Silyl dichromate

The typical Phillips catalyst comprises chemically anchored chromium species on a silica support. The formation of a surface silyl chromate, and eventually silyl dichromate [scheme (29)], is significant during the catalyst preparation, because at the calcination temperature chromium trioxide would decompose to lower-valent oxides. Chromium trioxide probably binds to the silica as the chromate initially, at least for the ordinary 1% loading. However, some rearrangement to the dichromate at high temperature may occur. It is incorrect to regard only one particular valence state of chromium as the only one capable of catalysing ethylene polymerisation. On the commercial CrOs/silica catalyst the predominant active species after reduction by ethylene or carbon monoxide [scheme (59)] is probably Cr(II), but other species, particularly Cr(III), may also polymerise ethylene under certain conditions ... 

General Considerations. The following chemicals were commercially available and used as received 3,3,3-Triphenylpropionic acid (Acros), 1.0 M LiAlH4 in tetrahydrofuran (THF) (Aldrich), pyridinium dichromate (Acros), 2,6 di-tert-butylpyridine (Acros), dichlorodimethylsilane (Acros), tetraethyl orthosilicate (Aldrich), 3-aminopropyltrimethoxy silane (Aldrich), hexamethyldisilazane (Aldrich), tetrakis (diethylamino) titanium (Aldrich), trimethyl silyl chloride (Aldrich), terephthaloyl chloride (Acros), anhydrous toluene (Acros), and n-butyllithium in hexanes (Aldrich). Anhydrous ether, anhydrous THF, anhydrous dichloromethane, and anhydrous hexanes were obtained from a packed bed solvent purification system utilizing columns of copper oxide catalyst and alumina (ether, hexanes) or dual alumina columns (tetrahydrofuran, dichloromethane) (9). Tetramethylcyclopentadiene (Aldrich) was distilled over sodium metal prior to use. p-Aminophenyltrimethoxysilane (Gelest) was purified by recrystallization from methanol. Anhydrous methanol (Acros) was... 

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

Alternatively, pyridinium chlorochromate in dichloromethane has successfully been used to obtain both aldehydes and ketones, 516,517,540,547,607,785 - 788,790-802 most cases from the corresponding alcohols, although the silyl ether of a secondary alcohol has been oxidized to the corresponding ketone, In order to facilitate the reaction, sodium acetate, molecular sieves, " alumina, and Celite have been added (Table 2), The closely related reagent pyridinium dichromate, however, has been far less frequently utilized. ... 

The silylated methyl ester was then a-methylated with lithium diisopropylamide and methyl iodide in tetrahydrofuran. Reduction of methyl 10-( erl-butyldimethylsilyloxy)-2-methyldecanoate with DIBAL in ether at -78°C afforded the corresponding aldehyde. The 10- tert-butyldimethylsilyloxy)-2-methyldecanal was subsequently coupled in a Wittig reaction with 1-hexyltriphenylphosphonium bromide and n-butyllithium affording (Z)- and ( )-1 -(teri-butyldimethylsilyloxy)-9-methyl-10-hexadecene in a 9 1 ratio, respectively. Deprotection with tetrabutylammonium fluroride (TBAF) in tetrahydrofuran and final oxidation with pyridinium dichromate (PDC) in dimethylformamide resulted in a 9 1 mixture of (Z)- and ( )-9-methyl-10-hexadecenoic acid as shown in Fig. (7). As was also the case with acid 6, the stereochemistry at C-9 in 7 is not known. The key step in the synthesis of the allylic methyl group was a-methylation of a methyl ester, followed by reduction to the corresponding aldehyde, which was used in the subsequent Wittig reaction. 

Due to the relative instability of silylated corticosteroids, alternatives have been developed by Her and Watron [52] and Courtheyn et al. [47]. For dexamethasone and structurally related analytes, an oxidation reaction with pyridinium chloro-chromate has proven to be useful in several laboratories. Later, the procedure was improved by changing the oxidizing reagent to dichromate and reducing the reaction time. Furthermore, the application of a hydrolysis step permitted triamcinolone acetonide to be included in the analytical procedure [46]. As described above, special attention should be given to the identification of dexamethasone and betamethasone because of their similar mass spectra and inadequate peak separation. 

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