With Meerwein’s reagent

Minisci reactions have also been applied to these compounds. formation by exposure to w-CPBA and O-methylation with Meerwein s reagent converted 54 into 55. Nucleophilic attack of the hydroxymethyl radical, generated with ammonium sulfate, provides an alternate route to 2-hydroxymethyl pyridines 56. 

Allylic bromination, with A -bromosuccinimide (NBS), of 4-ethoxy-l,2-dihydrocyclo-pent[rf]azepine (8), formed by ethylation (with Meerwein s reagent) of the corresponding 4-oxo derivative, followed by in situ dehydrobromination of the resulting bromo compound with basic alumina, produces a low yield of l-bromo-2-ethoxycyclopent[c/]azepine (9) as a photosensitive, sapphire-blue, crystalline solid.55... 

Methyl-5-oxo-l-phenyl-4,5-dihydropyrazol-4-ylidene)-2,3-dihydro-l//-azepine (3), formed in 62% yield by the condensation of 2-amino- or 2-butoxy-37/-azepine with 3-methyl-l-phenylpyrazol-5(4//)-one, with Meerwein s reagent yields the tetrafluoroborate salt 4 which, on treatment with sodium dihydrogen phosphate, liberates the free base 5.64... 

The corresponding 8/7-azepino[l,2-a]indol-8-one (18) with hydrogen bromide yields the deep-blue, fully conj ugated 8-hydroxyazepino[l, 2- ]indolinium bromide (19) which with Meerwein s reagent yields 8-ethoxyazepinof 1,2-a]indolinium tetrafluoroborate (20).218... 

Alkylation of 1//-azepin-2(3//)-ones, e.g. 17. with Meerwein s reagent provides ready access to 2-ethoxy-3//-azepines.72,81,228... 

Ethyl 6,7-diaryl-3-hydroxy-2-oxo-2,5-dihydro-l//-azepine-4-carboxylates, e.g. 20, with diazomethane methylate solely at the 3-hydroxy group, whereas with Meerwein s reagent the 7-ethoxy-4/7-azepines, e.g. 21, are formed.48... 

Methylation of indeno[l,2-r/]azepin-4(3//)-one (22) with iodomethane under phase transfer conditions produces the A-methyl derivative, whereas ethylation with Meerwein s reagent yields purple crystals of the indeno[l, 2-r/Jazepine 23, a 147r-aromatic system.57 1 l-Bromo-4-ethoxyin-deno[l,2-r/]azepine (69% mp 143-144°C) can be prepared similarly. 

Attempts to prepare 2-aminoazepinium salts by the action of trityl tetrafluoroborate on azepin-2-amines, e.g. 18, resulted in ring contraction to stable 2-azabicyclo[4.1.0]hepta-2.4-dienium salts, e.g. 3-(pyrrolidin-l-yl)-7-trityl-2-azabicyclo[4.1,0]hcpta-2,4-dicnium tetrafluoroborate (19), the valence isomers of 7-(dialkylamino)-3-trityl-3//-azepinium tetrafluoroborates 64 with Meerwein s reagent the 2-ethyl derivative (77%) is formed. 

X-ray analysis of 2-methoxy-4-hydroxy-5//-l-benzazepin-5-one (a benzazatropolone), prepared by methylation of the corresponding 4-hydroxy-l-benzazepin-2,5-dione with Meerwein s reagent, demonstrates the presence of a planar seven-membered ring but, in contrast to tropolone, little 71-electron delocalization.17 Likewise, ll//-dibenz[f>,e]azepin-ll-ones display no significant aromatic character.18 In contrast, 7-chloro-8//-thieno[3,2-c]azepin-8-one (12) has azepine ring hydrogen resonances at 8.7 and 9.02 ppm that indicate a substantial contribution from the polar zwitterionic mesomer 13.19... 

It is very interesting, however, that in alkane potassium diazoate alkylations with Meerwein s reagent (triethyloxonium tetrafluoroborate, Et30+BF4) in CH2C12 suspensions or with alkyl halides in hexamethylphosphoric triamide solutions, azoxy compounds (6.4) are formed, i.e., alkylation takes place at the (3-nitrogen (Moss et al., 1972). 

There are several examples of alkyl halides reacting with 1,2,3-thiadiazoles at nitrogen to yield either salts or mesoionic compounds <1996CHEC-II(4)289>. Similarly, with Meerwein s reagent, several substituted thiadiazoles yielded various 2- and 3-methylated 1,2,3-thiadiazoles (Scheme 4 Table 8) <1993JHC301>. The isomer ratios were determined by integrating the methyl singlets in the H NMR spectra and the compounds were further studied by 1SN NMR spectroscopy (Section 5.07.3.4). 

Table 8 Product distribution for the methylation of 1,2,3-thiadiazoles with Meerwein s reagent...

Hirsutine (58), the pseudo isomer of corynantheidine and dihydrocory-nantheine, has been elegantly synthesized by Wenkert and his collaborators (161). The reaction of keto diester290 with Meerwein s reagent, followed by hydrogenation, resulted in pseudo-type diester 321, which on reduction and methylation gave ( )-hirsutine (161). 

After (presumed). S-alkylation of diphenyl sulfide with Meerwein s reagent, heating the resultant material at 175°C gave a small quantity of thianthrene as a component of a complex product mixture (71JOC1513). 

Valine-derived (5)-2-ethoxy-4,5-dihydro-4-isopropyloxazole, obtained by alkylation of 4-isopropyl-2-oxazolidinone with Meerwein s reagent, has been used to convert secondary tion and alkylation occurs smoothly to give alkylated derivatives I, which can be deprotected to give tetrahydroisoquinolines 2 with recovery of the chiral auxiliary28. 

Piperazine-2,5-diones, in which both amino acid units are primary, lead to bislactim ethers on O-alkylation with Meerwein s reagents. No selectivity in this reaction has been demonstrated so far. Such bislactim ethers (171) have been prepared and extensively used by Schollkopf and his school [79AG(E)863, and later papers]. During the preparation of these bislactim ethers, neutralization of the initially formed bis-tetrafluoroborate salt is carried out with phosphate buffer to avoid racemization. 

The cis- and frans-sulfoxides (551) and (552) have been O-methylated with Meerwein s reagent. Reaction of the methoxy derivative with MeMgBr proceeds with inversion of configuration (Scheme 211) (74JA8026). The stereochemical course of the interconversions of sulfoxide, sulfimide and sulfoximide in the 2,3-dihydrobenzo[6]thiophene series has been investigated (73JA1916). The reaction cycle (Scheme 212) involves both nucleophilic and electrophilic substitution at chiral sulfur. Inversion of configuration takes place in the conversion of (553) to (554) in pyridine. 

Nitrones.1 O-Trimethylsilyloximes undergo N-methylation on reaction with Meerwein s reagent or methyl trifluoromethanesulfonate. The products are converted into nitrones on addition of KF or Bu4NF. Although both (E)- and (Z)-nitrones are formed initially, purification results in isolation of the more stable (Z)-nitrones. 

The addition-elimination reaction between 2-chlorocarbonyI-lfl-pyr-rolizin-l-one and chlorine has been described (see Section III,B,2,c). The 2,5-dichloro derivative 21b isolated in the same reaction must presumably be derived from an electrophilic substitution on the initially formed 2-chloro-pyrrolizinone.26 The iminopyrrolizine 270 can be methylated, first with Meerwein s reagent and then by methyl iodide to give the quaternary salt 271, which reacts readily with nucleophiles (see Section III,B,5). 

The addition of alkenyl imidates 623 (R, R = Ph, Me, H) to (l-alkynyl)carbene complexes 624 afforded 2,4-diethoxyazetine complexes 625 (Equation 237) <19970M2571>. Treatment of 2-thioxoazetidines 626 with Meerwein s reagent yielded 2,4-disubstituted 1-azetines 579 (Equation 238) <2006TL425>. 

Methylation with Meerwein s reagent affords the imino ethers which are treated in situ with aryl hydrazines and consequently cyclized with triethyl orthoformiate to yield both 5/5 and 6/5 bicyclic chiral triazolium scaffolds (Kerr et al. 2005). 

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