Homoallylic silyl ethers, formation

The a-siloxyalkyl radical intermediate can be used for carbon-carbon bond formation by intra- and intermolecular trapping (Scheme 10.233). In the BUjSriH-mediated system, a bromoalkenoylsilane is efficiently converted into a bicyclic compound by tandem radical cyclization [601]. When allyltributylstannane is used instead of BuaSnH, the a-siloxyalkyl radical generated from a 5-bromoalkanoylsi-lane undergoes homolytic allylation to provide a homoallyl silyl ether [602]. 

Employing a silyl ether instead of 38 provided a connective assembly of homoallylic ethers. This three-component reaction leads to the formation of homoallylic ethers 45 via activation of carbonyl 6 by Lewis acid 17. The in situ generated oxo-nium cation 43 can then be trapped by the nucleophilic silyl ether 42 affording 44. The new species can then react with allyltrimethylsilane 1, to form the desired ether 45 with subsequent regeneration of the catalyst and loss of TMSOTMS 47 (Scheme 13.16). 

Admixture of alkynylstannanes, aldehydes, trimethylsilyl chloride, and InClj in acetonitrile at room temperature results in the formation of propargyl silyl ethers. In the synthesis of homoallylic alcohols, simple allylic halides can be used to form the tin halides in situ in water. ... 

Homoallylic alcohols with a silyl group attached to the terminal alkene carbon were cyclised to oxetanes in high yields by reaction with bis(.sy z-collidine)bromine(l) hexafluoroantimonate (e.g., Equation 31) <2001TL2481>. This reaction exclusively gave the four-membered cyclic ether, with the silyl group directing formation of the electrophilic intermediate for the subsequent 4-fvo-/rrg -cyclization. When the carbon /3 to the silyl group on the double bond was unsubstituted, the reaction was diastereospecific. 

From an extensive examination of the addition of allylindium reagents to a-oxygenated aldehydes 291 it has been established that the stereochemical outcome is dependent on both the a-alkoxy substituent and the solvent employed (Table 10-33) [197]. The silyl and benzyl ethers favor the formation of the anti homoallylic alcohol, whereas the MOM and the hydroxyl aldehydes favor the syn alcohol. The rate of the reaction is dependent upon the solvent (faster in a mixed THF/water solvent) and the pH (faster at lower pH). 

In the absence of Lewis acids, allylsilanes react only with very electrophilic ketones like hexafluo-roacetone, when they undergo ene reactions rather than electrophilic replacement of the silyl group. In the presence of Lewis acids, allylsilanes react with aldehydes and ketones with clean allylic transposition (Scheme 1) and the formation of homoallylic alcohols. The range of Lewis acids used is wide, but titanium tetrachloride and boron trifluoride etherate are the most common. Typically, reaction takes place somewhere between -78 °C and 0 C in dichloromethane solution, and a molar proportion of Lewis acid is used. 

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