Oxygen-containing tricycles

The most recent contribution in this area is by Shea who prepared tricyclic oxygen containing heterocycles from acyclic enynes 82 using a combination of intramolecular Nicholas and PKRs. They constructed [5.7.5] and [5.8.5] (83-84) systems involving the formation of a complexed cyclic alkyne. They used several PKR conditions that give different yields and diastereo-selectivities. Due to the strain of the intermediate, [5.6.5] systems (85) were obtained in poor yields (Scheme 24) [121]. 

In connection with the aforementioned reaction, Shea et al. reported the synthesis of [5,8,5]- and [5,7,5]-tricyclic oxygen-containing heterocycles via tandem Nicholas [35] and Pauson-Khand [23,24] reactions of acyclic enynes [36]. A typical Nicholas/[2 + 2 + 1] sequence is depicted in Scheme 12. 

The Pschorr reaction also works with substrates containing a bridge other than oxygen. Thus various tricyclic products containing a biaryl subunit are accessible, e.g. carbazoles and fluorenes. 

J-Oxygen-functionalised sp3 organolithium compounds react with alkenyl-carbene complexes to generate the corresponding cyclic carbene complexes in a formal [3+3] process (see Sect. 2.8.1). In those cases where the organolithium derivative contains a double bond in an appropriate position, tricyclic ether derivatives are the only products isolated. These compounds derive from an intramolecular cyclopropanation of the corresponding cyclic carbene complex intermediate [89] (Scheme 83). 

Trisubstituted X -phosphorins or X -phosphorins can be isolated unchanged even after long periods of irradiation when oxygen is excluded. Stade discovered that cyclic phosphinic acid esters 208 a-c which contain a cyclic butadiene (1.3)-moiety, photochemically rearrange smoothly to the tricyclic compounds 214 a-c. 

Among radical cations of n-donors we mention briefly those of 1,4-diazabicy-clo[2.2.2]-octane (99) and of the tricyclic tetraaza compound (100). For the bicyclic system a perfect correspondence has been reported between the AEs of the radical ion and the AIs of its precursor [276], The radical cation of the tetracyclic system, on the other hand, is significantly distorted. While the parent system has D2d symmetry and a b2 HOMO, the radical cation is distorted towards two equivalent structures of C2y symmetry (2E), with a two-center three-electron N — N bond [281, 282]. The dioxetane radical cations (101), invoked as intermediates in oxygenations via oxygen capture (Scheme 6), and characterized by ESR spectroscopy [8] contain analogous three-electron O—O bonds. 

Heteroaromatic cations containing oxygen, sulfur, or selenium ring heteroatoms are far more susceptible to pseudobase formation than the corresponding AT-methyl cations. A comparison of data for structurally related cations is given in Table IV. It is clear that in most cases pKR + values fall in the order O < Se < S NMe, and a linear relationship of unit slope exists between pKk+ values for at least two series (20 and 21) of cations (Fig. 2). The pKR + values reported81 for the linear tricyclic system 22 are unusual in that they fall in the order Se < O < S and furthermore suggest that in these cases 22 are less susceptible to pseudobase formation than their isomers 21, which is unexpected in view of the relative pKR,... 

Tricyclic heterocyclic betaines containing a benzotriazole moiety as one of its component ring systems as in (375) are photolyzed in the presence of oxygen and Rose Bengal ( 2) to give both IH- and 2H-benzotriazoles (376a, b) (8icci089>. The products arise formally from addition of 2 to the two mesomeric forms of (375). 

Granting that a tricyclic ring system takes precedence over a tetracyclic ring in simplicity, the other structural modifications, namely introduction of oxygen functions and iinsaturation, are high order deviations from the simple hydrocarbon. However, we are concerned only with compounds containing the steroid skeleton. 

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