Zinc carbenoids stereoselective

Enol ethers may also be cyclopropanated using zinc carbenoids stereoselectively. Furukawa cyclopropanation of enol ether 32 proceeds with high stereoselection, and the obtained cyclopropyl ether 33 can be easily transformed into the enantiomerically pure cyclopropyl alcohol 35 [30]. In this case, high stereoselectivity is achieved by employing the chiral diol 36, which is not commercially available. Using the commercially available enantiopure diol 37, the level of stereoselectivity is significantly lower (Scheme 7). 

Functionalized zinc carbenoids have been prepared from diiodoalkanes and diethylzinc and used in stereoselective transformations, but their use is limited by the availability of the diiodoalkane" ° and the stability of the resulting zinc carbenoid. Alternatively, the reaction of a diazoalkane with Znl2 can be used to access complex zinc carbenoids, but with modest efficiency (equation 9) . ... 

The stereoselective cyclopropanation of chiral alkenes can be divided into two classes cyclic and acyclic alkenes. Furthermore, within each class, a subdivision exists involving those that contain a proximal basic group that can direct the cyclopropanation reaction of zinc carbenoids and the others that do not. The discrimination of reactivity between alkenes that possess a proximal basic group and those that do not was first highhghted early on when Simmons and Smith noticed that the cyclopropanation of l-(o-methoxyphenyl)-l-propene was more efficient than that of the related meta and para isomers (equation 46). ... 

Stereoselective and stereospecific addition of the zinc carbenoid Solutnn... 

Synthesis of model compounds and structural units are being investigated. A double Simmons-Smith reaction on the l,3-dioxolane-4,5-diylbis(alkene) 107 afforded the product 108 with excellent stereoselectivity. The required asymmetry in the double cyclopropanation was the result of coordination of the zinc carbenoid reagent by the Lewis basic dioxolane ring oxygen prior to each cyclopropanation event. The cyclopropanated product was converted to ( )-l,2-bis[(l 5,25)-2-methylcyclopropyl]ethene, a relevant model for the complete structural assignment of FR-900848. 

On the subject of stereochemistry, note that the Simmons-Smith zinc carbenoid behaves like a singlet carbene— its additions to alkenes are stereospecfifc (the product cyclopropane retains the geometry of the alkene) as well as stereoselective (the carbenoid adds to the same face as the hydroxyl group). 

Other interesting selectivities are observed in the stereoselective cyclopropanation of acyclic chiral nonracemic allylic alcohols. For example, cyclopropanation of substrate 72 gave the syn isomer 73 as the major product in the case of the zinc carbenoid and the anti isomer 74 in the case of the samarium reagent. ... 

Charette and coworkers investigated the stereoselectivity of gem-zinc carbenoids in the reaction with allylic alcohols 100 and 101 (Scheme 1.52). Configuration at the allylic stereogenic center and alkene geometry affected the stereoselectivity of cyclopropanation [87]. 

The synthesis of vinylaziridines through reactions between allylic carbenoid reagents and imines (i.e., Darzen-type reactions) was first reported by Mauze in 1980 [13]. Treatment of aldimines or ketimines 16 with gem-chloro(methyl)allyllithium (17) afforded N-substituted vinylaziridines 18 (Scheme 2.6). Similarly, 2,3-trans-N-diphenylphosphinyl-2-vinylaziridines 21 were prepared with good stereoselectivities (trans cis= 10 1 Scheme 2.7) by treatment of a-bromoallyllithium (20) with N-diphenylphosphinyl aldimines 19 in the presence of zinc chloride [14]. 

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