Organozinc reagents cyclic

This reaction was successfully applied to various hydrazones derived from acyclic or cyclic ketones and extended to a variety of unsubstituted vinylsilanes as acceptors (trialkyl, triphenyl and alkoxy vinylsilanes). In all cases, the regioselectivity of the addition reaction was consistent with the formation of a silicon-stabilized organozinc reagent. 

Nucleophilic displacement of anomeric sulfones. 2-Benzenesulfonyl cyclic ethers undergo nucleophilic substitution with various organozinc reagents, particularly with those formed by reaction of ZnBr2 with Grignard reagents, rather than an alkyllithium. 

An unusual syn addition to epoxides occurs when 1,3-diene monoepoxides are treated with organozinc reagents. Thus, the cyclic vinyl epoxide 72 was converted to the cis-ethyl-cyclohexenol 75 with diethyl zinc in methylene chloride and trifluoroacetic acid. The syn addition is believed to derive from an initial coordination of the oxiranyl oxygen to the organozinc compound, which then delivers the alkyl group to the same face. This transfer is facilitated by a relaxation of the sp3 hybridization brought about by the Lewis acidic zinc center and the allylic character of the incipient carbocation <020L905>. 

The new binaphthyl ligands 49 [22] and 50 [23] have led to a breakthrough in the copper catalyzed 1,4-addition of organozinc reagents to a,jff-unsatu-rated ketones, a reaction which has proved to be difficult if carried out enantioselectively. Selectivities up to 96 % ee have been reached with cyclic substrates, while acyclic substrates still leave room for further improvement. 50 has also been successfully employed in palladium-catalyzed alkylations of allyl acetates [24]. 

Progress with regard to this undesired substrate specificity was achieved with the phosphorus amidite 10, which catalyzes not only Michael additions of organozinc reagents to cyclic enones but also to chalcone (11 R = Ph) and related acyclic substrates. In the case of the addition of diethylzinc to chalcone, a good enantioselectiv-ity of 87 % ee was observed [8]. 

The reaction of simple cyclic vinyl aziridines with organozinc reagents typically provides the 1,4-addition product <2003TL8559>. Several binaphthyl phosphoramidate ligands were examined. In all cases, the /ra r-product 159a was the major product. When no catalyst was used a roughly 1 1 mixture of the trans. cis product was obtained. The use of the chiral phosphoramidate catalyst provided 159a with moderate % ee values (Equation 43). 

Enantioselective 1,4-Addition of Organozinc Reagents to Enones. Phosphite-oxazoline copper complexes are highly efficient catalysts for the 1,4-addition of organozinc reagents to 5-, 6- and 7-membered cyclic enones. Both the chiral oxazoline and the chiral phosphite unit have a significant influence on the enantioselectivity. 

Addition to C O bond. Excellent regioselective addition of organozinc reagents to one of the C=0 group of a cyclic anhydride (see equation below) can be attributed to precoordination to the electronically more favorable double bond. ... 

Nucleophilic Displacement of Anomeric Sulfones 2-Benzenesulfonyl cyclic ethers 106,108, and 110 undergo nucleophilic substitution with various organozinc reagents (Brown et al. 1989) (Scheme 7.41). 

Scheme 60 Copper-peptide-catalysed ECA of organozinc reagents to activated cyclic ketoesters by Hoveyda [92]...

Application of the organozinc reagents (237) (diethylzinc and/or di-methylzinc) to the Cu-catalysed asymmetric 1,4-addition to enones (236) gave cyclic products (238) using Cu(OTf)2 as a catalytic precursor and the phosphoric acid (239) (Scheme 62). ... 

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