2-Aryl-27/-chromenes, rearrangement

A particularly useful chromene synthesis involves the thermal rearrangement in an inert solvent of aryl propargyl ethers (33) (62CPB926,63CPB1042) which are conveniently prepared from a phenol and a chloroalkyne. An indication of the ease of cyclization is apparent from the presence of chromene in the ethers prepared in this manner. 

Propargyl aryl ether is also an attractive substrate of the Claisen rearrangement for the construction of heterocycles such as chromene derivatives. 

Rearrangement of propargyl aryl ether 19 smoothly proceeds under thermal conditions to afford chromene derivatives 22 [23, 24]. The mechanism involves the formation of ortho allenyl phenol 20 followed by a 1,5-hydrogen shift and elec-trocyclic ring closure sequence via 21. For example, the Claisen rearrangement of propargyl aryl ether 23 prepared by the Mitsunobu reaction smoothly took place at 180 °C to give a flav-3-ene derivative 24 in excellent yield [25]. 

Cyclopropanation. Rhodium complexes have been extensively employed for catalytic carbenoid cyclopropanation of alkenes. In the presence of [Rh2(S-TBSP)4], treatment of 2-aryl-substituted 2//-chromenes with dimethyl diazomalonate furnished the desired cyclopropanes in good yields (eq 41). The cyclopropane product with a terf-butyl ester group underwent rearrangement upon treatment with Sn(OTf)2 to from a y-lactone. OMe... 

Aryl-2//-chromenes in the presence of HAUCI4 have been reported to give 3-(2-hydroxyaryl)-l-arylpropan-l-ones through a hydration-rearrangement reaction sequence with the key step a [l,5]-hydride shift followed the hydrolysis (Scheme 126). ... 

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