Reductive opening of epoxide

With an operating catalytic cycle for the reductive opening of epoxides at hand, we decided to investigate the preparatively more important formation of carbon—carbon bonds. Inter-molecular addition reactions to a,(3-unsaturated carbonyl compounds have been described for the stoichiometric process of Nugent and RajanBabu [5], The general concept behind the catalytic conditions is outlined in Scheme 12.16. 

Table 12.2. Reductive opening of epoxide 1 with various titanocene complexes.

In summary, our studies have revealed that Sml2 is not a suitable reagent for the reductive opening of epoxides. The high Lewis acidity of this metal combined with the high nucleophilicity of the iodide ions leads to the for-... 

Apart from these mechanistic considerations, the Cp2TiCl/H20 couple has emerged as an excellent reagent for the reductive opening of epoxides, avoiding the use of conventional hydrogen-atom donors such as 1,4-CHD, A rt-butyl thiol, or BusSnH, which are toxic, expensive, and/or foul-smelling. 

Radical A-exo cyclizations can also be catalyzed without recourse to the Thorpe-Ingold effect by applying the concept of template catalysis (Fig. 24) [157]. Here, 20 mol% of a cationic titanocene(IV) precatalyst 82 is applied, which contains a coordinating neutral tether. After reductive opening of epoxide 81, a cationic titanocene(III) complexed radical 83 is formed, in which both the epoxide and the xfi-un saturated carboxamide radical acceptor are coordinated. This provides the template to accelerate the slow A-exo radical cyclization step considerably. Cyclobutanes 84 were isolated in 46-84% yield with mostly good / / [Pg.145]

Scheme 8.7 Proposed mechanism of Ti-induced deoxygenation and reductive opening of epoxides.

Reduction-addition. On reductive opening of epoxides in the presence of pyridines the radicals are intercepted, therefore 2-(P-hydroxyalkyl)pyridines such as chiral ligands 1 are readily accessible on the basis of this reaction. ... 

Tributyltin hydride-organotin iodide. Reductive opening of epoxides. The <... 

Reductive opening of epoxides. The selective opening of a, 3-epoxy ketones leads to (3-hydroxy ketones. A phosphine oxide is used as ligand. 

To convert enantiopure epoxides into secondary or tertiary alcohols, techniques for the regioselective reduction of epoxides are in demand. Again aluminum compounds have to be considered for chelation and complex formation. Being available as Lewis acids (e.g., DIBAH) as well as a nucleophile (e.g., Red-Al), they are particularly convenient for this endeavor. Their special properties come nicely into play in the reductive opening of epoxide 81 [27]. 

The borate (1) is a new reagent for the reductive opening of epoxides to alcohols, with reduction occurring at the least substituted carbon of aliphatic epoxides, and at the most substituted carbon when an aryl substituent is present (Scheme 2). This provides a pattern of regioselectivity which contrasts with other methods. 

SCHEME 12 Redox manipulations for the reductive opening of epoxide 2. 

Gansauer A, Bluhm H, Pierobon M. Emergence of novel catalytic radical reactions titanocene-catalyzed reductive opening of epoxides. J. Am. Chem. Soc. 1998 120(49) 12849-12859. 

Dilithium derivatives 102-105 were generated by reductive opening of epoxides derived from D-glucose, D-fmctose, estrone and cholestanone, respectively, and trapped with different electrophiles. In this way, 6C-substituted 6-deoxy-D-glucose, 3C-substituted D-psycose [85], 17C-substituted-17/8-estradiol and 3C-substi-tuted-3a-cholestanol [91] derivatives were prepared. 

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