Naphthyl propargyl ether

The cyclisation of naphthyl propargyl ethers occurs efficiently under microwave irradiation leading to naphthopyrans, but naphthofurans are formed in the presence of base <96JCR(S)338>. The thermal rearrangement of naphthyl 3-trimethylsilylprop-2-ynyl ethers yields the 4-trimethylsilyl derivatives of naphthopyrans <96H(43)751>. 

The pyrolytic reactions of pentafluorophenyl and heptafluoro-2-naphthyl propargyl ethers give product mixtures containing benzo- and naphtho[6]furan derivatives, resulting from initial Claisen rearrangement followed by further intra- and intermolecular reactions.31 34... 

Rearrangement of aryl propargyl ethers.1 Claisen rearrangement of the naphthyl propargyl ether 1 at 215° results in the benzopyran 2 as the only isolable product (40% yield). Addition of 1 equiv. of CsF results in the benzofuran 3 as the major product, presumably formed via an a-allenyl ketone (a). Related salts such as KF, RbF, or BaF2 are completely ineffective. This modified Claisen rearrangement provides a route to o-hydroxy aldehydes such as 4 from a phenol. 

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

Irradiation of propargyl naphthyl ethers in A -methylformamide (NMF) either alone or in the presence of sodium methoxide in an MW oven gave naphthopyrans or naphthofurans, respectively, in good to excellent yields. Thus, conversion of 1,1-dimethylpropargyl naphthyl ethers 37 and 38 proceeded smoothly to produce 2,2-dimethylnaphtho[l,2-6]pyrans 39 (92%) and 40 (96%), respectively, by MWI for 1 min. Under conventional heating, ether 38 produced 40 in lower yield (80%) and longer reaction time (2 h) (Scheme 9) (96JCR(S)338). 

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