Yukihiro Fujita

SmI2-promoted conjugate reduction of α,β-unsaturated esters and ketones studied in comparison with Mukaiyama-Michael reaction of ketene silyl acetal

Abstract SmI2-promoted conjugate reduction of α,β-unsaturated esters and ketones proceeds in a manner quite similar to Mukaiyama-Michael reaction of ketene silyl acetal. The more substituted esters are reduced more preferentially than the less substituted ones. The substrates with a 12 or 16 membered-ring structure undergo reduction smoothly. On the other hand, the 6-membered substrates completely fail to react under the same conditions. These results indicate generation of intermediate enolate radicals to be a key step for the conjugate reduction.

Activation and synthetic applications of thiostannanes. A new method for synthesis of thio- and selenoglycosides

Abstract Exposure of thiostannane to acetyl or methyl glycosides in the presence of a catalytic amount of Bu 2 Sn(OTf) 2 provides thioglycosides in good yields. Selenoglycosidation is achieved in a like manner by use of selenostannane.

Diastereocontrol in Lewis acid-catalyzed Michael reactions of 4-siloxycyclopentenone with ketene silyl acetals: Stereoelectronic vs. steric effect

Abstract The title reactions are shown to proceed with sterically unfavorable syn preference (to the siloxy group) when steric demand of the acetals is trivial, whereas β-substitution of acetals results in reversal of diastereoselection. These results are discussed in terms of the stereoelectronic vs. steric effect.

The design of diastereoselective Mukaiyama-Michael reaction of ketene silyl acetal

Abstract The highly diastereoselective Mukaiyama-Michael reaction has been designed. Suppression of an electron-transfer process is crucial for this purpose and the following conditions should be satisfied. (1) TiCl 4 is employed as a Lewis acid. (2) Ketene silyl acetals have bulky siloxy and/or alkoxy group(s). (3) α-Enones have a bulky acyl group. The excellent syn -selectivity up to a 99:1 ratio is attained under these conditions. The selectivity is reversed to the anti -preference upon decreasing the size of the alkoxy group.

Diastereoselective Mukaiyama-Michael reaction of O,S-ketene silyl acetal

Abstract The highly diastereoselective Mukaiyama-Michael reaction has been realized by use of O , S -ketene silyl acetals. Of special importance in this reaction is the effective tuning of the stereochemical course simply by changing the double bond geometry of ketene silyl acetals. Thus, both syn - and anti -Michael adducts can be obtained on the synthetically useful levels of stereochemical purity.

Mukaiyama-Michael reaction of cyclic ketene silyl acetals and enones. Importance of ring flexibility in electron transfer process

Abstract Michael reaction of macrocyclic ketene silyl acetals or α-enones occurs smoothly under electron transfer conditions while the reaction of 6-membered analogs is more sluggish, indicating the importance of the ring flexibility to allow the α,β-carbon-carbon bonds of both reaction components to rotate when the radical species are generated.