Trans-Hydroxylation

As noted at the end of Section 7.8, the prefixes cis- and trans- would be ambiguous when naming the diols derived from 1-methylcyclohexene because the ring has three substituents. Instead, a reference substituent r is chosen and other substituents are either cis (c) or trans (f) to that reference. For the two l-methyl-l,2-cyc ohexanediol isomers, the -OH group at Cl is the reference (r-1), and the -OH at C2 is either cis (c-2) or trans (t-2) to that reference. Thus, the diol isomer derived by cis hydroxylation is named l-methyl-r-l,c-2-cyc ohexanediol, and the isomer derived by trans hydroxylation is named l-methyl-r-l,t-2-cyclohexanediol. 

There are four stereoisomers of the diol epoxide of BP (XVIII -XXI). Two (XVIII and XIX) have the epoxide oxygen atom below the plane of BP and the other two (XX and XXI) have the oxygen above this plane. In addition the 7-hydroxyl group may be on the same (syn) side (XIX and XXI) as the epoxide oxygen atom or on the opposite side (anti) (XVIII and XX). Also the trans hydroxyl groups may be diaxial or diequatorial, and this may be demonstrated by crystallographic and other, such as NMR, studies (92, 93). 

As shown in this report, the CDA protecting group is easily introduced, and the present procedure allows facile protection of the trans-hydroxyl groups of a range of sugars. 

Until recently, the protection of the trans-hydroxyl groups of sugars has been an inefficient process.2 This protection has now been simplified by the introduction of the dispiroketal protecting group,3 and the cyclohexane diacetal (CDA) protecting group.4... 

The ease of selective protection can be explained in terms of maximum anomeric stabilization such that both anomeric methoxy groups of the product are oriented in the axial position with respect to the central dioxane ring. All four sterically demanding alkyl substituents are then placed in the favored equatorial positions. The poor selectivity for the protection of glucose results from the existence of two pairs of trans-hydroxyl groups in this sugar. 

Further research within in our laboratory has illustrated that certain substrates bearing trans-hydroxyl groups can be directly protected using cyclic and acyclic diones, thus alleviating the requirement for prior formation of the tetramethoxydiacetals.6 For example, butane-2,3-dione has proved particularly useful for the preparation of butane diacetal (BDA) derivatives, which serve as useful alternatives to cyclohexane diacetal (CDA) derivatives. 

Mono- und Ditosyl-Derivate der 1,6-Anhydroaldohexopyranosen mit einer vicinalen trans-Hydroxyl-Gruppe cyclisieren im alkalischen Medium zu entsprechenden Dianhydro-Derivaten mit Epoxidringen, wobei im Falle des Laevoglucosans die Reaktivitat der Tosyloxy-Gruppen in der Reihe 3 4 2 (mit der numerischen Auswertimg 180 23,3 1,0) sinkt 37). 

A 1,2-diol arising from a trans-hydroxylation process is formed from an alkene by way of an intermediate epoxide which is subjected to a ring-opening reaction and hydrolysis. The epoxides may be isolated when the alkene is reacted with perbenzoic add or m-chloroperbenzoic acid (Section 4.2.56, p. 457) in a solvent such as chloroform or dichloromethane the preparation of epoxides by this method and by other important procedures are discussed and illustrated... 

The -)-erythro-isomer of 2,3-dihydroxy-3-methylpentanoic acid (73) has been synthesized by Crout and Whitehouse, and shown to be identical with the naturally occurring (-)-acid obtained from strigosine.45 trans-Hydroxylation of E-3-methylpent-2-enoic acid with tungstic oxide-hydrogen peroxide gave the ( )-erythro-acid, which was resolved as the quinine salts. The absolute configuration of the (-)-erythro-ac d (73) was determined by degradation (Scheme 4). Methylation,... 

Protection, for diequatorial vicinal diols, 77, 218 of trans-hydroxyl groups in sugars, 75, 173 table, 75, 174... 

The trans-hydroxylation of l-benzoyl-3-piperideine (123) with peracetic acid led to [Pg.85]

The first poly-O-acetylaldosyl azide was synthesized by Bertho by treating tetra-O-acetyl-a-D-glucopyranosyl bromide with a metal azide. In addition to the aldosyl azide derivatives that are known (see Table IV), derivatives of 2-amino-2-deoxy-D-glucosyl azide have been prepared - (see Table V). If the trans hydroxyl group at C-2 is not acetylated, an intermediate epoxide of the type suggested for the aldosyl fluorides can form, and the azide obtained has the. same anomeric configuration as the original halide, as. shown by the formulas. The acetylated azides, like the... 

Trans hydroxylation of an alkene involves sequential conversion to an oxirane (epoxide), and its conjugate acid, followed by nucleophilic opening of the oxirane by water... 

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