1, 1-Dimethyl propargylation of 7-hydroxyflavanone (5) with 2-chloro-2-methyl-3-butyne in the presence of K2CO3, Me2CO, KI and DMF yielded a mixture of four compounds. Its 1, 1-dimethyl propargyl ether (6) and the corresponding chalcone (7) resulted in major amounts; their corresponding cyclised products (2 and 1) also formed albeit in smaller amounts. These cyclised products, lonchocarpin (1) and isolonchocarpin (2) were the sole products when the above mixture was heatedN, N-dimethylaniline. Lonchocarpin (1) is best synthesized by 1,1-dimethyl propargylation of 2′ 4′-dihydroxychalcone (8) followed by heating inN, N-dimethylaniline, because it was noted that in the above two reactions, the flavanone ring tends to open out whereas the chalcone does not cyclise.
Volume 134, 2022
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