Construction of Highly‐Functionalized Cyclopentanes from Silyl Enol Ethers and Activated Cyclopropanes by [3+2] Cycloaddition Catalyzed by Triflic Imide

Abstract

[1] Donor-acceptor (D-A) substituted cyclopropanes [2,3] are utilized as equivalents of three carbon 1,3-dipole unit for the formation of 5-membered carbocyles. Kuwajima and his co-workers reported that [3 + 2] cycloadditions of D-A cyclopropanes, such as 2-alkoxy-and 2-phenylthiocyclopropyl carbonyl compounds, with silyl enol ethers were promoted by SnCl 4 [4] and Me 2 AlCl, [5] respectively, to furnish multi-functionalized cyclopentanes. Although several such [3 + 2] cycloadditions have been documented using various substances after their findings, [2,6,7] most of them require a stoichiometric amount of Lewis acid, except for a few exceptions. [4,8] As a part of our ongoing research program on catalytic [2 + 2] cycloaddition of silyl enol ethers to construct highly-substituted cyclobutanes, [9] we recently found that triflic imide (Tf 2 NH) [10] efficiently catalyzes the [2 + 2] cycloaddition.[11] We have demonstrated that silyl triflic imide (R 3 SiNTf 2 ), [12] which is generated in situ from Tf 2 NH with silyl enol ethers, acts as the actual catalyst. We expected that Tf 2 NH would be a new possibility for good catalysis of At the outset of this study, the [3 + 2] cycloaddition of TBS enol ether (1a) with trans-ethyl 2-butoxycyclopropanecarboxylate (trans-2a) [4] as D-A cyclopropane was examined using several fluorinated Brønsted acid catalysts (Table 1, entries 1-4). Treatment of trans-2a with TBS enol ether 1a in the presence of Tf 2 NH (1 mol %) in CH 2 Cl 2 at À78 8C for 2 h afforded the highly-substituted cyclopentane product 3a in 69 % yield as a mixture of four diastereomers (entry 1). [13] In contrast, the reaction using a stoichiometric amount of Tf 2 NH resulted in only a trace yield of 3a and decomposition of 1a (entry 2). Imide 4, [14] which is the cyclic analogue of Tf 2 NH, also acts as a good catalyst (entry 4), whereas triflic acid (TfOH) does not promote the cycloaddition (entry 3).As shown in Table 1 (entries 5-9), [3 + 2] cycloaddition of 1 with trans-2a proceeded smoothly by addition of a catalytic amount (1 mol %) of Tf 2 NH to afford highly-functionalized cyclopentanes 3 in good yield. In contrast, the reaction of TMS enol ether 1b afforded 3b in 9 % yield, along with desilylated product in 60 % yield (entry 5). We have examined the 2376

Stronger Brønsted Acids: Recent Progress

“…73 Takasu also reported a Tf 2 NH-catalyzed domino reaction. Upon treatment of 3-oxymethyl-2-siloxy-1,3-butadienes 40, which are prepared from Baylis−Hillman adducts, and α,β-unsaturated ketones 39 with Tf 2 NH, [4 + 2] cycloaddition and elimination took place smoothly to furnish substituted 2-alkylidenecyclohexanones 41 in good yields (Scheme 12).…”

Synthesis and Reactivity of Donor-Acceptor Substituted Aminocyclopropanes and Aminocyclobutanes

“…In 2006, Ihara and co-workers exploited the ability of triflic imide to generate a powerful Lewis acid [69] in presence of silyl enol ethers (Scheme 1.14) [70]. The group used this catalyst to promote the [3 + 2] annulation of DA cyclopropanes with silyl enol ethers.…”

Glyco‐pseudopolyrotaxanes: Carbohydrate Wheels Threaded on a Polymer String and Their Inhibition of Bacterial Adhesion

“…The reaction affords tricyclic ketone 63 , which corresponds to a sterprane sesquiterpenoid skeleton. A similar auto‐tandem catalysis has been also developed with the Tf 2 NH‐catalyzed domino [3+2]–[4+2] cycloaddition to give bicyclo[4.3.0]nonanes 39…”