6,7-Dihydro-57/-cyclopentapyrazines

The flavoring interest of 6,7-dihydro-57/-cyclopentapyrazines was revealed for the first time in patents by Polak s-Douwe Egberts (1968, 1969), and by Mookherjee et al. (International Flavors and Fragrances,... 

Cyclotene is the precursor of dihydrocyclopentapyrazines after having reacted with ammonia respectively amino acids to form cyclotene imine. Figure 2 demonstrates not only the isomerization and transformation of this compound but also the formula of bis-dihydro-cyclopentapyrazines as special reaction products. They are formed by condensation of cyclotene-imine and represent a mixture of 4 di-astereomeric compounds. These compounds produce identical mass spectra (Shibamoto et a 1,16). We have carried out corresponding experiments by heating Furaneol with phenylalanine or serine in an autoclave at 180 0 C. As Figure 3 demonstrates some monocyclic pyrazines are formed by reaction of the degradation products from Furaneol. 

Dimethy1-3-ethyl pyrazine 6,7-Dihydro-2-ethyl-5(H)-cyclopentapyrazine + +... 

The formation of some pyrroles, pyrrolines, pyrrolidines, pyridines, and tetra-hydropyridines were considered above. Next come the pyrazines, a very important group of odorants. Pyrazines have been reviewed periodically by Maga.231-233 Vemin and Parkanyi216 have tabulated 26 pyrazines, as well as 11 6,7-dihydro-(5//)-cyclopentapyrazines and 9 5,6,7,8-tetrahydroquinoxalines from 15 systems. 

V ch3 5-methyl-6,7-dihydro[5//]cyclopentapyrazine [23747-48-0] nutty, roast odor... 

Manley et al. (1974) suggested that cyclopentapyrazines may result from the condensation of 2-hydroxy-3-methylcyclopent-2-en-l-one (D.68) with glyoxal or pyruvaldehyde and amino acids, and a mechanism of formation has also been suggested from a study on popcorn flavor by Walradt et al. (1970). Vitzthum and Werkhoff (1975) reported 17 new alkylated five-and six-membered bicyclic pyrazines in roasted coffee the mass spectra of these new constituents and particularly the fragmentation pathways of 2-methyl-(O.50) and of 6,7-dihydro-5-methyl-5//-cyclopentapyrazine (0.51) are discussed in detail. 

A more original pathway for formation of dihydrocyclopentapyrazines has been proposed by Flament (1981). The reaction of 2,3-dihydropyrazines with aldehydes and ketones allowed the preparation of numerous and original trisubstituted pyrazines. When a, (3-unsaturated carbonyl compounds were used, the formation of bicyclic pyrazines was observed. Transitory 2,3-dihydropyrazines which certainly result from the trimolecular condensation of an a-dicarbonyl fragment with a diol in the presence of ammonia can condense with a, 3-unsaturated compounds, giving 6,7-dihydro-5//-cyclopentapyrazine (0.49) and various alkylated homologs. 

Cyclopentapyrazine, 6,7-dihydro-, 6,7-dihydro-5H-cyclopentapyrazine, 6,7-dihydro-5/7-cyclopenta[6]pyrazine 23747-47-9 ... 

Cyclopentapyrazine, 6,7-dihydro-5-methyl-, 6,7-dihydro-5-methyl-51i-cyclopentapyrazine [23747-48-0] FEMA 3306... 

Cyclopentapyrazine, 6,7-dihydro-2,6-dimethyl-, 6,7-dihydro-2,6-dimethyl-5Y -cyclopentapyrazine [61928-63-01... 

Cyclopentapyrazine, 2-ethyl-6,7-dihydro-3-methyl-, 2-ethyl-6,7-dihydro-3-methyl-5H-cyclopentapyrazine [41330-35-2]... 

Apart from two recent papers very little data have been published on this ring system. The fully unsaturated heterocycle (1) has not been reported, and all known cyclopentapyrazines are dihydro or tetrahydro compounds. Various 5H-6,7-dihydrocyclopentapyrazines have attracted attention as components of the flavor and aroma of roasted peanuts. It has been pointed out, however, that a great deal of this work is unsubstantiated. Confusion has arisen because certain examples of this ring system were mistakenly identified as isopropenylpyrazines. ... 

The main synthetic route to cyclopentapyrazines involves formation of the six-membered ring by condensation of appropriate 1,2-diamines and a-dicarbonyl compounds. For example, reaction of diacetyl with cyclopentane-1,2-diamine yielded the tetrahydro compound 2, which was dehydrogenated to the dihydroderivative 3. Suitable dehydrogenation methods include heating under reflux in xylene with palladium on charcoal or heating to 300° over copper chromite. Condensation of ethylenediamine or 1,2-propanediamine with cyclopentanediones such as 8 afforded tetrahydro compounds of formula 9, which on dehydrogenation with copper chromite at 300° afforded the dihydro derivatives 10. As expected, mixtures of isomers are formed when appropriate unsymmetrical starting materials are used. 

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