Hydroxyl groups equatorial, acylation

A less obvious instance is the acylation of l,5-anhydro-4,6-0-benzylidene-D-glucitol. The 1,5-anhydride has a chair conformation, and the 2- and 3-hydroxyl groups are equatorially attached trans to each other, and are, apparently, essentially equivalent91 However, the 2-benzoate and 2-p-toluenesulfonate were obtained almost exclusively on using the acyl chlorides in pyridine, although in the former acylation, an appreciable proportion of the dibenzoate was formed. 

The hydroxyl in (42-6) is then acylated with p-toluenesulfonyl chloride exposure of this to a base leads to elimination to form the 9,11 olehn (43-1). It should be noted that the hydroxyl group in the hrst-obtained fermentation product is equatorial and would eliminate only with great difficulty as it lacks a transoid proton at the adjacent position. Reaction of (43-1) with A -bromosuccinimide in an aqueous base leads to the addition of the elements of hypobromous acid. The stereochemistry of the reaction... 

Some partially esterified derivatives can also be obtained by using the decomposition of acyloxonium salts (see Section IV,5) as a result of a fair degree of stereospecificity, the preponderant isomer obtained is that having the axial hydroxyl group acylated and the equatorial one unsubstituted. An analogous regioselectivity is exhibited by the lithium aluminum hydride-aluminum chloride cleavage of 1,6-anhy-dro-2,3-0-benzylidene-/3-D-mannopyranose or its 4-methyl ether, which yields the 3-O-benzyl derivative only.677... 

As described above, the l-(or 3-) hydroxyl group of myo-inositol has higher reactivity over other equatorial hydroxyls. This tendency was observed in the following examples. Phosphorylation of 155 with diphenyl phosphorochloridate produced the 1,3-diphosphate 156 predominantly in moderate yield together with 1,5-diphosphate 157 (Scheme 3-24).15,55 Benzylation of the triol 155 under PTC conditions led to the 1-benzyl ether 158 in 71% yield (overall yield from the orthoester 95 in three steps). The resultant tetrabenzyl ether 158 was again regioselectively acylated at C-3 with camphanic acid chloride for resolution (Scheme 3-24).42... 

A lipoprotein lipase from Pseudomonas sp. effected not only regioselective acetylation of the orthoester 95 without observing the acylation of the axial hydroxyl groups but also enantioselective butyroylation (>95% ee) of symmetrical 4,6-dibenzoate 203 which was derived from the acetate 202 by benzoylation and acidic methanolysis (Scheme 4-10). 79 Regioselective chemical acylation of the equatorial hydroxyl group in 95 was also reported using benzoyl (64% yield) and p-nitrobenzoyl chlorides (51%).79... 

Preferentiai acylation of equatorial hydroxyl groups Carbohydrate derivatives... 

N-, and S-Acylations. 0-Ethoxycarbonylation is observed in some instances. Aliphatic alcohols, phenols, and cyanohydrins react in the presence of tertiary amines to give carbonates. Equatorial oriented steroid hydroxyls are selectively acylated in the presence of axial hydroxyl groups. Enol carbonates (eq 11 ) are readily derived from ketones by formation of the lithium enolate using Lithium 2,2,6,6-Tetramethylpiperidide in THF/HMPA at —78°C followed by an ethyl chloroformate quench at room temperature. The use of HMPA prevents competitive C-acylation. LiTMP is necessary to avoid side reactions com-... 

Enzymatic acyl transfer reactions are also practical processes for the acylation of hydroxyl groups in steroids [25]. The lipase from Chromobacterium viscosum (CVL), for instance, selectively transfers butyric acid from trifluoroethyl butyrate to equatorial 3p-alcoholic functions being present in a variety of sterols, such as 107, 110, and 113 (Table 9). Axially oriented alcohols at C-3 and secondary alcohols at C-17 (107 and 110) or in the sterol side chains (113) are not derivatized. In addition to the equatorial alcohols, the... 

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