Dehydration-cyclization

A key step in the approach to 3(2//)-furanone ring systems via the acid-catalyzed cyclization-dehydration of appropriately substituted a -hydroxy-l,3-diketones involves the acylation of a-hydroxy-ketone dianions 11141... 

In a similar fashion, hydroformylation of N-allyl-pyrrols leads to 5,6-dihydroindolizines via a one-pot hydroformylation/cyclization/dehydration process (Scheme 27) [81,82]. The cyclization step represents an intramolecular electrophilic aromatic substitution in a-position of the pyrrole ring. This procedure was expanded to various substrates bearing substituents in the al-lyl and in the pyrrole unit. 

Chapt. 5 is entirely devoted to the hydrolytic ring opening of lactams. The reverse reaction, namely lactam formation by cyclization-dehydration, has been seldom reported in the literature, suggesting that the reaction is, indeed, uncommon. However, it is also conceivable that lactam formation has sometimes been overlooked, and that much remains to be discovered regarding structural conditions for lactam formation, the factors that influence the lactam/amino acid equilibrium, and the role of enzymes. 

Scheme 7.1 Biosynthesis of heterocycles by cyclization-dehydration. Cy = cyclization domain.

A series of the regioisomeric benzofuran derivatives are produced from a base-catalyzed cyclization/dehydration sequence of dihydroxydibenzoylbenzenes (Equation 89) <2002HC0179, 2005T545>. The cyclization of the related derivatives containing benzyl or bromoalkyl substituents (R = benzyl or Br(CH2) -i, where = 8 or 12) has been reported to occur in benzene under photochemical conditions <1995JOC1303, 2002J(P1)310>. 

Cyclohydrocarbonylation of 1-allylpyrroles 74a-d catalyzed by Rh4(CO)i2 gave the corresponding 5,6-dihydro-indolizines 77a-d in good yield and excellent regioselectivity (Scheme 12)." This reaction proceeded through a cascade hydroformylation-cyclization-dehydration sequence (Scheme 12). Exclusive introduction of a formyl group... 

Trifluoracetic acid-promoted cyclization/dehydration is used to construct a dihydropyran-4-one ring during the total synthesis of (—)-maurenone, establishing the relative stereochemistry of the natural product <2006JOC117>. A double aldol reaction forms two dihydropyran-4-one rings during the total syntheses of (—)- and (+)-membrenone C <20030L1729>. 

JOC3904>. Pyrazole thioamides 22 were transformed into pyrazolothiazoles 23 via a cyclization/dehydration sequence <02S1079>. l-Acetyl-2-methoxyazulene (24) reacted with arylhydrazines in refluxing ethanol to give 1 -aryl-3-methylazuleno[ 1,2-r/]pyrazoles 25 in moderate to high yields <02H(56)497>. 

As mentioned earlier, hydrolysis of the aldobiuronic acid unit usually involves decomposition of the uronic acid portion. Neutral monosaccharides may also decompose by the action of mineral acids. Pentoses and hexoses form furfural and hydroxymethylfurfural, respectively, by the cyclization-dehydration reaction shown below (32). 

Michael addition, geometrical isomerization, cyclization, dehydration, isomeriza tion, and 1,2 hydride addition, in which each single step is efficiently catalyzed by chiral phosphoric acid catalyst Ig. The method enables direct, rapid, and efficient access to a diverse set of tetrahydropyridine and azadecalinone derivatives with excellent enantioselectivities. 

The scheme shows that here too, /9-oxidation scission with subsequent desulphonization and cyclization dehydration occur. 

The simplest method is sulfurization followed by cyclizing dehydration of 1,4-dicarbonyl compounds 14 analogous to the Paal-Knorr synthesis of furans. This cyclocondensation is carried out with P4S10 or HiS, and furnishes 2,5-disubstituted thiophenes 15 (Paul synthesis) ... 

Dioxane, mp -12°C, bp 101 °C, is a colourless liquid of pleasant odour, hygroscopic, and miscible with water and most organic solvents. It is widely used as a solvent. It is prepared by a cyclizing dehydration of ethylene glycol or diglycol, and by dimerization of oxirane, under acid catalysis in each case ... 

Another aspect of the 8-lactone chemistry is the interconversion between 8-lac-tone and 5-hydroxy fatty acid. Under basic aqueous conditions, the equilibrium favors the formation of the 5-hydroxy fatty acids. In contrast, catalytic amounts of acid will cause rapid cyclization/dehydration of the 5-hydroxy fatty acid into the 8-lactone. By utilizing the equilibrium between these two species, 5-hydroxy fatty acids were synthesized directly from meadowfoam oil (Scheme 13). 

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