2.6- Diphenyl trimethylsilyl ether

The formation of other mono- [27-29] or even bis[alkoxy(alkenyl)allenylidene[ ruthenium complexes [28, 30] from the corresponding ruthenium chlorides and 5,5 -diphenyl-penta-1,3 -diynyl alcohol or trimethylsilyl ether in the presence of methanol (Scheme 3.13) and of the allenylidene complex 18 in the absence of methanol (Scheme 3.13) [30, 31] was also suggested to proceed via pentatetraenylidene intermediates. Neither one of these pentatetraenylidene complexes could be isolated or spectroscopically detected although their formation as an intermediate was very likely. 

Oxazoles with a free 2-position are metallated thereat by n-butyllithium, while 2,4,5-trimethyloxazole forms the anion (142) (81JOC1410). In the presence of a carboxyl or methoxycarbonyl group at C(4) the metallation is directed to the 5-position, even in the absence of a 2-substituent, to give, for example, the lithio compounds (143) and (144) (81TL3163). 2-Lithiooxazoles can equilibrate with open-chain isocyanides and the diphenyl compound has been trapped as a mixture of cis- and /rans-trimethylsilyl ethers (equation 9). 

An asymmetric cyclocondensation of S-SFs-salicylic aldehyde 114a or its bromo-substituted analog 114d with 4,4,4-trifluoro-2-butenal in the presence of a catalytic amount of (S)- or (R )-(—)-a,a-diphenyl-2-pyrtohdine-methanol trimethylsilyl ether and 2-nitrobenzoic acid gave related (S)- or... 

Conjugate addition of the y-butyrolactam to enals was promoted by diphenyl-prolinol trimethylsilyl ether 12 via the iminium activation process (Scheme 32, second hne) [53]. A satisfactory level of enantioselectivity was generally observed irrespective of the solvent polarity, although the use of aqueous acetonitrile was superior for optimizing the chemical yield and enantioselectivity. In addition, acidic additives had apparent effects on the reaction profile and the highest diastereoselectivity was attained with 2-fluorobenzoic acid. The synthetic utility of this site- and stereoselective transformation was demonstrated in a series of product derivatizations, including the three-step synthesis of a cAMP-specific phosphodiesterase (PDE IV) inhibitor (Scheme 32, third hne). 

Scheme 2.32 Tandem Diels-Alder-benzoin reaction catalysed by chiral diphenyl-prolinol trimethylsilyl ether and an Af-heterocyclic carbene.

Since the Michael adducts 24 possess a stereogenic sp -center in the a-position, conjugate addition can be conducted asymmetrically in the presence of chiral catalysts, for example, with a,P-unsaturated aldehydes in the presence of diphenyl-(S)-prolinol-trimethylsilyl-ether/NEt3 [142]. 

The etiolate intermediate, generated by the addition of higher-order cyanocuprates to enones, has been trapped with several electrophiles. Thus the addition of trimethylsilyl chloride, diethyl or diphenyl phosphorochloridate and iV-phenyltrifluoro methane-sulphonamide affords the corresponding vinyl silyl ethers, vinyl phosphates and vinyltri-flates. " ... 

The reaction is carried out under a dry nitrogen atmosphere. To a mixture of 7.32 g (40 mmol) of ( )-bromophenylethcnc and 0.20 mmol of the palladium complex are added 100 mL (80 mmol) of a 0.8 M solution of [a-(trimethylsilyl)benzyl]magnesium bromide in diethyl ether at —78 °C. The mixture is allowed to warm and stirred at 0 "C for 2 d and then hydrolyzed with 10% HCI at 0 C. The organic layer and ether extracts from the aqueous layer are combined, washed with aq NaHCG3 and then water, and dried over anhyd MgS04. The solvent is evaporated and the product isolated by distillation yield 10.1 g (93% ) bp 135-139 JC/0.9 Torr [a]p° —43.9 (c = 1.0, benzene) 95% op (determined by hydrogenation and direct comparison with an authentic sample prepared via asymmetric hydrosilylation and correlated with 1,3-diphenyl-t -propanol). 

The established routes to organo-selenoketones have been reviewed." New methods of preparation of these important synthetic intermediates include the oxidation of alkenes, e.g. using diphenyl diselenide, bromine, and hexabutyl-distannoxane (Scheme 2)." For terminal alkenes alternative procedures involve the intermediate formation of the corresponding bromoselenide or ethoxy-selenide the former species is converted into a phenyl selenomethyl ketone using silver hexafluorophosphate in DMSO followed by reaction with triethylamine." /3-Phenylselenoketones protected as the trimethylsilyl enol ether are available by reaction of an a/8-unsaturated ketone with phenyl trimethylsilyl selenide."... 

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