Polycyclic structure

The simplest polycycles stem from two triply-bridged points. These systems are exemplified by Trost et al. s 4,8-dihydrodibenzo[oi,g7z]pentalene (69) [87, 88], a precursor for a purturbed [12]annulene dianion, and Mislow et al. s double-bridged biphenyl derivatives (generally shown as 70) [89], where X is methylene, carbonyl, or various heteroatoms. Other longer bridged biphenyls include the triple-bridged cyclophanes 71 made by Hubert and Dale [90], their unsym-metrical relatives by Cram and Reeves [91], and the recent polyalkynyl cyclo-phane (72) made by Rubin et al. [92], a proposed fullerene precursor. 

Homeomorphs of the two-point bicycle show up in an interesting collection of structures, ranging from the simple cage-type bicyclophane (73) [93, 94] to the extended homeomorphs by Lehn et al. [95], Vogtle et al. [96], and Moore et al. [97]. Other molecular representations are Moore and Bedard s flat molecular turnstile (74) [98], Hart and Vinod s cuppedophanes (75) [99-102], and Okazaki et al. s bowl-shaped bicyclic cyclophane (76) [103,104]. 

The polycyclic version of the star graph is represented by Diercks and Voll-hardt s triangular [4]phenylene (87), or starphenylene [121]. Some earlier examples are trifoliaphane (88) by Hopf and Psiorz [122], its meta-linked isomer by Vogtle and Kissener [123], and decacyclene (89) for which the crystal structure was only recently reported [124]. 

PAHs are attractive structures for chemists working with graph theory. Pyrene (106) and coronene (63) [80] are among the simplest polycycles with the graphic presentations or inner duals [2,3] as shown. Coronene is a special case that could also be a monocycle of six points depending whether the central hexagon is counted as a unit. The dual concept, as described in Sect. 1, has been 

Large polybenzenoids have been extensively studied by Mullen et al. [ 143 -145]. Among various sizes and shapes of these superacenes , as he named them, are 107,108, and the 50-benzene-unit PAH (109). These compounds are generally constructed from their dendritic polyphenylene precursors in good yields, albeit difficult to characterize conclusively. Even larger sheets have been proposed and where the limits lie is still unknown. 

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When stereochemical complexity is embedded in topological complexity, such as in complex polycyclic structures, the stereochemical strategies which are most effective are those which are linked to both complexities. A decidedly different strategic approach is appropriate for topologically simpler systems. 

A number of bi- or polycyclic structures have been prepared over the years which are somewhat difficult to classify within the narrow confines of crown and cryptand nomenclature. Nevertheless, these molecules deserve mention and are noted here. 

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

Representative examples of the recent applications of controlled microwave heating in solid-phase synthesis of heterocychc compounds are summarized. The preparation of monocyclic compoimds is presented hrst, followed by a description of the synthesis of polycyclic structures. 

The natural products aristone, 190, bisaristone A, 191, and bisaristone B, 192, all contain an indoloindolizine unit within their polycyclic structures. These natural products have been isolated from the leaves and branches of the Australian mountain wine berry. Structural assignments are derived from spectroscopic data, and as yet no syntheses have been reported <1987JOC4527>. 

Reaction pathways leading to other polycyclic structures and spiroheterocydes are summarized in Scheme 6.277 [466, 467]. 

Several ingenious syntheses of natural products have been developed by exploiting benzcyclobutene ring opening to o-quinodimethane. Particularly, the intramolecular Diels-Alder strategy employing o-quinodimethane intermediates has been very effective for the construction of polycyclic structures. Selected examples are gathered in Table 12. 

As mentioned above (66, 393), (see Scheme 3.150) silylation followed by intramolecular enantioselective cycloaddition with five-membered cyclic nitronates, containing the hydroxyl group at C-4, can produce chiral polycyclic structures (293), which are direct precursors of chiral hydroxyamino acids (294) and aminopolyols (295) (Scheme 3.179). 

The polycyclic structure of manzamine A 74, an alkaloid with promising antitumor activity, constitutes an ideal testing ground for probing the effciency of RCM. Although no total synthesis of 74 has yet been reported, various approaches to this complex target rely on RCM-based strategies. 

Fluorination of3 and 4 appears to have produced the saturated analogues 5 and 6, respectively. Characterization ofthe perfluorinated products provides evidence that the fused polycyclic structures were retained. 

Two examples of polycarbonyl and polyamino compounds are shown in Figure 5.7. Ethylenediaminetetraacetic add (EDTA) is a synthetic ligand. The other, a siderophore, is biologically derived. Other polycyclic structures that coordinate with iron are also common. 

Quinophthalone pigments have a polycyclic structure derived from quinaldine and phthalic anhydride. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

Although this synthesis provides the most direct entry into the twistane polycyclic structure, the adequate balance between the problem of framework construction and the subsequent functional group manipulations, required by the "principle of maximum simplicity", is missed. However, the synthesis represents without doubt an outstanding contribution to the synthesis of polycyclic non-natural products. 

Intramolecular N-alkylation of tertiary amines, followed by Stevens rearrangement, enables rapid preparation of elaborate polycyclic structures [1235-1238]. Illustrative examples are given in Table 4.16. 

Asphalt or petroleum coke Non-volatile esidue Polycyclic structures... 

The combination of ring opening and ring closing metathesis is used for the synthesis of polycyclic structures by Lee et al. [265]. The metathesis on solid-phase includes not only the chemical transformation of resin bound intermediates but as well as the cleavage of final products from the support [181]. 

Let us return to the cases of myoglobin and hemoglobin and recognize something explicitly these two proteins contain a nonprotein constiment. This constituent, or prosthetic group, is heme, a complex polycyclic structure, protoporphyrin IX, containing an atom of iron (as Fe (II) or Fe +) at its center. 

The term diversity-oriented synthesis (DOS) is relatively new and, as mentioned above, is usually defined as the synthesis of complex, natural product-like molecules using a combinatorial approach and employing the full palette of modern organic reactions. It may be a subject of discussion what exactly qualifies a molecule as being natural product-like [4], and in most cases the similarity to an actual natural product seems reciprocal to the number of synthesized compounds. However, even in less complex cases, the products may be highly substituted polycyclic structures with defined stereochemistry, reminiscent of natural products [19, 20]. In these cases, a moderately complex backbone structure is subsequently modified with a well-established set of selective reactions to introduce diversity. 

This article deals with the conformational analysis of substituted oxanes (tetrahydropyranes) and derivatives in which ring methylenes are replaced by further oxygen atoms (di-, tri-, tetroxanes, pentoxanes, and O ) or by carbonyl group(s) (oxanones, Meidrum s acid derivatives) and, if conforma-tionally of interest, systems incorporating these rings in polycyclic structures... 

Besides the different regiodirections described earlier for the MCRs involving amizoazoles and cyclic CH-acids, a problem of positional selectivity in some cases also arises. In the most reactions analyzed, the formation of fused heterocycles with linear polycyclic structure is described, while angular products (such as compound 45 at Scheme 20) were not observed. In some publications an absence of such heterocycles was noted especially [84, 90-93]. 

Beckmann fragmentation of polycyclic structures can be of great utility and was a key step in the two synthetic routes for the synthesis of propellane 510 (equation 224). Both fragmentation processes are oxygen assisted. 

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