Osmium tetraoxide selectivity

Exercises 15 and 17 show that rule 2 should be applied cautiously in nucleophilic reactions of unsaturated ketones. This is true also for electrophilic reactions for example, osmium tetraoxide selectively oxidizes 40 at the isolated double bond (yield -90%),52 despite the fact that its HOMO is lower in energy. 

Unlike palladium(II), osmium tetraoxide and ruthenium tetraoxide catalyze the dihydroxylation of one or both double bonds of an allene. The osmium tetraoxide-catalyzed dihydroxylation of unsymmetrically substituted allenes 45 can lead to two different a-ketols, 46 and 47, depending on which of the double bonds is oxidized. David et al. studied this reaction using NMO as a stoichiometric oxidant and found good product selectivity in a few cases, but the yields were only moderate (Scheme 17.15) [16]. They showed that the most substituted double bond was oxidized preferably when the bulkiness of the allene substituents did not interfere. 

An alternate entry to the narciclasine class of alkaloids has provided access to compounds related to isonarciclasine (263) (Scheme 24). In the event, the aryla-tion of p-benzoquinone with diazonium salts derived from the aryl amines 250 and 251 yielded the aryl-substituted benzoquinones 252 and 253, respectively (146). The selective hydroxylation of 252 and 253 with osmium tetraoxide provided the corresponding m-diols 254 and 255. Catalytic hydrogenation of 254 and 255 using Pd/C or Raney Ni and subsequent lactonization gave the triols 256 and 257 together with small amounts of the C-2 a-epimers 258 and 259. Aminolysis of 256 and 257 afforded the corresponding racemic tetrahydrophen-anthridones 260 and 261, whereas similar treatment of the a-epimers 258 and 259 led to the formation of ( )-isolycoricidine (262) and ( )-isonarciclasine (263), respectively. 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

Compound 191 was transformed into the exo-alkene 193 via the respective spiro epoxide the enone 192 (11%) was obtained as a side product (Scheme 24).97 Compound 193 was deprotected, and the triol obtained was selectively mesylated at the allylic position to give, after acetylation, compound 194 (68%). Treatment of 194 with sodium acetate resulted in the inversion of configuration at C-l to give the tetra-N,O-acetyl derivative 195. Oxidation of 195 with osmium tetraoxide in aqueous acetone, followed by acetylation, afforded 196 (87%) and 197 (13%), whose acid hydrolysis provided the free bases 5 and 37, respectively. 

Alternatively, synthesis of compound 215 (4-epimer of 208) started by initial inversion of the OH group at C-l of 207 (Scheme 27).35,96,99 101 Acid hydrolysis of 207 gave the triol 209 (100%), which was identified as its tetraacetate 210, whose allylic hydroxyl group was selectively sulfonylated with mesyl chloride to afford 211, which was then converted into the acetate 212 (65%). On treatment with an excess of sodium acetate in DMF, 212 afforded 213 (60%). Oxidation of 213 with osmium tetraoxide gave, after acetylation, 214 and 216. Furthermore, epoxidation of 213 gave a single spiro epoxide 214 (64%), which was transformed exclusively into 216 (83%)... 

Hydroboration of the methyl enol ether in (275) and subsequent oxidation provided crystalline (276). The selective hydrolysis of the protected lactol followed by treatment with methanesulfonyl chloride generated the sensitive dihydropyran (277), which was treated directly with osmium tetraoxide to give (278) and (279). 

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