Solvent-free protection

The 0-silylation reaction of alcohols is important as a protection method of hydroxyl groups. 0-Silylations of liquid or crystalline alcohols with liquid or crystalline silyl chlorides were found to be possible in the solid state. For example, when a mixture of powdered L-menthol (26), ferf-butyldimethylsilyl chloride (27), and imidazole (28) was kept at 60 °C for 5 h, 0-tert-butyldi-methylsilyl L-menthol (29) was obtained in 97% yield [8] (Scheme 4). Similar treatments of 26 with the liquid silyl chlorides, trimethyl- (30a) and triethylsilyl chloride (30b), gave the corresponding 0-silylation products 31a (89%) and 31b (89%), respectively, in the yields indicated [8] (Scheme 4). However, 0-silylation of triisopropyl- (30c) and triphenylsilyl chloride (30d) proceeded with difficultly even at 120 °C and gave 31c (57%) and 31d (70%), respectively, in relatively low yields. Nevertheless, when the solvent-free silylation reactions at 120 °C were carried out using two equivalents of 30c and 30d, 31c (77%) and 31d (99%) were obtained, respectively, in relatively high yields. 

The protection-deprotection reaction sequences constitute an integral part of organic syntheses such as the preparation of monomers, fine chemicals, and reaction intermediates or precursors for pharmaceuticals. These reactions often involve the use of acidic, basic or hazardous reagents and toxic metal salts [30], The solvent-free MW-accelerated protection/deprotection of functional groups, developed during the last decade, provides an attractive alternative to the conventional cleavage reactions. 

Aldehydes and ketones have been protected as acetals and dioxolanes using orthoformates, 1,2-ethanedithiol or 2,2-dimethyl-l,3-dioxolane by Hamelin and coworkers. This acid-catalyzed reaction proceeds in the presence of p-toluenesulfonic acid (p-TsOH) or KSF clay under solvent-free conditions (Scheme 6.2). The yields ob-... 

General procedures for the synthesis of the imidazole core have been published in 2000. Solvent-free microwave assisted synthesis of 2,4,5-substituted imidazoles 64 from aldehydes 62 and 1,2-dicarbonyl compounds 63 in the presence of ammonium acetate and alumina has been reported <00TL5031>. V-protected a-amino glyoxals 65 were utilized as potential chiral educts for the synthesis of amino acid-derived imidazoles 66 <00TL1275>. 

An epoxy resin formulation completely free of organic solvent was applied to carbon steel coupons using supercritical carbon dioxide (CO2) as the volatile solvent. Sleet coupons coated in this manner were compared to coupons coated with a standard military specification marine primer (MlL-P-24441) during exposure to aerated 0.5 nrl sodium chloride (NaCl). The solvent-free coating protected the steel from corrosion in this environment as well, if not better than, the solvent-based primer (Kendig et al., 1999). 

Type of reaction protection Reaction condition solvent-free... 

Solvent-Free Protection under Microwave Irradiation... 

Waste prevention and environmental protection are major requirements in an overcrowded world of increasing demands. Synthetic chemistry continues to develop various techniques for obtaining better products with less environmental impact. One of the more promising approaches is solvent-free organic synthesis this book of Koichi Tanaka collects recent examples in this field in a concise way so that their performance and merits can be easily judged. This endeavor is very welcome, as most recent syntheses and educational textbooks largely neglect solvent-free techniques. 

Fluoroboric acid supported on silica (HBF4-silica) has recently been found to be a highly efficient catalyst in the protection of various functional groups. Structurally diverse alcohols, phenols, thiophenols, and anilines can be acylated under solvent-free conditions at room temperature.669 Even acid-sensitive tertiary alcohols (1-alkylcyclo-hexanols) and sterically hindered compounds, such as endo-borneol, give the acylated products in high yields. A triflic acid-silica catalyst also shows high activity in the (9-acetylation with Ac20 of alcohols and phenols.359... 

Aliphatic aldehydes typically provide only moderate yields in the Biginelli reaction unless special reaction conditions are employed, such as Lewis-acid catalysts or solvent-free methods, or the aldehydes are used in protected form [96]. The C4-unsubstituted DHPM can be prepared in a similar manner employing suitable formaldehyde synthons [96]. Of particular interest are reactions where the aldehyde component is derived from a carbohydrate. In such transformations, DHPMs having a sugar-like moiety in position 4 (C-nucleoside analogues) are obtained (see Section 4.7) [97-106]. Also of interest is the use of masked amino acids as building blocks [107, 108]. In a few cases, bisaldehydes have been used as synthons in Biginelli reactions [89, 109, 110]. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

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