Stereoselective cis-Vinylcyclopropanation via a Gold(I)-Catalyzed Retro-Buchner Reaction under Mild Conditions

Abstract

A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia–Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones. On the basis of mechanistic investigations, a stereochemical model for the cis selectivity was proposed. An unprecedented gold-catalyzed isomerization of cis- to trans-cyclopropanes has also been discovered and studied by DFT calculations.

Generation of Gold(I) Carbenes by Retro‐Buchner Reaction: From Cyclopropanes to Natural Products Synthesis

“…Although some 7‐styryl‐1,3,5‐cycloheptatrienes can be prepared using tropylium tetrafluoroborate as shown in Scheme , a more reliable and general method is based on the use of the Julia‐Kocienski olefination employing sulfone 39 (Scheme ) …”

“…Sulfone 39 can be prepared in multi‐gram scale and stored indefinitely. Treatment of this reagent with LiHMDS followed by the addition of commercially available aldehydes or ketones allows the synthesis of a wide range of 7‐vinyl and 7‐styryl‐1,3,5‐cycloheptatrienes, which can be also employed to generate gold(I) carbenes by retro‐Buchner reaction (Table ) …”

“…In the presence of 5 mol % of a cationic gold complex such as [( t BuXPhos)Au(MeCN)]SbF 6 , these substrates undergo a retro‐Buchner reaction at 75 °C releasing benzene and generating vinyl or styryl gold(I) carbenes that can cyclopropanate styrenes and other related activated alkenes, such as indenes, chromenes, or N ‐vinylphthalimides. This reaction generally proceeds efficiently and with good diastereocontrol, favoring the formation of the corresponding cis ‐vinylcyclopropanes (Table ) …”

Gold(I)‐Catalyzed Synthesis of Indenes and Cyclopentadienes: Access to (±)‐Laurokamurene B and the Skeletons of the Cycloaurenones and Dysiherbols

“…[15] (AE)-Laurokamurene B( 10)w as readily synthesized in good yield by simple hydrogenation of cyclopentadiene 7h in the presence of the Wilkinson catalyst (Scheme 3). Considering that (E)-7-(4-methylstyryl)cyclohepta-1,3,5-triene (2b), the cycloheptatriene required for the preparation of 7h,c an be readily obtained by the treatement of potassium (E)-(4-methylstyryl)trifluoroborate with tropylium tetrafluoroborate in almost quantitative yield, [3] this total synthesis requires just three steps and provides (AE)-10 in 39 %overall yield from commercially available starting materials,w hich compares favorably with previous syntheses of (AE)-10. [17] Cycloaurenones A-C (8a-c)feature a cis-decalin moiety, whereas the dysiherbols (9a-c)s how trans fusion of the A/B rings ( Figure 1).…”

“…Wep reviously found that cationic gold(I) complexes promote the retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes,s uch as 1 and 2,v ia their norcaradiene tautomers,leading to reactive metal carbenes [1] [LAu = CHR] + 3 and 4 (Scheme 1), which react with alkenes to give cyclopropanes [2,3] or undergo intramolecular Friedel-Craftstype reactions. [4] Cyclopentenes were obtained by the formal (4+ +1) cycloaddition of gold(I) carbenes 3 with cyclobutenes or methylenecyclopropanes.…”