Tricylic Systems

In the period covered by this review 8 bicyclic and 11 benzo-fused tricylic systems have been reported and these are shown in Table 1. For each system, one tautomeric form only is shown. 

The alcohol portion in hyoscyamine is tropine in hyoscine it is the epoxide scopine. Tropine is an example of an azabicyclo[3,2,l]octane system with a nitrogen bridge, whereas scopine is a tricylic system with a three-membered epoxide ring fused onto tropine. Note that systematic nomenclature considers an all-carbon ring system with one carbon replaced by nitrogen hence, tropane is an azabicyclooctane (see Section 1.4). 

More extended polyenes can cyclize to tricylic systems ... 

An analogous reaction via the carbon analogue of 72 [with NTs replaced by CH(C02Me)2 has given the corresponding tricylic system. In the latter case the intermediate organopalladium compound was trapped by tetraphenylborate or hydrogen (from HCOOH) [90-92]. 

It is generally assumed that the unusual CH acidity of 6,7-dimethyl-8-ribityllumazine is important for the understanding of the enzyme-catalyzed as well as the uncatalyzed reaction. Briefly, the position 7 methyl groups of 8-substituted 7-methyllumazine have pA values in the range of 8—9. " The anions are characterized by their 7-exomethylene structure. In cases where the position 8 substituent carries hydroxyl groups in the 2 or 3 position, a nucleophilic attack of C-7 of the exomethylene anion by the hydroxyl group is conducive to the formation of tricylic systems (Figure 9). 

When a suitable electrophile is positioned at C-3 of 277-benzo-l-thia-2,4-diazine 1,1-dioxides such as 86, bromo-nium-induced intramolecular alkylation occurs at N-2 rather than N-4 to produce the cycles 87 (Equation 2) <1998JME3128>. Similarly, the pyrazolo 4//-benzo-l-thia-2,4-diazine 1,1-dioxide analog 84 is considered to afford the linear tricylic system 88 through N-2 alkylation despite evidence that the precursor 84 is mainly the 4//-tautomer in solution (Equation 3) <2005JHC755>. 

INOC has been used for the synthesis of tricylic compounds having the taxane A/B ring system with an aromatic C ring (Eq. 8.68).105... 

Intramolecular nitrone or nitrile oxide cycloadditions have been used to construct tricylic ring systems 156 and 157 rom furanoside templates (Scheme 39). ... 

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