Molecular skeleton

A classification based first on ion specificity, then on stmctural features has been suggested for the polyethers (7). Another method uses the presence of unsaturation or of aromatic groups in the molecular skeleton (8). In this review the compounds are classified based on the number of carbons in the backbone according to the numbering system proposed in reference 9. The carbon backbone or skeleton refers to the longest chain of contiguous carbons between the carboxyl group and the terminal carbon. 

The existence of alternative bond paths through a molecular skeleton as a consequence of the presence of cyclic subunits gives rise to a topological complexity which is proportional to the degree of internal connectivity. Topological strategies are those aimed at the retrosynthetic reduction of connectivity. 

Select benzene from the molecules on screen, and select Surfaces. Potential Map refers to an electrostatic potential map. Select Transparent to present it as a transparent (actually translucent) solid. This will allow you to see the molecular skeleton underneath. The surface is colored red in the n system (indicating negative potential and the fact that this region is attracted to a positive charge), and blue in the a system (indicating positive potential and the fact that this region is repelled by a positive charge). 

Molecular Orbital. A function made of contributions of Atomic Orbitals and delocalized throughout the entire molecular skeleton. 

The number of reaction sequences required for liberation of trimethylamine 4 indicates the degree of incorporation of a particular nitrogen into the molecular skeleton. Because of that feature, the Hofmann elimination has been used for the structural analysis of natural products, e.g. alkaloids. 

Hemin is shown on the right in Figure 22-7. It is shown beside the model of chlorophyll A to emphasize the astonishing similarity. The portions within dotted lines identify the differences. Except for the central metal atom, tl.e differences are all on the periphery of these cumbersome molecules. We cannot help wondering how nature managed to standardize on this molecular skeleton for molecules with such different functions. We cannot avoid a feeling of impatience as we await the clarification of the possible relationship, a clarification that will surely be provided by scientists of the next generation. 

The key intermediate 25 was prepared efficiently from aldehyde 23, obtained by reduction of nitrile 22 with Dibal-H. Treatment of 23 with the lithium salt of frans-diethyl cinnamylphosphonate furnishes compound 24 in 75 % yield and with a 20 1 ratio of E Z olefin stereoisomers. The stage is now set for the final and crucial operations to complete the molecular skeletons of endiandric acids A and B. 

However, the proposed in vivo transformation of anhydrovobasinediol to gelsemine might involve a series of more extensive alterations of the molecular skeleton which presumably would include (1) oxidation of the 2,7 double bond, (2) an acid-induced 2,7 shift, and (3) allylic rearrangement of the C-20 ethylidene side chain to a C-20 vinyl, so that a humantenine-type precursor... 

To probe the range of application of the new inclusion strategy (coordination-assisted clathrate formation) in different ways, directed structural modifications were undertaken starting from the basic constitution 1, for instance as to the molecular skeleton (basic structure) and/or the sensor section (functional groups). The formulae 7-24 show different possibilities of such variations. 

Special features which characterize the prototypical roof-shaped compound 26 are a rigid molecular skeleton and the type of sensor groups used. They render 26 a potential coordinatoclathrand 50). The capability of 26 in forming inclusion compounds is evident from Table 5. Here, nearly thirty different crystal inclusions of 26 are specified including as guest molecules various alcohols, acids, aprotic dipolar, and rather apolar species. 

Although theoretical estimates for neutral branching fractions of reactions such as 10 have been attempted, their reliability is suspect.32 In the absence of measurement, modelers have typically assumed equal branching fractions between channels in which one and two hydrogen atoms are separated from the molecular skeleton (e.g. 0.50 for reactions 10a and 10b). New assumptions based on recent experimental work have also been made.33... 

Figure 3. A sketch of the molecular skeleton of [Me2(rBuO)Si]2N 2Ca as derived from X-ray analysis...

Figure 3. Half-shell structure model for organosilicon compounds with different spacers along the C3 symmetric substituent axis and dihedral angles go(X3Y-YX3) determined for the molecular skeletons.

Obviously (Fig. 3), at distances Y" Y below 333 pm different dihedral angles u(X3Y-YX3) between the half-shell substituents are observed, and above 414 pm they are identical. The additional torsion D3J - D3 within the molecular skeletons, therefore, is a criterion for steric overcrowding, which via drastic cogwheel-meshing of the methyl groups in between the two molecular halves causes their extremely short non-bonded C "C distances (Fig. 2). 

The unique molecular skeleton of the C-nor-D-homosteroids represents significant challenges for organic chemists. The structure elucidation and the total synthesis of cyclopamine (3, also known as 11-deoxojervine), jervine (ll-oxo-3), and veratramine (4) are important milestones in steroid chemistry. Many synthetic strategies were developed in the 1960s-1970s for these targets. Notably, Masamune... 

The impulse to the study of these cycloadditions came from the discovery that 5-spirocyclopropane isoxazolidines (or isoxazolines) undergo a thermal rearrangement resulting in the production of selectively substituted tetrahydro-(or dihydro) pyrid-4-ones (Scheme 42) [64], In particular, cyclic nitrones gave ultimately N-bridgehead bicyclic ketones, molecular skeleton of many alkaloid families [65]. 

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