Osmylation, diastereoselectivity

Osmylation of the olefmic ester 151 in an acetone-water mixture proceeds with high diastereoselectivity to produce the lactone 152 in a one-pot procedure (Equation 52) <1999TL5791>. 

Other comparisons of diastereoselective osmylations in the absence and in the presence of chiral ligands have been reported. A study of the dihydroxylation of unsaturated side chains... 

Diastereoselective epoxidations.6 Epoxidation of the ergoline 1 with m-chloro-perbenzoic acid provides the a-epoxide (2) with high diastereoselectivity and yield. In contrast, epoxidation via the bromohydrin provides the (1-oxide (2) in equally high diastereoselectivity. Osmylation of 1 also provides the 5a,6a-diol selectively (7 1) with similar preference for the a-face. 

Further explorations should be done in the stereoselective synthesis of chiral fullerene derivatives, a field initiated by the kinetic resolution of D2-C through diastereoselective osmylation with 0s04 in the presence of alkaloid-derived ligands. Besides, a few diastereoselective macrocyclizations by bridging of C6o with enantiomerically pure, bifunctional reagents have been achieved with high d.e. s. The use of optically pure fullerene derivatives as bulky chiral auxiliaries... 

E)- and (Z)-Allylsilanes with bulky alkyl groups are osmylated with the same sense of induced diastereoselectivity as observed for allylic alcohols and derivatives. In contrast to allylic alcohols, the preferred ground state conformation of these substrates with the allylic hydrogen atom in the plane of the double bond is also expected to be the most reactive one. Assuming electrophile approach from the opposite side to the silicon atom, the outside position is usually preferred by the alkyl groups3 32,38,39. 

These results have been interpreted in terms of the stereodivergent nature of the two fast-equilibrating and differently reactive envelope conformations of such cyclopentenes. Specifically, torsional effects may dictate the ul topicity of the former substrates, whereas normal steric factors may account for the opposite diastereoselection of the latter cases. A similar syn selectivity has been observed in the osmylation of other cyclopentene derivatives103 and of f/.v-disubstituted cyclobutenes102. 

From the synthetic point of view, satisfactory cis dihydroxylations with these reagents are best achieved with electron-poor alkenes such as oc,/ -unsaturated esters and lactones. Permanganate ion mediated dihydroxylations of chiral alkenes usually afford the same sense of diastereoselection as the osmylation reaction, a result suggesting comparable steric and electronic requirements in the corresponding transition states. 

Poll and Sames [112] have converted L-serine into A -methylfucosamine (Scheme 13.63). The method relies on the diastereoselective addition of propenyllithium to the aldehyde derived from protected L-serine derivative 183, giving allylic alcohol 184. Catalytic osmylation of 184 gives a 6 1 mixture of anti,syn 185 and syn,syn-2iminotnols. Protection of the triol 185 as its triacetate, reductive methylation and desilylation provides 186. Successive Swern oxidation, methanolysis and benzhydryl group hydrogenation leads to A -methylfucosamine 187. 

The diastereoselective synthesis of higher sugars was accomplished by bZi-osmylation of sugar derived dienes using OsO4-NMO4°. As shown in equation 23, osmylation of diene 26 afforded diastereomeric sugars 27, 28, and 29 in 90% overall yield in a 6.6 1.2 1.0 ratio, respectively. 

Scheme 8.20. (a) Kishi model for acyclic control in osmylation reactions [90]. (b) Double diastereoselectivity in an AD reaction [92],... 

Contrary to the results described above (Scheme 4-19), highly diastereoselective osmylation of cyclohexene derivatives 258 with the more bulky substituent, TBDMS, on one side of the double bond, which were derived from oxanorbornenone 257, was reported. This afforded the suitably protected chiral myo-inositol derivatives 259 (Scheme 4-20 ). 9 ... 

The highly diastereoselective osmylation of 694 or 700 with osmium tetroxide (5 mol%) and two equivalents of A/-methylmorpholine-A/-oxide (NMO) provides either 701 in 94% yield or 702 in 87% yield. When four equivalents of NMO are utilized, the corresponding 2,3,6,7-tetrahydroxylated product 703 or 704 is isolated in 88% or 93% yield, respectively. All products are obtained as single isomers [213] (Scheme 153). 

In the osmylation of 1,2-disubstituted allylic alcohols and derivatives, cw-alkenes provide higher diastereoselectivity than the corresponding fran -alkenes (eqs 7 and 8). Opposite se-lectivities have been observed in the osmylation of (Z)-enoate and ( )-enoate esters (eqs 9 and 10). High selectivity has also been observed in the osmylation of 1,1-disubstituted and ( )-trisubstituted allylic alcohols and derivatives and bis-allylic compounds (eqs 11-13). 

Sulfoxide groups direct the dihydroxylation of a remote double bond in an acyclic system perhaps by prior complexation of the sulfoxide oxygen with osmium tetroxide (eqs 16 and 17). Chiral sulfoximine-directed diastereoselective osmylation of cy-cloalkenes has been used for the synthesis of optically pure di-hydroxycycloalkanones (eq 18). Nitro groups also direct the osmylation of certain cycloalkenes, resulting in dihydroxylation from the more hindered side of the ring. In contrast, without the nitro group the dihydroxylation proceeds from the less hindered side (eq 19). ... 

Emmons condensations and addition of a silyloxyallylic stannane, and involving highly stereoselective osmylation of three double bonds is illustrated in Scheme 11. The selectively protected D-manno-D-manrm-D abino derivative 57 was obtained with excellent diastereoselectivity regioselective diol cleavage furnished lactol 58." ... 

In another lyiproach, diastereoselective osmylation of 120 introduced the two noncarbohydrate-derived stereocentres, and acid-catalysed trans-acetalation and further manipulations convoled the major isomer to core analogue 121 (Scheme 31). Monocyclic analogues of zaragozic acid ate discussed in Chapter 16. 

Diastereoselective osmylation and hydroboration of, y-unsaturated (V,(V-diisoprop-ylamides (100)-(1 2) have been reported to occur with useful diastereofacial selectivity (Scheme 11). The major diol isomer from (E)-alkenes (100) and (101), in the presence of TMEDA at -78 °C, corresponds to the facial preference shown in transition-state model (103) [(100) gave an 11 1 preference at -78 °C (101) afforded the same ratio at —78 °C, and 6 1 at room temperature], while the opposite preference for (104) is observed with the (Z)-alkene (102) (>20 1 at —78°C and 7 1 at room temperature). Hydroboration with 9-BBN does not show this inversion of diastereofacial selectivity for the (Z)-alkene all of the results correspond to the usual preference for a transition state such as (105). ... 

A route to (- )-162 devised by Kibayashi and co-workers began with the chiral hydroxylamine 186, prepared in eleven steps from D-malic add (123). In situ oxidation of 186 with periodate produced the transient acylnitroso species 187, intramolecular Diels-Alder reaction of vidiich yielded stereoisomeric oxazine cycloadducts 188 and 189 (Scheme 26). The observed ratio was 1 1.3 when the oxidation was performed in chloroform, but the dramatic improvement to 1 4.1 in aqueous medium was ascribed to a hydroj obic packing effect . Reductive cleavage of the N-0 bond of 189 and silylation of the exposed alcohol group gave (2)-alkene 190. A moderately diastereoselective osmylation yielded the (75,8R)-cu-diol 191 and its chipmatographically separable (7R,8 isomer in a 4 1 ratio. The synthesis of ( - ) 162 from isomer 191 was completed as shown in five straightforward steps. 

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