Stereoselective Reductive Opening of 2,3-Benzofuran − A Two-Step Synthesis of 2H-Chromenes Including Deoxycordiachromene

Abstract

The reaction of 2,3-benzofuran (1) with lithium and a catalytic amount of 4,4Ј-di-tert-butylbiphenyl (TDBB, 5 mol %) in THF at 0°C leads to the stereoselective ring opening of the heterocycle, yielding the (Z)-organolithium derivatives (2) which, by reaction with different electrophiles [H 2 O, D 2 O, tBuCHO, PhCHO, Ph(CH 2 ) 2 CHO, Me 2 CO, nPrCOMe,

Reactivity of Benzofuran Derivatives

“…CHEMISTRY OF BENZOFURANS 2909 and the formed vinyllithium 86 was able to further react with electrophiles [such as tBuCHO, PhCHO, Ph (CH 2 ) 2 CHO, Me 2 CO, n-PrCOMe, PhCOMe, (CH2) 4 CO] togive their corresponding (Z)-products. Further reaction of the allyl alcohol derivative 84 led to substituted 2H-chromenes 85 under acid-catalyzed cyclization[34] (Scheme 26). 3-Substituted-2-phenylbenzo[b]furans 87 were able to undergo a palladiumcatalyzed hydrogenation reaction to give their corresponding 2,3-dihydrobenzo[b]-furans 88 and 89[35] (Scheme 27).Reduction of benzo[b]furan 1a with sodium boronhydride in the presence rhodium chloride afforded 1,2-dihydrobenzo[b]furan 90[36] (Scheme 28).…”

Ring-expanding and Ring-opening Transformations of Benzofurans and Indoles with Introducing Heteroatoms

“…This result also indicated the dianionic intermediate 21 existed in the reaction mixture with stereoretention. Although the optimized reaction conditions were similar to those of Yus's reductive ring opening of benzofuran (Scheme 5), 15 an electron transfer catalyst, 4,4¤-di-tert-butylbiphenyl (DTBB), was not required in our system. Moreover, addition of phenylboronic acid pinacol ester (PhB(pin)) as an electrophile to the reaction mixture after filtration furnished azaborin 20aa in a good yield (Scheme 15b).…”

“…Treatment of lithium metal with benzofuran in the presence of an electron transfer catalyst results in selective cleavage of the C2O bond to afford o-hydroxystyrene after acidic workup (Scheme 5). 15 By employing other electrophiles instead of protonolysis, a series of substituents could be installed at the ¢-position of the styrene skeleton.…”

“…Introducing heteroatoms to benzofuran while cleaving its endocyclic bond had never been achieved, while Yus reported a formal insertion of a C=O moiety to the C2O bond of benzofuran. 15 Moreover, a skeletal transformation of an indole core with CN bond cleavage is extremely difficult due to the strong aromaticity of indole although it has a structure similar to benzofuran. Ring-opening or ring-expanding transformations of indoles are reported as very limited examples 16,17 in contrast with benzofurans.…”

Ring Opening of Heterocycles by an Arene‐Catalyzed Lithiation

“…The lithiation of phenylthiirane 37 was performed using a catalytic amount of DTBB under the abovementioned reaction conditions or in the presence of the electrophile (Barbier-type reaction conditions) [23], but incorporation of the electrophile was never observed, ethylbenzene (38) being the only reaction product obtained [24]. Probably, the ring opening takes place at the most stable benzylic position giving the intermediate 39, but then a β-elimination occurs (lithium sulfide is a good leaving group) yielding styrene (40), which in the final work-up suffers reduction (with the excess of lithium and water used in the final hydrolysis) affording ethylbenzene (Scheme 7).…”