Triflates, vinyl cuprates

It should be noted that direct substitution of sulfonate or iodide derivatives at C6 is sensitive to steric hindrance. Kishi reported a failure in the substitution of a primary tosylate by vinyl cuprate in the synthesis of mycalamides where C4 had two geminal methyl substituents. An alternate five step route was developed to introduce the vinyl substituent [119]. Direct substitution of a primary triflate by an aryl Grignard reagent is a key step in the formal synthesis of (+)-apicularen A from D-glucal [194]. Displacement of a triflate with an alkynyllithium has also been used in the total synthesis of (+)-panaxacol [195]. 

Stannyl cuprates couple with vinyl halides or triflates [16c-d, 85], and a vinyl stannane produced this way has been used in the synthesis of 7-[( )-alkylidene]-cephalosporins [117]. Vinyl substitution reactions starting from dihydrofurans are... 

Scheme 3.42. Substitution reactions between a-aminoalkyl-cuprates and acid chlorides [166], vinyl triflates [167], and vinyl iodides (Boc = t-butoxycarbonyl) [168].

B. H. Lipshutz and T. R. Elworthy, Intermolecular couplings between vinyl triflates and allylic cuprates a mild and rapid atypical route to 1,4-dienes, J. Org. Chem., 55 (1990) 1695-1696. 

Vinyllithiums cf. 13, 142).10 An improved route to vinyllithiums involves coupling of vinyl and aryl triflates with tributyl- or trimethylstannylcuprates of the formula (R3Sn)2Cu(CN)Li2, prepared from LDA, R,SnH, and CuCN in THF at -50° to -60°. The higher-order cuprates couple with vinyl and aryl triflates at -20° to provide a vinyltin intermediate. Work-up of the intermediate with AgOAc provides a vinyltin, with bromine a vinyl bromide, or with CF,COOH an aryltin. 

Recently, the direct transformation of allyl- 206272 and aryl tellurides 209273 into the corresponding organocopper species 207 and 210 by tellurium-copper exchange was described. The resulting allyl 207 or aryl cuprates 210 were captured by coupling with vinyl triflates 208272 or by 1,4-addition to enones 211,273 respectively (Scheme 112). 

In a similar manner to vinyl triflates (Scheme 6), vinylmercury chlorides also undergo direct coupling with vinylcuprates leading to butadienes in ca. 60-66% yield, but only when a higher order cuprate, R3CuLi2, is used. ... 

An improved method that uses stannyl cuprates instead of hexabutyldistannane and which react even with hindered vinyl triflates and give the coupling products in good yields121 is exemplified in equation 99. 

Good enantioselectivity has been demonstrated in the transmetallation of the enantioenriched a-lithio carbamates to configurationally stable a-(N-carbamoyl)allq l cuprates and their subsequent reactions with electrophiles such as vinyl iodides, vinyl triflates, a-iodo-a,p-enoates, propargyl mesylates and allyl bromide (Scheme 11.27). ... 

Unsaturation can be maintained in the products of 1,4-addition reactions through in situ elimination of a p-leaving group. Such a reaction of orga-nocuprates with activated vinyl triflates presented scientists at Bristol-Myers Squibb with an opportunity for the rapid synthesis of 3-substituted cephalosporin analogs from a readily prepared starting material (Table 11.6). L.O. and H.O. cuprates, with and without the Lewis acid additive BF3 OEt2, were studied the use of BF3 OEta was found to suppress an undesired olefin isomerisation and, in combination with H.O. cuprates, afford an increased yield. 

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