Hetero- -rearrangements

The term hetero rearrangement is not limited to those reactions in which only the initially electron-deficient atom is something other than carbon, but may be applied to rearrangements in which any of the carbon atoms of formula L have been replaced with other elements. 

The reader will no doubt have realized that the y-hetero rearrangement is well-known, being none other than the familiar neighboring group effect. The a-hetero rearrangement is known for all of the elements mentioned but boron, aluminum, and silicon. Perhaps a suitably constituted boron compound would rearrange to a zwitterionic product as follows ... 

The /3-hetero rearrangement is known only for silicon in contrast to the variety of instances of the reverse reaction, the a-hetero rearrangement.295... 

The generalized picture of hetero-rearrangements that Katritzky and co-workers177 pointed out in 1967 stimulated much research for other examples. In the five-membered ring (90), the bonding ABD gives way... 

Diaziridines, discovered in 1958, six years after the oxaziridines, were almost immediately realized to be structural analogs of oxaziridines. Like these they showed oxidizing properties unexpected for other classes of organic nitrogen compound. Properties in common with oxaziridines include the rearrangement to open chain isomers on heating above 100 °C (for several diaziridines), and their hydrolytic behavior in acidic media, which leads to carbonyl compounds with conservation of the hetero-hetero bond. 

Rearrangement ot 0-(2-pyrtdyi)oximes to 2-pyndones via a hetero-Cope rearrangement. 

Rearrangement by nucleophilic aromatic substitution and aromatic mgrallon trom one hetero atom to another (O lo N or S lo O)... 

Favorskii rearrangement, 159, 176 Formation of hetero-radicals, 238 2-Formyl-A-nor-5a-androstan-l 7 -oI, 415 2-Formyl-A-nor-5 a-androst-1 -en-17 S-ol, 416 6/3-Formyl-B-nor-5 -cholestane-3, 5 -dioI 3-acetate, 432... 

The hetero ring in 4-(l -hydroxyalkylidene)-5-oxazolones is cleaved by alcoholic HCl to form alkyl a-acylaminoacylacetates, which cyclize to oxazole-4-carboxylates [Eq. (25)]. This rearrangement occurs directly in alkali, and a carbon-14 tracer study has substantiated a mechanism involving ring opening followed by the alternative ring closure. ... 

The aza-Wittig rearrangement in synthesis and transformations of N-hetero-cycles 97S497. 

Homochiral thiiranium and aziridinium ion intermediates formed by Lewis acid-induced rearrangement of l-hetero-2, 3-epoxides 97SL11. 

Occasionally, addition products of 4//-l,2,4-triazole-3,5-diones or diazenedicarboxylic esters and oxepins have been obtained whose formation can be rationalized by an addition to the 2,4-diene system in the oxepin, e.g. formation of 10.190191 In these cases, the primary adduct usually cannot be isolated, because it undergoes a hetero-Cope rearrangement to a tricyclic or bicyclic structure in which the oxepin oxygen has become part of a carbonyl function.190 191,227... 

Peroxyesters (1), R(C03 R )n Peroxycarbonicesters (2), ROC(O)—0—0—R Diperoxyesters (3), C(0)(-0-0—R)a Peroxycarbamates (4), >NC(0)-0-0-R Areneperoxysulfonates (5), rso2-o-o-r Difficult to prepare because of ready rearrangement or decompn in polar solvents peroxy sulfonates decomp hetero lyrically (no free radicals) Peroxycarbamates are stable, distillable liqs or cryst solids rapid decompn at 80—140° and violent decompn at 140— 180°. Alkylareneperoxysulfon-ates have low stability and decomp violently at RT within 10 mins... 

Allyl sulfones formed from allyl sulfinates (cf. equation 1) can easily tautomerize to give a, /J-unsaturated sulfones26 in cases for which R1, R2 are part of an (hetero) aromatic system, this tautomerization occurs spontaneously. Similarly, sulfinic acid esters from jV-phenylhydroxamic acids as reactive intermediates rearrange to give o-(major part) and p-sulfonylanilines (minor part)27 ... 

Streith J., Defoin A. Aza Sugar Syntheses and Multi-Step Cascade Rearrangements Via Hetero Diels-Alder Cycloadditions With Nitroso Dienophiles Synlett 1996 189-200... 

Regarding the series of hetero aromatic pentacyclic compounds with three heteroatoms, an accelerated synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles 66 under microwave irradiation has been reported by thermic rearrangement of dihydro-1,2,4,5 tetrazine 65 (Scheme 22). This product was obtained by reaction of aromatic nitriles with hydrazine under microwave irradiation [53]. The main limitation of the method is that exclusively symmetrically 3,5-disubstituted (aromatic) triazoles can be obtained. 

Keywords. Phosphorus and sulfur derivatives, Dithioesters, Hetero Diels-Alder, Sigmatropic rearrangements. Chiral ligands... 

Scheme 1.52. Proposed catalytic cycle for the cationic domino rearrangement/hetero-Tishchenko reduction process of secondary a-hydroxy epoxides in the presence of Sml2.

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

In a clever application of the hetero-Cope rearrangement, Martin used a Pd-catalyzed coupling of W-arylhydroxamates 362 with vinyl acetate to set up the [3,3] sigmatropic rearrangement 363 to 364 and final cyclization 364 to 365 [432-435]. Applications of this novel indole ring... 

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