Protection of diols

Protection of Diols. 1,2-Diols are obviously alcohols, but the vicinal nature of the hydroxyls allows them to be protected as cyclic ketals. When a 1,2-diol such as 2,3-butanediol (96) reacts with a ketone such as acetone, in the presence of an acid catalyst, a 1,3-dioxolane (97) is formed. Ketals such as 97, which 

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For the selective protection of diols Bu2SnO, benzene, reflux MOMCl, Bu4N r, It, 87% yield. ... 

The use of chiral ketones for the protection of diols serves two purposes first, diol protection is accomplished, and second, symmetrical intermediates are converted to chiral derivatives that can be elaborated further, so that when the diol is deprotected, the molecule retains chirality. ... 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

Although several more examples of this type of ring system are known in the literature, the majority of these are cyclic diacetals, which have been used for the protection of diols. These have not been included in this chapter the reader is directed to the chapters dealing with the relevant diols. 

Bio-oxidation of bromobenzene 11 catalyzed by Pseudomonas putidae leads to diol 12. Protection of diol 12, followed by the addition of an acyl nitroso dienophile and subsequent reduction gives compound 14. This compound can be used as the key intermediate in the preparation of (+)-l-deoxy-galacto-nojirimycin (16) and related indolizidine compounds (15) (Scheme 8-5).12... 

Protection of diols. The reaction of diols with 1 and pyridine provides protected derivatives, which are cleaved by aqueous HF or R3NHF. These disiloxane derivatives are exceedingly useful for preparation of myo-inositol phosphates.2 Thus... 

In order to side-step potential problems with hydroxyl-directed hydro-metallation of the alkyne, ester 14 was reduced with sodium borohydride in methanol. This also led to a small amount of alkyne desilylation (5%-10%). Fortunately, however, this was the next step of the synthesis. After selective protection of diol 15 to obtain 8, the stabilised acetylenic anion... 

SCHEME 2.12 Protection of diols with acetals or ketals. 

Through Protection of Diols with Acetals or Ketals... 

After the simultaneous protection at 04 and 06, the differentiation of the remaining 2-hydroxyl and 3-hydroxyl is puzzling for glucosides because they are both secondary and equatorial. Conditions reported in the literature usually result in mixtures of regioisomers [6], For example, the regioselective protection of diol 59 via a copper complex intermediate gave the 3-0-acylated derivatives 61 and 63 as the major products in moderate yields (Scheme 2.20) [172], Nonetheless, 2-0-benzoylation... 

Acetals reveal virtually unlimited stability to basic conditions but they are quite fragile towards acid — a property that derives from participation of a lone pair on the adjacent oxygen atom in the cleavage of a protonated intermediate as depicted in Scheme 1.6. Moreover, there is a wide range of acid lability depending on the structure of the acetal — a subject that was aired previously in Chapter 2. In this chapter our discussion of acetals will only embrace matters pertinent to the protection of diols and polyols. 

Cyclic orthoesters. Alkoxy exchange catalyzed protection of diols using orthoesters such as MeC(OMe)j. 

This photolabile protective group was developed for the protection of diols, which could release caged biologically active molecules in biological systems. The acetal is prepared from the aldehyde and a diol (PPTS, toluene, MgS04, reflux) and is cleaved by photolysis at 348 nm in a pH 7.4 buffer. ... 

Propanone (acetone) carbon acidity, 10 protection of diols as "acetonides , 158, 266-267, 276-277, 321, 322 reaction with pyrrole, 250-251 Propanoyl chloride, 2- acetyloxy)-2-methyI-(Moffatt s reagent) O-protection, 160 chlorohydrtns from diols, 160, 327-328 2-Propenal (acrolein, acrylaldehyde) pr., 174... 

One year later, Kitazume et al. also described the synthesis of 6-deoxy-6,6,6-trifluoro-D-mannose and D-allose using intermediate (45,55)-76 (Scheme 5.10). Thus, oxidation of 76 with potassium permanganate in the presence of catalytic 18-crown-6 provided the diol 84 as the only isomer in 42% yield. Protection of diol moiety in 84 as its acetonide followed by DlBAL-H reduction afforded the lactol 85, which was subjected to the t-BuOK promoted isomerization to give the desired fluoro sugar 86 in 35% yield. The precursor 87 of fluoro sugar 89 was obtained by reduction... 

The formation of chiral 1,2-diols can be achieved by dihydroxylation of benzene derivatives by using appropriate enzymes or whole cell systems, in particular Pseudomonasputida (5.89). This biotransformation reaction provides cyclohexa-diene diols in essentially enantiopure form. Many of these diols are commercially available and can be converted to a variety of different enantiomericaUy enriched compounds. For example, protection of diol 90 as its acetonide and aziridination... 

In Situ Protection of Diols from Dihydroxylation. Sharpless recently examined the applicability of Narasaka s dihydroxylation reaction conditions (catalytic osmium tetroxide and NMO in the presence of phenylboronic acid). The boronic acid reagent replaces the water which is normally present in dihydroxylation reactions to hydrolyze the osmate ester and liberate the diol. In this case, however, the phenyl boronic acid promotes this reaction... 

Fig. 1 Selective protection of diol systems in mraiosaccharides as cyclic acetals...

Figurel.43 Examples of the use of boronic acids forthe protection of diol compounds.

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