Alkinmetathese im Aufwind

Abstract

Die frühe Entwicklung der Alkinmetathese war nicht frei von einer gewissen Paradoxie: Zwar konnten der vorgeschlagene Mechanismus der Reaktion schnell verifiziert und mehrere effektive Katalysatoren bereitgestellt werden, doch blieben die Auswirkungen auf die Synthesechemie für viele Jahre äußerst bescheiden. In jüngster Zeit scheinen die Dinge jedoch in Bewegung zu geraten: Die außerordentliche Aktivität und Verlässlichkeit der letzten Generation an Alkinmetathese Katalysatoren, im Verein mit ihrer hervorragenden Toleranz gegenüber einer Vielzahl funktioneller Gruppen, machen neuartige und wesentlich anspruchsvollere Anwendungen möglich. Werden die so zugänglichen (cyclischen) Alkine anschließend geschickt modifiziert, lässt sich ein breites Spektrum an Strukturmotiven aufbauen. Da zugleich einige der hoch aktiven und selektiven Katalysatoren ohne Schlenk‐Technik an Luft gehandhabt und gelagert werden können, sollte nunmehr weitergehenden Anwendungen dieser vielseitigen Reaktion kaum etwas im Wege stehen.

Divergent Total Synthesis of the Antimitotic Agent Leiodermatolide

“…[5] However, the segregated stereoclusters within the macrolac-tone [10,11] and the d-lactone terminus could not be correlated with each other, thus leaving it open as to whether structure 1 or 2 represents the natural product (Scheme 1). [10] Provided that A can be forged by ring-closing alkyne metathesis (RCAM) as envisaged, [13,14] the macrocyclization and the formation of the Z,Z diene become advantageously aligned. Moreover, Maier and co-workers cautiously questioned whether the unusual axial attachment of the side chain onto the d-lactone at the tertiary alcohol site C21 had been correctly assigned by the isolation team.…”

“…This outcome was independent of whether the reaction was performed at ambient temperature or at 100 8C. [13] Since the alkenyl iodide had survived the metathesis step uncompromised, the stage was set for the attachment of the side chain by a Suzuki-Miyaura reaction with boronate 28. [36] The transformation required harsh conditions, a high catalyst loading, and an unusually long reaction time; this fact likely reflects the challenge derived-collectively-from the strain of the incipient polyunsaturated ring, the severe steric hindrance about the reacting propargylic site, [17] and the electronic toll to be paid for the enyne motif.…”

Katalyse und Totalsynthese: eine persönliche Zwischenbilanz

“…[102,103] Abbildung 3. Vollständig charakterisierte Eisenkomplexe, die die mçgli-chen Zwischenstufen des in Schema 5 skizzierten Katalysekreislaufs nachbilden; Farbcode: Fe = rot, Li = grün, N = blau, Cl = orange.…”

ChemInform Abstract: A trans‐Selective Hydroboration of Internal Alkynes.

“…Thus, terminal alkynes were recently shown to lead to a net anti addition when reacted with catecholborane (cat-BH) or pinacolborane (pin-BH) in the presence of certain rhodium, iridium, or ruthenium complexes as the catalysts (Scheme 2). Thus, the reaction of cycloalkyne 1 [16,17] with the 9-H-BBN dimer [2,3] in the presence of [Cp*Ru(MeCN) 3 ]PF 6 (3; 5 mol %) gave a rather complex product mixture (Table 1, entry 1). As a consequence, it is the alkyne proton itself, rather than the hydrogen from the borane reagent, that ends up anti to the boron moiety in the product, as unequivocally shown by labeling studies.…”

“…Inspiration was provided by pioneering studies on trans-selective hydrosilylations (and -germylations) [12][13][14] and by a recent report on a trans-selective hydrogenation, [15] which rely on the use of cationic ruthenium complexes as precatalysts. Thus, the reaction of cycloalkyne 1 [16,17] with the 9-H-BBN dimer [2,3] in the presence of [Cp*Ru(MeCN) 3 ]PF 6 (3; 5 mol %) gave a rather complex product mixture (Table 1, entry 1). Thus, the reaction of cycloalkyne 1 [16,17] with the 9-H-BBN dimer [2,3] in the presence of [Cp*Ru(MeCN) 3 ]PF 6 (3; 5 mol %) gave a rather complex product mixture (Table 1, entry 1).…”