Fusion positions

If there is a choice between components of the same size containing the same number and kind of heteroatoms, choose as the base component that one with the lower numbers for the heteroatoms before fusion. When a fusion position is occupied by a heteroatom, the names of the component rings to be fused are selected to contain the heteroatom. 

This intramolecular Michael addition, when followed by an aldol condensation provides a useful route to tram-octahydroindenes methylated in the ring fusion positions. 

Orf/io-fused and ortho- and pen-fused ring compounds containing hetero atoms are named according to the fusion principle described in Rule A-21 for hydrocarbons. The components are named according to Rules A-21, B-l and B-2. When the name of a component in a fusion name contains locants (numerals or letters) that do not apply also to the numbering of the fused system, these locants are placed in square brackets (as are also the locants for fusion positions required by Rule A-21.5). The base component should be a heterocyclic system. If there is a choice, the base component should be, by order of preference ... 

The transformation of penicillin-derived diazo ketones 696 into compounds 697a and b with the nitrogen atom in the ring fusion position on treatment by Cu(acac)2 proceeds via a sulfonium ylide with subsequent rearrangement (77T547 80TL2451 82H1647). 

The 3-methoxy-substituted steroidal olefins 5 and 7 (R = OCh3) were hydrogenated to the saturated products 6, with 5a-configuration, and 8, with the opposite 5/(-configuration at the fusion position, respectively1617. Other 3a-substituted cholest-5-enes 7 were investigated and the influence of the substituent on product stereochemistry recorded16. 

Fusion" Positions of Further substitution Formula no. max 111 nm (log e) Solvent Reference... 

Clicking CH and C fragments into fused hexagons requires an even number of C fragments, which occupy the ring fusion positions. The HC fragments will occupy the rest of the positions. The so-constructed molecules are drawn below ... 

Bicyclic molecules in which two rings share two or more carbons are even more important than the spirocyclic compounds. The simplest way in which two rings can share more than one carbon is for two adjacent carbons to be shared in a fused structure. The fusion positions, or bridgeheads (p. 121), are shown in red (Fig. 5.50). 

The hydrogens attached at the ring junction, or fusion positions, can be either on the same side (cis) or on opposite sides (trans). In practice, both stereochemistries are possible for larger rings, but for the small rings only the cis form is stable (see Problem 5.23). Figure 5.51 shows two compounds that contain fused rings. 

It is easy to connect two equatorial positions with a chain of four methylenes the hydrogens at the fusion positions (bridgeheads) must be axial... 

A more detailed empirical rule for the entropy of melting is listed at the bottom of Fig. 3.7. Three types of disorder make up the change on fusion positional (pos), orientational (or), and conformational (conf). The approximate contributions to AS are listed in brackets. The first term represents Richards s rule. It is the only contribution for spherical motifs. Irregular motifs can, in addition, show orientational disorder, and thus gain an extra 20-50 J/(K mol) on fusion. Flexible molecules, finally, have a third contribution to the entropy of fusion of 7 -12 J/(K mol) for each flexible bead within the molecule. 

According to a recent rule revision the numbering of each interior atom is now related to its lowest numbered peripheral neighbour atom by a superscript denoting the number of bonds between the two atoms. The following example stands among others for the intricacies involved in the application of orientation rules a) through e). Here the decision for the correct orientation is only possible after lowest locants for the fusion positions have been evaluated. 

It should be noted once again that hetero atoms in fusion positions are numbered sequentially by plain numbers and taken into account in the names of both components to be fused together. 

---