Organometal addition

Heterocyclic Fused Tropylium Salts by Organometal Addition onto Tropones... 

Ketobases possessing a chiral center give rise to diastcreomeric mixtures of aminoalcohols upon organometal addition (Fig. 115). The same occurs when the asymmetric induction center is located on the reagent (see below). 

This reaction is highly stereoselective, usually to a larger extent than the corresponding reduction. The following are the main conclusions on the stereochemistry of organometal addition to a-asymmetric P-aminoketones that can be drawn from the great wealth of results reported in the literature "- ... 

Hydrogenation and organometal addition to the carbonyl group of Mannich bases of camphor and related compounds are also widely reported (see references in Chap. II,... 

MeLi-TiCU or PbEt4-TiCl4 ° reagents show high Cram selectivities in addition reactions to 2-phe-nylpropanal. An explanation was presented by Heathcock for the selectivities achieved by the Lewis acid promoted additions compared to simple organometal additions. In the latter cases, a trajectory is followed that brings the nucleophiles closer to H rather than to R, and asymmetry in R is transferred... 

Although the stereochemistry of the organometal addition reactions to carbonyls with a-alkyl ( i-cally a-methyl) substituents is explained mostly by the Felkin-Ahn model, different phenomena are observed with a-alkoxy- or a-hydroxy-caibonyls, which make the opposite ar-face sterically more... 

The stereochemistry of organometal additions to cyclic ketones has been extensively studied using various alkyl metals. The results are used to gain knowledge concerning the solution state of the particular organometals and to explore the driving force behind the steric course of the addition. 

The examples presented so far dealt with stereochemical issues of organometal additions to carbonyl groups. We now draw our attention to regioselectivity issues starting with Grignard additions to 3-substituted phthalic anhydrides. 

The diastereofaciality of organometal additions to carbonyl bonds is strongly influenced by precomplexation with suitable Lewis acids (chelate-Cram mechanism) [134]. 

Not only silcon, but also any other element of the second or a higher periodic row is able to stabilize electron excess and hence to facilitate organometal addition onto contiguous multiple bonds. For example, both dibutylvinylphosphine and diphenylvinylphosphine react with n-butyllithium or ter/-butyllithium to form the corresponding adducts. ... 

Numerous organometal additions onto ene-sulfones are reported. A sulfonyl group secures powerful activation, and moreover, it can be readily removed by reduction or elimination. This makes the sulfonyl function a cherished auxiliary in natural product synthesis. A rational entry to d-(+)-carbacyclin (in 4.7% overall yield by a triply convergent synthesis) exemplifies the approach. The addition of the silyl-protected trans-(5)-3-hydroxy-l-octenyllithium onto cyclopentenyl phenyl sulfone 162 triggers a condensation with the allyl chloride tail. The carbacyclin skeleton constructed, the sequence is terminated by protodesilylation (using tetrabutylammonium fluoride hydrate), reductive removal (with lithium) of benzyl and benzenesulfinyl, and selective oxidation of the primary alcohol site (Scheme 1-117). ... 

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