Skeleton of a polycycle

An acyl radical formed from Se-Ph selenoester, can be used for the construction of a polycyclic skeleton in one-pot reaction [315, 316]. In this way, triquinane (+)- 

Aromatic acylation. An acyl group is introduced to the skeleton of a polycyclic aromatic hydrocarbon such as naphthalene when it is treated with sodium and a carboxylic ester in THE at room temperature. Aldehydes are formed when formic esters or DMF are used. 

An advantage of this methodology is that a strained or complicated molecule of the natural alkaloids can be built from simple compounds, such as commercially available carboxylic acids or amines, from which enamides are prepared. In addition, on cyclization, the skeletons of complex polycyclic and multifunctionalized natural products or their precursors, which are easily converted to the target alkaloids, can be formed in one step. 

A novel approach towards the construction of the morphine skeleton is demonstrated by the total synthesis of ( )-desoxycodeine-D. One of the key steps for this synthesis is the palladium-catalyzed intramolecular Heck reaction. Therefore, this synthetic strategy for the construction of the polycyclic ring systems has provided an efficient access to the complete pentacyclic skeleton of morphine <00TL915>. 

Carbon skeletons of some polycyclic aromatic compounds of the (a) benzenoid type and (b) non-benzenoid type. 

The skeleton of a polycycle is the edge-vertex graph defined by it, i.e. we forget the faces. By Theorem 4.3.6, except for five Platonic ones, the skeleton has a unique polycyclic realization, i.e. a polycycle for which it is the skeleton. 

Assume b < 11. Given a ( 5, b], 3)-plane, the gonality of a face in the major skeleton is equal to the number of (5,3)-polycycles E and C3 to which it is incident. Clearly, there are at most five such incidences for each face. Since the major skeleton is 3-valent, we reach a contradiction by Euler formula (1.1) and (i) holds. 

These two compounds have in common the presence of a polycyclic steroidal skeleton, and although in both syntheses the same strategy is used to build up ring A, the synthesis of cortisone shows a greater simplicity than the synthesis of conessine. 

Studies on the ABC -I- E ring construction (189) of the pentacyclic triterpene skeleton were recently described by ApSimon et al. The route has the advantage of rapid provision of a polycyclic species with preformed stereochemistry, countered by the disadvantage of uncontrolled stereochemistry in the cyclization step. 

Theorem 43.6 Given a graph G that is the skeleton of an (r, q)-polycycle different from (one of 5) elliptic [r, q], then die polycyclic realization is completely determined byG. 

The condensation of several rings with one another leads to the appearance of additional interaction depending both on the nature of the individual rings and on the stereochemistry of their linkage. The rigidity of a polycyclic skeleton leads to the interdependence of conformational effects in 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

The thermally and Lewis acid-promoted transannular Diels-Alder reactions have proven to be a powerful tool for the synthesis of steroids and other natural products [24]. A research team led by Takamura, Arimoto, and Uemura utilized this reaction to assemble the polycyclic skeleton of nakiterpiosin (1) [25]. Heating macrolide 39 at 160 °C gave 40 and 41 as a mixture of diastereomers in good yields. 

The key step in the enantioselective synthesis of a polycyclic target is the establishment of the first cyclic stereogenic center. Further construction can then be chrected by that initial center. This principle is nicely illustrated by a synthesis of (+)-estrone methyl ether (145), reported by Taber (Scheme 8). The specifically designed naphthylbomyl ester (141) is used to direct C—insertion selectively toward one of the two diastereotopic C—H bonds. The new ternary center so created then directs the formation of the adjacent quaternary center in the course of the alkylation. Finally, the chiral skew in the product cy-clopentanone (144) directs the relative and absolute course of the intramolecular cycloaddition, to give the steroid carbon skeleton. 

---