Chelating groups chelation-controlled insertion

Conseqnently, the magnesinm chelate 71 can also react as a nucleophilic donor in aldol reactions. In the chemistry involving magnesium chelates, these two aspects model their mode of action as nucleophilic partners in aldol condensations. This is exemplified in aldol condensations of y-diketones . Thus, sodium hydroxyde catalyzed cyclization of diketone 73 to give a mixtnre of 3,5,5-trimethyl-cyclopent-2-enone 74 and 3,4,4-trimethyl-cyclopent-2-enone 75 in a 2.2/1 isomeric ratio (equation 100). When treated with magnesinm methanolate, the insertion of a a-methoxy carbonyl group as control element, as in 76, allows the formation of a chelated magnesium enolate 77, and the major prodnct is now mainly the aldol 78. This latter treated with aqueous NaOH provides the trimethylcyclopent-2-enones 74 and 75 in a 1/49 ratio. 

The thiourea functionality, inserted on the most frequently used chiral pyrrolidine scalTold, works excellently as reactivity and enantioselectivity control co-factor by chelating the nitro group of the acceptor. This solution, adopted in 25, provides a family of robust catalysts that afford high yields (up to 98%) and great stereoselectivities (up to 99 1 dr and 99% ee) in direct Michael additions of ketones to various nitroolefins in water. ... 

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