Crystal structures pyrimidine complexes

In contrast with the Schiff base salen, salicylaldehyde oxime (79) (salox) complexes of Co have received comparatively little attention, but a series of bis-bidentate divalent complexes of the form iraiis-Co(sa 1 ox)2( D M SO)2 have been reported.343 The heterocyclic bidentate oxime violurate (lH,3H-pyrimidine-2,4,5,6-tetrone 5-oximate, Hvi) (80) and its /V-methyl (mvi) and /V,/V -dimethyl (dmvi) derivatives form high-spin divalent [Co(vi)]+ and Co(vi)2 complexes, whereas [Co(vi)3] is low spin.344 The mixed-ligand Co(dmvi)2(phen) complex is also low spin. The crystal structure of m-Co(pxo)2Br2 (pxo = 2-acetylpyridine-l-oxide oxime) is isostructural with the Ni11 relative.345 The dichloro complex also adopts a cis configuration. The tridentate dioximes 2,6-diformyl-4-methylphenol dioxime and 2,6-diacetyl-4-methylphenol dioxime (Hdampo) form binuclear complexes of the type (81a) and (81b) respectively.346 Cobalt oxide nanoparticles were prepared by... 

Overlap Geometry A schematic representation of the proposed overlap geometry for proflavine intercalated into a deoxy pyrimidine(3 -5 )purine site is presented below with the (o) symbols representing the location of the phenanthridine ring protons. The mutual overlap of the two base pairs at the intercalation site involves features observed in the crystal structures of a platinum metallointercalator miniature dC-dG duplex complex (55) and the more recent proflavine miniature dC-dG duplex complex (48), as well as features derived in a linked-atom conformational calculation of the intercalation site in the proflavine DNA complex (51). [4]... 

The complexity of many of these molecules contributes to errors in assignment of their structure. The crystal structure of an inhibitor of starfish embryonic development led to a revision of its structural assignment. Instead of a 4-oxo compound, the correct structure is 4-amino-7-(/ -D-ribofuranosyl)-3//-furo[3,2-d]pyrimidine <92Ml 707-02). The crystal structure of compound (5), an intermediate in the enantioselective synthesis of 8-epicastanospermine has also been reported <93AX(C)40). 

The discovery of these base pairs, which imply the double helical structure of DNA, stimulated a series of crystal structural studies not only of complexes of purines and pyrimidines, but of other complexes involving related molecules and their derivatives. Although we can formulate a large number of possible heterocombinations in matrix form, as shown below these complexes are reluctant to crystallize even when there is spectroscopic evidence of hetero-complex formation in solution. This is presumably because self-(homo)-association is energetically more favorable and only in rare cases were crystals of hetero complexes actually formed. Because of their three hydrogen bonds, G-C complexes form and crystallize more readily. There have been many attempts to crystallize the Watson-Crick A-U base pair, but none was successful and it only formed when the dinucleoside phosphate adenylyl-3,5,-uridine (ApU [536]) or higher oligomers were crystallized (see Part III, Chapter 20). 

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