Tricyclic carbocycles

Where the diene function is attached to or is part of a carbocyclic system, the vinyl cyclopropane formation and subsequent rearrangement affords a reliable approach to the formation of tricyclic carbocycles. Some of the most elegant demonstrations of the use of this methodology in total synthesis of fused cyclopentanoid terpenes come from the work of Hudlicky and coworkers (equations 63-66)90 95"100. In these cases the diazoketones bear a carboxylate-substituted double bond of diene and the intramolecular cyclopropanation requires the combination of CuS04/Cu(acac)2 as catalyst. 

Bi- and tricyclic carbocyclic ring systems can be easily obtained from (1-alkynyl)carbene complexes in two or more reaction steps without isolation of intermediate products. For example, the bicyclic 1,3-dienone 44 has been generated from compound le in a two-step cycloaddition/annulation procedure involving [4+2] cycloaddition of a diene to the C=C bond of a (l-alkynyl)carbene complex and a subsequent Dotz reaction.68 Furthermore, a 1,4-dioxy dihydronaphthalene 45 was obtained from the silyl derivative lg in a similar reaction sequence. In contrast to the methyl derivative 44, the corresponding silyl derivative generated from lg undergoes facile... 

Tautomerism of benz- and dibenzazepines is much less common than with monocyclic azepines since, as pointed out in the introduction, with most of these bi- and tricyclic systems the number of tautomers in which the carbocyclic ring retains its benzenoid character is severely restricted. Rare examples in the benzazepine series are the thermal isomcrizations of butyl l-aryl-5//-2-benzazepine-5-carboxylates 1 (X = H, Cl, F) to their 3//-tautomers 2,7S and of 3-ethoxy-1-phenyl-5//-2-benzazepines 3 (R = Me, Bn) to the 1//-tautomers 4.240... 

When the cnolate of an enone is brought into reaction with an enone, usually a carbocyclic system is prepared by two consecutive Michael additions (M1MIRC reactions). Due to the lower temperatures employed and the absence of diene polymerization these reactions are useful alternatives for Diels-Alder reactions and proceed in general with high diastereoselectivities. When neither enolate nor enone is cyclic a monocyclic system is formed 338 which can be converted into a bicyclic system when the Michael addition is followed by an aldol reaction339. When, however, the enolate is cyclic a bicyclic or a tricyclic system is formed340 341. 

Grieco investigated the intramolecular Diels-Alder reaction of imi-nium ions in polar media such as 5.0 M lithium perchlorate-diethyl ether and in water129 to form carbocyclic arrays. They showed that water as the solvent provided good-to-excellent yields of tricyclic amines with excellent stereocontrol (Eq. 12.58). 

It is worth noting that product 443 was not obtained without silicon assistance, which means that the reaction is in fact initiated by an Si-[Rh] species (Scheme 112). Fused tricyclic benzenes such as 443 are formed exclusively using an exactly stoichiometric amount of silane. On the other hand, 2 equiv. of silane preferably lead to the formation of silylated benzenes such as 447 through a hydrosilyIation-carbocyclization-/ -hydride elimination cascade process. 

Also, the intramolecular [3+2] cycloaddition approach can be used to generate several tricyclic ring systems 66-68 when the azide and the cyanamide functionalities are bonded to a carbocyclic ring <20010L4091> (Scheme 14). The relative stereochemistry of the starting materials is preserved in the products. While the yield of the m-fused 5-5-6 tricyclic ring system 66 is very high, the yield of the trans-fuseA products 67 and 68 is considerably lower as expected based on the unfavorable conformation for the cycloaddition process. The even lower yield for the tosylated and therefore activated derivative 68 was rationalized by its decreased thermal stability. 

A radical tandem cyclization, consisting of two radical carbocyclizations and a heterocoupling reaction, has been achieved by electrolysis of unsaturated carboxylic acids with different coacids. This provides a short synthetic sequence to tricyclic products, for example, triquinanes, starting from carboxylic acids which are accessible in few steps (Scheme 6) [123]. The selectivity for the formation of the tricyclic, bi-cyclic, and monocyclic product depending on the current density could be predicted by applying a mathematical simulation based on the proposed mechanism. 

Another tetracyclic carbon skeleton, named cumbiane, has been isolated from Pseudopterogorgia elisahethae. Its representatives are the diterpenoids cumbiasin A (33) and B (34) [20] their structures and relative configurations were elucidated by interpretation of a combination of spectral data. The six-membered ring D was formed by connecting CIO and C16 of an elisabethane carbon skeleton. The carbocyclic skeleton of the cumbiasins is unprecedented and represents a new class of C20 rearranged diterpenes. The tricyclic seco-cumbiane skeleton is derived from the cumbiasins by cleavage of the C15-C16 bond. Due to intramolecular cyclizations two additional oxo-heterocycles are present in cumbiasin C (35) [20] (Fig. 7). 

Cascade silylcarbocyclization reactions tiave been developed based on the fact that it is possible to realize successive intramolecular carbocyclizations, as long as the competing reductive elimination is slower than the carbometallation. For example, the reaction of dodec-6-ene-l,ll-diyne 67 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at 50°C under 1 atm CO gives bis(exo-methylenecyclopentyl) 68 in 55% yield [44]. The reaction is stereo-specific that is, (6 )- and (6Z)-dodec-6-ene-l,ll-diynes, ( )-67 and (Z)-67, afford R, R )-68 and (S, R -68 respectively. A possible mechanism for this reaction is outlined in Scheme 7.20. It should be noted that none of the tricyclic product is formed even though a third carbocyclization in the intermediate III.2c is conceptually possible. 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

Tricyclic Systems Central Carbocyclic Ring with Fused Five-membered Rings... 

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