Fischer reactions

Problem 25.21 1 What product(s) would you expect from Kiliani-Fischer reaction of n-ribose ... 

What chemicals are required in a Karl Fischer reaction What is the reaction ... 

A method of synthesis which has been used generally in this series employed a Fischer reaction at an early stage to form the tetrahydrocarbazolone nucleus [59] (Scheme 7.2). We devised a second route employing the Fischer method in which the key intermediate was the cyclohexenone (22) (Scheme 7.3). This was readily prepared by treating the enolate of the methyl enol ether (21) with dimethyl(methylene)ammonium iodide [60] to form the Mannich base which was then condensed with 2-methylimidazole to give (22). 

About the same time a number of publications appeared on the complete synthesis of 7a, 7b, 30 and 31 using a different approach to build the alkaloid framework (Scheme 5 68M1364, 68M1584, 69JCA(C)2738). Phenylhydrazone 34 under Fischer reaction conditions was converted into indole 34a with subsequent successive transformations to pentacyclic derivative 35. Its acid-initiated rearrangement led to the formation of 7b and 30 in a ratio of 1 5. While 7b and... 

The study (05ZOR89) showed that thenoines actively react with thiols in acidic media at room temperature to give /1-keto sulfides 250 in good yields. On alkaline hydrolysis, the latter compounds are readily cleaved to ketones 251, which are valuable intermediates for subsequent syntheses. In 06MI2, they were involved in a Fischer reaction and indole- bridged dithienylethenes 252 and 253 were obtained. Hence, a convenient procedure was developed for the transformation of readily available acyloins into photochromic indole-bridged dithienylethenes (Scheme 71). 

Frovatnptan (6, SB-209509/VML-251), initially discovered by SmithKline Beecham, was licensed to Vanguard Medica/Vemalis for development. The marketing rights for frovatriptan have subsequently been licensed to Elan in the United States. Frovatriptan is a conformationally-restrained analog of the selective 5-HTi agonist 5-carbamoyltryptamine (5-CT). The key step in the syntheses involve Fischer reaction of 4-cyanophenylhydrazine (70) or 4-carboxamidophenylhydrazine (79) with an... 

Syntheses of Heterocyclic Compounds Based on E. Fischer Reaction Yu. P. Kitaev, Usp. Khim., 1959, 28, 336-368. 

Different results, depending on the structure of the initial 2-benzopyry-lium salt, were obtained also in reactions with phenylhydrazine. Thus, the 1,3-dimethyl-substituted cation 35 is converted into an isoquinolinium salt 174(R = Ph)(70KGS1308), whereas the 1,3-diphenyl derivative 178 forms an indole 180 via the bishydrazone 179 by the Fischer reaction (71CB2984). 

The use of arylhydrazones for the synthesis of indoles has been widely discussed in the literature [1,2, 169], One of the most useable methods is the Fischer reaction [169, 170, 171], modified by Arbuzov [172],... 

Arylhydrazones containing diethoxyphosphoryl group cyclize to 2-phosphorylindoles 167 according to the scheme of the Fischer reaction [175] ... 

When studying the course of reactions, l3C-n.m.r. spectra may be used to monitor the progress of a reaction,34 or to detect intermediates. The latter was achieved in a study of the Kiliani-Fischer reaction.88... 

Compound (1) and 2-methyl-1//-pyrrolo[2,3-Z>]pyridine (87) have been prepared by a heterogeneous catalyzed Fischer reaction. A cyclization reaction occurred with acetaldehyde and acetone 2-pyridylhydrazones in the presence of y-alumina and fluorinated aluminum oxide (Equation (20)). In addition to the desired pyrrolopyridine products, 2-aminopyridine (88) and a triazine derivative (89) were also formed. Use of the fluorinated aluminum oxide maximized formation of the desired products <72CHE594, 72CHE1037). 

Aminomethylene pyrrolidones are converted in a similar manner to / -carbolines by reaction with a substituted phenylhydrazine (equation 161). The reaction involves hydrazinolysis of the enaminone followed by the Fischer reaction of the intermediate spiro-compounds230. 

The products of Kiliani-Fischer reaction of D-ribose have the same configuration at C3, C4 and C5 as D-ribose. 

Pauson-Khand cyclization3k 143 of tV-allyl (l-alkynyl)carbene complexes 134 (M = Cr, W R = Ph, Et R1 = H, Me) affords cyclopentenone derivatives 136144 via cobalt complexes 135145 (Scheme 53), as well as chromium complexes.146 Cyclopentenones also have been derived from 7V-diallyl(l-alkynyl)carbene complexes.39 Stable cobalt complexes of type 135 are obtained from O-allyl (l-alkynyl)carbene complexes. Interestingly, the last-named compounds do not form a cyclopentenone on heating instead, they form an enyne by elimination of M(CO)6 in a retro-Fischer reaction. 147... 

Rh-catalysed intramolecular insertion into thiophene. 59, 204 cycloaddition to uracils, 55, 1% Carbene, 5,6-cycloadduct with 2H-thiopyran, 59, 225 Carbenes, a-keto-perfluoroalkyl-, generation, trapping. 60, 40 Carbenes, Fischer, reaction with N-(hexafluoroisopropylidene) amides,... 

The Fischer reaction has been used to prepare pyrrole intermediates of the current invention and is discussed (4). 

For a very limited number of biological product sample types the Karl Fischer method cannot be used. There may be a substance present in the product that interferes chemically with the Karl Fischer reaction such as a substance that binds iodine, or the sample may not dissolve adequately into the Karl Fischer reagent, or the sample moisture may not adequately extract into the Karl Fischer solvent. 

There is clearly a Fischer indole synthesis in the second step but the first step makes the usut. hydrazone in a most unusual way. The first reaction is diazotization so we have to combine fi diazonium salt with the enolate of the keto-ester. That creates a quaternary centre and the KC H deacylates it to give the aryl hydrazone needed for the Fischer reaction. 

Although the final equilibrium in the Fischer reaction favors the pyrano-sides, the furanosides appear to be formed first, and they can sometimes be isolated by performing the reaction under mild conditions and arresting it at an early stage. In the same way, advantage can sometimes be taken, at the expense of the yield, of the fact that the a and /3 anomers (furanose or pyranose) may be formed at different rates. If an alcoholic glycoside is refluxed with the alcohol and an acid catalyst, equihbrium between the different forms is re-established. 

With the exception of the Fischer reaction, the general methods employed for the formation of the glycoside bond result in the formation of glycoside acetates (or benzoates). Deacylation is accomplished by treatment with alkali, to which the glycoside bond is stable, unless the bond approximates to the ester type. The original method of deacetylation was by quantitative saponification with sodium hydroxide, potassium hydroxide, or barium hydroxide. It was noted, however, that, in anhydrous media, much less than the theoretical amount of alkali was required, and the reaction has since been made entirely catalyt.ic, with considerable gain in efficiency and convenience. The same methods have been employed for debenzoylation. 

ProMem 25.19 What producUs) would you expect from Kiliuni-Fischer reaction of u-rlbose ... 

Lipin ska, T. 1,2,4-Triazines in organic synthesis. 9. Synthesis of 3-(3-ethylindol-2-yl)-5,6,7,8-tetrahydroisoquinoline using the Fischer reaction under the usual conditions and with microwave irradiation. Chem. Het. Comp. (New York) (Translation ofKhim. Geterot. Soed.) 2001,37,231-236. 

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