Osmylations, directed

In the second case of osmylation directed by conformational control, the most promising conformation for this process was deliberately enforced. 

Despite the extensive utility of osmylation reactions, the mechanism remains controversial - generally focusing on either a concerted [3 + 2] cycloaddition to directly yield the cyclic osmate ester or a two-step mechanism where [2 + 2] cycloaddition forms a 4-membered osmaoxetane which then undergoes a ligand-assisted ring expansion to yield the cyclic osmate ester216 (Scheme 21). 

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... 

Directed osmylation.J One step in a synthesis of the carbonucleoside aris-teromycin (3) required dihydroxylation of the intermediate 1, in which a (nitro-phenylsulfonyl)methyl group is used as a carboxy equivalent. The desired diol (2)... 

The same features are observed in the osmylation of arene donors. Thus, osmium tetra-oxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair ANT+, 0s04, which then collapses to the osmium adduct (with a rate constant fc 109 s 1) in competition with back ET [59]. 

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. 

Transformation in the opposite direction, namely, from osmyl to osmyl oxy derivatives, is more difficult to effect, although it has been accomplished in th e preparation of potassium osmyl oxynitrite (see p. 228). 

Potassium Osmyl Oxychloride is apparently too unstable to exist, since its osmyl oxynitrite is converted directly into osmyl chloride by the action of hydrochloric acid. 

Potassium Osmyl Oxynitrite, K2(0s03)(N02)3.3II20, is readily obtained 1 as the result of the direct action of a concentrated solution of potassium nitrite on osmium tetroxide ... 

The direct observation of the reactive intermediates by the use of time-resolved picosecond spectroscopy and fast kinetics (Figure 4) enables the course of CT osmylation to be charted in some de. The analysis proceeds from the mechanistic context involving the evolution and metamorphosis of the CT ion pair, as summarized in Scheme 5 (the brackets denote solvent-caged pairs) for the critical initial stq> (equation 24) to form the 1 1 adduct to a benzene donw. 

An unusual directing effect of the nitro group has been claimed in the osmylation ofcyclopente-nes carrying an allylic 1-nitro-l-phenylsulfonyl alkyl group (as R1 substituent)4711. 

The osmylation of alkenes incorporated into rigid frameworks reveals that reagent approach is usually determined by steric factors. However, in some cases, the dihydroxylation is apparently directed by a functional group which is located close by. 

A similar effect is observed in the osmylation of allylamides bearing a bis-homoallylically located sulfoxide group94. In this case the asymmetric 1,5-induction of the stereogenic sulfur atom totally overwhelms the weak bias of the allylic chirality. The concomitant sulfoxide-to-sulfone transformation suggests sulfoxide involvement in the oxidation mechanism. In this example, as well as in the previous one, replacement of the sulfur-based directing group by a sulfone moiety leads to a drop in diastereoface differentiation. 

The C-glycoside 178 was used by Boyd and Sulikowski [87] in the total synthesis of enantiomerically pure urdamycinone B (182) and 104-2 (183) making use of the diene 107 derived from shikimic acid (Scheme 29) and the NMO oxidation to generate the C-5 phenols (Scheme 40). Thus, the bromonaphthoquinone 179 (prepared by treatment of phenol 178 with NBS) formed the tetracycle 180 through a Diels-Alder reaction with the diene 107 in analogy to sugar-free reactants. Osmylation to a cis-diol, deprotection, oxidation, and acetalization gave the acetonide 181. The decisive step in the aromatization to 182 and 183 was the reaction with NMO (Scheme 46). Aromatization was also effected by direct periodane oxidation of adduct 180 to derive 182 after deprotection. 

The combination of the chromatographic separation of enan-tiopure p-hydroxysulfoximine diastereomers and reductive elimination results in a method of ketone methylenation with optical resolution. The technique is illustrated in the synthesis of the ginseng sesquiterpene (—)-p-panasinsene and its enantiomer (eq 5). The addition of the enantiopure lithiosulfoximine to prochiral enones or the diastereoface selective addition to racemic enones results in the formation of two diastereomeric adducts. The hydroxy group in these adducts can be used to direct the Simmons-Smith cyclopropanation (eq 6 and eq 7). Catalytic osmylation of such adducts is directed by the anti effect of the hydroxy augmented by chelation by the methylimino group (eq 7). ... 

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