Rotameric forms

It is the tertiary amides that tend to be the most problematic in terms of proton NMR. They usually exhibit two rotametric forms, the relative proportion of each being determined by both electronic factors and by the relative sizes of the two groups, R1 and R2. Note this in no way implies that the rotameric forms of a tertiary amide could ever be physically separated as the inter-conversion rate between the two forms is generally in the order of seconds. A 50/50 ratio of rotamers is only guaranteed where R =R2 (as in the case of a primary amide where R1=R2=H). Consider the two compounds in Structures 6.14 and 6.15. 

However, in the limit of equal resonance weights (wj = w ) in the resonance hybrid (5.94), the rotameric forms la and Ila or lb and lib become equivalent, and the rotation barrier (A/ lb,hybj) of the resonance hybrid is expected to vanish,96... 

Treatment of either the sc or the ap atropisomer of the diester (97) with potassium hydroxide effected the hydrolysis of only one of the ester groups for steric reasons, to afford monocarboxylic acid sc-98 and ap-98, respectively. The sc isomer was converted into a nienthyl ester (99) for resolution into optical isomers. Thus the three rotameric forms of the monocarboxylic acid ( + sc, -sc, ap) were isolated (142). 

The presence of two OH groups separated from the nitrogen atom by a distance of 7-8 angstrom units is a common feature of all active compounds. We had hypothesized previously that the beta rotameric form of DA, as typified by A-6.7-DTN, was the preferred conformation. However, we recently reported that dipropyl A-5.6-DTN was a potent DA. agonist (10) and, thus, the rotameric conformation may be of secondary importance. 

Cannon, in 1975, suggested that dopamine might bind to Its receptors in two possible conformational extremes, which he designated the "alpha" and "beta" rotamers, XIa and Xlb, respectively (20). In this view apomorphine would represent a structure containing a dopamine fragment "frozen" into the alpha rotameric form. By contrast, 6,7-ADTN contains a dopamine moiety constrained in the beta rotameric form. In both cases the side chain is nearly coplanar with the aromatic ring and is... 

The alpha and beta nomenclature has been convenient to classify structures which clearly contain a dopamine like fragment. Ignoring for the moment subtypes of receptors, it seems the data have generally supported the idea that the alpha rotameric form of dopamine is more important for emetic activity and activity at DA2 receptors, while the beta rotameric form is more essential for action at the renal DA receptor. Some Important exceptions to this generalization will be presented later. 

The concept of alpha and beta rotameric forms of DA is covered in the section on rigidification. A summary of some recent findings in this area together with details of the X-ray structure of 6,7-ADTN has also been published by the present author (15). In connection with the above topic our group has published quantitative details of the penetration and distribution of DA agonists into the CNS as well as the effect of COMT inactivation (16, 17,... 

Very few structural studies have been concentrated on models of ApG adducts of cisplatin. Conformational analysis of two rotameric forms of the complex m-[Pt(NH3)2(9-MeA-A(7))(9-EtGH-A(7))]2+ has recently been described [40], One of the forms, crystallized as a PI 6 salt, can be characterized as a right-handed helicoidal model for the intrastrand ApG crosslink in double-stranded DNA. The bases in this compound assume a head-to-head orientation (Fig. 6) with the interbase dihedral angles of 81.8° and 87.5°. There are two independent complex cations in the unit cell. The left-... 

The NMR spectrum of the 1,4-oxazocine dione 101 showed no evidence of two equilibrium diastereomers thus, it was concluded that 101 exists in a single rotameric form <2004JOC4140>. 

Figure 2 Rotameric forms of 1 -(1 -methyl-2-pyrrolyl)-3,4-dimethylphosphole 36.

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

Functions were calculated from the constants given above using the rigid rotator harmonic oscillator method. The entropy was increased by R tn 2 because two rotameric forms are implied by use of the torsional frequency. Small but arbitrary adjustments were made in the assignment of the bending mode frequencies in order to reproduce the vapor pressure data of Scott et al. (1 ) as closely as possible. Calculated values of S (298.15 K) = 57.03 and S (340 K) = 58.70 cal k" raol" may be compared with 56.99 and 58.69, respectively, derived from the data of Scott et al. Internal rotation calculations would require a complex potential function in order to fit the data. The barrier to inversion (990 cm 2.8 kcal mol ) is slightly less than the barrier to... 

Figure 10.7 Rotameric forms of PNA and the structures of alkenic isosteres.

Figure 12.7. Rotameric forms of dopamine, a flexible jS-phenethylamine.

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