Ring transformation

Ring transformations of indoxazenes have been effected under acidic, basic, thermal, and photolytic conditions. 

As mentioned in Section II,C,2,b, attempts to reduce 3-(o-aminophenyl)-indoxazene with lithium aluminum hydride result in rearrangement to 3-(o-hydroxyphenyl)indazole. The rearrangement is equally successful with sodium hydride in boiling tetrahydrofuran and proceeds via the amine anion as shown in Eq. (3).29 

The thermal rearrangement of 3-hydroxyindoxazene to benzoxazolin-2-one is temperature-dependent. The indoxazene is inert at 250°C but rearranges quantitatively at 450°C.78 3-Phenylindoxazene under flash vacuum pyrolysis conditions (800°C, 0.1-1 torr) rearranges to 3-phenylbenzoxazole in 80% yield.79 In both reactions a spiroazirine intermediate [59 or 60 (R = Ph)], analogous to those isolated in some isoxazole-oxazole rearrangements,80 is considered likely. 

An oxygen-dependent photodimerization of indoxazene to bis(2,2 -benzoxazole) has been reported.82 

6-Nitrocoumarin and hydroxylamine in ethanol yield a 1 2 adduct (20) (see Section II,A,3) which with mineral acid cyclizes to 5-nitro-2,3-dihydro-indoxazene-3-acetic acid (63).21 With diazomethane, 63 yields the methyl ester and with acetic anhydride the JV-acetyl derivative. Oxidation to indoxazene-3-carboxylic acid is successful using selenium dioxide, whereas hydrogenation in the presence of palladium-charcoal brings about ring opening to j3-amino-j3-(2-hydroxy-5-nitrophenyl)propanoic acid. 

On heating in solution, azetidine (16 X = Cl) undergoes reversible ring contraction to the aziridine (17), presumably via the azabicyclobutyl cation (18). The tosylate (16 X = Ts) in ethanol gives aziridine (17 X = OEt) (B-73MI50901). 

Oxidation of A-aminoazetidines (19), deoxygenation of A-nitrosoazetidines (20) and direct deamination of azetidines (21) with difluoroamine leads to cyclopropanes (23) by extrusion of nitrogen from a diazine intermediate (22) (63JA97). A further interesting ring contraction occurs in the Ag catalysed solvolysis of the A-chloroazetidine (24), which appears to involve the intermediate cation (2S) (7ITLI09). 

Isomerization reaction involves a change in molecule s functional group without changing the chemical formula. An example here is the isomerization of D-glucose to D-fructose by way of enediol intermediate during glycolysis [1]. Cephalosporin antibiotic APIs will undergo isomerization of the olefin from the A3 position to the A2 position [28]. 

Organic Chemistry, McGraw-Hill, New York, 1987. 

Simmons, Sodium cephalothin, in K. Florey (ed.), Analytical Profiles of Drug Substances, Vol. 1, Academic Press, New York, 1972, p. 329. 

Since the generation of radicals from cyclo-S or -Sg requires an activation energy of more than 29 kcalmol the radical mechanism may account for the thermally or photochemically initiated processes, but it cannot prevail for the transformations that occur at ambient temperature. Hypervalent thiosulfoxides 

The photoisomerization of pyrazine and its methyl derivatives to pyrimidines has been described (632). Thus photolysis at 254 nm of 2-methylpyrazine gave 4-and 5-methylpyrimidine, 2,6-dimethylpyrazine gave 4,5-dimethylpyrimidine, and 

5-dimethylpyrazine gave 2,5- and 4,6-dimethylpyrimidine. 2-Methylpyrazine with 1-diethylaminopropyne and boron trifluoride in acetonitrile has been shown to give 5-diethylamino-2,4-dimethylpyridine, 3-diethylamino-2,4-dimethylpyridine, and other compounds (629). 

Treatment of chloronitro derivative 165 with 2 A NaOH at 75-80°C gave pyrido[3,2,l-y][3,l]benzoxazine-l,3-dione (102) (64M59). 

Ring expansion of 2,2-diphenyl-8-methylperhydroisoxazolo[2,3-a] pyridinium salt on the action of NaOH furnished 3,3-diphenylperhydro-pyrido[l,2-c][l,3]oxazine in good yield (91G393). 

The Lewis acid-promoted intramolecular cyclocondensation of piperidine epimers (166, R = H) led to the formation of a 2 1 mixture of epimers 167 (R = H) and 168 with a preference for cw-fused tricycles (167, R = H) (90JOC1447). The 3-methylpiperidine derivative (166, R = Me) gave only the c/s-fused epimer (167, R= Me). 

Ozonolysis of carbamate 169 and successive workup with dimethyl sulfide, followed by cyclization with formic acid, yielded hexahydro-3//- 

The intramolecular conjugate addition of either a 1 2 mixture of E and Z isomers or pure Z isomer of 171 proceeded smoothly with potassium fert-butoxide (0.3 equiv) in THF at low temperature (between -20 and -58°C) and gave an 8 11 mixture of pyrido[l,2-c][l,3]oxazines (33 and 34) in 87% yield (96SL100). 

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Examination of the pyrazino[2,3-rf]pyrimidine structure of pteridines reveals two principal pathways for the synthesis of this ring system, namely fusion of a pyrazine ring to a pyrimidine derivative, and annelation of a pyrimidine ring to a suitably substituted pyrazine derivative (equation 76). Since pyrimidines are more easily accessible the former pathway is of major importance. Less important methods include degradations of more complex substances and ring transformations of structurally related bicyclic nitrogen heterocycles. 

The rearrangement of thiophenes to the isomeric pyrroles has proven synthetically useful (Schemes 98a and 98b). In the absence of a suitable internal nucleophilic nitrogen, so-called degenerate ring transformations may occur (Schemes 98c and 98d). 

Although formally it could be classified with the ring transformations (Section 4.04.3.2.2), conversion of 2,4-diphenyl-l,3,4-oxadiazol-2-one (593) by flash vacuum pyrolysis at 500 °C into 3-phenylindazole (595) involves a C(3)—C(3a) ring closure of the diphenylnitrilimine (594) (79AG(E)721). 

The richness and complexity of the present section is considerable. Almost any heterocycle conveniently substituted can be transformed into another chosen ring system, and this is shown in the two excellent volumes of van der Plas ( Ring Transformations of Heterocycles ) (B-73MI4Q4Q2). The arrangement of the aforementioned book (from the starting heterocycle point of view) does not suit this section and for the purposes of this chapter an alternative classification has been selected. When no explicit references are given, the material has been taken from (B-73MI4Q4Q2). 

Figure 31 Synthesis of pyrazoles and indazoles by ring transformation...

Ring transformations of heterocycles leading to isoxazoles have been briefly reviewed (79AHC(25)147). The heterocycles undergoing such transformations may be divided into three classes. 

Other heterocycles which provide the CCC component in related ring transformations are benzodiazepine-3-carbonitrile (80CPB567), benzotriazepines (74T2765), azaxanthones (75JOC1734), acyloxiranes (61AP769) and oxetanones (65CPB248). 

Lumazine, 1,3,6,7,8-pentamethyl-5,6,7,8-tetrahydro-rearrangement, 3, 308 Lumazine, 5,6,7,8-tetrahydro-autoxidation, 3, 308 oxidation, 3, 306 Lumazine, 1,3,6,7-tetramethyl-ring transformations, 3, 308 Lumazine, 1,3,5,6-tetramethyl-5,6,7,8-tetrahydro-synthesis, 3, 316 Lumazine, 2-thio-reactions, 3, 300 sulfurization, 3, 296 Lumazine, 4-thio-methylation, 3, 299 reactions, 3, 300 Lumazine, 1,3,7-trimethyl-acylation, 3, 290 Lumazine, 1,6,7-trimethyl-bromination, 3, 302 synthesis, 3, 295 Lumazine, 3,6,7-trimethyl-... 

Purine, 2-methylthio-6,8-diphenyl-ring transformations, 3, 308 Purine, 9-methyl-6-thiomethyl-dipole moments, 5, 521... 

Ring transformations heterocyclic compounds reviews, 1, 70 Ring-chain tautomerism polyheteroatom six-membered rings, 3, 1056 Ripariochromene A synthesis, 3, 751, 755 Robinson-Gabriel synthesis oxazoles, 6, 216... 

Although development work on shellac in blends with other synthetic resins has been carried out over a period of time, the only current use in the plastics industry is in the manufacture of electrical insulators. At one time electrical insulators and like equipment were fabricated from mica but with increase in both the size and quantity of such equipment shellac was introduced as a binder for mica flake. For commutator work the amount of shellac used is only 3-5% of the mica but in hot moulding Micanite for V-rings, transformer rings etc., more than 10% may be used. The structures after assembly are pressed and cured, typically for two hours at 150-160°C under pressure. 

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