Esters, 1,6-keto zinc homoenolate

Internal nucleophilic cyclization is one of the most typical reactions of reactive metal homoenolates (equation 3 and Scheme 5),1,2 and provides a convenient route to silyloxycyclopropanes (e.g. 8) through cyclization of 3-halo esters.18 Zinc homoenolates (9) also cyclize to cyclopropanes under suitable conditions. Treatment of the zinc homoenolate in CHCI3 with an acid chloride at room temperature gives an O-acylation product (Scheme 6), instead of a 4-keto ester (see Section 1.14.7.1).19 The reaction of the zinc homoenolate with MejSiCl in a polar solvent gives a silyloxycyclopropane (Scheme 6), providing a very mild route to silyloxycyclopropanes.19... 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

Zinc homoenolates (9) react rapidly with acid chlorides in ethereal solvents containing a dipolar aprotic solvent to give 1,4-keto esters in high yield (Scheme 23). 9 palladium catalyst (or, less effective, a copper catalyst) ° accelerates the reaction, in contrast to cyclopropane formation in halometh-ane solvents (see Section 1.14.3.1). 

The zinc homoenolate undergoes copper-catalyzed allylation with allylic chlorides. The reaction is not only extremely Sn2 regioselective but stereoselective for 5-chiral allylic chlorides. Arylation and vinylation of the zinc homoenolates proceed in the presence of a palladium-phosphine complex. Similarly, palladium-catalyzed acylation reaction gives y-keto esters (eq 6). 

Acylation and Carbonylation. The zinc homoenolate reacts more rapidly with acyl halides than with a-/3-enones to give 4-keto esters in good yield (X = Cl) (eq The Ni-catalyzed... 

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