Retro- -Brook rearrangement

Retro-Brook Rearrangement Retro-Brook Rearrangement ... 

Leaving the (retro-)aldol addition-initiated threefold anionic domino processes, we are now describing sequences which are initiated by a SN-type transformation. In particular, domino reactions based on SN/1,4-Brook rearrangement/SN reactions are well known. For example, the group of Schaumann obtained functionalized cyclopentanols of type 2-461 by addition of lithiated silyldithioacetals 2-458 to epoxy-homoallyl tosylates 2-459 in 41-75% yield (Scheme 2.106) [248]. 

The first example of a retro-Brook rearrangement 158 159 was discovered by West and colleagues tor the [l,3]-variant (equation 91) °. Subsequently, the [l,2]-variant 160 —> 161 was reported by the same group (equation 92) °. The rearrangement is much faster than the corresponding Wittig rearrangement. 

Verlhac and colleagues have shown the destannylative lithiation-induced [l,2]-retro-Brook rearrangement of siloxyvinyltins (equation 99) ° . This rearrangement provides the corresponding acylsilanes (equation 99). 

Representative examples of [l,2]-retro-Brook rearrangements are summarized in Table 8. 

Retro-Brook rearrangement of the [l,3]-variant will readily take place in sp and sp carbanion systems. Kuwajima and Takeda and Corey and Rticker have developed the [l,3]-retro-Brook rearrangement of silyl enol ether anions which provide a-silyl ketones (equation 100 and 101). 

Representative examples of [l,3]-retro-Brook rearrangement are snmmarized in Table 9. 

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

The [1,4]-rearrangement of the allyloxysilane system is a potentially useful method for allylsilane synthesis (equation 106). However, in general, a [1,2]-rearrangement may occur predominantly in this system (see Section n.F.l). To this end, the author s group recently found that the rearrangement of 171 in the presence of excess amount of HMPA provides the [l,4]-retro-Brook rearrangement product 172 in excellent yield (equation 107). ... 

The enantioselective version of a retro-[ 1,4]-Brook rearrangement was accomphshed as the subsequent reaction of enantioselective cyclocarbolithiation by Hoppe and colleagues (equation 111) . The cyclization precursor 179 was treated with s-BuLi/(—)-sparteine (24) in ether, providing the cyclized and silyl-rearranged product (/f,/f,/f)-180 in 58% yield as a single stereoisomer. 

Jung and Nichols have reported a tandem [l,2]-Brook/[l,6]-retro-Brook rearrangement of a l-(trimethylsilyl)-2,4-pentadien-l-ol anion to the aldehyde 181 (equation 112) . Representative examples of [1,5]- and [l,6]-retro-Brook rearrangements are summarized in Table 11. 

The sulfur analogue of retro-Brook rearrangement is reported by Wright and West . Furthermore, silyl groups will migrate from carbon to the adjacent carbanion center . The driving force for these rearrangements is the formation of a more stable carbanion. Representative examples are shown in equations 113 and 114 . 

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