Tautomerism in ketomethyl quinolines. Part 3. 4-Ketomethylquinolines

Abstract

. 69,696 (1991).A series of 4-ketomethylquinolines has been prepared and their tautomerism investigated. The compounds were prepared from 4-methylquinoline deprotonated with lithium diisopropylamide or potassium ethoxide and treated with esters. Attempted deprotonation with tert-butyllithium or phenyllithium led to 2-substituted-4-methylquinolines. The simple alkyl or aryl4-ketornethylquinolines exist essentially as the keto forms, but the quinoline-4-pyruvates are exclusively or substantially in the en01 forms.Key words: ketomethyl quinolines, pyruvates, tautornerism, enols. Chem. 69, 696 (1991).On a prCparC une sCrie de 4-~Ctom~thylquinol~ines et on a examink leur tautornkrie. On a prCparC les produits h partir de la 4-mCthylquinolCine dCprotonCe, ? i l'aide du diisopropylamide de lithium ou de 1'Cthylate de potassium, qui a Ct C traitCe par des esters. Des essais de dCprotonation a I'aide du tert-butyllithium ou le phtnyllithium ont conduit h la formation de 4-mCthylquinolkines substitutes en position 2. Les 4-~CtomtthylquinolCines portant des groupements aryles ou alkyles simples existent essentiellernent sous la forme cttonique; toutefois, les quinolCine-4-pyruvates existent exclusivernent ou presque sous la forme Cnolique.Mots cle's : ~CtomCthylquinolCines, pyruvates, tautornkrie, enols.[Traduit par la rCdaction] Introduction We recently conducted detailed investigations into the tautomerism of 2-ketomethylquinolines (I), Scheme 1. By a combination of ir and nrnr spectroscopy it was clearly shown that the enaminone form B was strongly favoured over the ketone form A for 2-ketomethylquinolines where R' = H, CN, or COOEt (R2 = alkyl, aryl, CN, COOEt, etc.). For the compounds with R1 = H, R2 = Me, Et, i-Pr, or t-Bu, the tautomeric mixtures in chloroform had 12-20% of the ketone forms and signals for both forms A and B could clearly be seen in the ir and the 'H, 13c, and 15N nmr spectra. The rest of the structures investigated showed only the enaminone signals. However, compounds of Scheme 1 where R1 = Me or Br were found to be exclusively in the ketone form A.

Synthesis of 6-acylmethylphenanthridine enaminones

“…The respective bands in the spectra of the compounds carrying alkyl groups 6-10 are blue-shifted withrespect to those carrying aromatic groups 11-17. These data are consistent with previous reports [8,[14][15][16].…”

“…When R is a phenyl group, it has been proven that electron-donating substituents in the para position of the phenyl group reduces the amount of the form 1b but strong electron-withdrawing substituents cause the transformation of form 1a to form 1b due to strong internal hydrogen bonding [9]. Substituents R' in positions 3 and 4 also cause an increase in the ratio of form 1b (6), Et (7), n-Pr (8), i-Pr (9), t-Bu (10), Ph (11), p-Me-C 6 H 4 (12), p-F-C 6 H 4 (13), p-Me 2 N-C 6 H 4 (14), p-(CH 2 ) 4 N-C 6 H 4 (15), p-Cl-C 6 H 4 (16), p-MeO-C 6 H 4 (17).…”

Study of hydrazone-Hydrazoimine tautomerism in α-Azo-6-ketomethylphenanthridines

“…In 1 H NMR spectra a singlet at 10.75-10.99 ppm is due to N-H proton of hydrazoimine form B and the other downfield singlet at 12.76-14.27 ppm was assigned to N-H of hydrazoimine form C. Rotameric ratios were calculated for each compound from their 1 H NMR integrals by comparison of the H b signal of the hydrazoimine form B and H c signal of the hydrazoimine form C. The spectra do not show any more downfield signals around 15.5 ppm which is usually reported for the N-H of enaminone form D or E or around 4.5 ppm for hydrazone form A of the annulated similar compounds, respectively [14][15][16][17][18]. Therefore, 1 H NMR chemical shifts data can readily be employed to study the rotameric equilibria quantitatively ( Table 1).…”

Substituent and temperature controlled tautomerism: multinuclear magnetic resonance, X-ray, and theoretical studies on 2-phenacylquinolines

“…The E form non-stabilized by an intramolecular hydrogen bond is always less stable than the K form. 25 ii. The K form of 4-phenacylquinoline (solutions in chloroform, ethanol, and water) is more stable than its E form.…”