Exocyclic groups

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance stmctures, it might be expected that exocyclic groups that could strongly stabilize a positive charge would lead to a larger contribution from dipolar stmctures and enhanced stability. The stmctures 13 and 14 are cases in which a dipolar contribution would be feasible. 

Azoles with TWo Potentially Tautomeric Exocyclic Groups. 291... 

Cyclization of the intermediate, 182, by means of ethyl orthoformate takes a quite different course. The nitrogen on the aromatic ring in this case becomes one of the exocyclic groups to afford the chorazanil (185) a compound that shows diuretic activity. 

Each cluster atom has a lone pair or it is bonded to an exocyclic group. 

An electron-precise -atom cluster will have 5n (15n) electrons, of which 3n are used to bond the cluster (to form 3 /2 skeleton electron pairs). Notice that the total number of electrons in a cluster is given by the sum of the electrons on cluster vertex atoms plus the electrons, on vertex atoms, from covalent bonds with exocyclic groups. 

A simple, previously mentioned example may be represented by the tetrahedral molecule P4. In this structure there are 4 vertex atoms (n = 4) and there are no exocyclic groups. On the other hand, P has 5 valence electrons, so the number of cluster electrons is 4 X 5 = 20 electrons. This number (20) is therefore related to the number of vertices by the condition 20 = 5n. The cluster is electron-precise. 

A monosaccharide is flexible by virtue of the rotations of exocyclic groups, i.e.,... 

In Figure 5, we present a stereoview of the superposition of structures A", B and C" obtained when all atoms were included in the structural comparison. In Figure 5, we also present a stereoview of the superpositions of these three structures when unconstrained exocyclic groups were excluded from the comparison. 

Figure 5. Stereoview of the superposition of the NMR refined one-state structural solution. A" (—), B" (- ), and C" (—) (a) with unconstrained exocyclic groups included in the comparison and (b) excluded from the comparison.

In the case of X = C (CN)2 two nonequivalent CN groups are observed. Similarly, two nonequivalent ester groups are observed with X = C (COOEt)2 suggesting that the exocyclic group bond has double bond character <93MRC447> (see <76JCS(P1)744 . With X = C (CN)(COOEt) only one isomer was observed even at 373 K but it could not be identified <93MRC447>. 

Exocyclic Group Coordination and Direct Ring Phosphorus Interaction with... 

The following nomenclature has been used in this review The ring system is named as the appropriate heterocation and the exocyclic group carrying the negative charge is named as the appropriate anion (i.e., olate, thiolate, aminide, or methylate). Thus, compounds of type 9 are named as... 

Substitution reactions110 as well as certain rearrangements953 of exocyclic groups bonded to the heteroatom can occur via a five-coordinate anionic complex (108). 

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