Silyl ester migration

An entirely different approach to the introduction of isolated double bonds is illustrated by the selective silylation of the 3-hydroxy group of 155 and Swern oxidation at C-2 to give 156 following spontaneous elimination. Final conversion of the enone to the free sugar 157 is accomplished by carbonyl reduction that induces an ester migration (Scheme 17).176... 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

The cyclization of ethyl (5)-( ,)-6-tert-butyldiphenylsilyloxy-5-hydroxy-2-hexenoate (1) has been performed under thermodynamic conditions. Treatment of 1 with sodium ethoxide in ethanol, first at 20 °C, then at reflux for 4 hours, establishes an equilibrium and the silyl group migrates to the oxygen at C-5. Subsequent Michael addition of the primary hydroxy group to the a,/ -unsaturated ester gives the corresponding tetrahydro-2-furanacetate in 87% yield as a 67 33 (trans/cis) diastereomeric mixture, as determined by HNMR at 250 MHz. This result shows that the 1,3-asymmetric induction is poor compared to the iodocyclization reaction48- 4S>. 

The use of TBAF, a fairly basic reagent, for silyl ether cleavage can result in ester migration as the following example illustrates. ... 

Similarly, the reaction of the silyl ester 137 commences with a migration of the Me3Si group (reaction 59 137 - 138) the intermediate 138 serves as immediate precursor for the loss of C02 and formation of Me3SiO. Elimination of MeC02 is explained by transfer of a methyl group from 138 to 139 followed by dissociation. Loss of Me3SiOH may be accounted for by a mechanism comparable to that discussed above for 134 (reaction 59). 

The most thorough mechanistic investigation which supports an intramolecular silyl group migration has been carried out in connection with the method known as group transfer polymerization (GTP), f6r the polymerization of acrylic monomers182. This involves a catalysed Michael addition of silyl ketene acetals 89 to a,/ -unsaturated esters,... 

Aldol condensation. The formation of j8-siloxy ester derivatives from ketene silyl acetals and aldehydes may actually be an ene-type reaction involving silyl group migration. 

An extremely interesting development is the intramolecular silyl group migration from a silyl ester of thiophene-3-carboxylic acid to position 2 of the ring (Equation (54)) <9lSL33>. The yields are moderate. 3-Thienyllithium has been converted to 3-(tri- -butylsilyl)thiophene by reaction with n-BUjSiCl <94TL3673>. 

Acetyl-L-fucal underwent 0-3 to 0-i ester migration. A similar study of silylation reactions of these glycals was then undertaken. Several reagents gave 3-ethers selectively, the best selectivity... 

Acetates are the most commonly used of the esters for hydroxyl protection, particularly for peracylation (Figure 2.32) [47] although selective partial acetylation can be achieved, the tendency of acetates to migrate can lead to a loss in observed selectivity. In some cases, as with silyl ethers, migration can be sufficiently reliable to be synthetically useful for example, acetate migration can be used efficiently to lead to simultaneous release of the 4-OH and protection of the 6-OH in glucose derivatives (Figure 2.33) [48]. 

Silylacetic acids undergo a thermal rearrangement to afford the silyl ester (eq 3). The same migration of the silyl group is observed at lower temperatures when base catalysis is employed. ... 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

The reactions of the trimethylsilylketene 313 with silyl ketene acetals 314 provided /J,y-unsaturated esters 315 (equation 195) via a 1,5-silyl migration in the intermediate adduct 316461 462. 

---