Cyclopropenone acetals

To carry out tethered [3-i-2]-cycloadditions Nakamura et al. developed the new annulating reagents 123 [109, 110]. The reagents carry, in one molecule, two cyclopropenone acetals that are connected with an n-carbon methylene tether. Upon thermolysis in the presence of Cjq, the reagent undergoes [3-i-2]-cycloaddition reaction twice in a regio- and stereoselective manner to give Q- and C2-symmetrical bisadducts with cis-l - and cis-3-addition patterns, respectively. The selectivity varies... 

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). 

Nakamura and coworkers reported that the carbomagnesiation of cyclopropenone acetal 77 is significantly promoted by the addition of a catalytic amount of FeCls (Scheme 54) . ... 

The thermolysis of cyclopropenone acetals at 70-80 °C, generally in benzene solution leads to a ring opening which is formally described by the scheme below ... 

Primary allylmagnesium halides and allylzinc 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 acetals ... 

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