Isomerization-iodination

Isomerization-iodination of propargylic alcohols. 1-Phenylethynylcyclopen-tanols do not undergo ring expansion on reaction with NIS-PhI(OH)OTs. The phenyl group apparently changes the polarization of the alkyne unit, which favors formation of a-iodo enones. 

It is possible to conceive of compounds in which the two elements which enter into chemical combination have oxidizing potentials so close to each other in value that two combinations are possible in which the positive and negative elements are interchanged in the two compounds. As an example of this type of isomerism, the two isomeric iodine chlorides may be mentioned. Since chlorine has the higher oxidizing potential, it is highly probable that in the more stable of the two isomers the iodine atom is positive, while... 

On the other hand, 2-arylthiazoles are easily isomerized to 3-aryliso-thiazoles in 407o yield upon irradiation with a high-pressure mercury lamp, in benzene solution in the presence of iodine (738). A valence bond isomerization was proposed among several alternatives to account for these results. 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

Aromatic Ring Reactions. In the presence of an iodine catalyst chlorination of benzyl chloride yields a mixture consisting mostly of the ortho and para compounds. With strong Lewis acid catalysts such as ferric chloride, chlorination is accompanied by self-condensation. Nitration of benzyl chloride with nitric acid in acetic anhydride gives an isomeric mixture containing about 33% ortho, 15% meta, and 52% para isomers (27) with benzal chloride, a mixture containing 23% ortho, 34% meta, and 43% para nitrobenzal chlorides is obtained. 

Catalytic reduction of thiophenes over cobalt catalysts leads to thiolane derivatives, or hydrocarbons. " Noncatalytic reductions of thiophenes by sodium or lithium in liquid ammonia leads, via the isomeric dihydrothiophenes, to complete destructions of the ring system, ultimately giving butenethiols and olefins. " Exhaustive chlorination of thiophene in the presence of iodine yields 2,2,3,4,5,5,-hexachloro-3-thiolene, Pyrolysis of thiophene at 850°C gives a... 

The rate of iodine formation depends on the degree of A"-substitu-tion. Compounds which are unsubstituted on both the iV-atoms (35) and those wdth a single A -substituent (43) liberate instantly the calculated quantity of iodine in the cold. However, the 1,2-disubstituted diaziridines (44) need brief heating with the acid iodine solution they then give 95-100% of the calculated iodine. " This effect of substitution is so well defined that it can be used for a proof of constitution. The diaziridino-triazolidincs (37) prepared from aldehydes, ammonia, and chloramine give complete iodine liberation only on heating. Thus the structure 57 which is isomeric with 37 can be eliminated. ... 

Chlorpromazine (33) can probably be considered the prototype of the phenothiazine major tranquilizers. The antipsychotic potential of the phenothiazines was in fact discovered in the course of research with this agent. It is of note that, despite the great number of alternate analogs now available to clinicians, the original agent still finds considerable use. The first recorded preparation of this compound relies on the sulfuration reaction. Thus, heating 3-chlorodiphenylamine (30) with sulfur and iodine affords the desired phenothiazine (31) as well as a lesser amount of the isomeric product (32) produced by reaction at the 2 position. The predominance of reaction at 6 is perhaps due to the sterically hindered nature of the 2 position. Alkylation with w-C3-chloropropyl)dimethylamine by means of sodium amide affords chlorpromazine (33). ... 

The isomeric imidazo[l,5-a]pyrimidine (161) gave the 1,3-dibromo derivative when exposed to bromine or NBS (72BSF2481). Iodination of benzimidazo[2,l- >]quinazolin-12(5 or 6//)-ones was accomplished by boiling them with bromine in the presence of sodium or potassium iodide. Substitution took place at the 9-position in the benzimidazo moiety (91MI3). 

Omura, K. Iodine oxidation of alpha -tocopherol and its model compound in alkaline methanol unexpected isomerization of the product quinone monoketals. J. Org. Chem. 1989, 54, 1987-1990. 

The localized-valence halogen-bridged platinum(II)/platinum(IV) complexes (175) are of interest because of their highly anisotropic electrical and optical properties. The complexes are characterized by broad and intense intervalence bands in their electronic spectra.542 Partial oxidation of [PtCl(dien)]Cl with iodine yields the mixed valence species [PtI(dien)][PtI3(dien)]I2,543 but when 4Me-dien is used, the mixed valence compound is only obtained if it is trapped quickly, before isomerization of the meridional platinum(IV) complex to a facial form.544 Although these species nominally have localized valences, the extent of delocalization increases as the bridging halide... 

The photosensitized transformation of carotenoids has been studied using several sensitizer molecules, such as chlorophylls, iodine, rose bengal (RB), and methylene blue (MB) and in general terms isomerization is the major pathway of reaction. 

There is an absence of cis-to-trans isomerization with conversion or time for the C8 (1,5-cyclooctadiene) polymer. This is shown from 52 to 58% conversion after 1 to 16 hours reaction time in Table II and III. The above review (A0, A2, A3, A5) shows that the cis structure in polymers from 1,5-cyclooctadiene using various chloride catalysts fell below 50% cis even to 20% cis units this means that the second cis double bond from the monomer underwent extensive cis-to-trans isomerization following the ring-opening of the first cis bond. Where cis-2-butene isomerizes to trans structure using other catalyst preparations, there is no evidence of this for cis-2-butene using the iodine system. However, polymer molecular... 

It was envisioned that hydrindanone 83 and cyclopentene 85 could be used as intermediates in the synthesis of e f-retigeranic acid A (1) and e f-retigeranic acid B (2), respectively. To prepare the building block 90, cyclopentene 85 was reduced with diimide (93 %) in order to prevent isomerization and subsequently deprotected with PPTS to yield hydrindanone 90 (quant.), which could provide access to <77/-retigeranic acid B (2) (Scheme 10.7). Hydrindanone 83 was reduced via an enol triflate and then subjected to Pd-catalyzed reduction to provide cyclopentene 91 (87 % from 83). Upon hydrogenation of 91 with Pd/C and cleavage of the acetal with iodine, protected hydrindanone 92 (95 % from 91) was obtained. The deprotection of 92 provided ent-60, whose enantiomer was used in previous syntheses of retigeranic acid A (1) by Corey [14] and Hudlicky [46, 47]. 

It will be seen that the enediolic system can theoretically be written in the isomeric 2-keto (II) or 3-keto (III) forms and these in turn are seen to be derived from the 2-keto and the 3-keto acids IV and V, respectively (compare with benzoin which reacts with iodine in an analogous fashion to L-ascorbic acid). Consequently the synthesis of L-ascorbic acid and of its analogs has consisted in devising methods for the formation of 2-keto or 3-keto hydroxy acids followed by their enolization and lactonization. Four main methods are available for the synthesis of analogs of L-ascorbic acid containing the characteristic five-membered unsaturated enediolic ring. 

Iodine-transfer cyclization. Irradiation of unsaturated a-iodo carbonyl compounds in the presence of a hexaalkylditin (5-10%) can result in isomerization to cyclic y-iodo carbonyls.1 The reaction is very slow in the absence of an initiator. Thus under these conditions 1 isomerizes to a mixture of 2 and 3 in which 2 predominates. The reaction is particularly useful for formation of fused bicyclic systems (4 - 5). 

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