Hydroxyl groups selective

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

The necessity to protect hydroxyl groups selectively is frequently encountered in a total synthesis. Therefore an all cases scenario set of reactions has been created for this important functionality. Some of the more commonly encountered methods are summarized in Scheme 2.90. The list of protected compounds includes such derivatives of alcohols as esters (201-203), acetals (204, 205), ethers (206-209), and organosilicon ethers (210, 21The methods used to introduce these protecting groups vary tremendously. All of them actually represent different versions of the same reaction type, electrophilic substitution of the hydrogen of the hydroxyl group. The main differences are in the reaction conditions, e.g. acidic, neutral, or basic, required to form these derivatives. The... 

The introduction of an olefin at the anomeric position is synthetically interesting as it permits, via ozonolytic cleavage, access to the anomeric aldehyde, useful for further manipulations. Furthermore, the cleavage of the tether after cyclization releases one hydroxyl group selectively, which may be used chemoselectively in subsequent reactions. This reaction was also shown to be applicable to furanose sugar derivatives [2]. 

Finally, it s necessary to bring in the amino group, and this can be done by tosylating the less hindered primary hydroxyl group selectively, closing to an epoxide in base, and then reopening the epoxide at the less hindered terminal position with methylamine. 

The synthesis of 1-deoxytaxol analogs was eventually achieved by a sequence starting from baccatin VI (3.5.4). Baccatin VI has most of the key structural features of baccatin III, but importantly it lacks the C-1 hydroxyl group. Selective hydrolysis of baccatin VI followed by acylation with the taxol side chain by the / -lactam method gave various 1-deoxytaxol analogs such as 3.5.5 and 3.5.6 126). Both analogs were approximately one half as active as taxol in tubulin assembly and cytotoxicity assays. 

A C-nor derivative that retained the oxetane ring was formed by oxidation of 2 -protected diol with tetrapropyl-ammonium perruthenate (TPAP), which oxidized the C-6 hydroxyl group selectively to give an unstable 7-hydroxy-6-keto derivative. This derivative then underwent a retro-aldol reaction followed by an aldol reaction to give the C-nortaxol 4.2.2.4 unfortunately the deprotected final product 4.2.2.S was unstable and could not be completely characterized (779). 

The free OH can be modified by further chemical manipulation, and reaction with benzoyl chloride (BzCl) and pyridine (Py) gives 146 (the 0-benzyl group is in blue in the illustration). Treatment of 146 with 10% aqueous acetic acid cleaves the orthoester and gives a 1 1 mixture of hydroxy acetates 147 and 148, where the acetate units are marked in cyan. If 147 is isolated and treated with dihydropyran (see Chapter 26, Section 26.4.2) and an acid catalyst, the 2 -hydroxyl is converted to a dihydropyranyl ether (149) where the P3rranyl ether unit is marked in green. It is therefore possible to protect and deprotect the 5 -, 3 -, and 2 -hydroxyl groups selectively, although the abihty to differentiate the 2 - and 3 -hydroxyls is more difficrdt to achieve. 

So another circuitous intramolecular 8 2 attack strategy was proposed as shown in Fig. 3.19. The cii-diol 3.5a and 2-bromoacetyl bromide reacted in pyridine to obtain product 3.43 with primary hydroxyl group selectively converted to pyruvate in a yield of 65 % using DCM as a solvent. The hydroxyl group on... 

A mixture of 2, 3 -0-isopropylideneuridine, 2-mercaptopyrimidine, and dimethyl-formamide dineopentyl acetal refluxed in benzene under anhydrous conditions -> thioether (Y 73%) refluxed 0.5 hr. with Raney-Ni W4 in ethanol 2, 3 -0-isopropylidene-5 -deoxyuridine (Y 70%). - This reaction affects prim, hydroxyl groups selectively therefore, protection of the functional groups is not required. F. e. s. A. Holy, Tetrah. Let. 1972, 585. 

Five-membered cyclic dibutylstannylene acetals formed on vicinal cw-axial-equatorial pairs of hydroxyl groups selectively enhance the nucleophilicity of the equatorial oxygen in 0-alkylation reactions [179, 180], On the basis of this rule Schuerch surmised that alkylation of the tin complex 103, having the anomeric oxygen locked in the equatorial position, should lead to P-mannosides [181]. This assumption was proved by treating the mannose diol 102 with Bu2SnO (—> 103) followed by in situ exposure to alkyl halides whereby P-mannopyranosides 104 were formed stereospecifically (Scheme 30) [181],... 

Tsuda, Y. Regioselective manipulation of carbohydrate-hydroxyl groups (selective activation of a hydroxyl group by tin compounds). J. Synth. Org. Chem. Jpn. 1997, 55, 907-919. 

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