The thio-adduct facilitated, enzymatic kinetic resolution of 4-hydroxycyclopentenone and 4-hydroxycyclohexenone
作者:Aisling O'Byrne、Cian Murray、Dearbhla Keegan、Carole Palacio、Paul Evans、Ben S. Morgan
DOI:10.1039/b916506a
日期:——
The addition of 3,4-dimethoxybenzyl thiol 8, as a benzyl thiol surrogate, to racemic 4-hydroxycyclopent-2-enone 2 and 4-hydroxycyclohex-2-enone 15 gave the corresponding cis-adducts (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclopentanone 4b and (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclohexanone 16 with good diastereocontrol. In both cases, subsequent treatment with vinyl acetate, in the presence of a lipase enabled enantiomer resolution. Thus, (+)-16 and the acetate of its enantiomer, (–)-(1R,2S)-2-(3,4-dimethoxybenzylthio)-4-oxocyclohexyl acetate, (–)-17 were isolated in 98% enantiomeric excess. Based on the 1,4-dioxygenation pattern, (–)-17 can be used to prepare both enantiomers of 4-(tert-butyldimethylsilyloxy)cyclohex-2-enone 19. Firstly, saponification, with a sub-stoichiometric amount of NaOMe, followed by a one-pot silyl ether formation–sulfide elimination sequence gave (+)-19. Then using the same starting material a 6-step sequence, featuring a diastereoselective NaBH4 reduction and a Cope-type sulfoxide elimination, gave (–)-19.
将 3,4-二甲氧基苄基硫醇 8(作为苄基硫醇的代用品)加到外消旋 4-羟基环戊-2-烯酮 2 和 4-羟基环己-2-烯酮 15 中,可得到相应的顺式加成物 (±)-3-(3、(±)-3-(3,4-二甲氧基苄硫基)-4-羟基环戊酮 4b 和 (±)-3-(3,4-二甲氧基苄硫基)-4-羟基环己酮 16。在这两种情况下,随后在脂肪酶存在下用乙酸乙烯酯进行处理,都能实现对映体的解析。因此,(+)-16 及其对映体(-)-(1R,2S)-2-(3,4-二甲氧基苄硫基)-4-氧代环己基乙酸酯(-)-17 的对映体过量率为 98%。根据 1,4-二氧代模式,(-)-17 可用于制备 4-(叔丁基二甲基硅氧基)环己-2-烯酮 19 的两种对映体。首先,使用亚几何量的 NaOMe 进行皂化,然后通过硅醚形成-硫化物消除的一锅式顺序制备出 (+)-19。然后使用相同的起始原料,经过 6 个步骤,包括非对映选择性 NaBH4 还原和 Cope 型亚砜消除,得到了 (-)-19。