Zinc homoenolates preparation

One particular aspect of this chemistry is in the preparation of zinc homoenolates prepared from P-iodoesters (Scheme 2.33). This chemistry becomes particularly significant with p-iodoesters 2.95 derived from the amino acid serine 2.94. These are useful for the synthesis of novel amino acids by Negishi coupling with aryl and acyl halides (Scheme 2.34). Coupling of a complex peptidyl aryl iodide 2.98 with a zinc reagent 2.99 of this type was a late step in a synthesis of a cyclic tripeptide and aminopeptidase inhibitor OF494-III 2.100 (Scheme 2.35). ... 

The zinc homoenolate prepared by the treatment of the siloxycyclopropane with zinc chloride is a versatile synthetic reagent (eq 4). Reduction of 3-iodopropionate with activated zinc also produces a zinc homoenolate species. ... 

Carboalkoxycyclopentenones.3 The zinc homoenolate 1, prepared as shown (13, 349-350), can undergo a formal [3 + 2]cycloaddition to acetylenic esters in the presence of CuBr-S(CH3)2, ClSi(CH3)3, and HMPA to give 2-carboalkoxycyclopen-tenones. The reaction probably involves conjugate addition to give an allenolate followed by intramolecular cyclization. 

Amino acids are also prepared by adding zinc homoenolate to an arylazo tosylate, followed by a Raney nickel reduction of N-N hydrazine bond in ethanol at reflux." ... 

DeCamp et al.t19l synthesized the lactone intermediate of the 1-hydroxyethylene isostere with high yields and stereoselectivity. As summarized in Scheme 10 (Section 10.6.2), the titanium homoenolate is prepared from ethyl 3-iodopropionate. The iodide is metalated with zinc/copper couple to give the iodozinc homoenolate species. The alkyltitanium homoenolate is then generated by transmetalation of the iodozinc precursor with one of the several chlorotitanium isopropoxide species. The resulting titanium homoenolate reacts with a N-protected a-amino aldehyde, leading to a mixture of 45-diastereomers. In the last step, the product is lactonized. 

Zirconium ester homoenolates 43 or 44 can be prepared from the triethyl orthoacrylate 42 with the zirconocene complex of but-l-ene 41 (Cp means cyclopentadiene). These resemble the zinc and titanium species we have been discussing but are not derived from cyclopropanes.11... 

Reduction of 3-iodocarbonyl compounds (equation 2, n = 1) with a low-valent metal, - such as metallic zinc, generates homoenolates of esters, nitriles and ketones, and represents a convenient new entry to homoenolates. This reductive method also allows the preparation of higher homologs (n > 2). 

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