Fragmentation, fluoride-induced

The trimethylsilyl group can rigorously control and direct the Beckmann fragmentation leading to the regio- and stereo-specific formation of unsaturated nitriles (equations 41 to 43). Oxime acetate (57) fragments to the cij-alkenic nitrile (58) in 90% yield, whereas its epimer (59) affords the franj-alkenic nitrile (60) in 94% yield under the same conditions. Furthermore, fluoride-induced fragmentation can re-... 

Fluoride-induced fragmentation reactions were used in two stages of a synthesis of hexahydrocannabi-nol methyl ether (144 Scheme 52). One of the phenolic hydroxy functions in the resorcinol derivative (140) was selectively liberated from the SEM ether to give the diol (141), which was converted to the bis (trimethylsilyl) ether (142). Subsequent treatment with CsF resulted in a 1,4-elimination to the o-quinone methide (143) intermediate, which underwent an intramolecular [4 -i- 2] cycloaddition to give the product in good yield. 

This last section deals with the generation of o-quinodimethanes using fluoride-induced fragmentation reactions. Strictly speaking these are not vicinal desilylations but they bear a close affinity to the reactions discussed above. 

Ito and coworkers developed a mild and efficient procedure for generating o-quinodimethanes" as reactive intermediates in [4 -I- 2] cycloadditions. The key step in the sequence, illustrated here by the synthesis of Estrone methyl ether (146 Scheme 53) involves a fluoride-induced fragmentation of the o-[a-(trimethylsilyl)alkyl]benzyltrimethylammonium iodide (144) to give the o-quinodimethane (145), which underwent stereoselective intramolecular [4 + 2] cycloaddition to give the desired tetracyclic framework in (146). The entire process was conducted at room temperature. The annulation reaction is not limited to intramolecular cycloadditions intermolecular versions of the reaction proceed equally well. 

Whilst alkaline Iragmentation of hydroxysulfonates such as (160 Scheme 53) is accompanied by isomerization to give rrfl s-bicyclo[5.3.0]decenones, the fluoride-induced fragmentation of the silyl ether (161) leads exclusively to the cis isomer (162). This reaction type is limited to strained systems. Thus, the silyl ether (163) does not react at all under similar conditions. 

An interesting method for preparation and trapping of alkyl and aryl thioaldehydes was recently disclosed.23 Fluoride-induced fragmentation of a-silyldisulfides such as 8 gave thioaldehydes which were trapped in situ by cyclopentadiene in high yields (Scheme 5-V). In all these systems, endo adducts were the predominant stereoisomers. 

Fluoride ions induce a fragmentation reaction of SEM ethers resulting in loss of fluorotrimethylsilane, ethylene, and formaldehyde to give the deprotected alcohol (see section 1.2.4). However, compared with deprotection of ordinary silyl ethers, the reaction requires rather protracted reaction times, higher temperatures, or the presence of HMPA [Scheme 4.294]. Different combinations of fluoride source and dipolar aprotic solvent have been used. Deprotection of a SEM ether in a synthesis of Galbanolide [Scheme 4.295] was accomplished with tetraethyiammonium fluoride in hot DMSO whereas Ireland used excess cesium fluoride in HMPA at 115 C for 20 h to delete the SEM ether that completed the synthesis of Monensin [Scheme 4.296]. Fluoride-induced cleavage of a SEM ether in HMPA at temperatures as low as 45 has been re-ported. ... 

The fluoride ion-induced fragmentation of (328) is a mild and efficient way of generating of... 

The carbanion, formed by fluoride ion induced C-desilylation of 315, suffered fragmentation into ethylene, carbon dioxide and the cyanide 316, which on oxidation released... 

The authors explained the formation of these compounds as follows. The O-nucleophile initially attacks the C=C bond of the perfluoroolefin, forming a carbanion. The latter may be stabilized by elimination of the fluoride ion from the y-position of the CF2 fragment, giving a multiple bond. The olefin has a mobile fluorine atom in the allyl position, leading to intramolecular nucleophilic cyclization induced by the O- or N-nucleophiles. This process may be regarded as an example of the important role of the intermediate from the highly active perfluoroolefin, determining the direction of the subsequent attack of the O- and N-nucleophiles. 

---