F. Ono, H. Takenaka, T. Fujikawa, M. Mori, T. Sato, Synthesis, 2009, 1318-1322. For example, when formaldehyde (H2C=O) is dissolved in water, the geminal diol (H2C(OH)2, methanediol) is favored. [2] Primary alcohols can be protected in less than one hour while some hindered alcohols may require days of reaction time. (a) Propose a mechanism for the formation of this protected diol. On commercial scales, the main route to vicinal diols is the hydrolysis of epoxides. Both 1,2-diols and 1,3-diols can be protected by treatment with 2-methoxypropene according to the following reaction. In general, organic geminal diols readily dehydrate to form a carbonyl group. Some of the most common: OOOO 'acetonide' most stable least stable Usually, 1,2 >1,3>1,4 HO OH OH O OH O O O 5: 1 On: … A diol is a chemical compound containing two hydroxyl groups (−OH groups).
(b) Suggest an experimental procedure by which this protecting group can be removed to regenerate the unprotected diol. Fluoride-based deprotections deprotect electron-poor silyl groups faster than electron-rich silyl groups. h�b```b``b`a`�`f@ a�+sl *� 0000003301 00000 n
Although many methods are available for forming silyl ethers, two common strategies for the silylation of alcohols are: reaction of the alcohol with a silyl chloride using an amine base at room temperature and reaction of the alcohol with a silyl triflate using a hindered amine base at low temperature. So the first thing we could do, to do this transformation, is to form a thioacetal. ", Nelson, T. D.; Crouch, R. D. "Selective deprotection of silyl ethers. 0000001872 00000 n
Reaction with acids or fluorides such as tetra-n-butylammonium fluoride removes the silyl group when protection is no longer needed. This quenches remaining silyl reagent and protonates amine bases, removing them from the reaction mixture. 10 March, 2014 / by SK / in Reactions. A common hindered base for use with silyl triflates is 2,6-lutidine. 1,3-dioxanes cleave faster than 1,3-dioxolanes. v)�Kn=�du{�~rE�t͏0)(1)�1 Unfortunately, some optimization is inevitably required and it is often necessary to run deprotections partway and recycle material. Both the functional groups could react with a Grignard Reagent. 0000001794 00000 n
Examples include 2-methyl-2-propyl-1,3-propanediol and neopentyl glycol. One extremely reliable and rapid procedure is the Corey protocol in which the alcohol is reacted with a silyl chloride and imidazole at high concentration in DMF. This cannot be done without a protecting group because Grignard reagents react with esters and ketones. %PDF-1.3
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Common diols used to form ketals are show below in order of their relative rate of formation. The general structure is R1R2R3Si−O−R4 where R4 is an alkyl group or an aryl group. So we can use an acidic environment here. 2.
The epoxides are prepared by epoxidation of the alkene. Silyl triflate is more reactive and also converts ketones to silyl enol ethers. 29647 Views Silyl Protective Groups. CS1 maint: multiple names: authors list (, http://www.orgsynth.org/orgsyn/pdfs/CV3P0217.pdf, Ullmann's Encyclopedia of Industrial Chemistry, https://en.wikipedia.org/w/index.php?title=Diol&oldid=948056903, Creative Commons Attribution-ShareAlike License, This page was last edited on 29 March 2020, at 22:43. An example in the synthesis of trans-cyclohexanediol[3] or by microreactor:[4]. II. Contributors. Using alkaline potassium manganate(VII) produces a colour change from clear deep purple to clear green; acidic potassium manganate(VII) turns clear colourless. Following extraction, the product can be purified by flash chromatography. 0000002569 00000 n
A diol is a chemical compound containing two hydroxyl groups (−OH groups). 0000001397 00000 n
Prof. Steven Farmer (Sonoma State University) Compound 1.4.1.4A illustrates several important points in Protection / Deprotection protocol. Since R1R2R3 can be combinations of differing groups which can be varied in order to provide a number of silyl ethers, this group of chemical compounds provides a wide spectrum of selectivity for protecting group chemistry. ���u4(bi@Fa� ����A��@�&ǔ�@����Qf�e�a�&���=B�QWzÇ�*���Lk4ffPg`H��_��f��43�. Another method is the Woodward cis-hydroxylation (cis diol) and the related Prévost reaction (anti diol), depicted below, which both use iodine and the silver salt of a carboxylic acid.
1,3-Diols are described as syn or anti depending on the relative stereochemistries of the carbon atoms bearing the hydroxyl functional groups. This time we're going to use, instead of a diol, we're going to use something with two SH groups instead here, so a dithiol here. This reaction applies to glyoxal and related aldehydes. Another example is propane-1,2-diol, or alpha propylene glycol, HO−CH2−CH(OH)−CH3, used in the food and medicine industry, as well as a relatively non-poisonous antifreeze product. Other routes to vic-diols are the hydrogenation of acyloins[5] and the pinacol coupling reaction. Various types of hydroxyacetophenones are efficiently converted into the corresponding cyclic acetals in the presence of a diol, triisopropyl orthoformate, and a catalytic amount of cerium(III) trifluoromethanesulfonate under mild reaction conditions.