Allyl cyclopropenones

Allyl pyridinium betaines 441 isoelectronic with enol betaines 427 likewise reacted with diphenyl cyclopropenone by elimination of pyridine272,213 The product formation, different in aprotic and protic media (phenol 443 in aprotic solvent, A3,5-hexadienoic esters 445 in alcohol solvent), suggested that the diene... 

Bis(oxazoline) ligands have been shown to be useful in the many carbon-carbon bond-forming reactions previously listed. They have also been used in a myriad of other carbon-carbon bond-forming reactions. For example, Nakamura and coworkers used bis(oxazoline) ligands ent-2, ent-22, and ent-39 in ligand-induced enantioselective allylzincation. This reaction consisted of the transformation of the cyclopropenone acetal 198 into allylic cyclopropanone acetal 199 in yields ranging from 73 to 90% with selectivities from >98 2 for the isomer shown to 1 99 (Fig. 9.57). 

The first example of enantioselective allylzincation of an alkene was also reported for the cyclopropenone ketal 78 as substrate. The chiral allylzinc complex 135 was prepared from the corresponding bis-oxazoline derived from (,V)-valine by deprotonation with n-BuLi and transmetallation with allylzinc bromide. This reagent reacted with 78 and afforded the allylated product 136 with high optical purity ( = 99%) (equation 66)101. 

Primary allylmagnesium halides64-65 and allylzinc66 reagents react similarly, presumably by an ene-type reaction, to give selectively the cyclopropyl derivative in which the more substituted allylic carbon is attached to the ring (equation 24). Allylzinc compounds have been employed in the same manner in carbozincation of cyclopropenone acetals67. 

Variants of the Michael addition include the allylation of cyclopropenone acetals and the intramolecular Stetter reaction. So far, only moderate enantioselectivity for the latter reaction has been achieved. (Note that the same chiral catalyst is useful for benzoin condensation. )... 

Similarly, zincated hydrazone derivatives of type 56 undergo an intermolecular carbozincation of strained cyclopropene rings such as 57 leading to the adduct 58 with 92% yield [63]. This type of addition can be extended to ethylene ]63c]. It proceeds with an excellent stereoselectivity allowing the enantioselective synthesis of a-substituted ketones. Allylic zinc species also add to cyclopropenone acetals... 

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