Brook rearrangement, radical

It is worth mentioning that in a few cases the (3-elimination of the silyl radical from the a-silyl alkoxyl radical (47) with the formation of corresponding carbonyl derivative was observed [63,64]. Evidently the fate of a-silyl alkoxyl radical depends on the method of radical generation and/or the nature of the substrate. Two examples that delineate the potentialities of this rearrangements are reported in Reactions (5.33) and (5.34). In the former, the 5-exo cyclization of secondary alkyl radical on the carbonyl moiety followed by the radical Brook rearrangement afforded the cyclopentyl silyl ether [65], whereas Reaction (5.34) shows the treatment of an a-silyl alcohol with lead tetracetate to afford the mixed acetyl silyl acetal under mild conditions [63]. 

In addition to the radical Brook rearrangement from a-silylalkoxy radical 45 to the siloxyalkyl radical 46 shown in equation 37, several 1,2-silyl migrations to a radical... 

Tsai and coworkers89,91,246,247 reported the synthesis of cyclic silyl enol ethers and silyl ethers by using a radical cyclization followed by the radical Brook rearrangement (equation 111). The cyclization of 4-bromo-4-stannylbutyl silyl ketones 188 in benzene with a catalytic amount of tributyltin hydride and AIBN gave cyclic silyl enol ethers 18989 91 247. The whole catalytic cycle proposed is shown in equation 112. 

Paredes, M. D., Alonso, R. On the Radical Brook Rearrangement. Reactivity of a-Silyl Alcohols, a-Silyl Alcohol Nitrite Esters, and (i-Haloacylsilanes under Radical-Forming Conditions. J. Org. Chem. 2000, 65, 2292-2304. 

Acylsilanes also function as intramolecular radical acceptors, but, interestingly, a radical-Brook rearrangement takes place to give the silylated cycloalkanols (Scheme 4-20) [40]. 

Radical cyclization reactions have proven to be a very efficient approach for polycyclic natural product synthesis. In many cases, the last step involves a reduction of a cyclic radical with formation of a new stereogenic center. Very good stereochemical control has been achieved with such polycyclic radicals. For example, Beckwith has reported a highly stereoselective formation of a quinolizidine ring (Scheme 19, Eq. 19.1) [41b]. This process is the key reaction in a four-step synthesis of epilupinine and the stereochemical outcome results from a stereoselective axial reduction by tin hydride of a bicyclic radical. In a related process, Tsai has prepared silylated hydroxyquinolizidine by radical cyclization to an acylsilane followed by a radical-Brook rearrangement (Scheme 19, Eq. 19.2) [42]. 

Treatment of alcohols that have an accessible i5-hydrogen atom with lead tetraacetate facilitates a 1,5-hydrogen shift of the initially formed alkoxy radical, followed by cyclization to ethers. When a-silyl-substituted alcohols are used, however, the alkoxy radical is thought to undergo a radical Brook rearrangement affording an a-silyloxy carbon readical, which is trapped by acetate to give the products, mixed acetyl silyl acetals. ... 

Paredes, M.D. and Alonso, R., On the radical Brook rearrangement. Reactivity of a-silyl alcohols, a-silyl alcohol nitrite esters and P-haloacylsilanes under radical-forming conditions,. Org. Chem., 65, 2292, 2000. 

Radical Addition Followed by Brook-type Rearrangement... 

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