A new synthesis of benzofuro[3,2-c]pyridines

Rearrangement of N-Oxyenamines and Related Reactions

“…Thus, in most cases rearrangement of O -phenylsubstituted oximes 93 affords the benzofuran derivatives 96 (Scheme ). Typical reaction conditions for these transformations are reflux of the oxime ethers 93 with a strong acid such as hydrogen chloride, − methanesulfonic acid, , sulfuric acid, or boron fluoride etherate in acetic acid . This method was used for the synthesis of benzofuran-containing natural products as well as benzofuran-containing analogues of bioactive compounds such as naltrindole and betulinic acid .…”

“…Rearrangements can be performed in situ for synthesis of oxime ethers from carbonyl compounds and either O -phenylhydroxylamine ,, or O -aryl acetohydroximates . For unsymmetrical 1,3-dicarbonyl compounds such as the acetoacetic ester 102 or the azepinediones 103 , oximation is selective to the more reactive carbonyl group. Similarly, a keto-alkyl functional group is more active than a keto-aryl one.…”

“…It was only in the case of the pyridyl-substituted diketone 104 that a small erosion of selectivity was observed, giving an isomer ratio of 93:7. Not surprisingly, tautomerization of oximes containing electron-accepting substituents such as those derived from substrates 102 and 103 involves the more acidic protons. ,, A good catalyst for such substrates is the AuCl 3 /AgOTf system . Notably, with acetylacetone as a model compound the use of typical Lewis or Broensted acids (i.e., HCl, AlCl 3 , TfOH) afforded the target product only in low yields.…”

“…71,75,85,86 Formation of products resulting from enolization toward the methylene group has been reported for oximes of the type 100 (Chart 1), derived from unsymmetrical ketones. 67,70,76,80 It was found that for ketones 101 the methylene group that is most remote from the heteroatom participated in the reaction. 69 Rearrangements can be performed in situ for synthesis of oxime ethers from carbonyl compounds and either O-phenylhydroxylamine 68,69,76 or O-aryl acetohydroximates.…”