Riihlmann condensation

Preparation of l,2-bis(trimcthylsilyloxy)cyclobutene from diethyl succinate (Riihlmann condensation using Na and TMSC1), and of 2-hydroxycyclo-Ibutanone. 

Reductive lithiation, 68 Reductive silylation, 26 Reformatsky reaction, 71 Riihlmann condensation, 134,139... 

J. J. Bloomfield, D. C. Owsley and J M. Nelke, Organic Reactions 23, 259 (1976), Note. Includes Riihlmann condensation. 

Use of the trapping agent is recommended as the most efficient method for running acyloin condensations for many reasons. Among them are (a) the work-up is very simple filter and distil (b) the bis-(silyloxy)olefin is usually easier to store than the free acyloin and is readily purified by redistillation (c) unwanted base-catalyzed side reactions during reduction are completely avoided and (d) the bis-(silyloxy)olefin can be easily converted directly into the diketone by treatment with 1 mole of bromine in carbon tetrachloride.Other reactions are described in Riihlmann s review and in Organic Reactions ... 

Whereas condensation of a-hydroxy ketones such as benzoin and acetoin on heating with formamide [240] or ureas in acetic acid [239, 242] to form imidazoles such as 769 or 770 is a well known reaction, only two publications have yet discussed the amination of silylated enediols, prepared by Riihlmann-acyloin condensation of diesters [241], by sodium, in toluene, in the presence of TCS 14 [241, 242]. Thus the silylated acyloins 771 and higher homologues, derived from Riihl-... 

In this section primarily reductions of aldehydes, ketones, and esters with sodium, lithium, and potassium in the presence of TCS 14 are discussed closely related reductions with metals such as Zn, Mg, Mn, Sm, Ti, etc., in the presence of TCS 14 are described in Section 13.2. Treatment of ethyl isobutyrate with sodium in the presence of TCS 14 in toluene affords the O-silylated Riihlmann-acyloin-condensation product 1915, which can be readily desilylated to the free acyloin 1916 [119]. Further reactions of methyl or ethyl 1,2- or 1,4-dicarboxylates are discussed elsewhere [120-122]. The same reaction with trimethylsilyl isobutyrate affords the C,0-silylated alcohol 1917, in 72% yield, which is desilylated to 1918 [123] (Scheme 12.34). Likewise, reduction of the diesters 1919 affords the cyclized O-silylated acyloin products 1920 in high yields, which give on saponification the acyloins 1921 [119]. Whereas electroreduction on a Mg-electrode in the presence of MesSiCl 14 converts esters such as ethyl cyclohexane-carboxylate via 1922 and subsequent saponification into acyloins such as 1923 [124], electroreduction of esters such as ethyl cyclohexylcarboxylate using a Mg-electrode without Me3SiCl 14 yields 1,2-ketones such as 1924 [125] (Scheme 12.34). 

The Riihlmann modification (Bouveault-Blanc Condensation or Riihlmann Reaction) traps the dienolate as a TMS derivative. This protocol generally results in improved yields. 

Preparative Method can be prepared in one step from diethyl oxalate using the Riihlmann version of the acyloin condensation (eq 1). Although a yield of 64% of isolated product was reported, this could not be reproduced in a careful series of experiments. Rather, yields of 21-25% are consistently obtained (0.1-1.0 molar scale). The product consists of a ZJE) isomeric mixture which is of no consequence for synthetic applications. 

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