Methoxy migration

A most unusual result was encountered when the irradiation of 243 was carried out in pentane in the presence of both methanol and methyl trifluoro-acetate (excess).Under these conditions, the only cycloadducts observed consisted of a mixture of the two stereoisomers of 3-oxazoline, 260. This result suggests that 2-phenylazirinylidene (244) reacts with methanol to give mainly azirine 258, which is subsequently converted to nitrile ylide 259 (and thus cycloadduct 260) on further irradiation. The formation of 245 has been explained in terms of a photoinduced methoxy migration of 258 which competes with C-C bond cleavage of the azirine ring. Ciabattoni and Cabell have previously reported that 2-chloro-2/f-azirines undergo ready isomerization at room temperature via a 27r-electron azacyclopropenyl cation. A similar mechanism would rationalize the apparent photoconversion of azirines 258 and 245. The results obtained with this spiro azirine system indicate that cycloelimination of ethylene from 243 is much more efficient than C-C bond scis-... 

High-resolution studies (Howe and Williams, 1968) on a series of saturated dimethyl esters CH302C[CH2l C02 CH3 (where n = 1-8 and 12) have assisted in clarifying some of the complexities of these spectra, in particular in confirming the occurrence of methoxy migrations during the formation of certain ions. 

Howe, I. and Williams, D.H. (1968), Studies in mass spectrometry. Part XXIII. The mass spectra of dimethyl esters methoxy migrations in the mass spectra of dimethyl esters. J. Chem. Soc. C, 202. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

Acylated Corticoids. The corticoid side-chain of (30) was converted iato the cycHc ortho ester (96) by reaction with a lower alkyl ortho ester RC(OR )2 iu benzene solution ia the presence of i ra-toluenesulfonic acid (88). Acid hydrolysis of the product at room temperature led to the formation of the 17-monoesters (97) ia nearly quantitative yield. The 17-monoesters (97) underwent acyl migration to the 21-monoesters (98) on careful heating with. In this way, prednisolone 17a,21-methylorthovalerate was converted quantitatively iato prednisolone 17-valerate, which is a very active antiinflammatory agent (89). The iatermediate ortho esters also are active. Thus, 17a,21-(l -methoxy)-pentyhdenedioxy-l,4-pregnadiene-liP-ol-3,20-dione [(96), R = CH3, R = C Hg] is at least 70 times more potent than prednisolone (89). The above conversions... 

When alkoxypyridazine 1-oxides are heated alone or in the presence of p-toluenesulfonic acid the methyl group migrates from the methoxy group to the A-oxide group. In this manner, 4-methoxypyridazine 1-oxide rearranges to l-methoxypyridazin-4(l//)-one, 5-methoxypyridazine 1-oxide to 2-methylpyridazin-5(2//)-one 1-oxide and substituted 3,6-dimethoxypyridazine 1-oxides to l,3-dimethoxypyridazin-6(l//)-ones. 

A similar migration of the methyl group was found in 3-methoxy-l,2,4-triazine 2-oxides 18 under the same conditions to afford the 2-methoxy-1,2,4-triazin-3(2/f)-one 125 (84TL1677). 

This result is attributed to participation by the neighboring methoxy group at position 1 (formula 18) in the phosphorus pentachloride displacement reaction. A cyclic methoxonium ion (22) is postulated as the intermediate. The overall result is methoxyl migration from 1 to 6, with inversion of configuration at both positions (27). 

Under the catalysis of MeONa, MeOH can react with 1,2-allenyl ketones to give /i-methoxy-/3,y-unsaturated enones 441, which undergo migration of the C=C bond to afford the more favorable /3-methoxy-a,/3-unsaturated enones [197-199]. 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

When a phenyl group is linked at a vicinal position to tellurium, the replacement of tellurium by the methoxy group is accompanied by phenyl migration. - ... 

Shibuya H, Fujioka H, Yamamoto Y, Suzuki K, Kitagawa I, Chemical ttansfotmation of tetpenoids, IV, Acid tteatment of (3./ )-1-vinyl-, (3f )- 1-hydtoxyptopenyl and (3./ )-1 -epoxyethyl-5-methoxy-1,2,2-trimethyl-cyclopentane derivatives Ring enlargement relations and successive migrations of methyl residues, Chem Pharm Bull 30 1280—1288, 1982. 

Diastereoselective [2,3]-sigmatropic rearrangement of lithium O-allyl-A-benzylhy-droxylamides (195) bearing a stereogenic center adjacent to the migration terminus was reported 3" 3 (equation 57). When the (E) and (Z)-iV-benzyl-0-(4-methoxy-4-phenylbut-2-enyl)hydroxylamines (194) rearrange, a chelation by the lithium ion occurs and the (Z)-(lR5, 2R5 )-l-phenyl-l-methoxy-3-iV-benzylaminobut-3-ene (196) is the major product... 

An efficient process for one-carbon homologation to aldehydes is based on cyclic boronate esters.17 These can be prepared by hydroboration of an alkene with dibromobor-ane, followed by conversion of the dibromoborane to the cyclic ester. The homologation step is carried out by addition of methoxy(phenylthio)methyllithium to the boronate ester. The migration step is induced by mercuric ion. Use of enantioenriched boranes and boronates leads to products containing the groups of retained configuration.18... 

Hydride transfer or hydride migration is initiated by the electrophilic attack of the poly(oxymethylene) cation from the methylene bridge of its own or of a neighboring macromolecule. A hydride ion is thus split off, and a methoxy end group is formed. 

Methoxy[(trimethylsilyl)ethoxy]carbene - obtained by thermolysis of the corresponding oxadiazoline 337 - reacts with Cgo in an unprecedented reaction pathway [383,384]. The expected methanofullerene-[2-i-l]-cycloadduct could not be observed. Instead the two dihydrofullerene adducts 338 and 339 were isolated (Scheme 4.69). They are formed by an unusual addition-rearrangement mechanism that includes the migration of the trimethylsilyl group. 

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