The asymmetric synthesis of 2-benzopyrans and their quinones through intramolecular diastereoselective ring-closure of titanium phenolates of phenolic aldehydes

Treatment of the phenolic aldehyde (alpha'S,2S)-2-(5'-hydroxy-2'-methoxy-al pha'- methylbenzyloxy)propanal 15 with titanium tetraisopropoxide followed by ultrasonication led to its completely diastereoselective cyclisation in high yield to afford (1S,3S,4R)-3,4-dihydro-1,3- dimethyl-8-methoxy-2-benzo pyran-4,5-diol 18, which was characterised as its more stable 4,5-diacetate 19. The diastereomeric (alpha'R,2S)-2-(5'-hydroxy- 2'-methoxy-alpha'-methy lbenzyloxy)propanal 17 on similar treatment gave rise to a mixture of (1R, 3S, 4R)- and (1R, 3S, 4S)-3,4- dihydro-1,3-dimethyl-8-methoxy-2-benzopyran- 4,5-diols 21 and 23, isolated as the 4,5-diacetates 22 and 24, in a ratio o f 3:1. Oxidative dealkylation of the diol 18 with silver(II) oxide afforded (1S,3S,4R)-3,4- dihydro-1,3-dimethyl-4-hydroxy-2-benzopyran-5,8-quinone 2 in high yield, while the epimeric 1R quinone 40 was similarly obtained from diol 21.