A cyclodehydration

The same oximino acid (49) underwent a cyclodehydration on treatment with dicyclohexyl carbodiimide to form the unsaturated hydroxamic acid (52) in almost quantitative yield. 

Ueda and coworkers reported, in 1975, the preparation of polyamides 11 from unsaturated bisazlactones 10 and a variety of diamines (Scheme 3) (JL1). In a DTA study of one of these polyamides (11, R = CHj.Tr = (CHo)g), an endotherm was noted near 200dC. It was suggested, without supporting evidence, that this endotherm might be due to a cyclodehydration reaction leading to polyimidazolinone 12. 

Pyranopyrrolizines can be prepared from prolinylcoumarin derivatives. Treatment of the Weinreb-type amide 174 with methyllithium in THF gives a ketone intermediate, which when treated with silica gel in chloroform undergoes a cyclodehydration to give the triheterocyclic system 175 <1999T13211> (Scheme 52). 

Sotoca E, Allais C, Constantieux T, Rodriguez J (2009) User-friendly stereoselective one-pot access to 1,4-diazepane derivatives by a cyclodehydrative three-component reaction with 1,3-dicarbonyls. Org Biomol Chem 7 1911-1920... 

The first procedure to prepare unsaturated 5(4//)-oxazolones was the Erlenmeyer synthesis" " that was described more than one hundred years ago and is still used extensively with some variations in the experimental conditions. In general, the reaction employs an acylamino acid, for example, A-acetyl- or A-benzoylglycine are the most common, and a carbonyl compound, usually an aldehyde, in the presence of a cyclodehydrating agent such as acetic anhydride (Scheme 7.114). Hundreds of unsaturated oxazolones 363 have been obtained via this procedure and these compounds are valuable intermediates for the synthesis of many interesting organic compounds. 

Condensation of A -acylglycines with carbonyl compounds, the Erlenmeyer synthesis, continues to be exploited to prepare of a wide variety of unsaturated-5(47/)-oxazolones. The reaction is performed in the presence of a cyclodehydrating agent and recently bismuth(lll) acetate has been evaluated in this capacity. Alternatively, unsaturated 5(47/)-oxazolones can be obtained from hippuric acid and a carbonyl compound or from the appropriate dehydroamino acid derivative using 3-(aIkoxycarbonyl)benzotriazole-l-oxides as the cyclodehydrating agent. 

Treatment of the product with phosphorus oxychloride leads to a cyclodehydration reaction possibly via the imino chloride. There is thus obtained the antipsychotic compound loxapine (38-7) [39]. 

However, the explanation does not involve a rearrangement. Instead, the N—O bond was cleaved, the azo linkage was saturated, and the triazole ring was then formed by a cyclodehydration. 

In the case of 2,5-dimethylfuran (51) the dicarbonyl compound is revealed by the disconnection of one carbon-oxygen bond ring construction is of type (44). The forward synthetic reaction is a cyclodehydration reaction effected by the action of acetic anhydride in the presence of zinc chloride (Expt 8.12). 

The frequently reported dimerization of 1-azirines to 3,6-dihydro-pyrazines probably involves initial hydrolysis to the aminoketone which then undergoes a cyclodehydration to the dihydropyrazine, rather than a direct dimerization of the 1-azirine. Catalytic amounts of water absorbed on the acid surface of the glass are probably responsible for this reaction. 

Under acidic conditions enamines such as compound A in Figure 12.18 and aldehydes undergo condensation to form the conjugated iminium ions D. These will be deprotonated by the concomitantly formed hydroxide ions, In this way dienamines of type F are formed, which will then be hydrolyzed upon acidic workup to give a carbonyl group. The generation of the a,/i-unsaturated ketones E is thus completed. You will learn about type E compounds in Section 13.4.1 in connection with the so-called crossed aldol condensation products. It should be noted that it is not possible to form the same unsaturated ketone by reacting cyclopentanone or its equilibrium fraction of enol with an aliphatic aldehyde. Instead, a cyclodehydration of... 

Urea 1095 undergoes a cyclodehydration process in the presence of PhsP-CCU/EtsN to give 2-aminoimidazolones 1096, presumably through a transient carbodiimide that cyclizes spontaneously <2003TL6509>. Thiourea 1115 is more reactive than urea 1097 and can transform to imidazolone 1098 with the aid of EDC (Scheme 266) <2004SL2800>. 

Several variations have been proposed for the conversion of the enol of a 3-deoxyhexosulose (64) into a 2-furaldehyde. Hurd and Isenhour proposed that the intermediate is a vinyl ether (65), formed by a cyclodehydration. 

The tris(trifluoromethyl)-l,3,2,5-dioxathiazine 2,2-dioxide (228), obtained by the action of SO3 on Ar-[bis(trifluoromethyl)hydroxyniethyl]trifluoroacetaniide, on treatment with trifluoroacetamide undergoes ring scission accompanied by loss of SO3, which acts as a cyclodehydrating agent on the bis-amide (229) so-formed, to yield 2,4,4,6-tetrakis(trifluoromethyl)-4//-l,3,5-oxadiazine (230) (Scheme 38) <84JOU1213, 89JOU578>. 

Only a few indole syntheses make use of building blocks in which the N-atom is not directly bonded to an arene. The Nenitzescu synthesis is of this type. In this synthesis, 1,4-quinones are condensed with 3-aminoacrylic esters to give 5-hydroxyindole-3-carboxylic esters 44. The mechanism of this synthesis has not been completely clarified. It includes a Michael addition (-> 41) and a cyclodehydration (42 43) as well as a redox transformation (41 42 and 43 44) [53] ... 

Subsequent treatment of 86 with aqueous TFA allowed [122] a cyclodehydration in the polyhydroxylated chain to afford the a and -d-erithrofuranosyl-C-nucleosides 87 in a roughly 1 4 a//3 ratio (Scheme 20). Furthermore, prolonged treatment of imidazolidine-2-thione 86 with TFA in refluxing methanol gave access to S-methylimidazol derivatives 88 as a mixture of anomers, isolated in a 1 7 a/f) ratio. 

As alluded (vide supra), some confusion may arise with respect to this named reaction as there is reference in the literature to an alternative reaction with the same name. The Bradsher reaction forms aromatic rings but via an acid-catalyzed Friedel-Crafts-like process. Thus diaryl-methanes having a carbonyl group in the ortho position can undergo a cyclodehydration reaction to generate the corresponding anthracene derivatives. In this respect, the Bradsher reaction is related to the Elbs reaction, which involves the pyrolytic cyclization of diaryl ketones 6 having an ortho methyl or methylene substituent for the formation of polycyclic aromatics 7. 

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