A Simple and Efficient Access to Naphtho[b]furans by Claisen Rearrangement/Cyclization of Bromonaphthyl 3‐Phenylallyl Ethers

Abstract

At ransition-metal-free Claisen rearrangement/cyclization reaction was developed for the synthesis of naphthofuran derivatives from bromonaphthyl 3-phenylallyl ethers.T he nature of the base employed in this reactionp lays an important role in determining the ratio for the formation of naphthofurana nd naphthol products. By using K 2 CO 3 as base and DMFa ss olvent, we have synthesized av ariety of functionalized naphthofurans in good to high yields (49-92 %) and with satisfactory selectivities.Keywords: bromonaphthyl ethers;C laisen rearrangement;c yclization;n aphthofurans;t ransitionmetal-free Naphtho [b]furans are important structural motifst hat find presence in manyn atural products [1] and pharmaceutical agents.[2] As ac onsequence,n umerous efficient synthetic strategies have been developed for the construction of this structure.A mong the protocols reported to date,m ost involve the use of versatile starting substrates [3] in combination with as uitable transition-metal catalyst, such as Pd, [4] Pt, [5] In, [6] Ru, [7] Au, [8] Ag, [9] Fe, [10] Cu.[11] Despite of theirs ynthetic versatility,t he cost of transition-metal catalysts and the difficulty associated with removing transition-metal impuritiesf rom finalp roductsm ay restrict the practical applicability of these methods.Not surprisingly,t ransition-metal-free approaches to prepare naphthofurans have also been pursued. Fore xample,i n2 012, Thomas et al. [12] reported that naphtho [b]furans could be obtained in high yields by af acile microwave-assistedC laisen rearrangement of naphthyl 2-propynyl ethers (Scheme 1a). Unfortunately,e xpensive and highly hygroscopic CsF was used as base to promote this reaction. More recently, Rueping [13] and Yoshimi et al. [14] prepared as erieso f naphtho[b]furans in good yields,u tilizing at ransitionmetal-free Heck-type cyclization/isomerization reaction of halonaphthyle thers (Scheme 1b). However, these transformationsr equirede ither highly basic KOtBu or UV light, thus limiting their functional group tolerance.T herefore,t he development of as imple,s traightforward and cost-effective method to synthesize naphtho [b]furans is highly desirable. Herein, we report at ransitionm etal-free process for the synthesis of naphtho [b]furans from bromonaphthScheme1.Generals trategiesf or the constructiono ft he naphtho [b]furan skeleton through at ransition-metal-free system. 2442

An Approach to the Synthesis of 1-Propenylnaphthols and 3-Arylnaphtho[2,1-b]furans

“…HRMS (ESI): calcd for C 19 H 15 O [M – H] − 259.1123, found 259.1131. Spectral data obtained were in agreement with those reported in the literature …”

“…We initiated the investigations by using [[(2 E )-3-phenyl-2-propen-1-yl]­oxy]­naphthalene ( 1a ) as the model substrate, employing conditions similar to those reported by our group for the Claisen rearrangement/cyclization of bromonaphthyl 3-phenylallyl ethers . Gratifyingly, the desired transformation proceeded readily under basic conditions.…”

“…Gratifyingly, the desired transformation proceeded readily under basic conditions. In addition, this reaction performed well in controlling the stereoselectivity, and almost exclusively the Z product 2a was obtained, the structure of which was unambiguously assigned on the basis of single-crystal X-ray diffraction . Screening of bases for the reaction identified that K 2 CO 3 was the most effective base when N , N -dimethylformamide (DMF) was used as solvent (81% yield; Table , entry 5).…”

“…In contrast, the Claisen rearrangement of an allyl or propargyl naphthyl ether followed by intramolecular cyclization provides 3-arylnaphtho­[2,1- b ]­furan (Scheme , eq 6) . We have previously reported a transition-metal-free protocol for the synthesis of 3-arylnaphtho­[2,1- b ]­furan starting from 3-arylallyl bromonaphthyl ethers by employing such a strategy . Moreover, we found that the use of a suitable base could lead to formation of alkenylnaphthols as the major products (Scheme , eq 7).…”

B(C6F5)3‐Catalyzed Hydroarylation of Aryl Alkynes for the Synthesis of 1,1‐Diaryl and Triaryl Substituted Alkenes

“…The obtained alkenes containing phenolic hydroxyl and C=C group could be a versatile synthetic intermediate for further structural diversifications (Scheme 2b). For instance, 3 d could be converted into benzo( b )furan derivative 5 (88 % yield ) [13] . Besides that, 3 d can be easily converted to vinyl iodide 6 , [14] which may be used for further transformation through a transition‐metal‐catalyzed cross‐coupling reaction [15] .…”