A-Silylallyl alcohols

Magnesium ion-mediated nitrile oxide 1,3-DC reactions to allylic alcohols have been reviewed <02SL1371>. New examples have been recently reported, in particular, cycloadditions of aromatic and aliphatic nitrile oxides with optically active a-silylallyl alcohols in the presence of magnesium cations. The substituted isoxazolines, which were obtained with high diastereo- and enantioselectivity, were smoothly converted to [1,2]-oxazine derivatives by treatment with TBAF. For example, oxazin-3-one (S)-58 was obtained in 81% ee starting from dipolarophile (S)-55 <02T9613>. 

Temporary stereogenic centers derived from optically active a-silylallylic alcohols can serve as a chiral primary alcohol equivalent, Thus, the benzyl ethers 12 and 13 are prepared by rearrangement of 11 followed by protodesilylation478. The racemic a-silyl alcohol moiety of 11 is obtained by Brook rearrangement of the terf-butyldimethylsilyl crolyl ether479. 

A Brook rearrangement-mediated 5e2 protonation in y-carbamoyloxy- and y-siloxy-a-silylallyl alcohols has been used to examine chirality transfer from an a-silylalcohol to a-carbamoyloxy- and a-siloxyallyl-carbanions and provide an evaluation of the propensity for racemization of lithiocarbanions. The reaction is thought to involve either (1) a concerted protonation of a silicate intermediate or (ii) lithiation of the intermediate followed by protonation with retention or protonation after racemization (Scheme 163). 

RhH(PPh3)4 efficiently isomerized fv-silylallylic alcohols to cr-silyl ketones in excellent yields, but only in the presence of an a-silyl enone as co-catalyst (Equation (13)).48... 

Silyl homoallylic alcohols are obtained with high y-regioselection and E-stereoselection on reaction of chiral alkoxy- and aminomethyl-substituted a -silylallyl carbanions with aldehydes factors which influence the diastereomeric excess have been identified. 

The use of [Rh(COD)2]BF4 2PPh3 as the catalyst appears to be a better choice for this type of reaction. Thus, the reactions of sec- and /erf-propargylic alcohols 134 with HSiEt3 promoted by this rhodium catalyst system provide the desired ( )-y-silylallyllic alcohols 135 with 100% /-selectivity and virtually complete stereoselectivity (E/Z = > 99/1) in excellent yields (equation 54)136,137. 

J-Silylallylic alcohols undergo rhodium-catalyzed double bond migration to give the corresponding a-silyl-substituted ketones in excellent yields . Thermolysis of 243 gives the corresponding silacyclopentenes 244 (equation 196). The reaction may proceed via an olefin to a carbene isomerization . ... 

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