Collins reagent ethers

This is followed by hydrolysi.s of the ester moieties with potassium carbonate and reesterification of the carboxy moiety with diazomethane to produce intermediate 65. The solitary free alcoholic hydroxyl at C-9 is oxidized with Collins reagent and the silyl ether groups are removed with acetic acid to give enprostil (63) [15]. 

Protecting groups, including very labile ones, withstand the action of Collins reagent. The very labile primary TMS ethers are transformed into the corresponding aldehydes.103 As secondary and tertiary TMS ethers resist the action of Collins reagent, a protocol involving per-silylation followed by Collins oxidation allows the selective oxidation of primary alcohols in the presence of secondary ones.104... 

Although there are many published examples of silyl ethers resisting the action of Collins reagent, there is one report in which a diphenylmethylsilyl (DPMS) ether is transformed into the corresponding aldehyde by CrC>3 2Py in CH2CI2.105... 

Collins reagent is able to transform benzyl ethers into ketones and benzoates.110 Normally, this causes no interference with the oxidation of alcohols, because the oxidation of benzyl ethers demands more drastic conditions. 

Oxidation of 12 with excess Collins Reagent in methylene chloride yields the sensitive keto aldehyde 13. A pinacol cyclization of the keto aldehyde 13. to the tricyclic intermediate 14 is accomplisehd by reaction with finely powdered metallic Titanium inTHF. 2, 13 Oxidation to the ketol without appreciable glycol cleavage can be readily accomplished via an oxysulfonium intermediate. 14, 15 "phe ketol can then be converted to the MEM ether derivative by treatment with MEM chloride and diisopropylethylamine in methylene chloride at reflux to give the MEM ether derivative 15. ... 

Reagents i, NaBH4 ii, AcjO-pyridine iii, Collins reagent iv, Na-n-CsHuOH v, 0s04-pyridine-diethyl ether vi, 3,5-dinitrobenzoyl chloride-pyridine vii, OH ... 

In contrast to the Collins reagent, the complex formed by 3,S-dimethylpyrazoIe with chiomium(VI) oxide (2) is very soluble in dichloromethane. Hence, reactions (up to 0.1 mol scale) can be carried out in the minimum amount of solvent. Generally, 2.5 equiv. of complex, generated in situ, gives good yields of aldehydes and ketones. In addition, upon work-up most of the chromium salts may be precipitated by dilution with diethyl ether. 

Palladium-catalyzed cycUzation-carbonylation reaction of optically active bisallyl ether 209 was utilized by Aggarwal and coworkers [84] for the construction of 3-methylenotetrahydrofuranone framework 210 (Scheme 4.52). Subsequent oxidation of the C-2 carbon atom with Collins reagent afforded trans-f(,y-disubstituted-a-methylene-y-lactone 211 in good yield as single optically active diastereoisomer further exemplifying the feasibility of oxidation strategies in the synthesis of a-alkylidene heterocycles. 

The tritylsulfenamide can be prepared from an amine and the sulfenyl chloride (Na2C03, THF, H2O or Pyr, CH2CI2, 64-96% yield) it is cleaved by hydrogen chloride in ether or ethanol (0°C, Ih, 90% yield), " CuClj (THF, EtOH, 58-67% yield), MeaSil (77-96% yield),I2 (0.1 M, THF, collidine, HjO, 97% yield), BusSnH, 115°C, toluene, 5 min, 82% yield. " The tritylsulfenamide is stable to lA HCl, base, NaCNBHs, L1A1H4, tn-chloroperoxybenzoic acid, pyridinium chloro-chromate, Jones reagent, Collins oxidation and Moffat oxidation. The stability of this group is largely due to steric hindrance. 

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