Calcium‐katalysierte Cyclopropanierung

Calcium‐Catalyzed Formal [2+2+2] Cycloaddition

“…[6] In this transformation bond rotations are precluded, due to concertedness, and full transposition of the olefin geometry in the cyclization precursor into the relative configuration of the product was observed. The analysis of the origin of the high diastereoselectivity of the herein presented formal [2 + 2 + 2] cycloaddition hence started with the reaction of mechanistic probes 7 with 2 different substituents at the alkene moiety in either an E-(7 a) or a Z-configuration (7 b).…”

“…We were pleased to find that generation of the reactive intermediate C in presence of five equivalents of the aldehyde under the reaction conditions of the cyclopropanation reaction (Table 1, entry 1) readily yields the desired [2 + 2 + 2]-cycloaddition product 3 as a single diastereomer after only 30 minutes reaction time. Further optimization studies revealed that both, Bu 4 NPF 6 and Bu 4 NSbF 6 , are efficient additives for this reaction (Table 1, entry 1/3). In analogy to the cyclopropanation reaction even subtle changes in the dielectric properties of the solvent largely influenced the outcome of the reaction (Table 1, entry 4-6).…”

“…[6] Therefore, in pursuit of our program aiming at the development of carbocation cascade reactions for the generation of molecular complexity with simple non-transition metal catalysts, we reasoned that, following the same reaction pathway as the parent gold catalyzed reaction, trapping the intermediate C with carbonyl compounds will provide access to highly substituted formal [2 + 2 + 2] cycloaddition products.…”

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

“…Although ar elated calcium-catalyzed tandem indole annulation and cyclopropanation involving multiple cationic species was developed by Niggemann and co-workers,o nly intramolecular cyclcopropanation was reported by tethering the alkene to the propargylic alcohol. [9] Indole is one of the most abundant bioactive heterocycles in natural products and pharmaceutical agents. [10] Most previous indole syntheses focused on the construction of indole ring alone.…”

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

“…Thestructure of 8f [22] was unambiguously determined by X-ray analysis. Tr isubstituted cyclopropanes (8h-j)w ere successfully prepared from 1,1-disubstituted alkenes (entries [8][9][10]. Notably, the free alcohols in 7i and 7j were tolerated under the standard reaction conditions.H eterocycles such as indole could also be introduced to the diaryl cyclopropane products (entry 11).…”