Double Osmylation

Similar to the case of C60,105,223 some exploratory work on the regio-chemistry of single and double osmylation of C70 was done by Hawkins and co-workers.224 A major and a minor mono-adduct were assigned the Cs-symmetric C(l)-C(2)- and C(5)-C(6)-adduct structures (cf. Figure 1.6), respectively. After re-submission of the mono-adducts to osmylation, a total of nine compounds was reported, but no individual structural assignments were made.224... 

Full details (see Vol. 24, p. 197, ref. 34) on the thesis of racemic arabino- and altro-, 5 itnino sugars by double osmylation of 1,2-dihydropyridines has been reported. ... 

The axial O-An bonds are clearly very strong. They cannot be protonated and are nearly always shorter than the equatorial bonds. In the case of U02 ", for instance, it is likely that the U 0 bond order is even greater than 2, since the U-O distance is only about 180 pm in spite of the difference in the ionic radii of the metal ions (U = 73 pm. Os " = 54.5 pm), this is close to that of the 0s=0 double bond found in the isostructural, osmyl group (175 pm, see p. 1085). It is usually assumed that combinations... 

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... 

Dihydroxylation. The key step in the synthesis of a natural mycotoxin from the dehydropentacyclic precursor 1 requires dihydroxylation of the nuclear double bond. Direct osmylation with catalytic 0s04 and N-methylmorpholine N-oxide... 

Regio- and stereoselective dihydroxylation of dienes functionalized at the allylic position with a benzene sulfone group has been reported42. Osmylation of dienic sulfones 33, a potential key synthon for forskolin, occurred exclusively on the A6-7 double bound and preferentially from the a-face of the traws-fused bicyclic molecule, presumably due to a combination of steric and electronic factors (equation 25). While the reaction of diene sulfones proceeded sluggishly under catalytic conditions, treatment of 33a with a stoichiometric amount of OSO4 resulted in quantitative yield of diastereomeric diols 34a and 35 in a 9 1 ratio, respectively. Protecting the hydroxy group of the dienol as its t-butyldimethylsilyl ether (33b) affords diol 34b exclusively. 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

Osmylation of C—C double bonds adjacent to the (diene)Fe(CO)3 functionality has been reported (Scheme 57)205. This methodology has been used in the enantiospecific synthesis of 5,6- and 11,12-diHETEs. 

It is of interest to consider in retrospect the logic of the chirality transfers involved in this synthesis. In essence stereochemical information was passed from the stereogenic center of SS to control the sense of emergence of the pyranose ring in compound 8. Eventually, at the stage of Z-enoate IS, stereochemical information is pass from the ring back to the double bond in the all critical osmylation reaction (9). 

The osmylation of C q with 0.75 equiv of OSO4 in pyridine-toluene at 0 °C leads to the two regioisomers 15 (major product) and 16 (minor) (Scheme 8.7) [58]. The same regioselectivity was observed in the hydrogenation (Chapter 5) [59], nucleophilic additions (Chapter 3) [60] as well as in Ir-adduct formations (Chapter 7) [61] of C70 and is in line with the predicted stabilities [59,62] of C70 adducts. The reaction occurs at the most reactive [6,6] double bonds, with the closest geometric similarity to the [6,6] double bonds of C q. These are the double bonds between C-l/C-2 (bond 1, Scheme 8.7) and C-5/C-6 (bond 2, Scheme 8.7) respectively. 

A familiar type of mechanism-controlled stereochemistry are cis or turns additions to C-C double bonds, cis Additions (suprafacial) are concerted cycloadditions, e.g., osmylations, epoxidations, catalytic hydrogenations, etc., whereas trams additions (antarafacial) normally proceed via two steps. 

The problem of rotational disorder was solved by making an osmyl derivative of buckyball. Osmic acid, OsO. will add across double honds ... 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

The original synthetic plan contemplated the use of the diene of the type A (Scheme 14) as the pivotal intermediate for pancratistatin by selective modification of one of the double bonds. However, the propensity of the benzylether 101 to undergo facile aromatisation to the diphenylcarboxylic acid 102, necessitated hydroxylation of the olefin 101 prior to base-induced elimination of elements of hydroiodic acid. The cis-diol 103, thus secured, was converted via the olefin 104 into tavu-a-bromoacetate 105 in an unusual reaction studied by Moffat [28], involving the use of 2-acetoxyisobutyrylbromide. The new diol 106 secured from 105 by osmylation protocol was converted into the corresponding stannylene 107 that on successive... 

The ketone 446 also served as a suitable starting material for the syntheses of ( )-montanine(415) and ( )-coccinine (413). Accordingly, the olefin 448 admixed with the A1 2 isomer was obtained from the tosylhydrazone 447 by base catalysed elimination. Bis-hydroxylation of the double bond of purified 448 achieved by the usual osmylation protocol... 

Osmylation of 4 proceeded stereospecifically from the P face of the double bond to give diol 3 as the sole of the product reaction. The... 

The partial synthesis70 of macroline (136) from normacusine B (140) was inspired by its postulated biosynthesis from a sarpagine-type precursor. Normacusine B (140), prepared by a previously published route from perivine, was protected at the primary alcohol group and then methylated on Na. Direct epoxidation of the product (141) failed however, osmylation gave the desired diol together with the related oxindole obtained by simultaneous oxidation of the indole double-bond, followed by rearrangement. Conversion of these diols into the related epoxides gave a mixture of (142) and (143), from which the desired epoxide (142) could be separated satisfactorily by fractional crystallization. 

Potassium chlorosmate may also be obtained by heating the finely divided metal with potassium chloride in chlorine,1 by double decomposition of the sodium salt with potassium chloride,2 or by the action of hydrochloric acid on potassium osmyl oxynitrite.3 It is soluble in water, yielding a yellow solution from which alcohol precipitates it as a red crystalline powder. The crystals are deep red oetahedra, iso-morphous with the corresponding palladium, iridium, and platinum analogues. 

Of the various osmyl derivatives the potassium salts may be regarded as the most important, since from them it is easy to prepare the corresponding salts of silver, barium, etc., by simple double decomposition. 

Having once obtained a suitable osmyl oxy salt such as potassium or sodium osmyl oxynitrite by a method similar to the above, it is not difficult by suitably choosing the reagents to prepare many other derivatives. Thus, the ammonium salt may readily be obtained by double decomposition of ammonium chloride and the sodium salt. Thus ... 

Osmyl Tetra-ammine Hydroxide, 0s02(NH3)4(0H)2, is obtained by double decomposition of its sulphate with barium hydroxide,4 or of its chloride with moist silver oxide.5 The solution is not stable, and on concentration ammonia is evolved. The most important salt of this base is... 

Potassium hexacyanoferrate in combination with l,4-diazabicyclo[2.2.2]octane has also been reported to promote catalytic osmylation of the otherwise unreactive double bond of cholesterol3 u. 

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