Yamamoto’s reagent

Diastereospecific reduction of an enone. A key step in a synthesis of the lichen macrolide (+ )-aspicilin (4) is the reduction of the oxodienelactone 2 to the dienol 3 with Yamamoto s reagent (1) in 96% yield. Dihydroxylation of 3 with 0s04 and pyridine followed by H,S reduction gives the triol (4) in 40% yield. 

Introduction of an additive, e.g. TMS-Cl, did lead to good yields of an isolable product however, the product was shown by Alexakis to be the dialkylated ketone (e.g. 114) in all cases stuped (equation 30). Even the softer organometaUic, Yamamoto s reagent RCu-BFs , gave mixtures of mono- and dialkylated ketones with acrylonitrile and 1-cyclohexenecaibonitrile. ... 

The optimum results were obtained with Grignard reagents in the presence of 10 mol % of Cu(I)CN. The stereochemical course of this MIRC reaction can be explained by adopting Yamamoto s model for conjugate addition of cyano-cuprates to y-alkoxy-a,)5-unsaturated esters (Fig. 2) [35]. In this model, it is proposed that the larger substituent (L), in our case the tosyl group, will adopt the... 

At the exotic end of the Lewis acid scale is tetrafluorosilane (mp -90 5C, bp -86 UC) first proposed by Corey and Yi as a mild and selective reagent for the cleavage of silyl-protected alcohols with the reactivity order being EtiSi > f-Bu-Me2Si f-BuPhiSi/ 1 The substrate in dichloromethane or acetonitrile, is stirred at room temperature under an atmosphere of excess tetrafluorosilane provided by a gas-filled balloon. The reaction is slow in dichloromethane but quite fast (ca. 15 min) in acetonitrile. In the final step of Yamamoto s synthesis of the Hemibrevetoxin B [Scheme 4.40]61 the secondary TIPS and TBS ethers were removed from 40.1 with tetrafluorosilane. Identical conditions were used by Nicolaou et al to remove two TBS ethers in the final step of their synthesis of Hemibrevetoxin B.62 In the example shown in Scheme 4,41, deprotection with fluoride (basic) or cerium(lV) ammonium nitrate (CAN) in methanol (neutral) isomerised the angelate to the more thermodynamically stable tiglate.63 However, with tetrafluorosilane, no isomerisation occurred during the deprotection step. 

Yamamoto H., Yanagisawa A., Ishihara K., Salto S. Designer Lewis Adds for Selective Organic Synthesis Pure Appl. Chem. 1998 70 1507-1512 Keywords Lewis-acid reagents... 

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