Bisected structures

Extended Hiickel calculations have been carried out on the 1-cyclopropylvinyl cation 156 (122). These results show that the most favorable conformation for this ion is the linear bisected structure 156a. However, Hanack et al. (166b), by means of a modified CNDO technique, calculate the most stable geometry of the intermediate ion resulting from homopropargyl participation to be a bridged cyclobutenyl cation rather than 156a. 

The nmr spectrum of the 4,4-dimethylcyclopropylcarbinyl cation in superacid is only in agreement with the bisected structure of the ion (78). In Structure 78 the two methyl groups are not equivalent—one is cis to the cyclopropyl ring... 

Homans et al. [150] have analyzed the conformations of bisected structures by measuring the coupling constants 3J5 6. and 3J5 6. They conclude from a chemical shift analysis that the conformation of the 06 of the pentasaccharide is... 

From NMR spectroscopic analyses of bisected structures of the complex type [141, 164], it became clear that the relative orientation of the tx-(l-3) bond is only slightly affected in the tjr-angle, which then has a value of —20° after the attachment of a P-D-GlcJVAc at the 4-position of the P-D-Man. The p-D-GlcAAc at the 4-position of the P-mannose has cp = 60° and J/ = 10° (Table 10). 

The changes of the preferred conformation around the a-(l-6) bond at the p-man-nose was studied by NOE and coupling constant analysis in the NMR spectra of different bisected structures [146]. It is concluded that the preferred conformation of the co-angle is the gg conformation with coupling constants 3J56. = 3J5 6 = 2.0 Hz. This implies an interaction between the trisaccharide 6 -5 -4 - and the bisecting P-d-G1c7VAc 7. 

Figure 8.5. The methylcyclopropyl radical in various conformations, (a) The non-planar-bisected conformation is stable, (b) The non-planar-straddled conformation corresponds to methyl group rotation by 60°. (c) The planar eclipsed configuration is the transition state, (d) The planar-bisected structure corresponds to conversion without rotation. (From Zebretto et al. [1989].)...

As pointed out by the pioneer in those studies C4H has some of the characteristics expected for a very rapidly equilibrating mixture of classical cyclopropylmethyl, cyclobutyl, and 3-butenyl cations and yet other characteristics which wholly belie any description that implies conventional charge distributions or geometries derived from structural representations using solid lines representing two-electron bonds . The rapidly equilibrating ions referred to are shown in 59a, 59b and 59c respectively, and the structures with unconventional charge distribution are the bicyclobutonium ion (60) or the bisected structure (61). 

The bicyclobutonium ion (60) or the bisected structure (61) would both exist as a rapidly equilibrating group of three structures which would render the three CH2 groups equivalent over a short time span, a result well-documented by the H- and C-NMR spectra of the long-lived ions and by the scrambling in the products from isotopically labeled derivatives ... 

The cation 115 was directly observed by C-NMR and assigned the bisected structure shown The barrier to rotation was 7.2 kcal mol S as compared to 13.7 kcal mol for c-Pri Me2. 

H. Paulsen, M. Heume, and H. Nurnberger, Synthese der verzwiegten Nonasaccharid-Sequenz der bisected Structur von N-Glycoproteinen, Carbohydr. Res., 200 (1990) 127-166. 

Notice that the bisected structure of 15 is taken to reflect the stabilizing interaction " between the empty p orbital of the CHj moiety and the Walsh orbital of the cyclopropyl ring. 

NMR evidence indicates that it has a bisected structure (C symmetry), the most stable isomer having the methyl group in position trans relative to the ring (see Fig. 15). 

Evidence for the bisected structure of cyclopropylcarbinyl cations has been provided by solvolysis of deuterium-labelled derivatives. Attempts to determine the stereochemistry of the degenerate rearrangement of cyclohexenyl cations via cyclopropylcarbinyl intermediates have not been successful. ... 

Glycosylation of plant proteins diverges from the animal pathway after the action of Af-acetylglucosaminyltransferase II, by the addition of xylose and fucose residues (Figure 6). The xylose unit is attached in a P-1,2 linkage to the interior mannose residue of biantennary oligosaccharides to give bisected structures. Fucose is found... 

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