Osmium-catalyzed cyclizations

Scheme 5 Osmium-catalyzed cyclization Possible reaction pathways with (b) or without reoxidation (a) of the intermediate Os(VI)...

Scheme 9.4 Osmium-catalyzed oxidation/cyclization for the formation of THF derivative.

Excellent chemoselectivity in the enantioselective dihydroxylation of dienes has been observed with substrates such as 324 (Equation 51) and 326 (Equation 52) [218], An interesting expansion of this methodology was recently reported by Donohoe, who discovered that 1,5-dienes participated smoothly in highly diastereoselective osmium-catalyzed oxidative cyclization reactions [219]. This mild oxidation method was combined with an initial asymmetric catalytic dihydroxylation reaction to give optically active tetrahydrofurans, as illustrated by the conversion of 328 into 330 (95 % ee. Scheme 9.42) [220]. 

Furo[3,2-c][l]benzopyran-4-ones have been prepared either by acid catalyzed condensation of 4-hydroxycoumarin with a benzoin derivative (equation 11) (81IJC(B)614) or from 3-allyl-4-hydroxycoumarins on oxidation with osmium tetroxide/potassium periodate followed by cyclization of the intermediary aldehyde with PPA (equation 12) (79G109). 

The most impressive methodology utilizing CT, which has been developed by the group of Sharpless, is the vicinal aminohydroxylation of olefins catalyzed by osmium tetroxide [15]. The method has been elegantly extended to a practical asymmetric synthesis [16]. The reaction system was employed to the achiral aminohydroxylation of a,P-unsaturated amides to afford two hydroxysulfonamide regio-isomers. The crude mixtures were cyclized to the aziridines in a one-pot procedure, without the need for purification of the intermediates [17] (Scheme 10). 

A cobalt(II)-catalyzed oxidative cyclization converted a secondary alcohol to the trans-2,5-disubstituted tetrahydrofuran <03JA14702>. Oxidants such as vanadium(V) complex <03EJO2388>, ruthenium tetroxide <03TL5499> and osmium tetroxide <03AG(E)948> were all employed to convert either homoallyl alcohol or polyenes to molecules that contain... 

The same group also described a highly stereoselective synthesis of pyrrolidines utilizing a Os04-catalyzed oxidative cyclization strategy. Preferential mode of cyclization is most likely due to the robust intramolecular coordination of osmium with nitrogen and oxygen atoms of the iV-tosylated aminoalcohol (eq 54). ... 

According to B3LYP/6-311-i-G(d) calculations, the osmium tetroxide-catalyzed oxidative cyclization of 1,5-dienes proceeds on the singlet hypersurface via the... 

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