Nuclei, dehydration

The shapes of dehydration nuclei and rates of interface advance at different crystal surfaces of MgS04.7H20 are different [72], Reaction rates at 323 K for several crystals of equal size, but varied habits, were compared [72] and it was shown that the rate of penetration of the reactant-product interface normal to the (111) face was considerably greater than that normal to the (110) face. This anisotropy was attributed [73] to the influence of the crystal lattice. The rate of dehydration measured [74] by microscopy from the advance of the interface agreed with that measured from mass-losses. 

The Pictet-Hubert reaction describes the construction of the phenanthridine nucleus (2) by dehydration of acyl-o-xenylamines (1). The application of zinc chloride at high temperature facilitates the dehydration/ This reaction is also referred as the Morgan-Walls Reaction. 

Alkylated sulfonamide groups have proven useful additions to the phenothiazine nucleus. The same seems to hold true in the thioxanthene series. Chlorosulfonation of the benzoic acid, 38, followed by displacement with dimethylamine affords the sulfonamide, 39. This is then taken on to the substituted thioxanthone (41) by the sequence of steps shown above Grignard condensation followed by dehydration gives thiothixine (42). 

Topley and Hume [453], in a study of the dehydration of CaC03 6 H20, assumed the rapid initial formation of (on average) a single nucleus on the surface of each particle of reactant, represented as a sphere of radius a. In the absence of preferential surface development, the reaction interface penetrates the reactant at equal rates in all inward directions (kG = dr/df) and the volume of material reacted at time t is that volume of a sphere, having its centre at the site of surface nucleation and of radius kGt, which falls within the reactant. The fractional reaction, the zone of interpenetrating spheres, at time t is... 

Although the exact mechanism of the Tschitschibabin cyclisation has not been elucidated, it is reasonable, as shown in Scheme 4, to assume a series of reversible steps from the vinylogous ylide (or methylide) to a methine and an enol-betaine intermediate and then finally an irreversible dehydration to the indolizine nucleus. The reaction might be related to the modern electrocyclic 1,5 dipolar cyclization. 

Thomas J et al (2004) The effect of dehydration history on PVA/PVP hydrogels for nucleus pulposus replacement. J Biomed Mater Res B Appl Biomater 69(2) 135—140... 

More recently, a catalyst-free aqueous version of this strategy was proposed with simple acyclic 1,3-dicarbonyls, formaldehyde, and styrene or anilines derivatives (Scheme 40) [131], In the first case (Scheme 40), the very reactive 2-methylene-1,3-dicarbonyl intermediate reacts smoothly at 80°C with a variety of substituted styrenes to give the corresponding dihydropyrans in moderate to good yields. Remarkably, when styrenes were replaced by A-ethylaniline, a novel five-component reaction involving twofold excess of both formaldehyde and 1,3-dicarbonyl selectively occurred (Scheme 41). The result is the formation of complex fused pyranoquinolines following a Friedel-Craft alkylation - dehydration sequence to furnish the quinoline nucleus, which suffers the Hetero-Diels-Alder cyclization in synthetically useful yields. 

Thiophenes have served as important four-carbon templates for molecular elaboration. Both functionalized cyclic and acyclic chains can be generated by carrying out conventional reactions on the thiophene nucleus and then performing a Raney nickel desulfurization at or near the very end of the scheme. Thus, the major constituent (279) of the Panamanian ant, Atta columbica, was made by Friedel-Crafts acylation of the thiophene derivative (277), followed by Grignard addition to the carbonyl group of (278), dehydration, and reductive desulfurization (Scheme 57) (60JOC1754). 

An extensive study on the use of benzothiazoles as carbonyl equivalents has been conducted (78TL5, 9). The 2-lithiobenzothiazole (576) readily adds to carbonyl compounds to furnish on dehydration of the carbinol a vinylbenzothiazole (578). The benzothiazole nucleus can then be unmasked by a sequence of reactions involving (a) Af-methylation with methyl fluorosulfonate, (b) addition of either hydride (NaBKU) or an organometallic (MeLi) to afford the iV-methylbenzothiazoline and (c) hydrolysis of the crude Af-methylbenzothiazo-line (e.g. AgN03). A representative example is shown in Scheme 126. 

Fig. 1.—Nucleus (N) from D-Galactose-treated Orchid Seedling (Phalaenopsis cv. Doris Fi) Showing Dispersed Chromatin with Nuclear Envelope Envaginated (Arrows) into the Cytoplasm. [After treatment with D-galactose, seedlings were fixed in 2% glu-taraldehyde for 2 h followed by 2% OsO, for 12 h. Tissue was dehydrated in a graded concentration, embedded in Epon 812, sectioned with a diamond knife, and photographed with a Zeiss EM9A electron microscope x 13,340 (reproduced, by permission, from Ref. 533).]...

In this section, consideration will be given to the actual processes of acetal- or ketal-formation and not to the more indirect methods by which acetals and ketals of the polyhydric alcohols may be synthesized from compounds (e.g. derivatives of the monosaccharides) containing preformed alkylidene or arylidene groupings. The condensation of a carbonyl compound with a glycol is facilitated by acidic catalysts, and, since the reaction is reversible, by dehydration. The catalysts most frequently employed are concentrated sulfuric, hydrochloric and hydro-bromic acids, gaseous hydrogen chloride, zinc chloride and cupric sulfate others are phosphorus pentoxide, sulfosalicylic acid, and anhydrous sodium sulfate. The formation of benzylidene compounds is promoted less efficiently by phosphorus pentoxide than by either concentrated sulfuric acid or concentrated hydrochloric acid 1" the reaction is assisted by chloro- and nitro-substituents on the aromatic nucleus, but hindered by methyl- and methoxy-groups.18... 

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