1-Trimethylsilyloxy- -3-oxid

Pteridine, 2,4-bis(trimethylsilyloxy)-synthesis, 3, 297 Pteridine, 7-/-butyl-oxidation, 3, 305 Pteridine, 2-chloro-hydrolysis, 3, 291 Pteridine, 6-chloro-structure, 3, 266 Pteridine, 7-chloro-reactions, 3, 291 structure, 3, 266... 

In contrast, the titanium(IV)-catalyzed condensation of l-(trimethylsilyloxy)cydohexene and ( )-3-(2-nitro-l-propenyl)furan proceeded in the reverse stereochemical sense and gave three diastereomeric 1,2-oxazine 2-oxides lb, 2b and 3b in a ratio of 1 4 1. The major diastereomeric adducts 2b and lb were converted to the. syn-ketonc 4b upon hydrolysis16. 

The analogous reaction with 1,1,3-trimethyl-2-(trimethylsilyloxy)cyclohexene employing titani-um(IV) as the Lewis acid also proceeded in the same stereochemical sense and gave 1,2-benzox-azin-2-ium 2-oxide 6 as the only diastereomeric product in 75% yield16. 

Oxidative cleavage of l,2-bis(trimethylsilyloxy)cyclobutene with Br2, to give cyclobutane-l,2-dione. 

Whereas ethylene oxide gives with 17 at ambient temperature a quantitative yield of l-trimethylsilyloxy-2-iodoethane [5, 31], substituted epoxides such as 846b react with 17 to give 848 as the main product [32]. Excess 17, however, leads to the bis-iodo compounds 849 and HMDSO 7 [4, 5]. In the presence of DBU the epoxides 850 are converted by 17, which is generated in situ from hexamethyl-disilane 857 and I2, into the allyl alcohols 851 [4, 32] (Scheme 6.14). Cycloctene epoxide 852 is opened by SiCl4 at -78 °C in the presence of catalytic amounts of the asymmetric catalyst 853 to give 61% of the chlorohydrin 854 in 98% ee [33]. 

Because 2-trimethylsilyloxy sulfides such as 1154 and 1157 are hemiphenyl thioacetals of aldehydes, they are readily hydrolyzed to aldehydes [8-12] or ketones [13]. Thus alkylation of the lithium salt 1162 with cyclohexyhnethylbromide 1163, gives in nearly quantitative yield, the sulfide 1164, which, after oxidation with m-chloroperbenzoic acid and hydrolysis, rearranges in 70% yield to cyclohexylacetal-dehyde 1165 [8] (Scheme 8.2). A more detailed discussion of the formation of aldehydes is given in Section 8.5. 

Next is the construction of the D ring. The TMS enol ether of compound 111 undergoes oxidation with m-CPBA, providing the C-5a trimethylsilyloxy ketone 112. Addition of methyl Grignard reagent to the ketone group and subsequent dehydration provides compound 113. Osmylation of the C=C double... 

The trimethylsilyloxy (TMSO) group is stable under the coupling conditions in acetonitrile (Table 12, number 6). After oxidative dimerization the TMS-ether can be mildly hydrolyzed (H+ and H2O) to the phenol or converted to a dibenzofuran. 1,2-Dialkoxybenzenes have been trimerized to triphenylenes (Table 5, numbers 7, 8). The reaction product is the triphenylene radical cation, which is reduced to the final product either by zinc powder or in a flow cell consisting of a porous anode and cathode [188]. Anodic trimerization of catechol ketals yields triphenylene ketals, which can function as a platform for receptors, for example, in an artificial caffeine receptor [190]. 

Next, the TMS enol ether of 53c underwent oxidation with MCPBA to trimethylsilyloxy ketone 57. in 86% yield (86% conversion). Addition of methylmagnesium bromide in methylene chloride proceeded in almost quantitative yield (95%) to give tertiary alcohol 58. Dehydration with Burgess reagent [19] and acidic workup provided the allylic alcohol 59a in 63% yield, which was converted... 

Oxidation of 2-(trimethylsilyloxy)furan (301) with iodosobenzene in the presence of boron trifluoride etherate and alcohols or acids results in the formation of 5-substituted 2(5//)-furanones 303. The first step of this conversion gives intermediate 302, which on nucleophilic substitution by alcohols or acids affords the products (89TL3019) (Scheme 75). 

Analog erhalt man z.B. 3,4-Bis-[l-trimethylsilyloxy-cyclohexyl)-furazan-2-oxid [Schmp. 85-86 (Methanol)]. 

Bis-[l-methyl-l-trimethylsilyloxy-ethyl]-furazan-2-oxid setzt sich in Gegenwart von Alkenen bzw. Propinsaure-methylester ebenfalls zu 4,5-Dihydro-l,2-oxazolen um442 ... 

O-Silylation of 2-substituted pyrazole 1-oxides also activates lateral protons at the 3- and 5-positions. Thus the 5-methyl derivative 189 upon silylation furnishes the silyloxypyrazolium ion 190, which is deprotonated at the methyl group by PMP giving rise to a neutral species 191. Next, the iodide ion replaces the trimethylsilyloxy group of the intermediate 191 in an allylic type substitution to give iodomethyl-substituted pyrazole 192. The whole sequence 189 -> 192 takes place in one pot (1992JCS(P1)2555) (Scheme 54). 

The isomeric 2-benzyl-3-methyl-pyrazole 1-oxide 193 reacts similarly to give the 3-iodomethyl-pyrazole 196. In this case, the 3-trimethylsilyloxy-methyl compound 197 is formed as a by-product. Most likely, the sily-loxymethyl compound 197 arises when the liberated silyloxyanion acts as a nucleophile replacing the silyloxy group in a regenerative fashion. The silyloxy compound 197 is readily hydrolyzed to hydroxymethyl-pyrazole 198. The 3-methyl-pyrazole 1-oxide 193 reacts five times faster than the 5-methyl isomer 189 as shown by a competition experiment (1992JCS(P1)2555). The iodomethyl-pyrazoles 192 and 196 are versatile starting materials for further transformations in the pyrazole side chain (Scheme 55). 

Oxidation of 2-(trimethylsUyloxy)furan.1 Oxidation of 2-(trimethylsilyloxy)-furan with iodosylbenzene-BF3 etherate can afford 5-substituted 2(5//)-furanones. 

Very recently, chiral tricarbonylchromium complexes have been introduced as novel chiral auxiliaries for aza Diels-Alder reactions [192, 193]. Using the brominated imine 3-8, Kiindig s group was successful in efficiently generating enantiopure polycyclic compounds such as 3-10 by cycloaddition of 3-8 to l-methoxy-3-trimethylsilyloxy-l,3-butadiene (Danishefsky s diene), subsequent radical cyclisation of the cycloadduct 3-9 and oxidative metal removal from 3-11 (Fig. 3-3). 

Hydroxyketones are versatile intermediates in the synthesis of pharmaceutical intermediates and heterocyclic molecules. a-Aryl hydroxyketones have been prepared by reaction of aryl aldehydes with 1,4-dioxane followed by reduction with lithium aluminum hydride (LAH) and by the selective LAH reduction of a-silyloxy a,P-unsaturated esters." WissneC has shown that treatment of acid chlorides with tris(trimethylsilyloxy)ethylene affords alkyl and aryl hydroxymethyl ketones. 1-Hydroxy-3-phenyl-2-propanone (3) has been generated by the osmium-catalyzed oxidation of phenylpropene and by the palladium-catalyzed rearrangement of phenyl epoxy alcohoP both in 62% yield. 

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