Desily lation

The stereochemical outcome of the reaction of 1-43, formed from 1-40 by desily-lation, can be explained by assuming a pseudoequatorial orientation of the epoxide moiety in a pseudo-chair-chairlike transition state 1-44 which, after being attacked by the phenolic oxygen, furnishes the correct trans-fused stereoisomer 1-41 (Scheme 1.12). The conformation 1-45, which would lead to 1-46 seems to be disfavored. 

Further evidence for the intermediacy of a chiral metal enolate in the aldol process was provided in a subsequent publication (255). The authors found that this reaction could be equally well catalyzed by a Cu(I) complex (generated from the phosphine) and TB AT. Further, Tol-BINAPCuOf-Bu is also a competent catalyst for this reaction, underscoring the ability of the copper alkoxide to mediate desily-lation of the dienolsilane. The authors suggest that the dienolsilane effects the reduction of Cu(II) to Cu(I), although in light of the work of Lectka and co-workers (249) in this area, it seems equally likely that the phosphine mediates this reduction prior to introduction of the dienolsilane. Nevertheless, the intermediacy of a metal bound enolate seems assured. 

The Pandey group has developed a silver fluoride-promoted desily lation of tertiary bis(silyl)amines as an interesting alternative method to access azomethine ylides (Scheme 2.10).18 Notably, this method allows the generation of nonstabilized azomethine ylides under essentially neutral conditions. The starting materials are prepared by a three-step process, sometimes coupled into a single operation. For example, Boc-protected pyrrolidine 36 can be sequentially deprotonated and silylated twice in a one-pot reaction (Scheme 2.10). Removal of the Boc group and alkylation of the free amine leads to bis(silyl)amine 37. When this compound is treated with 2 equiv of silver fluoride in the presence of phenyl vinyl sulfone, rapid formation of products 39 as single stereoisomers results. 

Reduction of the ketone moiety in 24 with NaBH4 provides the corresponding alcohol as a 1 1 mixture of diastereoisomers, which is then transferred into the pentalene derivative by a Barton deoxygenation using thiocarbonyl diimidazole and tributyl tinhydride. Finally, desily-lation with AgNOs and KCN in EtOH provides the desired deprotected alkyne 25. 

The readily available diethyl 1,1 -dilluoro-1 -(irimcthylsilyl)mcthylphosphonate -- is particularly suited to fluoride-ion-catalyzed reactions. In the presence of CsF, it reacts with aromatic or heteroaromatic aldehydes in THF at room temperature to produce silylated adducts, which are readily hydrolyzed to produce diethyl l,l-difluoro-2-hydroxy-2-arylethylphosphonates in 57-87% yields. " Further applications of diethyl l,l-difluoro-l-(trimethylsilyl)methylphosphonate have been demonstrated in the synthesis of [l- C]2,2-difluoroethene. The fluoride-ion-induced desily-lation of diethyl l,l-drfluoro-l-(trimethylsilyl)methylphosphonate with catalytic amount of anhydrous CsF has been accomplished in the presence of conunercial [ C]formaldehyde. The key intermediate undergoes a Horner-Wadsworth-Emmons reaction to give the highly volatile [l-"C]2,2-difluoroethene, which is collected in 10-15% yields with a purity exceeding 97% (Scheme 2.16). "... 

Diene 2 was converted by five steps into 3, which (as a trimethylsilyl ether) underwent an intramolecular Diels-Alder reaction in refluxing toluene. Desily-lation and chromatography afforded 4 as the major cyclization products (endol exo = 5 1 total yield 84%). Alcohols 4 were elaborated by a lengthy (16 steps) sequence into the natural product. 

Aryl and alkyl-substituted diynes and tetraynes have been synthesized in good yields (82-99%) by TBAF-promoted desily-lation and Cu-catalyzed oxidative dimerization of triisopropylsi-lyl (TlPS)-protected acetylenes (eq 38). Copper acetate was used as oxidant in this reaction. Aryl- and alkenyl alkynes were made under similar conditions (eq 39). Pd/C with TBAF was used in ligand- and copper-free, one-pot, domino Halex-Sono-gashira reactions. Similarly, TBAF promoted the synthesis of 2-substituted indoles by a tandem Sonogashira/cyclization reaction of 2-iodoanilines and terminal alkynes. ... 

The diaIkeny1cyclopropano1 (403) is available from cyclo-propanone hemi-ketal (402) (Scheme 75). Thermal ring expansion gave the 3-a IkenyIcyc lopentanone silyl enol ether (404), which was regiospecifically alkylated to give, after desily lation, 11-deoxy-PGE methyl ester... 

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