The asymmetric synthesis of 2-benzopyrans and their quinones through intramolecular diastereoselective ring-closure of titanium phenolates of phenolic aldehydes
作者:Robin G. F. Giles、Cynthia A. Joll
DOI:10.1039/a901457h
日期:——
Treatment of the phenolic aldehyde (α′S,2S)-2-(5′-hydroxy-2′-methoxy-α′-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-benzopyran-4,5-diol 18, which was characterised as its more stable 4,5-diacetate 19. The diastereomeric (α′R,2S)-2-(5′-hydroxy-2′-methoxy-α′-methylbenzyloxy)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 of 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.
对酚醛(α′S,2S)-2-(5′-羟基-2′-甲氧基-α′-甲基苄氧基)丙醛15进行钛四异丙氧化物处理,随后进行超声波处理,导致其高产率完全读者选择性的环化,得到(1S,3S,4R)-3,4-二氢-1,3-二甲基-8-甲氧基-2-苯并吡喃-4,5-二醇18,该化合物以其更稳定的4,5-二乙酰基形式19进行表征。通过类似的方法对立体异构体(α′R,2S)-2-(5′-羟基-2′-甲氧基-α′-甲基苄氧基)丙醛17的处理,产生了(1R,3S,4R)-和(1R,3S,4S)-3,4-二氢-1,3-二甲基-8-甲氧基-2-苯并吡喃-4,5-二醇21和23的混合物,以4,5-二乙酰基形式22和24分离,比例为3∶1。用银(II)氧化物对二醇18进行氧化去烷基化,得到(1S,3S,4R)-3,4-二氢-1,3-二甲基-4-羟基-2-苯并吡喃-5,8-醌2,且以高产率得到。与此类似,从二醇21获得的表异构体1R醌40。