Retrosynthetic preservation

The most useful general criteria for the assessment of acyclic strategic disconnections are summarized below. Most of these are based on the retrosynthetic preservation of building blocks and expeditious reduction of molecular size and complexity. 

Preserve Bond. A bond which is not to be broken retrosynthetically. 

Preserve Ring (or Other Substructure). A ring (or other substructure) which for strategic reasons is to be preserved during retrosynthetic simplification. 

Topological Strategy. The use of a particular bond, pair of bonds, set of bonds, or subunit as eligible for disconnection to guide retrosynthetic analysis conversely the designation of bonds or cyclic subunits as ineligible for disconnection (i.e. to be preserved). 

In contrast to Nicolaou s synthetic plan, the retrosynthetic analysis of Holton s approach preserves the non-synthetically significant B ring and proceeds through disconnection of bonds which are involved in the D and C rings, to arrive finally to the bicyclo[5.3.1]undecane derivative 32, as the starting material. 

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