Synthesis bridged polycyclic systems

These transformations were applied to develop a new promising method for synthesis of various bridged polycyclic systems , viz. ketones 160 and 161. Tropone reacts with butenyl magnesium bromide (—78 °C, 75%) to form a mixture of 2-(3-butenyl)dihydrotropones 158 and 159, the pyrolysis of which (200-210 °C, neat or in heptane solution) leads to 60% total yield of the isomeric homoprotoadamantenones 160 and 161 and the tricyclic ketone 162 in a ratio of 58 18 24, respectively (equation 49) . 

The intramolecular Diels-Alder (IMDA) or hetero Diels-Alder (IMHDA) cydoad-dition reactions are often the key step in the synthesis of functionalized bicyclic bridged and polycyclic systems with high regio and stereoselectivity. The application of microwave irradiation to IMDA and IMHDA reactions may have significant advantages compared with conventional heating methods. Early examples of microwave activation in these reactions for synthesis of polycyclic frameworks, have been extensively reviewed by de la Hoz and will not be discussed herein [3jj. 

Abehnan, M.M., Oh, T. and Overman, L.E. (1987) Intramolecular alkene arylations for rapid assembly of polycyclic systems containing quaternary centers. A new synthesis of spirooxindoles and other fused and bridged ring systems. J. Org. Chem., 52, 4130-3. 

General.—Recent publications include a chapter on thiophens in a comprehensive treatise an article on recent advances in the synthesis of benzo[6]thiophens a published plenary lecture on condensed thiophens, especially thienothiophens, selenolothiophens, and related systems and a review on the synthesis of polycyclic thiophens from the direct insertion of heterosulphur bridges into vinylarenes, biaryls, and angularly condensed arenes, ... 

As in the case of acyclic compounds, the level of difficulty of the synthesis of a cyclic compound depends upon whether the molecule is a consonant or a dissonant system. However, some additional difficulties may be encountered in molecules with medium-sized rings as well as in polycyclic bridged compounds, which are treated in the next Chapter. On the other hand, as we have seen in Heading 4.3, even simple monofunctionalised cyclic molecules may require a FGA operation before bond disconnection of the cyclic network at the ipso-, a- or 13-positions can be effected. 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

Polyprenylated acylphloroglucinols, such as garsubeUin A (155) and hyperforin A (156), are bioactive molecules characterized by a polycyclic bridged system [32]. In 2005, Shibasaki et al. described a RCM-based total synthesis of ( )-garsubelhn A (Scheme 1.25) [33]. 

---