Synthetic applications of gold-catalyzed ring expansions

Abstract

The development of new methodologies catalyzed by late transition metals involving cycloisomerizations of strained rings can open new venues for the synthesis of structurally complex molecules with interesting biological activities. Herein we summarize, from both a synthetic as well as a mechanistic point of view, the most recent developments in gold-catalyzed ring expansions. 767

Gold(I)-Catalyzed Activation of Alkynes for the Construction of Molecular Complexity

“… 334 The gold(I)-catalyzed reaction of alkynylcyclopropanols and cyclobutanols proceeds through ring expansion to give α-alkylidene cyclobutanones. 335 …”

Adventures and Detours in the Synthesis of Hydropentalenes

“…Both N,N-dimethylhydrazine (112) and the corresponding N-tosylhydrazine were reluctant reactants in the asymmetric deprotonation/alkylation sequence when alkyllithi-um reagents were used for deprotonation. In contrast, deprotonation of 112 with LDA and subsequent electrophilic trapping with methyl, allyl, or TEG iodides provided the desired -substituted ketones 105 after oxidative cleavage of the hydrazone unit with NaIO 4 , albeit in low yields (Scheme 27).…”

“…Gold-catalyzed ring expansions have been successfully employed for a variety of hydropentalene-containing natural products. 112 For example, the Toste group reported an Au-catalyzed ring expansion of cyclopropanol 161, followed by a semipinacol shift, to give the hydropentalene subunit of the angular triquinane ventricos-7(13)-ene (163; Scheme 46). 113 As already mentioned in Section 14, the laborious synthesis of starting materials containing small rings often compromises the efficiency of subsequent catalytic steps.…”

A Hofmann Rearrangement–Ring Expansion Cascade for the Synthesis of 1‐Pyrrolines: Application to the Synthesis of 2,3‐Dihydro‐1H‐pyrrolo[2,1‐a]isoquinolinium Salts

“…The ring expansion reaction of small and strained molecules has attracted the interest of organic chemists for decades 1. However, unlike the ring expansion of three‐membered rings which have been well studied and widely applied in natural products synthesis,2 ring expansion reaction of four‐membered rings remains relatively underutilized 2d,e. 3…”