Enamines heterocyclic

A similar sequence of reactions takes place with the enamlnes of cyclic ketones (55-57) the initially formed unstable cyclobutene rearranges with insertion of two carbon atoms into the ring. A wide variety of cyclic ketones have been allowed to react in this way. For instance, the enamine (75) gave 76 on reaction with dimethyl acetylenedicarboxylate in refluxing toluene (55) and the heterocyclic enamine (77) obtained from dihydro-3-(2H)-... 

Acylation of the heterocyclic enamines e.g., l,3,3-trimethyl-2-methyl-eneindoline (126) and 2-benzylidene-3-methylbenzothiazoline (42) takes place normally at the methylene carbon (42,69), with both acyl and aroyl chlorides in the presence of base. 

A stable chloroiminium chloride (173) has been isolated from the chlorination of the heterocyclic enamine (174) (42). 

The pyrrolidine enamine of 2-tetralone (177) was converted to l-cyano-2-tetralone, which exists almost entirely in the enolic form (178), by reaction with cyanogen chloride (J23). Reaction of 177 with cyanogen bromide gave N-naphthylpyrrolidine (179), presumably via the unstable bromoenamine (180). The latter observation is in accord with the mode of reaction of the heterocyclic enamine (126) with cyanogen bromide, which resulted in the... 

Heterocyclic enamines often undergo two-step 1,3 cycloaddition with methyl vinyl ketone. This involves electrophilic attacks by an olefinic carbon and by a carbonyl carbon (24,25). For example, 1,2-dimethyl-Zl -pyrroline (14), when treated with methyl vinyl ketone, produces 1,6-dimethyl-2,3,4,5-tetrahydroindole (15) (24). The requirement which must be met so that this type of cyclization reaction can take place is that the a position of the heterocyclic enamine be carbon substituted. This provides... 

V. Aldol Reactions of Heterocyclic Enamines and Their Importance for the... 

As heterocyclic enamines we designate compounds containing an enamine grouping... 

Preparation of Heterocyclic Enamines from y- and h-Amino Ketones... 

Preparation of Heterocyclic Enamines by Means of Organometallic Reagents... 

The study of structure and reactivity of tertiary heterocyclic enamines is associated with the problem of equilibrium of the cyclic enamine form (70) and the tautomeric hydration products 173,174) quaternary hydroxide (71), pseudo base (so-called carbinolamine) (72) and an opened form of amino aldehyde or amino ketone (73). 

The double bonds of either enamines or their salts readily undergo many reactions. We shall divide the reactions of heterocyclic enamines on the basis of the mechanism involved. 

The first reported alkylations at the /3-carbon atom of a heterocyclic enamine were observed with the alkylations of dihydroberberine (102) (190,191) and l,3,3-trimethyl-2-2-methyleneindoline (192,193) (104) to yield monomethylated products 103 and l,3,-trimethyl-2-isopropylidene-indoline (105), respectively. 

Acylation of heterocyclic enamines is to a great extent similar to alkylation, and usually occurs on the /3-carbon atom of the enainine grouping. 

Read ions of Heterocyclic Enamines with a,p-Unsaturated Compounds Enamines react readily with compounds containing a double bone activated by electronegative groups. Addition of acrolein to 1-methyl-2 ethylidenepyrrolidine, followed by dehydrogenation, leads to 1,7-dimethyl indole (133) (Scheme 9) (215). 

Reactions of Heterocyclic Enamines with Other Electrophilic Reagents... 

Tertiary heterocyclic enamines are reduced with metals in acidic media 142) or electrolytically (237,238) and their salts are reduced with lithium aluminum hydride or sodium borohydride (239,240) to the corresponding saturated amines. 

Heterocyclic enamines A -pyrroline and A -piperideine are the precursors of compounds containing the pyrrolidine or piperidine rings in the molecule. Such compounds and their N-methylated analogs are believed to originate from arginine and lysine (291) by metabolic conversion. Under cellular conditions the proper reaction with an active methylene compound proceeds via an aldehyde ammonia, which is in equilibrium with other possible tautomeric forms. It is necessary to admit the involvement of the corresponding a-ketoacid (12,292) instead of an enamine. The a-ketoacid constitutes an intermediate state in the degradation of an amino acid to an aldehyde. a-Ketoacids or suitably substituted aromatic compounds may function as components in active methylene reactions (Scheme 17). 

All heterocyclic enamines readily undergo condensation with o-amino-benzaldehyde. The quinoxaline derivatives thus formed have a characteristic yellow color. Therefore, this reaction can serve as evidence of the presence of an enamine in plants (295,309). 

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