Boron tribromide ether cleavage

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). 

Finally, cleavage of the ethyl ether of 741 using boron tribromide afforded carazostatin (247). Similarly, cleavage of the ethyl ether of 742 led to the hydroxy-carbazole 728, which, on O-methylation, provided hyellazole (245) (536,537) (Scheme 5.74). 

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). 

Dimethoxycinnamic acid is a good source of 6-hydroxycoumarin and the preparation illustrates the value of boron tribromide in the cleavage of ethers (69MI22401). 

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],... 

If boron tribromide is used for the ether cleavage, adduct 41 (from a combination of ether 39 and the electrophilic boron reagent) is attacked by bromide ion. The resulting intermediate 42 can than react further to phenol 43. 

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-... 

Pearlman catalyst <67TL1663>. Cleavage of both methyl and benzyl ether linkages in 42 using six molar equivalents of boron tribromide afforded lamellarin H. 

The formation of cyclopropanols by C-O bond cleavage has been achieved under a variety of conditions (Table 1). In most cases the syntheses were carried out under acidic conditions, in the presenee of sulfuric acid, hydrochloric acid, pyridinium p-toluenesulfonate, boron tribromide, or an aqueous acid, to cleave esters, ethers, ° and acetals. Other acid-sensitive moieties in the substrate can react concomitantly under such conditions. Thus, when rer/-butyl-(l-ethoxycyclopropyl)carbamate was heated in hydrochloric acid at 60 "C both the ether and the carbamate functions reacted to give 1-aminocyclo-propanol hydrochloride. 1-Acetoxycyclopropanecarbonitrile, on the other hand, was converted to 1-hydroxycyclopropanecarbonitrile under similar conditions. ... 

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 . 

CLEAVAGE OF ETHERS Aluminum chloride. Boron tribromide. Bromine. Dimethylformamide. Methylmagnesium iodide. Triphenylphosphine dibromide. a-GLYCOLS Silver iododibenzoate KETOXIMES Chromous acetate METHYL ESTERS Lithium n-propyl mercaptide. 

---