A. C. Jain

Synthesis of (±)-7,3′- and 7,4′-di-O-methyleriodictyol and of velutin and pilloin

Abstract Synthetic 2′-hydroxy-3,4′,6′-trimethoxy-4-benzyloxychalcone (I) affords (±)-7,3′-di- O -methyleriodictyol (II) and 7,3′-di- O -methylluteolin (or velutin, VII) identical with natural samples. Similarly synthetic 2′-hydroxy-4,4′,6′-trimethoxy-3-benzyloxychalcone (X) gives natural (±)-7,4′-di- O -methyleriodictyol (XI) and 7,4′-di- O -methylluteolin (or pilloin, IX). However, attempts to partially etherify II with one mole of prenyl bromide to obtain the natural prenyl ether failed; only the corresponding diprenyloxychalcone (IV) was obtained.

Constitution of luteone and parvisoflavones-A and -B and synthesis of their methyl ethers and related isoflavones

The reaction of 5,7-dihydroxy-2′,4′-dimethoxyisoflavone (7) with 2-methylbut-3-en-2-ol in the presence of BF3–ether affords the 6-(3-methylbut-2-enyl) derivative (13) and its 8-isomer (9). Partial methylation of 7-hydroxy in (13) yields luteone trimethyl ether (15) and subsequent acid cyclisation gives α-isoluteone trimethyl ether (17) derived from natural luteone (3). Cyclodehydrogenation of (13) with DDQ followed by methylation afforded the fully methylated ether (19) of parvisoflavone-B, and the same two steps with (9) yielded the fully methylated ether (21) of parvisoflavone-A.

A new general synthesis of hydroxy- and methoxy-isoflavanones

A new general synthesis of hydroxy-(5e–h) and methoxy-(5a–d) isoflavanones has been accomplished in overall yields of 47–73% from the corresponding 2-hydroxydeoxybenzoins (1a–h). The first step involves reaction with appropriate amounts of ethoxymethyl chloride in the presence of dry K2CO3 and acetone, which gives the corresponding α-hydroxymethyldeoxybenzoins (4a–d) and (4i–l). The explanation for the formation of unexpected alcohols has been provided on the basis of an elimination–addition mechanism. Subsequent refluxing with 4% aqueous ethanolic Na2CO3 afforded protected isoflavanones (5a–d) and (5i–l) respectively. Final removal of the ethoxymethyl groups of (5i–l) with 10% MeOH–HCl afforded the corresponding hydroxyisoflavanones (5e–h).