Direct Transformation of Silyl Enol Ethers into Functionalized Allenes

Abstract

The first elimination reactions of silyl enol ethers to lithiated allenes are reported. These reactions allow a direct transformation of readily available silyl enol ethers into functionalized allenes. The action of three to four equivalents of lithium diisopropylamide (LDA) on silyl enol ethers results in the formation of lithiated allenes by initial allylic lithiation, subsequent elimination of a lithium silanolate, and finally, lithiation of the allene thus formed. Starting with amide-derived silyl imino ethers, lithiated ketenimines are obtained. A variety of reactions of the lithiated allenes with electrophiles (chlorosilanes, trimethylchlorostannane, dimethyl sulfate and ethanol) were carried out. Elimination of silanolate is observed only for substrates that contain the hindered SiMe2tBu or Si(iPr)3 moiety, but not for the SiMe3 group. The reaction of 1,1-dilithio-3,3-diphenylallene with ketones provides a convenient access to novel 1,1-di(hydroxymethyl)allenes which undergo a domino Nazarov-Friedel-Crafts reaction upon treatment with p-toluenesulfonic acid.

Recent developments in homobimetallic reagents and catalysts for organic synthesis

“…Langer and co-workers [6] carried out the first direct transformation of silyl enol ethers to the geminal lithiated allene 1 (Scheme 1), which was trapped by a ketone to give the corresponding diol 2. A subsequent domino reaction with nitriles gives the corresponding dihydropyrimidine 3 [7].…”

Efficient Synthesis of Nitrogen Heterocycles by Cyclization of Bis(nucleophiles) with Oxaldiimidoyl Dichlorides

“…Cyclization of phenylacetonitrile and 3d (reaction type 1); acetonitrile was used for the UV/Vis measurement Cyclizations of dilithiated arylmethyl cyanides and sulfones 90 with oxaldiimidoyl dichlorides 3 enabled a great variety of 2-alkylidene-3-iminoindoles 92 to be synthesized efficiently (Scheme 25). [37,38] Substituents could be introduced both at the indole moiety and at the exocyclic double bond by variation of the substituents on 3 and 90, respectively. The cyclizations proceeded with very good (E) diastereoselectivities for products with sterically distinct substituents R 1 and R 2 .…”

“…Amide-substituted indoles could be prepared with high (E) diastereoselectivity by treatment of 3 with monolithiated carboxylic amides. [38] Scheme 25. Scope of cyclizations of lithiated cyanides, sulfones, and amides with oxaldiimidoyl dichlorides (reaction type 1); R 1 ϭ Ar, SO 2 Ph, SiMe 3 ; R 2 ϭ CN, Ph, CONR 2 ; R 3 , R 4 , R 5 ϭ H, Me, OMe; for all products, (E)/(Z) Ͼ 98:2, except R 1 ϭ SO 2 Ph, R 2 ϭ Ph, (E)/ (Z) ϭ 2:1 To extend the preparative scope of our methodology further, we systematically studied the reaction conditions and eventually developed a stepwise deprotonation procedure using sodium hydride (Scheme 26).…”

One-Pot Synthesis of Functionalized Indoles by Cyclization of Lithiated Amides and Nitriles with Oxaldiimidoyl Dichlorides

“…The structure of 3-iminoindole 5a was confirmed independently by crystal structure analysis. [8] To study the preparative scope of the cyclization reaction, the substituents of the amide and the bis(imidoyl)chloride were varied systematically (Scheme 3, Table 3 (imidoyl)chlorides containing phenyl, 4-tolyl, 2-tolyl, 4-methoxyphenyl, and 2-methoxyphenyl groups afforded the 2-alkylidene-3-iminoindoles 5aϪf in acceptable yields and with very good stereoselectivities. Variation of the amide was studied next.…”