Desilylation rearrangements

Cohen has used a soluble version of the Simmons-Smith reagent to promote the 3,2-rearrangement of allylic sulfides (Scheme 52). He has also applied this methodology to the synthesis of sarkomycin (225). The allylic sulfide (221), easily available via a Petrow reaction, was first alkylated and the sulfo-nium ylide was then generated by a fluoride-promoted desilylation. Rearrangement of the ylide (223) gave the homoallylic sulfide (224) which was eventually transformed into sarkomycin (Scheme 53). 

After the formation of tautomeric anions A=A. the anion A a rearranges to give the anion B, which reacts with the second nitroso acetal molecule to form a mixture of stereoisomers of silyl derivative 509a. After desilylation of 509a, oxime 510a is isolated. The reaction with the fluoride anion proceeds at low temperature, whereas the use of triethylamine is efficient only at room temperature. The yield of oxime (510a) is virtually independent of the reaction conditions, whereas the diastereomeric ratio varies substantially. 

The isopropenyl side chain may derive by elimination of a tertiary alcohol or ether as in 202. Such a masking of the olefin avoids a possible competing vinylcyclopropane rearrangement. The correspondence of the cyclopentene of 202 with the vinylcyclopropane in 203 now becomes obvious. The presence of the dimethylcarbinol side chain now also offers the opportunity for its introduction by addition of a cyclopropyl anion to acetone. The feasibility of creating such an anion by fluoride initiated desilylation... 

The first step is addition of Cl 1 to C4. We still need to form C10-C1 and break C4-C3. Since we have a 1,5-diene (Cl 1=C10-C4-C3-C2=C1), we can do an oxy-Cope rearrangement. This gives a 5-8 system in which we only have to form the C4-C2 bond. C4 is neither nucleophilic nor electrophilic, while Cl 1 is nucleophilic (conjugation from OSiMe3). Upon quenching with water, however, C4 becomes an electrophilic carbonyl C, whereupon Cl 1 attacks with concomitant desilylation of O to give the product. 

Ferrier-type rearrangement to give glycal 74 [25] (Scheme 18). Spiroketals 75 are then obtained after desilylation. A similar sequence of reactions based on cyclic sulfate 76 furnishes adduct 77 which, after desilylation to give 78, provides spiroketals 79 upon N-iodosuccinimide (NIS) treatment. 

Formation of a silyl enol ether will generate an allyl vinyl ether which after rearrangement can be desilylated to give a carboxylic acid. 

A related process involves fluoride-induced desilylation in HMPA at room temperature of benzyldi-methyl(trimethylsilylmethyl)ammonium halides (20) to (21 62-84% yields R = H, 2-Me, 4-Me, 2-C1,4-Cl, 4-OAc X = Cl, Br).9 100 Compunds (20) were prepared by reaction of the corresponding benzyl halide derivatives with (dimethylaminomethyl)trimethylsilane, Me3SiCH2NMe2. The product of Stevens rearrangement (22) formed competitively and predominantly from precursors (20) having strong electron-withdrawing substituents (R = 2-COMe, 2-CN, 4-CN, 2-NO2, and 4-NO2).100... 

It is notable that allyloxylation can also be performed in relatively good yields (Table 6) although allyl alcohols are easily oxidized anodically12. The allyloxylated sulfides thus obtained are easily converted into the corresponding ft, /-unsaturated ketones by a [2,3] Wittig rearrangement using bases as shown in equation 21. Anodic desilylation/carboxylation of a-thiomethylsilanes also takes place similarly as shown in an example in Table 6. 

The synthesis of a-hydroxy ketones is achieved by reaction of silyl enol ethers with mCPBA, with subsequent rearrangement. Aqueous work up or reaction with TBAF (fluoride ions) gives the desired product after desilylation... 

The salts 139 and 140a were desilylated with CsF to generate transient selenadiazoliumylmethanide (ylide) intermediates 245 and 248 at ambient temperature in dichloromethane (Scheme 32) <2001J(P1)394>. 4,5-Diphenyl-l,2,3-selenadiazol-3-ium-3-ylmethanide 245 undergoes cycloaddition-rearrangement reaction with excess... 

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