Amethod for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles,and the reaction is catalyzed by copper(I) salts.E nantioenriched benzylic ammonium salts react stereospecifically through an S N 2-type displacement of the ammonium group to afford achiral silanes with inversion of the configuration. Ac yclopropyl-substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate.
The power of cation-initiated cyclizations of polyenes for the synthesis of polycyclic terpenoids cannot be overstated. However, a major limitation is the intolerance of many relevant reaction conditions toward the inclusion in the substrate of polar functionality, particularly in unprotected form. Radical polycyclizations are important alternatives to bioinspired cationic variants, in part owing to the range of possible initiation strategies, and in part for the functional group tolerance of radical reactions. In this article, we demonstrate that Co-catalyzed MHAT-initiated radical bicyclizations are not only tolerant of oxidation at virtually every position in the substrate, oftentimes in unprotected form, but these functional groups can also contribute to high levels of stereochemical control in these complexity-generating transformations. Specifically, we show the effects of protected or unprotected hydroxy groups at six different positions and their impact on stereoselectivity. Further, we show how multiply oxidized substrates perform in these reactions, and finally, we document the utility of these reactions in the synthesis of three aromatic abietane diterpenoids.
We report the total synthesis of the furanobutenolide-derived diterpenoid (+)-ineleganolide. The synthetic approach relies on a convergent strategy based on the coupling of two enantioenriched fragments, which are derived from (−)-linalool and (+)-norcarvone, respectively. A high-yielding, one-step Michael addition and aldol cascade furnishes a pentacyclic framework as a single diastereomer, thereby overcoming previous challenges in controlling stereochemistry. The endgame features an O 2 -facilitated C−H oxidation and a samarium diiodide-induced semipinacol rearrangement to furnish the highly rigid central seven-membered ring.
The synthesis of a helically chiral carbenium ion is reported. The new motif is essentially a trityl cation embedded into a [7]helicene-like framework. The key step of its preparation establishes the π-extended fluorenone system in one step by an unprecedented palladium-catalyzed carbonylative annulation of a 4,4’-biphenanthryl-3,3’-diyl precursor. The racemic form of the new carbon Lewis acid was found to catalyze a representative set of reactions typically promoted by the trityl cation.
volligen Auflosen gleicher Mengen M,. In NaC103-Losungen, die mit Na,SO, verunreinigt sind, verzogert sich die Auflosung gegenuber den nicht verunreinigten Losungen, die Auflosefunktion behalt aber ihren geradlinigen Verlauf: erst bei einer 5proz. Na,S04-Losung als Losungsmittel und hohen Vorkonzentrationen der NaC10,-Losung weicht die Auflosefunktion vom gradlinigen Verlauf ab, so daR sich die Zeiten bis zur endgultigen Auflosung betrachtlich erhohen. Aus einem Vergleich der Geschwindigkeitskoeffizienten in den reinen una den verunreinigten Losungen lieB sich die Grenzflachen-Diffusionsgeschwindigkeit von den Kristallabbaustellen zu den Stellen berechnen, an denen die Verunreinigung adsorbiert ist. D a m miissen Annahmen iiber die Zahl der Kristallabbaustellen pro cm2 und die Grenzflache einer Abbau-*) Leiter: 0. Prof. Dr.-Ing. W. Mialki. ") Symbolverzeichnis am SchluB der Arbeit.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.