Synthese und Reaktionsverhalten 2′‐substituierter Isoflavone

Synthesis and Behaviour of Isoflavones Substituted in 2′‐Position The protected chalcones 6–8 prepared from acetophenone and benzaldehydes rearranged to the dimethoxypropanone derivatives 9–11 in the presence of trimethyl orthoformate by Tl(NO 3 ) 3 . 3 H 2 O. These compounds could be cyclized to the isoflavones 12–14 in high yields ( Scheme 2 ). The conversion of these isoflavones to the corresponding isoflavanes (model compounds of the phytoalexin glabridin; see Scheme 1 ) was the main goal of this work. Hydrogenation of 13 and 14 gave the isoflavanes 15 and 16 , respectively and their deprotection the racemic natural product 4′‐ O ‐demethylvestitol ( 17 ). Reduction of 13 and 14 yielded different compounds depending on the reducing agent ( Scheme 3 ). The saturated alcohols 20–23 could be obtained with NaBH 4 or LiBH 4 . They were transferred into the racemic 9‐O ‐demethylmedicarpin ( 24 ) and haginin D ( 25 ) under acidic conditions. The ketones 26–28 ( Scheme 4 ) were obtained in high yields by reduction of 12–14 with DIBAH. Deprotection of 26 yielded the racemic 2,3‐dihydrodaidzein ( 29 ). Compounds 13 and 27 as well as 20 and 22 showed different behaviour under reduction conditions with Li in liquid ammonia. An efficient method for the introduction of the MeOCH 2 O and the MeOCH 2 CH 2 OCH 2 protecting groups into hydroxylated benzaldehydes and acetophenones ( Scheme 5 ) is described. The appropriate experimental conditions depend on the regioselectivity and on the number of the protected groups. The protected aldehydes, especially those with a protected ortho OH group, show an extraordinary ionization behaviour in chemical‐ionization mass spectrometry (isobutane; Scheme 6 ).