Protection diols

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. ... 

For protection of glucose derivatives, which possess two sets of 1,2-trans diequatorial diols, protection is best achieved by way of chiral bis(dihydropyrans).6 In this way, complete regioselectivity can be achieved as a result of chirality "matching" of the C-2, C-3 diol pairs with that of the bls(dihydropyran). 

Y. Oikawa, T. Yoshioka, and O. Yonemitsu, Protection of hydroxy groups by intramolecular oxidative formation of methoxybenzylidene acetals with DDQ, Tetrahedron Lett. 889 (1982) Y. Oikawa, T. Nishi, and 0. Yonemitsu, Kinetic acetalatioa for 1,2- and 1,3-diol protection by the reaction of p-methoxyphenylmethyl methyl ether with DDQ, Tetrahedron Lett 4037... 

Tips A diol protecting group is prepared from the protecting group al-... 

Alcohols protected as esters, and diols protected as cyclic acetals resist Jones oxidation. 

An intermediate in the synthesis of laulimalide by Davidson8 illustrates the differential protection of alcohols. The starting materials 56 and 57 already have an alcohol protected as a TBDMS silyl ether and a diol protected as an acetal. The alcohol in 58 is protected as a p-methoxybenzyl ether and the acetal hydrolysed by acetal exchange to give the free diol 60. Selective protection of the primary alcohol by a bulky acyl group (pivaloyl, i-BuCO ) 61 allows silylation of the secondary alcohol with a TIPS group 62. Finally the pivaloyl group is selectively removed by DIBAL reduction to release just one free alcohol 63. 

Cyclic diol protecting groups are not always conducive to a successful RCM. For example, when Banwell and McRae submitted acetonide-protected 1,3-diol 28 to the Grubbs first-generation catalyst, none of the desired macrocyle was produced, but cyclohexene 29 was obtained in 81% yield, probably because the acetonide protecting group facilitated interaction of the double bonds of the carboxylic acid portion of the molecule (Scheme 2.11) [24]. To circumvent this problem, substrate 30 was synthesized, where the diol was protected as two silyl ethers, and RCM of this compound led to the desired 18-membered lactone 31 in 70% yield under the same reaction conditions. 

The Arylmethylene Acetal Family of 1,2- and 1,3-Diol Protecting Groups... 

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