Sulfur and oxygen atoms

Fig. 14. Choleic acid inclusion chemistry (a) crystal stmcture of DCA inclusion compound with phenanthrene (b) view along a DCA inclusion helix accommodating DMSO and water guest molecules (oxygen and sulfur atoms and methyl groups are represented by open circles and large and small black...

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

Unlike the products of condensation of 1,2-diaminoethane and 1-amino-2-hydroxyethane with aminobutynones, which exist almost exclusively in the enole form, benzoxazoles 359,361 and benzothiazoles 360,362 are present in the solution as a tautomeric mixture of the ketone (359, 360) and enole (361,362) forms. This seems to be related to weakening of the imine nitrogen basicity due to adjacent oxygen and sulfur atoms. 

We have already pointed out that the reduction in conjugation efficiency in PCSs is followed by a short-wave shift of the CTC transfer band. This accounts for the fact that poly(schiff base)s and polyazines having conjugated sections separated by oxygen and sulfur atoms are characterized by a short-wave shift of the transfer band of CTC with all acceptors compared to the respective polymers having no interruption of the conjugated chain. This shift may reach 20-50 nm. 

Forty-four five-membered heterocycles of type A (13, 19) have been described (Table I). If the atoms or groups a, b, c, d, e, and f are selected from suitably substituted carbon, nitrogen, oxygen, and sulfur atoms, then with these conditions it can be shown that 144 structural possibilities are provided by the general formula 19. The number of structural possibilities can be deduced in various ways, but a very useful approach is to regard type A meso-ionic molecules (19) as being derived by the union (-<—u— ) of 1,3-dipoles (34) and heterocumulenes (35). 

Radical A group of atoms with distinct features when combined with other elements, but which do not exist on their own, e.g. sulfate, a combination of oxygen and sulfur atoms. 

The fully unsaturated compounds containing two oxygens (e.g., 1,4-dioxin 61), two sulfur atoms (e.g., 1,4-dithiin 62), and one each of oxygen and sulfur atoms (e.g., 1,4-oxathiin) are electron-rich and highly reactive toward electrophiles. Benzo derivatives are usually most reactive in the fused heteroring (84M113). 

S. K. Burley and G. A. Petsko cover the field of noncovalent interactions of proteins, with particular emphasis on weakly polar interactions. Their presentation of the whole field of electrostatic interactions should be of value to many workers in protein chemistry, but their special concern is with the weaker, but very important, interactions involving aromatic side chains, their orientation relative to one another, to oxygen and sulfur atoms, to amino groups, and to aromatic ligands that may bind to the protein. These interactions, only recently recognized for their influence on protein structure, play an important part in the formation of aromatic clusters in the interior of globular proteins and in other features of structure. The authors provide numerous illustrations of the principles involved, from recently determined structures, of both small molecules and proteins. 

The extension of this synthesis to a wide variety of 1-azabicyclic compounds containing in some cases oxygen and sulfur atoms in the ring has been promised for future publications. 

Like pyrrole-type nitrogens, ring oxygen and sulfur atoms also have a deshielding effect on pyridine-type nitrogen atoms with the effect greatest on adjacent atoms. For examples see Table 23b and structures 166-173 in Figure 20. 

Ten-membered Rings or Larger with One or More Oxygen and Sulfur Atoms ... 

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