Jüngste Entwicklungen bei der gekreuzten Olefinmetathese

Connon, Stephen J.; Blechert, Siegfried

doi:10.1002/ange.200200556

Cited by 263 publications

(66 citation statements)

References 185 publications

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“…[17] Preparation of allylic phosphate 26 (or its trans isomer) required access to allylic alcohol 25. Attempts to synthesize 25 (or its trans isomer) through catalytic cross-metathesis [18] of 23 (or its protected derivatives) with various olefin partners and catalysts resulted in less than 20 % conversion. [19] An alternative approach, involving catalytic Si-tethered ring-closing metathesis, [20] was therefore pursued.…”

Section: Methodsmentioning

confidence: 99%

Chiral N‐Heterocyclic Carbenes in Natural Product Synthesis: Application of Ru‐Catalyzed Asymmetric Ring‐Opening/Cross‐Metathesis and Cu‐Catalyzed Allylic Alkylation to Total Synthesis of Baconipyrone C

2007

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Natural product synthesis and development of catalysts and methods benefit from a critical relationship. [1] A new process provides access to alternative, and often more efficient, routes-it renders a previously untenable scheme feasible. Total synthesis, an important testing ground for a new catalyst and the transformation that it promotes, is particularly valuable when it necessitates the discovery of a method that might otherwise remain unknown. Herein, we report an enantioselective synthesis of the unusual siphonariid metabolite baconipyrone C.[2] The total synthesis demonstrates the utility of recently developed N-heterocyclic carbene (NHC) complexes (Scheme 1); it provides the first application of Rucatalyzed asymmetric olefin metathesis. [3][4] Completion of the synthesis necessitated the development of a new protocol for catalytic asymmetric allylic alkylation (AAA) as well-the first with an alkylaluminum reagent. [5,6] The retrosynthesis for baconipyrone C is presented in Scheme 2. We envisioned that pseudo-C 2 -symmetric diketone I might be prepared via 1,6-diene II; this approach would allow us to investigate whether chiral (NHC)Cu complexes developed for AAA reactions [5,6] provide efficient access to 1,6-diene II via III. Segment IV would be synthesized by reductive cleavage of pyran V, secured by asymmetric ringopening/cross-metathesis (AROM/CM) of VI.[3e] The chiral catalyst-based approach in Scheme 2 thus differs fundamentally from the well-established chiral auxiliary-based diastereoselective aldol strategies [7] employed in the only other recorded total synthesis of this target.[8]The catalytic double AAA proposed for conversion of III to II establishes, in a single operation, the two stereogenic centers in I, but would present a number of challenges as well. One set of complications is inherent to processes that are promoted by a single chiral catalyst and that involve diastereo-and enantioselective formation of proximal stereogenic centers. The initially established center can strongly influence, often in competition with the chiral catalyst, the sense of stereocontrol in the subsequent bond formation. Thus, as illustrated in Scheme 3, addition of the first Me unit to diene III would generate two new stereogenic centers. The first alkylation delivers VII (or the corresponding syn isomer), wherein the central carbon, unlike III or the desired final product II, is a stereogenic center. Selective formation of II requires that the second alkylation occur preferentially with the opposite sense of relative stereochemistry (vs. III!VII); otherwise, meso-VIII AAA would be generated. That is, II can only be obtained selectively if the chiral catalyst-not the stereogenic centers in VII-dictates the course of the second alkylation.The substitution pattern of the olefins in III poses another challenge. This class of olefins represents a difficult and relatively unexplored set of substrates for catalytic AAA, [5] the first examples of which were only recently reported. [9] Existing disclosures do not, how...

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Section: Methodsmentioning

confidence: 99%

Chiral N‐Heterocyclic Carbenes in Natural Product Synthesis: Application of Ru‐Catalyzed Asymmetric Ring‐Opening/Cross‐Metathesis and Cu‐Catalyzed Allylic Alkylation to Total Synthesis of Baconipyrone C

2007

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“…Spiroketal 4 possesses a sterically small C19 alkyne substituent that would allow for introduction of the C20ÀC24 diene and a C11 hydroxyethyl side chain for the formation of the C1ÀC11 triene. The spiroketal precursor ketone 5 could then be accessed via a cross-metathesis [10] between enone 6 and alkene 7.…”

Section: Introductionmentioning

confidence: 99%

Stereoselective Total Synthesis of (−)‐Spirofungin A by Utilising Hydrogen‐Bond Controlled Spiroketalisation

Lynch

Zanatta

White

et al. 2010

Chemistry A European J

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The stereoselective total synthesis of the spiroketal containing Streptomyces metabolite (-)-spirofungin A (1) is described. A key step involved a spiroketalisation controlled by an intramolecular H-bond which favoured the desired spiroketal 4 (13:1 ratio). The presence of the intramolecular H-bond in 4 is possibly due to a 1,5-alkyne-oxygen interaction. Other key steps include an efficient cross-metathesis to form the spiroketal precursor, a tin mediated syn-aldol reaction and a Stille cross-coupling reaction to create the C22C23 bond. A final Wittig extension followed by deprotection gave (-)-spirofungin A (1).

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“…[2] We have recently reported an efficient four-step synthesis of the potent phytoestrogen 8-prenylnaringenin (2) from commercially available naringenin (1) consisting of chemoselective diacetylation of the C-7 and C-4' phenolic hydroxy groups, prenyl ether formation at C-5 À OH, domino Claisen-Cope rearrangement and deprotection (Scheme 1). [3] Here we disclose a related route that allows a completely regioselective C-6 prenylation of flavonoids belonging to different subgroups of these natural products via europiumA C H T U N G T R E N N U N G (III)-catalyzed Claisen rearrangement [4] followed by cross-metathesis (CM) [5,6] as the key events. [7] Using this approach, the flavanone 6-prenylnaringenin (3) first isolated from Humulus lupulus, [8] the isoflavone 6-prenylgenistein (wighteone, erythrinin B) (4) first isolated from Glycine wightii [9] and Erythrina variegata [10] as well as a protected derivative of the flavonol 6-prenylquercetin (gancaonin P) (5) first isolated from Glycyrrhiza uralensis [11] have been synthesized in short reaction sequences (Figure 1).…”

mentioning

confidence: 98%

Selective C‐6 Prenylation of Flavonoids via Europium(III)‐ Catalyzed Claisen Rearrangement and Cross‐Metathesis

Tischer

Metz

2007

Adv Synth Catal

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Jüngste Entwicklungen bei der gekreuzten Olefinmetathese

Cited by 263 publications

References 185 publications

Chiral N‐Heterocyclic Carbenes in Natural Product Synthesis: Application of Ru‐Catalyzed Asymmetric Ring‐Opening/Cross‐Metathesis and Cu‐Catalyzed Allylic Alkylation to Total Synthesis of Baconipyrone C

Chiral N‐Heterocyclic Carbenes in Natural Product Synthesis: Application of Ru‐Catalyzed Asymmetric Ring‐Opening/Cross‐Metathesis and Cu‐Catalyzed Allylic Alkylation to Total Synthesis of Baconipyrone C

Stereoselective Total Synthesis of (−)‐Spirofungin A by Utilising Hydrogen‐Bond Controlled Spiroketalisation

Selective C‐6 Prenylation of Flavonoids via Europium(III)‐ Catalyzed Claisen Rearrangement and Cross‐Metathesis

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