Copper‐Catalyzed Cyclization–Fragmentations of Enynols

Abstract

Pt and Au are going out of fashion: The [Cu(CH3CN)4][BF4] species effectively catalyzes the cyclization–fragmentation of 5‐en‐1‐yn‐3‐ols. The reactivity of 6‐en‐1‐yn‐4‐ols greatly depends on the catalyst used (see scheme). An unusual fragmentation and metathesis reactions of 6‐en‐1‐yn‐4‐ols have also been detected.

Platinum(II) Catalyzed Enantioselective Cycloisomerizations of 3‐Hydroxylated 1,5‐Enynes

“…This mechanistic hypothesis implies that the stereogenic carbon of the substrate will be lost in step (c) only, and therefore its configuration might affect the stereochemical course of the reaction at the stereodetermining cyclopropanation step (b). The postulated mechanism has been supported by Fehr’s experiments showing that a degree of chirality transfer takes place in the cycloisomerization of an enantiomerically enriched substrate: a hydroxylated 1,5‐enyne with 70 % ee was converted into the corresponding bicyclo[3.1.0]hexanone in 37 % ee , under PtCl 2 catalysis 8g. These results also suggest that the configuration of the stereogenic center of the substrate does not simply translate into the final product 7a.…”

Copper-Catalyzed Cycloisomerizations of Enynols and Their Esters

“…To our surprise, both diastereomers 60 and 61 overwhelmingly led to the rearranged dienals 62 and 63 (>94%), whereas the expected cyclopropanation-1,2-H shift reaction, leading to 64, represented only a very minor pathway (0-6%; Scheme 17). 15 Evidently, the intermediate 65 (or its diastereomer) can also undergo a Grob-type fragmentation, followed by protodemetalation and C-C double-bond isomerization (see also Scheme 1). The cyclopropanation route could, however, be favored to some extent by the use of PtCl 2 .…”