Cyclopentanone enamine

Keto acids are obtained by acylation of cyclopentanone enamines (see p. I3f.) with acid chlorides and subsequent base-catalyzed mro-aldol cleavage (S. Hdnig, 1960). 

The preparation of long-chain fatty acids has been carried out in this way because cleavage of 115 with strong sodium hydroxide gives the ketoacid (116), which is easily reduced by the Wolf-Kishner method to the saturated acid. A similar sequence of reactions can be carried out starting with the cyclopentanone enamine, and this method allows lengthening the chain... 

Scheme 33 illustrates the difference in reactivity between triazolines obtained from cyclohexanone and cyclo-pentanone enamines. Thus, the reactions of azidophosphonates 239 with cyclohexanone enamines produce unstable aminotriazolines 240 that cannot be isolated due to their spontaneous elimination of amines to provide triazoles 241. Contrary to that, triazolines 242, derived from cyclopentanone enamines, are isolated in good yield (76-88%) and cannot be converted to the corresponding triazoles even by thermolysis <1995H(40)543>. Probably, introduction of a double bond between two five-membered rings would involve too much molecular strain. 

Cyclopentanones. Enamines (2) react with 1 at 25° to give in high yield the keto nitriles 3, which undergo the expected addition of lithium phenylacetylide to provide 4. On treatment of 4 with (C6H5)3SnH (AIBN) a 5-acetylenic radical is formed, which cyclizes to a benzylidenecyclopentane (5). Ozonolysis of 5 provides cyclopentanone 6. 

The Vilsmeier reaction,103 which has been successfully applied104 to the formation of 3-acylindoles, has yielded the 4-formyl-1,2-dihydroisoquinolines (76)—(78).105—10 d. Reactions with Aldehydes. Cyclopentanone enamine gives the... 

The steric influence on the enamine-imine tautomerism has also been observed in the cyclic ketone derivatives. Cyclohexanone imines of w-propylamine, cyclohexylamine and 2-bornylamine show no signals ascribed to the enamine tautomer in their NMR spectra, but the /-butylamine38 does display signals of the enamine. In DMSO-d6 it comprises 38% of enamine 56 at equilibrium. The proportion of the 3,3,5,5-tetra-methylcyclohexanone and cyclopentanone enamines 57 and 58 is even higher, rising to 52% and 58%, respectively, in DMSO-d663. 

Miscellaneous Azides. Ethyl (A-methanesulfonyl)azidoformimidate [N3C (0Et)=NS02Me] has been used to aminate chiral cyclopentanone enamines but the yields are low and the reaction could not be extended to the corresponding cyclohexanone enamines.303 Trimethylsilyl azide (TMSN3) transfers the TMS rather than the azide group to a lactam enolate.339... 

Aza-annulation of enamine substrates with acrylamide (20) occurred under milder reaction conditions. Treatment of 19 with 20 in dioxane at reflux (100 °C) resulted in the efficient formation of 4b with reported yields of 70%7 and 83%8 (eq. 4). Subsequent study of this reaction led to the quantitative formation of 4b and 5b reported as a 50 50 mixture of the regioisomeric alkenes.9 This methodology was also extended to aza-annulation of the cyclopentanone enamine with 20 (72%).9 The analogous reactions have been performed by the combination of 20 with the benzyl imine (77%)8 or the propyl imine (18%)10 of cyclohexanone (lb). Limitations arose when the acrylamide was / -substituted or had a substituent at the [3-position (crotylanilide).9 When reagents with these features were used, the reaction did not proceed. 

There are occasional reports in the literature on the use of special reactions to form polyamides. One is a synthesis via enamines. In this case diisocyanates are condensed with cyclopentanone enamines of morpholine or piperidine ... 

Some aspects concerning the transition state of this (4 + 2)-cycloaddition with inverse electron demand follow from the reaction of 3- or 6-phenyl-l,2,4-triazine 5 or 8 with cyclopentanone enamines [323]. 

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