Synthesis of 2', 6'-Dihydroxychalcones by Using Tetrahydropyran-2-yl and Trialkylsilyl Protective Groups; the Crystal Structure Determination of 2',6'-Dihydroxy-2,4,6-trimethoxychalcone

Abstract

Two improved general routes to 2′,6′-dihydroxychalcones are described in which the final step is protective-group removal from O 2′ under mild acid conditions. The first involves base-catalysed condensation of benzaldehydes with 2′-hydroxy-6′-tetrahydropyran-2-yloxyacetophenone, the second ring-opening of 5-hydroxyflavanones with 1,8-diazabicyclo[5.4.0]undec-7-ene in the presence of a trialkylchlorosilane to trap out the chalcone as a bis silyl ether. Chalcones prepared by the first route are 2',6'-dihydroxychalcone (1), and its 4-methoxy (2), 3,4-dimethoxy (3), 3,4,5-trimethoxy (4), and 2,4,6-trimethoxy (5) derivatives. The 4-chloro derivative (6) and the chalcone from hesperetin are prepared by the second method.

.

The X-ray crystal structure of 2',6'-dihydroxy-2,4,6-trimethoxychalcone (5), the first for a 2',6′-dihydroxychalcone, is reported, the hydrogen involved in intramolecular hydrogen-bonding between the carbonyl and phenolic oxygens being located for the first time for any 2'-hydroxychalcone derivative. The O 6' involved in the intramolecular hydrogen-bonding is also hydrogen-bonded intermolecularly to the hydrogen of the other (2'-)hydroxy group of a neighbouring molecule in the lattice.

¹³C n.m.r. data are the first reported for a series of 2',6'-dihydroxychalcones.