Deprotection desilylation

In order to establish the bridgehead carbon configuration with certainty and to demonstrate the usefulness of the method, desilylation and deprotection of 438b in acidic conditions have been performed. Since they are accompanied by double bond migration and loss of chirality, a dimethylcuprate alkylation has been carried out before acid hydrolysis (Scheme 59) [112]. 

Enyne 7a having a silyloxy group on the alkyne gives cyclic compound 8a having a vinyl silyloxy moiety, which is converted into methyl ketone 9a by desilylation. In a similar manner, enyne 7b affords bicyclic methyl ketone 9b in 68% yield after deprotection of the silyl group. However, ynoate 7c and yne-phosphonate 7d do not give cyclized compounds. Ene-ynoate 12, which is obtained by treatment of enol ether 11 with BuLi affords cyclic enol ether 13 in good to moderate yields " (Scheme 4). 

Appropriately functionalized resins can sequester excess nucleophiles from solution-phase reactions. Thus the calcium sulfonate resin 4 captures tetra-n-butylammonium fluoride (TBAF) from a variety of desilylation reactions.22,24 Polymer-bound tetra-n-butylammonium sulfonate and insoluble calcium fluoride are formed. The applicability of this strategy was illustrated for deprotection of (3-trimethylsilylethyl esters as well as silyl ethers. 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

The C-Si bond of an SMA can also be cleaved by oxidizing reagents like cerium ammonium nitrate (CAN). Starting from (V-bis(trimethylsilyl)methylazetidinones, treatment with CAN probably leads to the oxidation product of the two C-Si bonds, i.e., the corresponding disilylketal that is hydrolyzed into the formamide to give the N-H azetidinones (yields >80%). This constitutes an alternate and more efficient way to sequential fluoride-induced desilylation. Peterson olefination, ozonolysis, and formamide decomposition when deprotection of bis(trimethylsilyl)methylated azetidinones into NH-azetidinones is required.228,230... 

Oxasilacyclopentenes were shown to be competent substrates for a scandium triflate-catalyzed Mukaiyama aldol process (Scheme 7.35).104 Exposure of silacy-clopentene 121 and benzaldehyde to substoichiometric amounts of scandium triflate produced ketone 122 diastereoselectively.105 This ketone was proposed to form by addition of enolate 123, resulting from desilylation of 121,106 to benzaldehyde. A 1,3-Brook rearrangement then afforded 122 from 124.107 This ketone could be further functionalized through lithium aluminum hydride reduction followed by deprotection to afford triol 125 containing four contiguous stereocenters. Thus, the molecular complexity of silyloxyalkynes can be increased dramatically in just three operations. 

Compound 32 (82 mg, 0.12 mmol) was desilylated by treatment with TBAF in TH F at room temperature for 30 min. Following workup, the deprotected polyyne was oxidatively coupled in the presence of Cul (0.18 g, 0.92 mmol), TMEDA (0.35 mL. 2.3 mmol), and air in dry CH2C12 (220 mL) for 3 h at room temperature followed by 12 h at 4°C. Following aqueous workup, the solvent was reduced by approximately 80%, resulting in the formation of a bright yellow precipitate. Isolation of the precipitate afforded 33 (29mg, 51%) as a bright yellow solid [51]. 

The key [4+2] glycosylation followed by desilylation of the two phosphorylation sites resulted in a moderate 39% yield over two steps (Scheme 13.23). Phosphoramidite 153 was used to phosphorylate both of the resulting free hydroxy groups. A three-step protocol was applied for final deprotection, starting with azide reduction with Zn and acetic acid, then removal of cyanoethoxy phosphate protection with DBU, and finally... 

---