Aromatic Patterns

The minor component from T. glandulosa was shown to be 12-demethox-ytabemulosine (43) on the basis of UV and H-NMR spectra, which showed an aromatic pattern compatible with the presence of protons in the I, 2, and 4 positions. 

One more structural dilferentiation that can influence functionalization is the tip vs. sidewall region. The tips are the two ending parts of the tube in these two regions, the curvature is increased and the shape more resembles that of a hemisphere, with reactivity expected to be similar to that of fullerenes. In contrast, sidewalls present reduced pyramidalization angles and therefore different behaviors towards functionalization. Reactions involving the use of harsh conditions can result in a fracture of the tubes, enabling production of shorter tubes with open tips, where the aromatic pattern is interrupted and carbon atoms are more reactive. 

The NMR-spectra (Table IV) showed that pleiocarpine and aspidofractine were unsubstituted in the benzene ring (note the characteristic downfield doublet with fine structure due to the C-17 aromatic proton which lies close to the V l-acyl group), while refractine had a C-17 methoxyl (position from UV- and NMR-aromatic patterns which resemble those of aspidospermine). The remaining two oxygen atoms so far unaccounted for in each of the alkaloids were located in a carbo-methoxyl group (IR-, 5.74-5.80 /a NMR-, ca. 3.7 S, methyl singlet) (82, 91, 92). 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

E. Brenna, G. Fronza, C. Fuganti, F.G. Gatti, M. Pinciroli, S. Serra (2004) Differentiation of extractive and synthetic salicin The aromatic pattern of natural 2-hydroxybenzyl alcohol. J. Agric. Food Chem. 52, 7747-7751... 

In summary, then, analysis of the structures of product alkyl-aromatics, patterns of substrate reactivity, and side reaction pathways in modified faujasite-catalyzed alkylations reveals great similarity to the corresponding features commonly reported for electrophilic aromatic substitutions in the presence of strong protonic acids or promoted Lewis acids. 

O (f> 2)io 3 Rat (liver). H 250 MHz COCI3 Aromatic pattern similar to that of 4-benzyloxy-benzolc acid. Spectrum also similar to that of cholesterol. 

Some spectra display extremely similar aromatic patterns for these sym-metrically substituted amidine groups. It can be inferred that one is observing an "averaged" structure via resonance in which the double bond is shared by the nitrogen atoms endowing them with identical shielding/ deshielding effects. This phenomenon is not observed in the unsymmetrical structures of this type. 

The sulfone substituted phenyl groups produce an aromatic pattern characteristic of a substituent which strongly deshields the ortho protons. The para and meta hydrogens overlap to produce a complex multiplet in the range from about 7.2-7.8 ppm while the ortho hydrogens appear as a higher order doublet of doublets at about 7.9 ppm. 

The aromatic pattern produced by the Tertiary Amide nitrogen atom depends primarily on whether there are one or two phenyl groups bonded to the nitrogen atom. The aromatic protons of the N-phenyl compounds appear as a complex band over the chemical shift range from 7.-7.6 ppm while those of the N,N-diphenyl compounds appear as a single peak centered at about 7.3 ppm. 

Fig. 3.4. Incorporation of the 2,4-dihalo-5-alkoxy aromatic pattern of oxadiazon into new phenyl tetrahydrophthalimide ring systems.

Sangster AW, Stuart KL (1965) Ultra-violet spectra of alkaloids. Chem Rev 65 69-130 Schripsema J, Verpoorte R (1991) Rapid identification of trace amounts of indole alkaloids analysis of the aromatic pattern from the H-NMR spectrum. In Atta-ur-Rahman (ed) Studies in natural products chemistry, vol 9. Elsevier, Amsterdam, pp 163-199 Schripsema J, Verpoorte R, Baerheim Svendsen A (1986) Trifluoroacetic acid, a H-NMR shift reagent for alkaloids. Tetrahedron Lett 27 2523-2526 Scopes PM (1975) Applications of the chiroptical methods to the study of natural products. Fortschr Chem Org Naturst 32 167-265... 

Recrystallisation from toluene gave beige needles, m. p. 186-188°. NMR 8 7.30-8.42 - complex aromatic pattern IR 1740 cm ... 

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