Lewis acid chelates

Dialkylboron enolates cannot accommodate an additional aldehyde ligand group and chelated TSs are not expected. When BF3 is used as the Lewis acid, chelation is... 

With a-benzyloxypropanal, using MgBr2 as the Lewis acid, chelation control is observed. The stereospecificity is determined by an anti orientation of the C-Sn bond... 

Hydride Sponge and Other Lewis Acid Chelates... 

When the same kind of reaction is carried out in the presence of a Lewis acid, instead of intramolecular hydrogen bonding, the formation of addition product with high diastereoselectivity can be achieved [18-23]. Eq. 10.10 shows the chiral addition of an isopropyl radical to Lewis acid-chelated chiral a,(3-unsaturated-A-enoyloxazolidinone 6-membered rigid ring [IY] in the presence of Lu(OTf )3 (Tf = CF3S02). 

Fig. 3 (a) Lewis acid chelation induces rigidity and electron-deficiency into the iV-acylhydrazone radical acceptor (LA = Lewis acid), (b) Benzyl substituent of 4-benzyl-2-oxazolidinone provides facial differentiation of the C=N bond... 

The most feasible pathway to coupled products is intramolecular ketyl addition to the Sm -coordi-nated ketone (see Scheme 6). Several examples of ketyl addition to Lewis acid activated carbonyls have been reported in the literature. Clerici and Porta have demonstrated in detailed experiments that intermolecular addition of ketyls to carbonyls can be a rapid process. Generally, ketyl addition to carbonyls is a reversible reaction. However, reversibility can be greatly affected by Lewis acid chelation of the complex, and further reduction of the radical intermediate (8) by the second equivalent of Smh would serve to make the process irreversible. ... 

In studies towards the synthesis of 4-deoxy-y-rhodomycinone (114a), the steric course of the cyclization of aldehyde 110 was reexamined [118]. Aldehyde 110 was obtained as before, via condensation of aldehyde 109 with leu-coquinizarine (86) [116] (Scheme 22). Reaction of 110 with tin tetrachloride gave tetralin 111, with the (R) configuration at C-10, as the major product (epimeric ratio 5 1). The observed stereoselectivity was attributed to the prevalence of the complex (Fig. 11) with the Lewis acid chelated by the... 

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

A series of polycyclic aromatic hydrocarbons (PAH) can be synthesized using a slight excess of pyridazine (1.1 equiv) with Lewis acid chelators such as 2,2 -di-bora-diphenyls generating the... 

The presence of additional Lewis base sites within the molecule can result in the formation of chelates with SnCU or TiCU, which can lead to 1,2- or 1,3-asymmetric induction with the appropriate substitution at the C-2 or C-3 centers. NMR studies have provided a basis for explaining the levels of diastereofacial selectivity observed in nucleophilic additions to Lewis acid chelates of... 

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