Boron tribromide, ether cleavage with

The mechanism for ether cleavage with boron tribromide involves attack of bromide ion on an adduct formed from the ether and the electrophilic boron reagent. The cleavage step can occur by either an Sn2- or Swl-type process,... 

R2 = COOEt). Reduction of (26) with LAH and ether cleavage with boron tribromide yielded ( )-(26) (Rj = H, R2 = CH3). Dehydration of the carbamate (26) with phosphorous oxychloride in pyridine afforded the interesting 8,14-dehydromorphinan (27), connecting the trans series with the 14-hydroxy-substituted cis-series. Only few isomorphinans were resolved into optical isomers and adequately tested. 

It should be noted that one of these diols, the hydroquinone, did not provide any oligomer in the first step. This was due to the formation of the quinone structure which made it impossible to use hydroquinone directly in the substitution reaction. An alternate method was used to overcome this problem which involved the use of 4-methoxyphenol to obtain the sulfone product, followed by cleavage of the methyl ether to the diol (VIII) with boron tribromide. This set of reactions is outlined in Figure 5. 

Ether cleavage of 4-heptyl-3-methylveratrole 121 using boron tribromide affords 4-heptyl-3-methylcatechol 122 (Scheme 38). Oxidation of the catechol 122 with o-chloranil to 4-heptyl-3-methyl-l,2-benzoquinone 123 and subsequent immediate addition of aniline leads to 5-anilino-4-heptyl-3-methyl-l,2-benzo-quinone 124. Unlike the very labile disubstituted ort/zo-quinone 123, compound 124 is stable and can be isolated. Palladium(II)-mediated oxidative cyclization of the anilino-l,2-benzoquinone 124 provides carbazoquinocin C 51. 

Oxidation of 48 using manganese dioxide in methanol in the presence of potassium cyanide provides clausine H (clauszoline-C) (50) quantitatively, which on ester cleavage affords clausine K (clauszoline-J) (51). Cleavage of both methyl ethers of 51 on treatment with boron tribromide led to clausine O (72) (588). 

Reaction of the iron complex salt 602 with the arylamine 921 in the presence of air led directly to the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex (923) by a one-pot C-C and C-N bond formation. Demetalation of complex 923 and subsequent aromatization by catalytic dehydrogenation afforded 3,4-dimethoxy-l-heptyl-2-methylcarbazole (924), a protected carbazoquinocin C. Finally, ether cleavage of 924 with boron tribromide followed by oxidation in air provided carbazoquinocin C (274) (640) (Scheme 5.120). 

The relay compound 1025 required for the synthesis of all of these 7-oxygenated carbazole alkaloids was obtained starting from commercially available 4-bromo-toluene (1023) and m-anisidine (840) in two steps and 72% overall yield. Buchwald-Hartwig amination of 4-bromotoluene (1023) with m-anisidine (840) furnished quantitatively the corresponding diarylamine 1024. Oxidative cyclization of 1024 using catalytic amounts of palladium(ll) acetate afforded 3-methyl-7-methoxycarbazole (1025). Oxidation of 1025 with DDQ led to clauszoline-K (98), which, on cleavage of the methyl ether using boron tribromide, afforded 3-formyl-7-hydroxycarbazole (99) (546) (Scheme 5.149). 

Cleavage conditions for alkyl benzyl ethers prepared from acid-labile benzyl alcohols are similar to those for the corresponding benzyl esters (Table 3.30). Aryl benzyl ethers, however, are generally cleaved more easily by acidolysis than esters or alkyl ethers. Phenols etherified with hydroxymethyl polystyrene, for instance, can even be released by treatment with TFA (Entry 1, Table 3.31). It has also been shown that Wang resin derived phenyl ethers are less stable than Wang resin derived esters towards refluxing acetic acid [29]. Alternatively, boron tribromide may be used to cleave aryl ethers from hydroxymethyl polystyrene [573],... 

Cleavage of the iron complex and the ben/yl ether functions is accomplished with boron tribromide solution. 

MET-CAMO was prepared from 5/ -methylthebaine (57) [109,110] via 14/ -amino-7,8-dihydro-5/ -methylcodeinone (59) which was obtained by the Kirby-McLean procedure [111]. Thus, oxidation of 2,2,2-trichloroethyl A-hydroxycarbamate with sodium periodate in the presence of 5/ -methylthebaine gave the adduct (58). Catalytic hydrogenation using a Pd/C catalyst in methanol in the presence of sodium acetate-acetic acid buffer yielded amine (59). Reaction with 4-nitrocinnamoyl chloride furnished amide (60) and ether cleavage using boron tribromide yielded MET-... 

Intramolecular reaction with nucleophilic groups can also lead to heterocycles. For example, good yields of 3-acylbenzofurans result from cyclization caused by intramolecular substitution of the tertiary amino group by a phenol formed by cleavage of a phenol ether by boron tribromide (equation 182). o-Hydroxybenzyl alcohols were used to obtain 4 f-chromenes by their reaction with 4-morpholino-3-buten-2-one in acetic acid-acetic anhydride . 

A new synthesis of rotenoids has been developed from 2,2 -dihydroxydeoxybenzoins the unwanted nucleophilic sites were blocked by treatment with sodium-ethyl formate, and after four further steps, ( )-isorotenone (65) was obtained. A method of labelling the 7 -methylene group of (-)-rotenone with 13C and 1 has also been described. Selective cleavage of one or more of the ether linkages of rotenones has been achieved by means of boron tribromide or trimethylsilyl iodide and is a valuable method of... 

Direct cleavage of the ally lie methyl ether in 106 with boron tribromide afforded codeine in only minor amounts. Better yields were obtained when 106 was converted to the corresponding carbamate before a selenium dioxide mediated oxidation delivered ketone 107. Stereoselective reduction of the ketone and concomitant generation of the iV-methyl group concluded the synthesis of codeine [60, 68]. This synthesis reported by Stork and co-workers provided a closure to several years of research, some of which has been reported in Ph.D. dissertations [69]. 

Ether cleavage of glycozoline (128) with boron tribromide provided the... 

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