Exendo bond

Cocyclic vicinal exendo bonds, especially in centrally-located rings may be selected for topologically strategic disconnection. Structures 124 and 125 are provided for illustration. One reason for the effectiveness of such disconnections is that it can signal the application of various annulation transforms. The broken bonds may involve heteroatoms such as N, O and S. 

Fused ring structures with sequences of contiguous exendo and fusion bonds in alternation may be strategic for disconnection. Such structures may be converted to linear or nearly linear precursors by cleavage of the successive exendo bonds, as shown in 126 127. Disconnections such as this can serve to guide the application of... 

A strategic bond must be an exendo bond within a primary (i.e. non-peripheral, or non-perimeter) ring of 4-7 members and exo to a primary ring larger than 3-membered. 

Exendo Bond. A bond which is directly attached to a ring and within another ring. 

Offexendo Bond. A bond connected to an exendo bond. 

A bicyclic system consisting of two aneUated rings can be viewed as a monocycle, which carries two substituents that happen to be closed to the other ring. In terms of retrosynthetic considerations, the four exendo bonds are important, i.e., those bonds that are endocyclic in one ring and exocyclic with respect to the other ring (Scheme 6.17). 

Scheme 6.17 Illustration of exendo bonds in an anellated hicycUc system...

The formation of an exendo bond rapidly increases complexity during synthesis, as it creates a branch (a substituent) on one ring and at the same... 

Scheme 6.18 Bond-sets for the synthesis of aneUated bicycles focusing on exendo bonds...

Of lower ranking would be disconnections in which only one exendo bond is targeted (Scheme 6.19). 

Annelation schemes provide a viable route to obtain bicycles. A much more rapid increase in complexity is achieved in bicyclization reactions. Here, bicycles arise directly from open chain precursors. Scheme 6.22 shows that again exendo bonds are considered in this two-bond disconnection, but it is a different pair of the exendo bonds than in anellation schemes ... 

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