Asymmetric osmylation

J. M. Hawkins, A. Meyer, Optically Active Carbon -Kinetic Resolution of C76 by Asymmetric Osmylation , Science 1993,260,1918-1920. 

Asymmetric osmylation of alkenes.3 In the presence of 1 equiv. each of 1 and 0s04, alkenes undergo highly enantioselective ris-dihydroxylation. Highest enantiofacial selectivity (90-99%) is shown in osmylation of trans-di- and trisub-... 

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

An asymmetric osmylation method has been developed by Sharpless and coworkers. 0s04 modified by a dihydroquinidine auxiliary (cinchona alkaloid derivatives)449,455 158 or chiral diamines449,457 160 such as 59 and 60 used in stoichiometric oxidation may yield cis diols with excellent optical purity 460... 

In contrast to the aforementioned fullerenes, C76 is a chiral molecule containing 30 different types of carbon-carbon bond. In this molecule five different pyracylene-type carbon-carbon bonds repeat to form chrysene-shaped units. Kinetic resolution of this fullerene has been achieved via asymmetric osmylation in the presence of a cinchona based chiral ligand (see Section 4.4.4.1.1., ligand 1 d/2 d, Table 5). The calculated enantiomeric excess of the recovered material (after 95% conversion) is >97%, whereas the regenerated C76, formed by tin(II) chloride reduction of the osmylated material (after 33 % conversion), is enriched in the opposite enantiomer. Analysis of the local curvature of the C76 molecule indicates that Os04 should selectively add to two of the 30 types of bonds86. 

In summary, the asymmetric osmylation of alkenes catalyzed by derivatives of cinchona alkaloids represents a very elegant method which enables the enantioselective cis dihydroxylation of several types of alkenes in high enantiomeric excess and with predictable selectivities. The design of specific chiral ligands for substrates that still do not afford enantiomeric excesses over 90% would be desirable for the near future. 

Two classes of ligands have been successfully used in asymmetric osmylation of prochiral olefins chiral diamines and cinchona alkaloids. The diamines have been developed mainly by Corey, Koga and Tomioka, Hirama and their coworkers [559, 750, 754, 755, 756], and cinchona alkaloids have been designed and implemented by Sharpless and coworkers [750, 753-756],... 

Asymmetric osmylation. Chiral ligands of /V,/V -dialkyl bispiperazines linked by two carbons (I, n = 2) can effect highly enantioselective dihydroxylation of trans-disubstituted alkenes. 

When n = 3 or 4, these ligands suppress the reaction completely. If R is an acyl group, the cnantiosclcctivity is depressed. These ligands (I) can also effect asymmetric osmylation of mono- and disubstituted terminal alkencs, but not of civ-disubstituted alkcncs. 

This desilylation provides a key step in an asymmetric osmylation of a double bond joined by one carbon to a five-memberd siloxane. Thus the aldehyde 3, obtained by deprotection of a (H2,Pd/C) and Swern oxidation, undergoes a Wittig-Horner reaction to provide the unsaturated amide 4 in 70% overall yield. This product undergoes osmylation to give essentially one product (5). After protection of the two hydroxy groups, the... 

In a total synthesis of mycalamide B, the aminal unit (63) was prepared from glucose derivative (61) via the key intermediates indicated in Scheme 14. Amongst much ingenious chemistry, the introduction of the axial dimethyl acetal grouping into (62) by the use of propargyl trimethylsilane-TMSOTf, followed by ozonolysis and acetalization, is noteworthy. Hydroxyl-ation of the allyl group involved asymmetric osmylation the synthesis was completed by acylation of (63) with a non-sugar-derived acid unit, and mycalamide A was made by a variation on the route.4... 

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