Enamines ring closures with

The second type of cyclodehydration to be presented is the formation of cyclic enol ethers and cyclic enamines. Here, an alcohol or amino group reacts with an enol, resulting in ring closure with loss of H20. Such reactions... 

Treatment of (16-3) with sodium borohydride leads to the selective reduction of the enamine bond to lead to the tetrahydropyridine (16-4). This intermediate undergoes ring closure with a strong acid to give the benzomorphan (16-5) in direct analogy to the more complex morphinans. The product consists predominantly of the isomer that bears the equatorial secondary methyl group [18],... 

Earlier it was observed that the treatment of pyrylium salts (14) with enamines of cycloalkanones (15) in boiling acetonitrile results in the formation of substituted cycloalkenobenzophenones (17) if the pyrrolidino residue in (15) is used33. However, with enamines of morpholine preferentially the cycloalkenobenzenes (18) could be isolated. The heptatrienones (16), although not isolated, were supposed to be intermediates consistently with our experience, and a disrotatory ring-closure with c/s-elimination of the corresponding benzamide was proposed for the formation of (iS)33 (Table 4). 

Following the interaction of propargylamine amino group with a ketone, producing an enamine, ring closure can be effected with a gold catalyst, the whole process being conducted in one pot. ... 

Dimethylformamide Multi-component reactions Ring closures with enamines 2-Thiazolidones... 

A soln. of 2,2-bis(chloromethyl)acetophenone in acetonitrile added dropwise during 0.5 hr. to N-cyclohex-l-enylpyrrolidine, triethylamine, and acetonitrile, stirring continued 1 hr., the solvent removed, first at atmospheric pressure and finally in vacuo, water added to the resulting immonium salt, and stirred 12 hrs. at room temp. -> 3-benzoylbicyclo[3,3.1]nonan-9-one. Y 81%. F. e. s. H. Stetter, K. D. Ramsch, and K. Elfert, A. 1974, 1322 bicyclo[4.n.l]enones cf. W. C. Still, Synthesis 1976, 453 ring closure with 3-chlor-2-enamines cf. Synth. Meth. 31, 838. 

Without additional reagents Ring closures with sulfur 2-Aminothiazole ring from enamines... 

In this procedure, a hydrazone of an enolizable aldehyde or ketone is heated in strong acid, causing ring closure with simultaneous expulsion of ammonia to furnish the indole nucleus. [Hints The mechanism of the reaction proceeds in three stages (1) an imine-enamine tautomerization (recall... 

In the case of unsubstituted BFO 1 reacting with an enamine, the following mechanism is generally accepted in the literature. The first step is nucleophilic addition of an enamine 2 to electrophilic BFO 1 to form the intermediate 12. Ring closure occurs via condensation of the imino-oxide onto the iminium functionality to give 13. Finally, P-elimination of the dialkyl amine produces the quinoxaline-1,4-dioxide 4. 

Reaction of tryptamine with simple ketones has not been widely explored. Acetone in the presence of benzoyl chloride has been reported to yield 2-benzoyl-1,1 -dimethyl-1,2,3,4-tetrahydro-j8-carbo-line. That the keto group is much less reactive than the aldehyde group is indicated by the fact that j8-keto aldehydes, in the form of their acetals or sodium salts, react with tryptamine at the aldehyde function to yield the conjugated enamine 24, which undergoes ring closure via an intramolecular Michael addition. The potentialities of this interesting modification of the Pictet-Spengler reaction have not yet been fuUy explored. 

A novel ring closure was discovered by Stork (6) in which the pyrrolidine enamine of a cycloalkanone reacts with acrolein. The scheme illustrates the sequence in the case of 1-pyrrolidino-l-cyclohexene, and the cyclopentane compound was found to undergo the reaction analogously. The procedure details the preparation of the bicyclo adduct and its cleavage to 4-cyclooctenecarboxylic acid. 

The reaction of 2-phenyl-1,1-dicyanoallyl anion 20 with Af-methy 1-2,2-dimethylpropionitrilinm ion in acetonitrile solution resulted in formation of enamine 21, which on heating undergoes a ring closure to pyridine 22 <96SC A(50)623>. 

The spiroindolinobenzopyran 2 is a classical example of spiropyran and is easily prepared by the condensation of l,3,3-trimethyl-2-methyleneindo-line (Fischer s base) and salicylaldehyde in anhydrous ethanol or benzene (Scheme 2).ia The nucleophilic attack of Fischer s base on the carbonyl group (like an enamine) gives an aldol product, which undergoes ring closure followed by dehydration. This condensation is reversible therefore, an exchange of the salicylaldehyde component of spiropyran with a different salicylaldehyde is possible. For example, when a solution of spiropyran 2 (Scheme 2) was refluxed with 3,5-dinitro-substituted salicylaldehyde, the open form of 6,8-dinitro-BIPS was obtained.2... 

The enamine 141 can be cyclized to the [l,2,4]triazolopyridopyrimidine 142 upon treatment with sodium ethoxide (Scheme 40) <2002M1297>. This fused tricyclic system may also be obtained, like the pteridine analogue (cf. Scheme 38), from the reaction of hydrazonoyl halides and pyridopyrimidines such as 143, and also by treatment of the triazolopyrimidine 144 with dimethylformamide dimethylacetal (DMF-DMA) dimethylacetal and subsequent ring closure <2003MOL333, 2003HAC491> (Scheme 41). Another series of triazolopyridopyrimidines, for example, 146, can be prepared from a hydrazine-substituted pyridopyrimidine 145, in two ways either directly by reaction with an acid chloride, or via a derived hydrazone (Scheme 42) <1996MI585>. 

Pyrido[2,3-, pyridazine derivatives 48 have been synthesized by refluxing equimolar amounts of an appropriate 5-benzylidene-2,2-dimethyl-l,3-dioxane-4,6-dione 47 with 5-amino-6-phenylpyridazin-3(2/7)-one 46 in methanol or a methanol acetic acid mixture. The electron-poor carbon atom of the polarized carbon-carbon double bond of 47 is the electrophile attacking C-4 of the 5-aminopyridazinone 46. Imino-enamine tautomerization of the intermediate is followed by ring closure and subsequent loss of acetone and carbon dioxide affording the reaction products 48 as stable crystalline solids in 70-90% yield (Scheme 9) <2000T2473>. 

The disulfide sulfur of 263 acts as an electrophile with the enolate anion resulting in ring closure (Equation 54) <1980LA1917>. DMSO oxidizes the sulfur of 264 which then reacts with the enamine double bond (Equation 55) <2001JFC(108)51>. 

N-Methyl-2-hydroxypyrroIidine (246) is derived biosynthetically from ornithine (245). It functions as a source of the N- methylpyrrolinium ion (247), which in turn functions as a precursor of alkaloids such as tropine (248). The pyrrolidine enamine of cyclopentanone undergoes an interesting ring closure reaction with DMAD, resulting in the formation of a pyrrolizine (Scheme 92) (78TL1351). 

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