Ketene aminals cyclic

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

Tetrakis(dimethylamino)titanium converts DMF to tris(dimethylamino)methane (529 equation 236). This compound is also formed in the reaction of V-tetramethyloxamide with the titanium reagent, whereas amides which have a-CH bonds are converted to ketene aminals. Cyclic and spitocyclic compounds, containing the substitution pattern of a trisamino compound, are formed in the reaction of IVA -dialkylformamides or A/ -aryl-(VJV-dialkylformamidines with aryl isocyanates. Similar compounds, e.g. (530 equation 237), are produced in the reaction of imidazolines or lactamid-ines with isocyanates. 

In context with the formation of peraminosubstituted 1,4,5,8-tetraazaful-valenes of type 85 it must be mentioned that the bis-vinylogous compounds 94 can be easily prepared by reaction of acetamidine with bisimidoylchlo-rides derived from oxalic acid (96S1302). In the course of a complex reaction a cyclic ketene aminal was produced it immediately underwent an oxidative dimerization to yield deeply colored TAFs. Tlieir high chemical stability can be compared with that of indigoid dyes and manifests itself, for example, by the fact that they are soluble in hot concentrated sulfuric acid without decomposition. Tire same type of fulvalene is also available by cy-... 

Treatment of the cyclic ketene aminal 267 with diethyl azodicarboxylate results in formation of the reduced ring system 268 (Equation 52), probably via an initial aza-ene reaction, followed by fragmentation and ring closure <2002T7791>. 

A two-step (4 -t- 2) cycloaddition mode was proposed by Hartmann and Heuschmann for the reaction of the cyclic ketene aminal (198) with 3,6-diphenyltetrazine (8), as a crystalline zwitterionic intermediate (199) could be isolated, which ring-closed in a second step to form (200) (Scheme 36) <89AG(E)1267>. 

No single procedure of wide applicability has emerged. The sequences reported are largely divided into annotations of imidazoles or reactions of cyclic ketene aminals. 

Synthesis of Triazoles from Cyclic Acyl Ketene Aminals. 50... 

The Eschenmoser-Claisen rearrangement is a closely related transformation which delivers carboxylic amide rather than ester products, via the intermediacy of the analogous ketene aminals. Scheme 13.30 depicts the reaction of a cyclic, unsaturated antz-1,2-diol which was... 

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

Amination of ketene has been studied by ab initio methods.Reactions of ammonia, its dimer, and its (mono)hydrate with ketene have been calculated and compared with earlier smdies of ammonia (at lower levels of theory), of water, and of water dimer. In general, the results favour initial addition of ammonia to the C=0 bond (giving the enol amide), as against addition to the C=C bond (which gives the amide directly). Amide formation is compared with the corresponding hydration reaction where enol acid and acid are the alternative immediate products. Most of the reactions, i.e. both additions and tautomerizations, are suggested to involve cyclic six-membered transition states. 

This chapter covers the kinetic resolution of racemic alcohols by formation of esters and the kinetic resolution of racemic amines by formation of amides [1]. The desymmetrization of meso diols is discussed in Section 13.3. The acyl donors employed are usually either acid chlorides or acid anhydrides. In principle, acylation reactions of this type are equally suitable for resolving or desymmetrizing the acyl donor (e.g. a meso-anhydride or a prochiral ketene). Transformations of the latter type are discussed in Section 13.1, Desymmetrization and Kinetic Resolution of Cyclic Anhydrides, and Section 13.2, Additions to Prochiral Ketenes. 

Ozonolysis of ketene dimer gives a very unstable compound that can be observed only at low temperatures (-78 °C or below). It has two carbonyl bands in the IR and reacts with amines to give amides, so it looks like an anhydride (Chapter 12). Can it be the previously unknown cyclic anhydride of malonic acid ... 

Cyclic ketene acetals can also react with amines to give 1,1-enediamines with elimination of ethylene glycol76. Treatment of 2-[(methoxycarbonyl)cyanomethylene]-1,3-dioxolane (38) with 1,3-diaminopropane or with 4,5-dimethyl-1,2-phenylenediamine gives the hexahydropyrimidine and the benzimidazoline derivatives 39 and 40, respectively (equation 11). Similarly to the reaction of ketene dithioacetals with amines, the reaction between ketene acetals and amines proceeds via monoamino-substituted intermediates and ketene 7V,Oacetals can be isolated when one molar amine is used72. 

Cyclic ketene 7V,S-aminals (187) with different ring sizes were used by Marcelis and van der Plas to form (188) (Equation (5)) <87JHC545>. Cyclic lactim ethers (189) and donor substituted 2,5-dihydropyrazines (191) react in their tautomeric ketene (9,A -aminal forms (190) and (192) (194) and (193) are accessible as has been shown in a number of examples (Scheme 34) <77AP(310)936, 90AP(323)89>. The sulfur analogue reacts in the same way. Finally, lactam acetals (195) can be used as an in situ source of cyclic ketene 0,A -aminals (196) and form pyridazines (197) on reaction with... 

A possible borderline case is observed in the reactions of tetrazines with cyclic ketene A,A-aminals (Scheme 51) <89AG(E)1267>. Hartmann and Heuschmann offered some evidence that the dipolar intermediate (199) isolable in their system might be a true intermediate in a two-step (4- -2) cycloaddition. There are a few examples <90TH 621-02,93UP 621-01 > which contradict their assumption. Tetrazine (166) and aminal (290) react to form the dipole (291), which can be protonated to (292) (NMR) but the cycloaddition forms (294), the product with we/a-orientation, via (293). 

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