1.4- Diazabicyclo pyrazines

The high selectivity that the system shows to pyrazine 20 compared to the stronger base pyridine, indicates that the diamine is chelated between the carboxylic acid functions as in 21. Spectroscopic evidence in the form of upfield shifts in the NMR spectra of the complexes supports such structures. Not only aromatic diamines are accommodated but also aliphatics such as l,4-diazabicyclo[2.2.2]octane (DABCO) in complex 22. Typically, exchange rates into and out of these complexes are such that they appear fast on the NMR time scale at ambient temperature, but exchange can be frozen out at low temperatures20. For DABCO, an activation barrier of 10.5 kcal M 1 was observed at Tc = 208 °K. 

Earlier work on the photolytic or thermal rearrangement of polyhalogenated pyridazines to corresponding pyrazines has been continued,14,161,774,1690 but the fascinating results offer little of preparative value. It has been reported that 300-nm irradiation of 3,4,5,6-tetra-tert-butylpyridazine (66) gave a quantitative yield of the Dewar isomer (3,4,5,6-tetra-tert-butyl-l,2-diazabicyclo [2.2.0]hexa-2,5-diene 67] that subsequently afforded 2,3,5,6-tetra-tert-butylpyrazine (68) in 18% yield on 254-nm irradiation.1464... 

One such epidioxypiperazinedione has been reduced to a regular pyrazine. Thus l,4-dibenzyl-2,3-dioxa-5,7-diazabicyclo[2.2.2]octane-6,8-dione (86) underwent reduction by sodium borohydride in ethanol at 20°C during 1 h to afford 3,6-dibenzyl-3,6-dihydroxy-3,6-dihydro-2,5( l//, 4//)-pyrazincdione (86a) in 65% yield, confirmed in structure by dehydration to 3,6-dibenzylidene-3,6-dihydro-2,5(1 H, 4//)-pyrazincdionc (86b).5... 

Key to Figures 16 and 17 1, Pyrazine 2, 2-Methylpyrazine 3, 2,6-Dimethyl-pyrazine 4, Quinoxaline 5, 2-Methylquinoxaline 6,, 4-Diazabicyclo, 2,2 -octane 7, Pyridine 8, Dioxan 9, Tetramethylammonium bromide 10, Tetror ethylammonium bromide 11, Tetrcq>ropylaminonium bromide 12, Tetrabutyl-ammonium bromide 13, n-propanol. 

The pyrolysis of human hair [15] showed that diketopiperazine formation could provide information on the structure of proteins. This study utilized a Py-GC/MS/MS approach, assigning structures to the compounds generating different chromatographic peaks using the MS/MS capability with both El and Cl ionization techniques. Besides compounds of the type hexahydro-3-(2-methylpropyl)-pyrrolo[1,2-a]pyrazine (1,4-diazabicyclo[4,3,0]nonane-3-methylpropyl-2,5-dione, see Figure 3.5.6d), a second series of compounds with the same mass were noticed. The daughter ions of the two series of compounds were different, showing that pyrolysis of proteins does have in fact... 

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

However, more detailed study of the products showed that perfluoro-1,4-di-isopropy 1-2,5-diazabicyclo [2.2.0] hexa-2,5-diene was formed (Scheme 41).103 This result excluded the mechanism in Scheme 40. A similar isomerization of perfluoro-3,6-methylpyridazine to the 2,5-substituted pyrazine was reported.104... 

FIGURE 3.8 Eight metal-organic supramolecular isomers formed with the flexible ligand TA with transition metals Zn, Cd, Co, Cu, and Mg. All the isomers showed the rigid framework while structure A (TetZB) showed flexible framework towards guest sorption [21, 22, 47], Py = pyrazine, DB = l,4-diazabicyclo[2.2.2]octane, and BP = bipyridine. (The stmctures reproduced from Motkuri et al. in referenced RSC and ACS journals, with permission). 

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