Temporary directing group

In 2011, by employing a triazolium catalyst, Zeitler and Connon found for the first time highly chemoselective cross acyloin condensation reactions between aliphatic and ortfto-substituted aromatic aldehydes, of which the ort/to-substituted group can serve as a temporary directing group to ensure... 

While this strategy achieves the objective, two steps are added attachment and removal of the temporary ligand. A shorter process is to introduce a reagent to the hydroformylation mixture that will attach and detach itself in situ (Scheme 4.66). Provided that the rate of hydrofomylation of the attached form 4.171 of the substrate is much faster than that of the detached form 4.170, then an efficient process will be achieved. As the temporary directing group is likely to also accelerate coordination to rhodium, this rate enhancement is to be expected. Several systems have been developed to put this into practice (Schemes 4.67 and 4.68). ... 

Reactions of Ketones Using a Temporary Directing Group... 

A chiral auxiliary is a temporary chiral group on a molecule that directs the stereochemical outcome of a reaction on another part of the molecule. Almost all synthetic routes involving chiral auxiliaries follow three steps (1) covalently attach the auxiliary to the molecule, (2) perform the reaction that forms the new stereocenter, and (3) remove the auxiliary. Under this model, use of a chiral auxiliary adds two steps to a synthesis because the auxiliary must be added and removed. Additional steps in a synthetic scheme require additional time, increase cost, and decrease the overall yield. Despite these disadvantages, chiral auxiliaries are fairly common in drug synthesis. 

As (—)-pulegone is not readily available, subsequent work addressed the natural enantiomer by different methods [12], but all completed their synthesis via Snider s enone (17). Asaoka utilised a TMS group as temporary directing volume (Scheme 5) [12a]. Cossy applied a photore-ductive cyclopropane ring opening, which was obtained by a diastereoselective Simmons-Smith reaction [12b]. Schmalz finally used a planar-chiral arene complex to synthesise enone 17 with interesting CrfCO), chemistry [12c]. 

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