Synthesis thin-layer chromatography

The nitration of the 2-anilino-4-phenylselenazole (103) is much more complicated. Even careful nitration using the nitrate-sulfuric acid method leads to the formation of a mixture of variously nitrated compounds in an almost violent reaction. By the use of column chromatography as well as thin-layer chromatography a separation could be made, and the compounds could be partly identified by an independent synthesis. Scheme 33 shows a general view of the substances prepared. Ring fission was not obser ed under mild conditions. 

Thin-Layer Chromatography TLC) The function of TLC in organic synthesis is primarily one of allowing the experimenter to follow the progress of the reaction without actually interrupting the reaction. Since successful TLC can be carried out on a minute scale, only a very small fraction of the reaction mixture need be withdrawn and subjected to analysis. The following example of the TLC analysis of the chromic acid oxidation of borneol, described by Davis (3), is a useful model. 

Finally, the number of steps in the strigol synthesis could be reduced by the direct conversion of Xa to Xlla. To this end, Xa was treated with 1.2 equivalents of NBS in refluxing carbon tetrachloride under the irradiation of a 150-watt lamp. Purification of the crude product mixture by thin layer chromatography afforded two products Ila (27%) and the unanticipated bromoketone XV (31%). The reaction... 

The epothilone synthesis in Scheme 13.49 has been used as the basis for a combinatorial approach to epothilone analogs. 167 The acyclic precursors were synthesized and attached to a solid support resin by steps A-E in Scheme 13.58. The cyclization and disconnection from the resin were then done by the olefin metathesis reaction. The aldol condensation in step D is not highly stereoselective. Similarly, olefin metathesis gives a mixture of E- and Z-stereoisomers so that the product of each combinatorial sequence is a mixture of four isomers. These were separated by thin-layer chromatography prior to bioassay. In this project, reactants A (3 variations), B (3 variations), and C (5 variations) were used, generating 45 possible combinations. The stereoisomeric products increase this to 180 (45 x 4). 

Monitoring reaction progress throughout a multistep synthesis is a relatively difficult task.22 Typical methods used for solution-phase synthesis, including thin-layer chromatography (TLC), GC, and most types of mass spectrometry (MS), are less informative for solid-phase methods. However, Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) are particularly useful in solid-phase strategies. 

Thin-layer chromatography (TLC) [5] is routinely used in dendrimer synthesis for purity checks and identification of individual constituents of a dendrimer sample and is suitable for monitoring column chromatographic separations. Since chemistry students are introduced to this analytical technique during their first semesters, we shall not go into further details here. 

Benzylacetoacetate (5) was reduced to be n z y I - (.S ) - (+) - 3 - h y dm x y b u ty rale (6) by the yeast Candida schatavii MY 1831 (Scheme 19.7), which is considered a possible intermediate in the synthesis of L-734,217 (7), an experimental fibrinogen receptor antagonist.80 Initially, 8 microorganisms were tested for their reduction ability using 5 as substrate. The best culture, C. schatavii MY 1831, yielded product with an ee of 93% and an estimated conversion yield of >95% (by thin layer chromatography [TLC]). 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

Fig. 4.5. Sphingomyelin synthesis in Hymenolepis diminuta distribution of label in various intermediates, separated by thin-layer chromatography, after incubation with cytidine-5 -diphospho [methyl-13C]choline. The position of the lipid standards is indicated by the arrows, (a) sphingomyelin (b) dihydrosphingosine (c) sphingosine (d) ketosphingosine (e) ceramide. The origin is at band O. d.p.m., disintegrations/min. (After Bankov Barrett, 1985.)...

Diacetate Derivative, Isopropylidene Formation, and Mass Spectrometry. A particularly facile route to proof of structure of the glyceryl ether components is to use a combination of (a) purification by thin-layer chromatography and (b) separation into individual species of the isopropylidene or acetate derivatives by gas-liquid chromatography. As an example, the experimental protocol for examination of the diacetates will be described at this point and later that of the isopropylidene derivative. Inherent in such an experimental approach is to have synthetic standards of high purity available. It is possible to accomplish this task, but it is not necessarily quick and neat. The synthesis... 

Furfural (18) was oxidized to give the butenolide 19, which on Michael condensation with diethyl ethylmalonate afforded lactone 20. Hydrolysis of 20 yielded the dilactone 21. This was treated with ethanol in the presence of sulfuric acid. The isomer mixture obtained was separated by preparative thin-layer chromatography, yielding 4-ethoxy-3-ethoxycarbonylmethyl-2-ethyl-4-butanolide (22). Elimination of ethanol, with the aid of p-amino-benzenesulfonic acid, gave 2-(3//)-furanone 23 the 2-(5//)-furanone isomer 24 was obtained with the aid of orthophosphoric acid. Both isomeric esters gave the acid 25 after hydrolysis, which on catalytic hydrogenation afforded m-3-carboxymethyl-2-ethyl-4-butanolide 26, identical to homopilopic acid. This synthesis of homopilopic acid differs from earlier syntheses because the less stable cw-2,3-disubstituted butanolide (26) is formed in the last step. 

The latter compounds are prone to an intramolecular nitroaldol (Henry) reaction, yielding nitrocyclohexanola (53) in less than 1 h. The formation of (53) can be easily observed by thin-layer chromatography (TLC), thus treatment of (53) with 4N hydrochloric acid favors the elimination of both water and a further molecule of nitrous acid, allowing the one-pot synthesis of target molecules (54) in 42-77% overall yields. It is important to note that this one-pot process formally includes five different transformations (i) Michael addition, (ii) nitrous acid elimination, (iii) intramolecular nitroaldol reaction, (iv) water elimination, and (v) elimination of a further molecule of nitrous acid. 

During a monumental synthesis of Strychnine, the Overman group encountered difficulties with the simple selective protection of the primary alcohol function in diol 87,1 as its TIPS ether [Scheme 4.89].143 The best method involved treatment of diol 87.1 with 2 equivalents of triisopropylsilyl chloride and 2.2 equivalents of 1,1,3,3-tetramethylguanidine at 0 °C in N-methylpyrrolidinone until the diol could no longer be detected by thin layer chromatography. This treatment... 

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