Ketones enolizable

Me3Si)2NH, Me3SiCl, Pyr, 20°, 5 min, 100% yield. ROH is a carbohydrate. Hexamethyldisilazane (HMDS) is one of the most common sily-lating agents and readily silylates alcohols, acids, amines, thiols, phenols, hydroxamic acids, amides, thioamides, sulfonamides, phosphoric amides, phosphites, hydrazines, and enolizable ketones. It works best in the presence of a catalyst such as X-NH-Y, where at least one of the group X or Y is electron-withdrawing. ... 

The in situ cyanosilylation of p-an1saldehyde is only one example of the reaction which can be applied to aldehydes and ketones in general. - The simplicity of this one-pot procedure coupled with the use of inexpensive reagents are important advantages over previous methods. The silylated cyanohydrins shown in the Table were prepared under conditions similar to those described here. Enolizable ketones and aldehydes have a tendency to produce silyl enol ethers as by-products in addition to the desired cyanohydrins. The... 

The present procedure illustrates the facile demethylation of methyl aryl ethers by lithium diphenylphosphide.3 This reaction is specific for methyl ethers and may be carried out in the presence of ethyl ethers in high yield.4 Use of excess reagent allows cleavage in the presence of enolizable ketones.5 In the present case, the cleavage may be performed without protection of the aldehyde, but two equivalents of reagent are required, and the yield is reduced to ca. 60%. 

Note. This sequence succeeds with both aldehydes and ketones, but fails with readily enolizable ketones and enones. 

Lucchi and coworkers68 have found a novel addition reaction of enolizable ketones to 1, l-bis(phenylsulfonyl)ethylene (102) under neutral conditions (equations 82 and 83). 

Oxazoles have attracted considerable interest due their presence as subunits of several biologically active compoimds or as rigid mimetics of a peptidic ring. A first synthesis of 2-phenyl-4,5-substituted oxazoles 54 [47] was described by microwave-assisted reaction of enolizable ketones with benzoni-trile in the presence of mercury(II) p-toluenesulfonate (Scheme 17). 

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

N,N-Bis(trimethylsilyl)carbodiimide 328, which is readily accessible in 81% yield on silylation of cyanamide with TCS 14/triethylamine [124] and which is apparently in equilibrium with N,N-bis(trimefhylsilyl)cyanamide 553, reacts readily with non-enolizable ketones such as 554 or 2,5-dimethyl-p-quinone in the presence of CsF or TiCL., probably via 553, to N-cyanoimines such as 555 or 556, in 47 and 89% yield, respectively, and HMDSO 7 [125, 126] whereas the enohzable ketone... 

Wolff rearrangements were also observed when most of the same acylsi-lyldiazoalkanes were photolyzed in acetone instead of benzene.21 The ketenes 185 resulting from a 1,3-methyl migration of the silene were detected in addition to the expected ene product 186 derived from the reaction of the silene with acetone (or other enolizable ketones) (Eq. 58). When R = Ad, only the cyclic siloxatene 187 was formed under the same... 

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

The yellow-green chemiluminescence of firefly luciferin is evidently dependent on the enol form of the thiazolinone 109a, for 5.5-dimethyl-luciferin 116a which does not yield an enolizable ketone does not exhibit a yellow-greenish emission on addition of excess base only red emission is observed. 

The oxidative carbonylation reaction of enolizable ketones follows the general routes already illustrated for simple alkenes. Thus, a-methoxycarbonyl-ation may occur either by addition of a Cl - Pd - C02Me species to the enolic... 

When bromination of the enolizable ketone is conducted in glycol, simultaneous monobromination and acetalization is observed [2]. 1,5-Dicarbonyl compounds react with benzyltrimethylammonium dichloroiodate to yield initially a monochloro derivative, which undergoes ring closure to produce a 2-substituted 5-acylfuran [26],... 

Selected examples of the bromination of enolizable ketones R COCLLR2... 

Trimethylsilylation of enolizable ketones using ethyl trimethylsilylacetate ... 

Base-induced cleavage of non-enolizable ketones leading to carboxylic amide derivative and a neutral fragment in which the carbonyl group is replaced by a hydrogen. 

Noteworthy features of these reactions are the high predominance of E stereochemistry in the case of aldehydes and the good results obtained even with highly enolizable ketones (such as cyclopentanone), a, -epoxy ketones (isophorone oxide) and a,j8-unsaturated compounds (benzalacetophenone and cinnamaldehyde). 

Scheme 41 Propargylic substitution reactions with enolizable ketones...

This is a fairly general reaction, and requires an amine plus an aldehyde (usually, but not necessarily, formaldehyde) together with an enolizable ketone, which together generate a P-aminoketone via an iminium system. The Mannich reaction is surprisingly... 

In addition, iodine snccessfnlly catalyzed the electrophilic snbstitntion reaction of indoles with aldehydes and ketones to bis(indonyl)methanes [225], the deprotection of aromatic acetates [226], esterifications [227], transesterifications [227], the chemoselective thioacetalization of carbon functions [228], the addition of mercaptans to a,P-nnsatnrated carboxylic acids [229], the imino-Diels-Alder reaction [230], the synthesis of iV-Boc protected amines [231], the preparation of alkynyl sngars from D-glycals [232], the preparation of methyl bisnlfate [233], and the synthesis of P-acetamido ketones from aromatic aldehydes, enolizable ketones or ketoesters and acetonitrile [234],... 

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