Rearrangement acylsilanes

Nucleophilic addition to an acylsilane followed by a 1,2-Brook rearrangement and final trapping of the resulting carbanion in either an acylation or intramolecular... 

The novel [6+2] annulation approach developed by the Takeda group has also been included in a threefold anionic/pericyclic process (Scheme 2.149) [340]. The reaction leads to functionalized eight-membered rings 2-659 in a highly stereoselective manner, starting from acylsilanes 2-656 and 3-(trimethylsilyl)vinyl-lithium (2-657). After 1,2-addition and 1,2-Brook rearrangement, the cyclobutane 2-... 

Palladium-catalyzed cross-coupling of 19 with allyl bromide occurs exclusively at the vinylstannane moiety to give 1-ethoxy-1-silyl-1,4-diene 20. The following ether exchange with allyl alcohol causes the Glaisen rearrangement to give an acylsilane derivative 21 (Scheme 64).2... 

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). 

Recently, analogues of nucleosides [60], natural products Huperzine-A [61] and Hydroartemisinin [62], and inhibitors of metallo-/ -lactamases have been synthesised [63]. With acylsilane electrophiles, the initial adducts undergo Brook rearrangement which is interrupted by -Si bond fission with loss of fluoride anion (Eq. 16), leading to the formation of extremely useful difluoro-enol silanes [64]. Of the various fluoride sources employed, the tetrabutylam-monium triphenyldifluorostannate described by Gingras appears to be particularly effective. The numerous other methods for trifluoromethylation formed the subject of an exhaustive review [65]. More recently, the Olah group described a chlorodifluoromethyl trimethylsilane which is expected to have a rich chemistry [66]. 

OL, -Unsaturated acylsilanes (3). Allyl trimethylsilyl ethers (1) on metalla-tion with /-butyllithium (- 78 to - 30°) rearrange to organolithiums that on protonation form (a-hydroxyallyl)silanes (2). These silanes are thermally unstable, but can be oxidized by DMSO-oxalyl chloride or by NCS-S(CH3)2 to a,p-unsaturated acylsilanes (3).1... 

Scheme 12.8. Mechanistic sequence for the tandem three-component nucleophilic addition/Brook rearrangement/ alkylation on acylsilanes TBS = t-butyldimethylsilyl.

Scheme 12.9. Synthesis of <5-araneosene using DMF = dimethylformamide, TMS = trimethyl-a three-component nucleophilic addition/Brook silyl, Tf = trifluoromethanesulfonyl, rearrangement/alkylation on acylsilanes, by Hu dba = dibenzylideneacetone, dppf = and Corey [30]. TBS = t-butyldimethylsilyl, TAS- 1,1 -bis(diphenylphosphino)ferrocene, DME = F = tris(dimethylamino)sulfur (trimethylsilyl)- 1,2-dimethoxyethane. difluoride, THF = tetrahydrofuran,...

Scheme 12.10. Synthesis of a-onocerin using butyldimethylsilyl, THF = tetrahydrofuran, a three-component nucleophilic addition/Brook Tf = trifluoromethanesulfonyl, DME = 1,2-rearrangement/alkylation on acylsilanes, by dimethoxyethane, TMS = trimethylsilyl, Corey and co-workers [33]. TBS = t- TBAF = n-tetrabutylammonium fluoride.

The dilithio derivative of 1,4-bisphenylsufonylbutane 61 was formed prior to the introduction of homochiral acylsilane 56 into the reaction mixture. The nucleophilic carbonyl addition/Brook rearrangement/elimination sequence delivered bis (fi)-vinyl silyl ether 64 in high yield and with very high selectivity through the putative intermediates 62 and 63. This short and effective synthesis of 55, this time made as the major isomer, was then completed as described above for 54. 

Claisen rearrangement.1 Claisen rearrangement of simple allyl vinyl ethers effected with (R)- and (S)-l shows low enantioselectivity, but the rearrangement of allyl vinylsilyl ethers results in acylsilanes in high optical yield (80-90%). 

Four-membered rings containing three silicon atoms (compounds of type 2) were prepared by reaction of acylsilane with Et3GeLi in THF <1999JA8118> or by thermal rearrangement of trans-silane 32 <2006AGE6371>. 

Acylsilanes have been synthesized by several other approaches including (i) a reverse Brook rearrangement (silyl-Wittig rearrangement) of allyltriinethylsilyl ether followed by oxidation (Figure Si6.5)... 

Phenyllithium (and lithiophosphites) also bring with them sufficient stabilisation of the organolithium for Brook rearrangement to occur readily.40 With acylsilane 49, intramolecular Michael addition leads to cyclic structures 50. 

Nucleophilic addition of anionic species to acylsilanes followed by the Brook rearrangement provides an efficient route to siloxy-substituted carbon nucleophiles valuable for further carbon-carbon bond formation. The reaction of... 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

S-Substituted a-lithiated silyl enol ether 557 has been prepared by reductive lithia-tion of vinyl tellurides834 and sulfides835,836 with lithium 1 -(dimethylamino)naphthalenide (LDMAN). This intermediate 557 gave, after inverse Brook rearrangement, the enolate 558 and after hydrolysis the corresponding acylsilane (Scheme 151). 

Notably, the asymmetric Claisen rearrangement of ci j-allylic ot-(trimethylsilyl)vinyl ethers with the chiral aluminum reagent produced optically active acylsilanes with the same absolute configuration as those from trans-allylic a-(trimethylsilyl)vinyl ethers (eq 5). ... 

---