Asymmetric (4+3) Cycloadditions of Enantiomerically Enriched Epoxy Enolsilanes

Abstract

Chiral bleibt chiral: Die intermolekulare (4+3)‐Cycloaddition enantiomerenangereicherter Epoxyenolsilane führt zu Cycloaddukten mit bis zu 99 % ee; demnach sollte die Reaktion nicht über ein achirales Oxyallylkation als Intermediat verlaufen, sondern über ein transient chirales Elektrophil, das die stereochemische Information des Epoxids bewahrt (siehe Schema; TES=Triethylsilyl, Tf=Trifluormethansulfonyl).

Palladium‐Catalyzed Asymmetric [4+2] Cycloaddition of 2‐Methylidenetrimethylene Carbonate with Alkenes: Access to Chiral Tetrahydropyran‐Fused Spirocyclic Scaffolds

“…[1] Developing concise approaches to synthesize spirocyclic molecules asymmetrically using readily available and inexpensive starting materials has attracted much attention. [2] As a reliable and powerful tool for the convergent construction of diverse carbo-and heterocycles, [3] Pd-catalyzed cycloaddition reactions involving zwitterionic allylpalladium intermediates are extremely valuable in the construction of spirocyclic scaffolds. Among various zwitterionic allylpalladium intermediates, the zwitterionic intermediates from g-methylidene-d-valerolactones (GMDVs) first documented as a new type of reactive species by Shintani, Hayashi, and co-workers served as a four-carbon unit in Pd-catalyzed asymmetric [4+3] cycloaddition with nitrones [4] and [4+2] cycloaddition of aryl isocyanates [5] (Scheme 1 a).…”

“…Using the optimal reaction conditions, the scope with respect to unsaturated pyrazolones 2 in the Pd-catalyzed [4+2] cycloaddition was investigated for the asymmetric synthesis of spiropyrazolone derivatives 3 ( Table 2). Alkenes bearing electron-donating or -withdrawing groups at the ortho, meta, and para positions of the aromatic R 1 group were converted into the expected products 3 aa-3 an in high yields with excellent enantioselectivity (80-99 % yield, 91-99 % ee, entries [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The generality of the cycloaddition reaction was further extended by incorporating unsaturated pyrazo-…”