Tailored Reagents

While the dehydration reagents discussed thus far in this Section are all well known, or at least are existing compounds, some reagents have been specifically developed for dehydration reactions. 

Burgess reagent (methyl carboxysulfiimoyl triethylammonium hydroxide inner sah) 

The Burgess reagent is a chlorine-free advancement of chlorosulfonyl isocyanate (see also Section 4.5.1), in which the two active sites are reacted with triethyl-amine and methanol, respectively, forming appropriate residues. This achieves the required level of reactivity and the right degree of selectivity, and thus enables consistent reaction mechanisms. 

Typical procedure. 3-lsocyano-3,7-dimethylocta-1,6-diene 1675 [1243] Burgess reagent (0.35 g, 1.5 mmol) was added in a single portion to a solution of the formamide (0.18 g, 1 mmol) in dry dichloromethane (25 mL). The solution was heated at reflux under a nitrogen atmosphere until TLC analysis indicated that the formamide 

2-Chloro-l,3-dimethylimidazoliniiim chloride (CDC) [1137, 1245] represents an alternative reagent to carboditmides such as dicyclohexylcarbodiimide 1681, which, although inexpensive and usable under mild conditions, is less reactive and the dicydohexylurea 1682 it produces presents difficulties in the purification of products. The reagent CDC is a solid, mp 95-100 °C, is stable to oxygen, but unstable in the presence of moisture, producing hydrochloric add. It is stable at room temperature for over a year when kept desiccated. CDC represents a powerful dehy- 

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The synthesis of tailor-made star-shaped polymers can be performed in several ways by means of a plurifunctional organometallic initiator, or by reacting a living precursor polymer with a plurifunctional reagent, to build the centra] body, or by block copolymerization involving a diunsaturated monomer (Scheme 3). 

Selection of a flotation technique for gold preconcentration depends very much on the ore mineralogy, gangue composition and gold particle size. There is no universal method for flotation of the gold-bearing minerals, and the process is tailored to the ore characteristics. A specific reagent scheme and flowsheet are required for each ore. 

The preparation of a combinatorial library requires the simultaneous manipulation and isolation of many different compounds. A uniform sequence of operations is required to efficiently prepare and isolate each member of the library. In contrast the traditional synthesis of compounds utilizes conditions tailored specifically to the compound desired. A key challenge for library preparation is the development of a robust synthesis sequence that cleanly incorporates chemical building blocks containing a diverse range of chemical functionality in high yields. Equally important are the development of uniform and efficient methods to isolate intermediates and products from solvents, reagents, and byproducts. 

There is a cost associated with the effort needed to prepare these nanostructured materials building blocks must be found and suitably functionalized tailored linking reagents are required air sensitive handling procedures developed and byproducts carefully removed. It is therefore appropriate to ask, has the effort and expense lead to new and better catalysts ... 

Carbohydrates are configurationally stable, easily available in enantiopure forms from the chiral pool, and they show a high density of chiral information per molecular unit. Their polyfunctionality and structural diversity fadhtate their tailor-made modification, derivatization, and structural optimization for a broad spectrum of synthetic applications. While derivatives of various saccharides have already been utilized as versatile starting materials and building blocks for chiral auxiliaries, ligands, and reagents [330] their obvious role as precursors for the... 

Hosami and co-workers (12,173,174) prepared alternative forms of nonstabilized ylides (Scheme 4.87). They generated tailor-made carbonyl ylides from substituted 1,3-dichloroethers through a 1,3-elimination pathway mediated with a samarium reagent. These simple carbonyl ylide intermediates are valuable for preparing tetrahydro- and dihydrofurans. To a mixture of bis(chloromethyl) ethers and... 

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