Efficient Access to All‐Carbon Quaternary and Tertiary α‐Functionalized Homoallyl‐type Aldehydes from Ketones

Abstract

b,g-Unsaturated aldehydes with all-carbon quaternary or tertiary a-centers were rapidly assembled from ketones through au nique synthetic operation consisting of 1) C 1 homologation, 2) Lewis acid mediated epoxide-aldehyde isomerization, and 3) electrophilic trapping.T he synthetic equivalence of avinyl oxirane and a b,g-unsaturated aldehyde is the key concept of this previously undisclosed tactic. Mechanistic studies and labeling experiments suggest that an aldehyde enolate is ac rucial intermediate.T he homologating carbenoid formation playsacritical role in determining the chemoselectivity.Aldehydes with all-carbon a-quaternary centers constitute an important motif across the chemical sciences. [1] From ar eactivity perspective,t he high electrophilicity of the aldehyde moiety represents an excellent tool for constructing sterically hindered quaternary centers.I nt his context, the a-allylation of aldehydes to generate the corresponding homoallyl derivatives has been extensively investigated (Scheme 1A1). Thep alladium-catalyzed (Tsuji-Trost) approach reported by Tamaru and co-workers in 2001 can be considered as abreakthrough in this field. [2] Fine-tuning of the reaction conditions enabled the development of highly enantioselective variants by the groups of List [3] and Yoshida, [4] which complement asymmetric organocatalytic (Jacobsen) [5] and stereodivergent dual-catalytic (Carreira) [6] approaches.T he process also benefits from switching to nickel catalysis,a sr ecently disclosed by Sauthier and coworkers. [7] Interestingly,f or simple linear aldehydes,t his reaction could be incorporated into at andem aldol condensation/allylation process.The development of these strategies also allowed overcoming the reluctance of the conceptually simplest strategy based on the use of aldehyde enolates as nucleophiles in alkylation chemistry. [8] Amajor advancement in the field was reported in 2016 by Evans and Wright, who developed an enantioselective rhodium-catalyzed allylation of prochiral a,a-disubstituted aldehyde enolates with allyl benzoate (Scheme 1A2). [9] Furthermore,t he selective ring fragmentation of diastereomerically pure and enantioen-riched cyclopropanols represents av ersatile process for synthesizing acyclic n-butenals with all-carbon quaternary centers,a ss hown in elegant work by Marek and co-workers (Scheme 1A3). [10] However,t he high efficiency of these methods in terms of their enantioselectivity or general applicability is counterbalanced by the limited availability of a,b-unsaturated and a-quaternary b,g-unsaturated aldehydes,a nd there are hardly any methods for the synthesis of differently substituted aldehydes with all-carbon a-quaternary centers.P rior to Evans work, [9] the a-derivatization of aldehydes by enolate-type chemistryalbeit limited to the synthesis of a-tertiary species-has been achieved by Hodgson and co-workers [11] by epoxide-aldehyde isomerization (Scheme 1B1). [12] Accordingly,the treatment of am onosubstituted epoxide with ah indered lithium amide base provided an ucle...

Progress in catalytic asymmetric α-functionalization of vinylogous nucleophilic species

“…In 2017, Pace and coworkers put forward a concept that a vinyl oxirane and a β,γ-unsaturated aldehyde could be regarded as synthetic equivalents. 48 In the presence of a Lewis acid catalyst, vinyl oxirane can transform into β,γ-unsaturated aldehyde that reacts with an electrophile to achieve the synthesis of functionalized aldehyde. 49 In this context, Feng and Lin disclosed an α-regioselective asymmetric Mannich reaction of in situ -formed β,γ-unsaturated aldehyde (Scheme 36).…”

Sterically Hindered and Deconjugative α‐Regioselective Asymmetric Mannich Reaction of Meinwald Rearrangement‐Intermediate

“…Meinwald rearrangement of epoxides has found diverse applications in the construction of valuable aldehydes and ketones, especially with α‐quaternary stereocenters [9] . As one of them, the Meinwald rearrangement of vinyl epoxides [10, 11] can generate β,γ‐unsaturated aldehydes, which can't be isolated from the system due to their easy isomerization to inert α,β‐unsaturated aldehydes, but can form active dienolates after enolization [10a, 12–14] . Several γ ‐ selective reactions to synthesize conjugate α,β‐unsaturated aldehydes, [12] and one α‐selective reaction to haloalkane [14] to synthesize non‐asymmetric α‐quaternary allyl aldehydes have been reported.…”

“…As one of them, the Meinwald rearrangement of vinyl epoxides [10, 11] can generate β,γ‐unsaturated aldehydes, which can't be isolated from the system due to their easy isomerization to inert α,β‐unsaturated aldehydes, but can form active dienolates after enolization [10a, 12–14] . Several γ ‐ selective reactions to synthesize conjugate α,β‐unsaturated aldehydes, [12] and one α‐selective reaction to haloalkane [14] to synthesize non‐asymmetric α‐quaternary allyl aldehydes have been reported. Recently, the Feng group accomplished the asymmetric γ‐selective addition to satins, 2‐alkenoylpyridines and methyleneindolinones using chiral N , N ′‐dioxide‐metal catalysts, [13a] while the Deng group applied Sc/Ir dual catalysts to realize the γ‐allylation with π‐allyl Ir III species [13b] .…”