Tautomerism, valence

Small unsaturated rings are usually very reactive undergoing ring opening in a number of ways, and this characteristic has been utilized in heterocyclic synthesis. In their role as dienophiles or dipolarophiles, the initial cycloaddition is usually followed by a valence tautomerism resulting in a six-membered or larger ring system. Several examples exist, however, where this does not occur, and these are described below. 

The addition of benzonitrile oxide to cyclooctatetraene produced a monoadduct which was induced to undergo valence tautomerism to produce a tricycloisoxazoline (Scheme 104). A similar reaction with tropone gave a minimum of eight adducts from which two monoadducts were isolated (Scheme 104) (70T5113). 

Any heterocycle containing the OCH=CH moiety can in principle extrude the superfluous fragment and form oxirene, as illustrated for a five-membered ring in Scheme 105. Probably the most propitious AB fragment would be nitrogen, but the required 1,2,3-oxadiazole (123) is unknown (see Chapter 4.21), probably because of ready valence tautomerization to diazoethanal (Scheme 106) (this approach has been spectacularly successful with the sulfur analogue of (2) (8UA486)). The use of (123) as an oxirene precursor is thus closely linked to the important diazo ketone decompositions discussed in Section 5.05.6.3.4(f). 

Azepine, 5-bromo- 1-ethoxycarbonyl-X-ray crystallography, 7, 520 1 //-Azepine, 5-cyano-4,5-dihydro-rearrangement, 7, 512 1 //-Azepine, 3-cyclopropyl-2-diethylamino-synthesis, 7, 536 1//-azepine, 2,6-dicyano-ring contractions, 7, 506 1//-Azepine, 2,3-dihydro-synthesis, 7, 42 1 //-Azepine, 2,5-dihydro-valence tautomerism, 7, 506 1 //-Azepine, 2,7-dihydro-rearrangement, 7, 507 1//-Azepine, 4,5-dihydro-... 

Valence shell electron pair repulsion theory, 1, 564 Valence tautomerism photochromic processes and, 1, 387 y-Valerolactone, o -allyl-a -2-(pyrido[2,3-6]-imidazolyl)-synthesis, 5, 637 Validamycin A as fungicide, 1, 194 Valinomycin... 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

Stetter and ReischP described the preparation of 2,6-diphonyl-P5Tylium perchlorate (57) from l,5-diphenyl-2-penten-4-yn-l-one (56a) and concentrated sulfuric acid in the cold, followed hy perchloric acid, in 60% 5rield. The process may involve hydration to a 1,5-enedione, or an ion 56b related to 52, and valence tautomerism. 

The phenomenon of tautomerism comprises many different types of which the prototropic tautomerism that we consider here is only one. Prototropic tautomerism exists when the two tautomers differ only in the position of a proton (this is, of course, an approximation there are other differences between two tautomers, for example, in precise bond lengths). Other important types of tautomerism include the following (1) anioniotropy, where the two tautomers differ only in the position of an anion, which moves from one place to another in the molecule (2) cationiotropy, where the two tautomers differ in the position of a cation (other than a proton), which moves from one place to another in the molecule (3) ring-chain tautomerism and (4) bond-valence tautomerism. 

Valence tautomerism of heterocyclic systems containing tetrazole rings has received much attention [76AHC(S1), p. 498 84CHEC-I(5)791 96CHEC-1I(4)621], The latest contributions to this area are considered in Section IV,C, which is concerned with the topic of azido-tetrazole tautomerism. 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and hve-membered heterocyclic azides. Tire tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

The knowledge of the valence tautomerization of benzene oxides to oxepins12 prompted several groups to synthesize oxepins by dehydrohalogenation of 7-oxabicyclo[4.1.0]heptane derivatives. Numerous examples have been described for the base-catalyzed elimination of hydrogen bromide from the 3,4-dibromo-7-oxabicyclo[4.1.0]heptane system. The reaction products are usually obtained as mixtures of oxepin 1 and benzene oxide 2. The 2,7-bis(hydroxy-methyl)oxepin 1 p obtained by this route can be converted to the 2,7-dicarbaldehyde with man-ganese(IV) oxide.23... 

When the valence tautomeric mixture of oxepin and benzene oxide is treated with singlet oxygen, the primary product is the 1,4-endoperoxide 3 which has proven to be too labile for isolation.219 Its formation can be rationalized by a 1,4-addition across the diene system of the benzene oxide structure 3 then rearranges to ba s-3,6,9-trioxatetracyclo[6.1.0.02 4.05 ]nonane (transbenzene trioxide, 4). 

Oxazepines are readily decomposed by heat and light and by acids and bases.8 Some of these reactions proceed by hydrolytic opening of the oxazepine ring, as in the formation of 1-benzoylpyrrole (1) from 2-phenyl-l,3-oxazepine and cold dilute hydrochloric acid,12 while others involve valence-tautomeric oxazanorcaradienes, as in the thermal conversion of 2-phenyl-l,3-oxazcpine into 2-phenylpyridin-3-ol (2).12... 

A mixture of ethyl 3,5-dimethyl-l/f-thieno[2,3-c]-1,2-diazepine-l-carboxylate (6) (10-15%) and the isomeric ethyl 2,5-dimethyl-3//-thieno[2,3-t/]-l, 3-diazepine-3-carboxylate (8) (50-60%) is obtained when a solution of the thienopyridine A-iminc 4 in benzene is irradiated with a 400-W high-pressure mercury lamp for 2-3 hours at 20rC. It has been suggested that product 6 is formed from the intermediate 5. The isomeric intermediate 7 undergoes two consecutive walk rearrangements, followed by valence tautomerization, to give the major product 8.168... 

The chloro groups of 4,7-dichloro-l,2-diazocines (vide supra) can be sequentially substituted by O-, S- or W-nucleophiles.25 27 The reaction most likely proceeds via an elimination-addition mechanism utilizing the valence tautomeric diazabicyclo[4.2.0]octatriene forms. 

An important method for the synthesis of 1,3-diazocines consists of the reaction of pyrimidines 1 with ynamines to give [2 + 2] cycloadducts, which rearrange to the eight-membered-ring compounds 2 by valence tautomerization.2-3... 

The 2- or 3-azidoquinoxalines exist in valence tautomerism with tetrazolo[l,5-u]quinoxalines indeed, they are often known and indexed in the hterature only under the latter name, irrespective of the dominant form. However, in this book all are known as azidoquinoxalines (see also Section 3.2.4). 

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