Triisopropylsilyl enol ethers

Reaction of triisopropylsilyl enol ether with a combination of iodosylbenzene 18 and trimethylsilyl azide at -15 °C gives directly the /J-azido triisopropylsilyl enol ether 38 in a high yield. A mechanism involving the reductive -elimination of a-iodanyl onium ion 37, probably produced by ligand exchange of in situ generated PhI(N3)OTMS with silyl enol ether, was proposed. Addition of azide to the resulting a,/l-unsaturated onium ion explains the formation of 38 [58,59]. 

Direct 1,2-bis azidation of triisopropylsilyl enol ethers was possible when catalytic amounts of 2,2,6,6-tetramethylpiperidine-/V-oxyl (TEMPO) were used, at - 45°C. The reaction under these conditions was often highly stereoselective, since only one (trans) adduct was obtained. In contrast to the other bis azidations of alkenes, which proceed ionically or through cycloaddition, this addition is a free radical process [98], A radical pathway occurred also when cyclohexene was treated with PhIO-Me3SiN3-TEMPO the yield of 1,2-bis azide was doubled (80%) in relation to the system PhIO-AcOH-NaN3 (in both cases the trans adduct prevailed). 

Allylic azides, e.g., 1, were produced by treatment of the triisopropylsilyl enol ethers of cyclic ketones with azidotrimethylsilane and iodosobenzene78, but by lowering the temperature and in the presence of the stable radical 2,2,6,6-tetramethylpiperidine-/V-oxyl (TEMPO), 1-triso-propylsilyloxy-l,2-diazides, e.g., 2, became the predominant product79. The radical mechanism of the reaction was demonstrated. A number of 1,2-diazides (Table 4) were produced in the determined optimum conditions (Method B 16h). The simple diastereoselectivity (trans addition) was complete only with the enol ethers of unsubstituted cycloalkanones or 4-tert-butylcy-clohexanone. This 1,2-bis-azidonation procedure has not been exploited to prepare a-azide ketones, which should be available by simple hydrolysis of the adducts. Instead, the cis-l-triiso-propylsilyloxy-1,2-diazides were applied to the preparation of cw-2-azido tertiary cyclohexanols by selective substitution of the C-l azide group by nucleophiles in the presence of Lewis acids. 

Table 4. 1,2-Bis-azidonation of Triisopropylsilyl Enol Ethers Promoted by TEMPO79...

Treatment of triisopropylsilyl enol ethers of cyclic ketones with ammonium cerium nitrate (3 equiv) and sodium azide (4.5 equiv) in acetonitrile at — 20 °C gave a-azido ketones in good yields124. By varying the ratio of the reagents the yields were lower or the formation of byproducts, difficult to separate, increased. Mixture of diastereomers (ratio not reported) were generally obtained from substituted substrates, except from the bicyclic ketones 16. 

X-ray analysis of the product from racemic (4-methylcyclohexenyl)triisopropylsilyl enol ether showed the toluenesulfonamino group to be locked in an axial position105 107,... 

Reaction with azidotrimethylsilane.1 The triisopropylsilyl enol ether 1 reacts with (CHj)jSiN.-, and Q,H5IO (1 cquiv.) to form the /if-azido adduct 2 as the major product. The azido group of 2 is displaced on reaction with a number of carbon nucleophiles... 

Magnus reported the direct a- and )9-azido functionalization of triisopropylsilyl enol ethers using trimethylsilylazide and iodosylbenzene. In this mechanistically complex reaction sequence, it is believed that azidation of a carbon centered radical is occurring [78],... 

Nitration of silyl enol ethers. BU4NNO3 is activated by (CF3C0)20 toward reaction with triisopropylsilyl enol ethers. 

Enones. Oxidative desilylation of cyclic triisopropylsilyl enol ethers with CAN in DMF leads to enones. - ... 

A synthesis of a-azido ketones consists of the oxidation of triisopropylsilyl enol ethers in the presence of NaNj. Nitroalkanes act as aldol acceptors when their anions are exposed to CAN in the presence of silyl enol ethers. Conjugated ketones are obtained on subsequent treatment with triethylamine. 

The (3-azidation reaction of triisopropylsilyl enol ethers (Schemes 3.186 and 3.187) has been effectively utilized in organic synthesis [563-565]. Magnus and coworkers have developed a mechanistically different enone synthesis that involves treatment of (3-azido TIPS enol ethers 469 and 471 with fluoride anion to effect desilylation and concomitant (3-elimination to give an a,(3-enone [563]. Alternatively, the (3-azido group in 469 or 471 can be ionized with MesAl or Me2AlCl and the intermediate enonium ion trapped by various nucleophiles, such as an allylstannane, electron-rich aromatics and trimethylsilyl enol ethers, to give various (3-substituted TIPS enol ethers. Reduction of the (3-azido TIPS enol ether provides access to the synthetically useful p-amino TIPS enol ethers [563]. 

Magnus P, Lacour J (1992) New trialkylsilyl enol ether chemistry. DirecLbeta.-azido functionalization of triisopropylsilyl enol ethers. J Am Chem Soc 114(2) 767-769... 

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