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Rayabarapu DK, Yang CH, Cheng CH: Regioselective synthesis of indenols via nickel-catalyzed carbocyclization reaction. J Org Chem. 2003 Aug 22;68(17):6726-31.
2-Halophenyl ketones 1a-e (1a, o-IC (6) H (4) COCH (3)) undergo carbocyclization with alkyl propiolates (2a, CH (3)(CH (2))(4) C [triple bond] CCO (2) CH (3); 2b, TMSC [triple bond] CCO (2) Et 2c, CH (3) C [triple bond] CCO (2) CH (3); 2d, CH (3) OCH (2) C [triple bond] CCO (2) CH (3); 2e, CH (3)(CH (2))(3) C [triple bond] CCO (2) CH (3); 2f, PhC [triple bond] CCO (2) CH (3); and 2g, (CH (3))(3) C [triple bond] CCO (2) CH (3)) in the presence of Ni (dppe) Br (2) and zinc powder in acetonitrile at 80 degrees C to afford the corresponding indenol derivatives 3a-m with remarkable regioselectivity in good to excellent yields. The nickel-catalyzed carbocyclization reaction was successfully extended to other simple disubstituted alkynes. Thus, the reaction of 2-halophenyl ketones 1a-e with disubstituted alkynes (2h, PhC [triple bond] CPh; 2i, CH (3) C (6) H (4) C [triple bond] CC (6) H (4) CH (3); 2j, CH (3) CH (2) C [triple bond] CCH (2) CH (3); 2k, PhC [triple bond] CCH (3); 2l, TMSC [triple bond] CCH (3); and 2m, PhC [triple bond] C (CH (2))(3) CH (3)) proceeded smoothly to afford the corresponding indenols 4a-t in good to excellent yields. For unsymmetrical alkynes 2k-m, the carbocyclization gave two regioisomers with regioselectivities ranging from 1:2 to 1:12 depending on the substituents on the alkyne and on the aromatic ring of halophenyl ketone. A possible mechanism for this nickel-catalyzed carbocyclization reaction is also proposed. |
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