Types of Rearrangements

The isomerisation of aldoses and ketoses by hydride transfer, although suggested for enzymic isomerisations of aldoses and ketoses (rather than their phosphates), and having precedent in the metal ion-catalysed isomerisations of ot-hydroxy ketones, was considered to be the less common route until the paradigmal aldose-ketose isomerisation, that of glyceraldehyde to dihydrox-yacetone, was examined by NMR spectroscopy. Identification and analysis of all products of the reaction in D2O permitted concentrations to be chosen which minimised side-reactions. A useful feature of the system was that it was possible to integrate the areas of proton resonances in the CHD and CH2 

In the absence of Zn, the similar dependence of both the hydride-shift pathway and the enolisation pathway on [OD ] established that the hydrate diol anion was inert to both types of rearrangement, and that the hydride-shift mechanism probably went through the glyceraldehyde anion formed by deprotonation of 02. 

In sterically hindered buffers such as hexafluoroacetone hydrate or pivalate in the presence of Zn, the hydride shift mechanism is completely dominant, and the second-order rate constant is proportional to [OD ], but in high concentrations of acetate the enolisation mechanism, with k hs = k3[Zn ][Base], becomes predominant. 

In the course of biological evolution, primitive enzymes which bind the appropriate transition states initially fairly poorly are slowly refined as a consequence of Natural Selection. Evolution by Natural Selection thus predicts that, wherever enzymes are found to catalyse equivalent reactions by very different mechanisms, as with aldose-ketose isomerisations by hydride transfer or proton transfer, the spontaneous, uncatalysed mechanisms will be found to proceed at comparable rates under ambient conditions.  

The stereochemistry of the enediols and enediolates involved in non-enzymic sugar isomerisations is unknown, but protein X-ray crystallography, particularly of isomerases in complexes with stable analogues of the enediolates, has established the hydroxyl stereochemistry to be cis. The argument from Natural 

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A different type of rearrangement occurs when suitable side chains are a to a pyridine-like nitrogen atom. In the monocyclic series this can be generalized by Scheme 43. For a given side chain the rate of rearrangement is l,2,4-oxadiazoles>isoxazoles> 1,2,5-oxadiazoles. Typical side chains include hydrazone, oxime and amidine. Some examples are shown in Table 9 (79AHC(25)147). Similar rearrangements for benzazoles are discussed in Section 4.02.3.2.4. 

A new type of rearrangement has been reported for certain l-(o-nitrophenyl)pyrazoles (169), giving cis- and trans-benzotriazole 1-oxides (170 Scheme 12) (73TL891). The reaction was rationalized in terms of an intermediate azo compound (171 formed in turn either from the diradical species (172) or from the intramolecular 1,4-adduct (173). Subsequently... 

A number of examples of acid catalyzed ring expansion of acyl and thioacyl azetidines to sbc-membered rings have been reported (B-73MI50903). This type of rearrangement (Scheme 2) is similar to the more general vinylaziridine to pyrroline ring expansion. 

The 1-oxide 3-oxide tautomerism [Eq. (3), p. 4] has been discussed earlier (Sections II and III,C) in connection with the problem of the structure of benzofuroxan. A second type of rearrangement involves the furoxan ring and an adjacent substituent group, and arose out of a suggestion of Bailey and Case that 4-nitro-benzofuroxan might be a resonance hybrid of type (57)-(-> (58), rather than 57. NMR ruled out this possibility the three protons present in... 

Sulfur and selenium analogues of 91 are also susceptible of this type of rearrangement. Its rate is defined by the strength of the C-chalcogen bonds in 91 and analogues and was found to increase in the order of S < Se < Te (94MI1). 

Treatment of 6-arylidenehydrazino-3-alkyl-5-nitrouracils 510 with etha-nolic KOH caused a benzylic acid type of rearrangement to give 511, which were alkylated to give 512, whose cyclization with diethyl azodicar-boxylate gave (80H1295) 513 by intramolecular cycloaddition through valence isomerization and then aromatization with diethyl azodicarboxylate (Scheme 107). 

Treatment of the derivatives of 7-azalumazines, 514, with alcoholic sodium hydroxide caused a benzilic acid type of rearrangement via 515, followed by decarboxylation and oxidation by air or potassium permanga-... 

Thermal rearrangement of 2-vinylaziridines yields 3-pyrrolines. When aziri-dines 192 were heated in decalin at 180 °C, 3-pyrrolines 193 were formed in 75-85% yields (Scheme 2.48) [75]. A similar rearrangement was also observed by Lwowski and coworkers [4]. This type of rearrangement can be effectively pro-... 

There are many other examples of this type of rearrangement particularly in the reactions of poly-alkylated benzenes163 for example, the sulphonation of durene (CXXII) gives prehnitenesulphonic acid (CXXIII) and octahydrophenanthrene (CXXIV) similarly yields the octahydroanthracenesulphonic acid (CXXV), viz. 

A further type of rearrangement occurs in 1,3-dicarbonyl compounds/48 ... 

Brewer and Heaney<45> observed the same type of rearrangement with tetra-fluorobenzobarrelene, while Friedman found that dibenzobarrelene formed dibenzosemibullvalene upon sensitized photolysis(46,47> ... 

Thiadiazoles are also obtained when the thermolysis is carried out in the presence of isocyanates and carbodiimides <1996CHEC-II(4)307>. There have been no new reports of this type of rearrangement since the publication of CHEC-II(1996). 

For acyl nitronates of the general formula RCH=N(0)OAc, two types of rearrangements were suggested (Scheme 3.102), which are associated, respectively, with 1,2-migrations of the N-oxide oxygen atom (223) or the OAc fragment (219). 

This type of rearrangement has been described previously Binger P, Muller P, Benn R, Mynott R (1989) Angew Chem 101 647 Angew Chem Int Ed Engl 28 610... 

The oxygen derivative 39a has its origins in the acyloin reduction product of diethyl succinate 45. A benzylic acid type of rearrangement of in situ formed... 

The operation time scale of molecular devices [point (iv)] can range from less than picoseconds to seconds, depending on the type of rearrangement (electronic or nuclear) and the nature of the components involved. 

The intermolecular reaction of imines with acceptor-substituted carbene complexes generally leads to the formation of azomethine ylides. These can undergo several types of transformation, such as ring closure to aziridines [1242-1245], 1,3-dipolar cycloadditions [1133,1243,1246-1248], or different types of rearrangement (Figure 4.9). 

For this reason unstable cyclopropanes or only rearrangement products are obtained when donor-substituted alkenes react with acceptor-substituted carbene complexes [1409-1416]. In reactions of acyl- and vinylcarbene complexes with enol ethers the most common types of rearrangement observed are those shown in Figure 4.23. 

Scheme 25). l-Oxo-2,8-diphenyl-2,5,8 triaza-1 X -phosphabicyclo[3.3.0]octane (68) formed by acid catalysis of the bicyclic phosphoric triamide (67) has been found to isomerize via a new type of rearrangement to yield the ring contracted 3-[2-(phenylamino)ethyl]-2-oxo-2-ethoxy-l-phenyl-l,3,2 A, -diazaphospholidine (69). The rearrangement has been explained in terms of intramolecular 1,5-nucleophilic attack... 

A hitherto unknown type of rearrangement of l-(l-alkynyl)cyclopropanols (321) to cyclopent-2-en-l-ones (322) mediated by octacarbonyldicobalt" and hexacarbonyldicobalt" complexes has been described. A possible pathway for the transformation is outlined in Scheme 101. A -proton transfer accompanied by a metal-mediated Stevens rearrangement, which converts a coordinated dimethylsulfane... 

Scheme IV.3). The general scheme of this type of rearrangements in aromatic five-membered heterocycles with an unsaturated side chain is known as the Boulton-Katritzky rearrangement [67JCS(C)2005]. 

The same type of rearrangement could be observed with 3-cyano-5-nitropyridinium salt (123) when it reacts with methylamine, affording 124... 

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