Ketene Experimental Procedure

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Keywords cis-1,2-dibenzoylalkene, photorearrangement, keten, enolether Experimental procedures ... 

A critical discussion of methods for the preparation of ketenes and ketene dimers including many experimental procedures has been elegantly presented, For the most part, the methods are modifications of those used for the synthesis of olefins. Ketenes are very reactive substances and are prepared for immediate consumption. The simplest member, ketene, reacts with various groups which contain hydrogen to form acetyl derivatives. Even t-butyl alcohol reacts readily to form t-butyl acetate when a small amount of sulfuric acid is present as a catalyst. ... 

Transfer of a formylcarbene unit from an a-diazo aldehyde to an alkene has never become popular. The copper-catalyzed cyclopropanation with diazoacetaldehyde to give 5 occurred in low yield, since ketene formation is competitive (for experimental procedure, see Houben-Weyl Vol.E19b, pi224). It appears that with other catalysts and at a lower temperature the cyclopropanation could be more effective. 

This method is an extension of the known ketene—imine procedure, which has been in use for -lactam formation since the beginning of the century [51]. Examples of this acid chloride - ketene route up to the end of 1971 have been reviewed by Mukerjee and Srivastava [52]. It is interesting to note that, depending upon the experimental conditions, both the concerted 1,2-cycloaddition path and another involving an acylation step and salt formation can be operative [53,54], The former is supported by the formation of by-products of dioxo-piperidine type [55,56]. Use of this procedure has provided a number of azaoctams of type (92) [46]. 

The isomeric 2-deoxy-2-C-methyl-pentonic acids (34) have been prepared by Lewis acid-catalysed aldol condensation of the thioester silyl ketene acetal (35) with 2,3-di-O-benzyl-D-glyceraldehyde either epimer can be obtained as the major product depending on the precise experimental procedure adopted. 

Recently, the enolate method has emerged as a widely and frequently employed method in the synthesis for reasons like the easy access to the starting materials, simplicity and mildness of the experimental procedures, accurate predictability of the stereochemistry of the products and good to excellent realization of the stereoselection. This method can be further sub-divided into sub-topics like the simple enolate method [11], the Ireland silyl ketene acetal method, the chelated ester enolate method, imidate method, and the N,0- and N,S-acetal method. The present discussion will be restricted to simple enolate method as the remaining sub-topics are covered separately in this book. 

Meerwein-Eschenmoser-Claisen reactions proceed through the intermediacy of ketene N,0-acetals, which usually cannot be isolated. The following paragraphs give an overview of the various synthetic routes leading to these intermediates and include representative experimental procedures. 

The various ketene dimers which have been reported are listed in Table VII. No attempt has been made to give all the references to the preparation of a particular dimer only those references which, in the authors opinion, give the best preparative methods are listed. Experimental procedures for preparing ketene dimers are described on pages f 137 and 140. 

Since sUyl ynol ethers have an electron-rich triple bond, they are useful for Lewis acid catalyzed synthetic reactions. Lithium ynolates 175 are silylated by TIPSCl or TIPSOTf and TBSCl to afford the corresponding silyl ynol ethers 176 and 177, which are thermally stable and isolable, but sensitive toward acids (equation 71) . See also equations 9 and 10 in Section ll.C. An experimentally improved procedure for the purification of 176 derived from Kowalski s method is described. Lithium ynolate derived from Julia s method is also used for the preparation of 176. TMSCl and TESCl provide silyl ketenes 179, however, by C-silylation. These small silyl chlorides primarily gave the silyl ynol ethers 178, but, upon warming the reaction mixture, isomerization to the more stable silyl ketenes takes place. The soft electrophilic silyl chlorides like PhsSiCl afford silyl ketenes. Disi-lyl ynol ethers, prepared from ynolate dianions, are rearranged to disilylketenes mediated by salts . 

---