annulation strategy [24—28] that was developed in our lab [29-32] and, in particular, an intramolecular annulation. While oxa-[3 + 3] annulations or related reaction manifolds [33, 34] are known and can be traced back more than six decades [35], an intramolecular variants of this reaction were not known [24—28]. There were no applications of intramolecular oxa-[3 + 3] annulations in natural product synthesis [27, 28, 36] until our approach toward phomactin A was disclosed. Subsequently, an account toward ( )-likonide B was reported by Trauner [36e],... [Pg.186]

CONTENTS The Synthesis of Seven-Membered-Rings General Strategies and the Design and Development of a New Class of Cycloaddition Reactions, Paul A. Wender and Jennifer A. Love. Recent Advances in Diels -Alder Cycloadditions of 2-Pyrones, Benjamin T. Woodard and Gary H. Posner. The Inter- and Intramolecular [4 + 4] Photocycloaddition of 2-Pyridones and Its Application to Natural Product Synthesis, Scott McN. Sieburth. 3 + 4 Annulations Between Rhodi- J s... [Pg.227]

Bromoetherification of alkenes can be achieved using NBS in the desired alcohol as the solvent. The reaction of 1,3-dichloropro-pene with NBS in methanol yields an a-bromo dimethyl acetal in the first step in a convenient synthesis of cyclopropenone. Using propargyl alcohol the reaction depicted in eq 22 has been extended to an annulation method for the synthesis of a-methy-lene-y-butyrolactones. Intramolecular bromoetherification and bromoamination reactions are generally very facile (eq 23). In natural products synthesis, bromoetherification has been used for the synthesis of cyclic ethers (by subsequent debromination, see... [Pg.45]

Chromium-carbenoids have been used in the field of natural product synthesis the scope of the annulation reaction has been demonstrated by the synthesis of vitamins E and D (Figure 7) [102] as well as of naph-toquinone antibiotics LXVI (Figure 6) [101]. [Pg.222]

Since an annulated six-membered carbocycle is a common structural element of natural products, the Robinson annulation is an important reaction in organic synthesis. [Pg.243]

Feringa and co-workers described the tandem addition-aldol cyclization protocol leading to the formation of 6,6-, 6,7-, and 6,8-annulated bicyclic systems (Scheme 68).39 Using Cu(n)-29 as catalyst and functionalized organozinc reagents as nucleophiles, the conjugate addition reaction followed by aldol cyclization can offer highly enantioselec-tive annulation products (up to 98% ee). This method can be used in the synthesis of carbocyclic compounds, such as steroids, terpenes, and other natural products. [Pg.397]

Another key event in the history of organocatalytic reaction was the discovery of efficient r-proline-mediated asymmetric Robinson annulation reported during the early 1970s. The so-called Hajos-Parrish-Eder-Sauer-Wiechert reaction (an intramolecular aldol reaction) allowed access to some of the key intermediates for the synthesis of natural products (Scheme 1.4) [37, 38], and offered a practical and enantioselective route to the Wieland-Miescher ketone [39]. It is pertinent to note, that this chemistry is rooted in the early studies of Langenbeck and in the extensive investigations work of Stork and co-workers on enamine chemistry... [Pg.5]

A convenient method for the synthesis of annulated 2-alkylthio-5-aminofurans has been described by Padwa et al. The reaction sequence involves the formation of a thionium group from readily available dithioacetals upon treatment with dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF). The thionium ion undergoes cyclization with the 7-carbonyl group followed by an elimination step to yield the 2,3,5-trisubstituted furans in good to excellent yields (Equation 29) <2002JOC1595>. The alkylthioaminofuran reaction products can be utilized to constmct polyclic frameworks of natural products in a subsequent Diels-Alder reaction. [Pg.509]

The indole nucleus is a common and important feature of a variety of natural products and medicinal agents.2 The traditional approach for preparing the indole nucleus is the Fischer indole reaction.3 As this reaction has shortcomings, the palladium-catalyzed coupling of ortho-haloanilines is becoming an excellent alternative.4 Recently, the submitters disclosed a new and efficient method for indole synthesis using a palladium-catalyzed annulation between o-iodoanilines and ketones (Scheme I).5... [Pg.164]

Dihydroxynaphthalene and 9,10-diacetoxyphenanthrene react with 1,2-diols and 1,2-dithiols in a one-pot synthesis of annulated 2,3-dihydro-1,4-dioxins and -1,4-dithiins (Scheme 26) <04TL1343>. The reaction of 2,3-dihydroxynaphthalene with 1,2-dihalogenated aromatic compounds leads to linearly annulated dioxins of particular interest are tri- and tetra-dioxins and various hetero-fused dioxins e.g. 62 (34%). Several examples yield cation radical salts on electrocrystallisation <04T8899>. Linear arrays of fused pyran-dioxin-cyclohexane rings as found in natural products derived from the milkweed family have been described e.g. 63 <04EJO4911>. [Pg.380]


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