Stereoselective dihydroxylation

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. 

Stereoselective dihydroxylation The Os04-catalyzed dihydroxylation of 5-vinyl-4,5-dihydroisoxazoles (1) (obtained by reaction of the nitrile oxide derived from nitroethane with 1,3-dienes) is anti-selective, and the anti-selectivity is markedly enhanced by a cw-substituent on the double bond. 

Few applications of cyclizations to form fused ring 8-lactones or tetrahydropyrans are found. Two consecutive bromolactonizations were used to effect stereoselective dihydroxylation of a cyclohexadi-enone system in a total synthesis of erythronolide B (Scheme S).64 Iodolactonization of an NJV-di-ethylbenzamide derivative to form a ds-fused benzolactone was a key step in a recent synthesis of pancratistatin.641 A di-fused tetrahydropyran was produced in good yield by intramolecular oxymercura-tion as shown in equation (17),59 although attempts to cyclize a more highly functionalized system have been reported to fail.65 Formation of a fused ring tetrahydropyran via an anti-Markovnikov 6-endo sel-enoetherification has been reported in cases where steric and stereoelectronic factors disfavor a 5-exo cyclization to a spirocyclic structure.38... 

As an example of conversion of complex 111 to 113, optically pure complex 126 was used for the enantioselective total synthesis of shikimic acid (129) [30]. The hydroxy-substituted diene complex 127 was prepared from 126. Silylation and decomplexation of 127 gave 128. Stereoselective dihydroxylation of the more reactive double bond of the decomplexed silyl ether derivative 128, followed by desilylation afforded (—)-methyl shikimate (129). 

Scheme 6.4.9. Regio- and stereoselective dihydroxylation and subsequent modification.

SCHEME 13.31 Asymmetric synthesis of L-arabinitol derivatives via stereoselective dihydroxylation of an enantiomerically pure ally lie alcohol. 

The C1-C13 segment 206 of halichondrin B (205) has been recently synthesized with improvements from 210 and 211. The Cl-Cl 1 segment 210 was prepared from L-mannonic-y-lactone (212) (O Scheme 24). Di-O-cyclohexylidene protection of 212 followed by DIBALH reduction and Wittig reaction gave 213. Stereoselective dihydroxylation and acetylation of 213 provided 214. C-Glycosidation of 214 with methyl 3-trimethylsilylpent-4-enoate and BF3 Et20 afforded a-glycoside 215 exclusively. This compound was transformed into 206 via the 7-step manipulation as shown in O Scheme 24. 

Red-Al) or the Z-olefin 341 after benzylation. Stereoselective dihydroxylation and regiose-lective protection of the diol provides protected polyol 342 that is transformed in three steps to terminal epoxide 343. Removal of the p-methoxybenzyl group followed by acid-catalyzed epoxide opening yields exclusively tetrahydropyran 344 after protection of the primary alcohol and deacetylation. Inversion of the C2 stereocenter by an oxidation-reduction sequence and deprotection furnishes the C-mimic of a,a-trehalose 345. 

Cha, J. K., Kim, N.-S. Acyclic Stereocontrol Induced by Allylic Alkoxy Groups. Synthetic Applications of Stereoselective Dihydroxylation in Natural Product Synthesis. Chem. Rev. 1995, 95, 1761-1795. 

Regio- and stereoselective dihydroxylation. This reagent adds 10 the oxepin 2 exclusively with the isolated cycloalkene double bond to provide 3 (84% yield). Whereas direct osmylation of 2 provides a mixture of diols in low yield, osmyla-... 

The preparation34 of the left half of cephalostatin 7 was more straightforward. Compound 56, the minor product of the stannane addition (Scheme 25, vide supra) was deoxygenated by Barton s xanthate procedure (Scheme 28) to give 60, which was stereoselectively dihydroxylated using the Sharpless AD-mix-a reagent35 to yield 61 and its inseparable C-25 epimer in a 2.5 1 ratio. 

A stereoselective [3 + 3] annulalion by a Michael-Wittig reaction on the enal 82 derived from 2,3-0-isopropylidene-(/ )-glyceraldehyde and ethyl-3-oxo-(triphenyl phosphoryU-dene) butanoate (69)" in one pot gave the cyclohexenone derivative 83. Enone reduction and stereoselective dihydroxylation led to formation of highly functionalized cyclohexane systans (85a to 85c) (Schone 22.19). 

A new route to 2-C-methyl-D-ribonolactone 69 has been described utilizing stereoselective dihydroxylation of 68 (Scheme 18). An efficient synthesis of the fiiranopyranone 70 in 94% overall yield from dicyclohexylidene-D-glucose has been described. ... 

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