Lewis Acid-Promoted Cross Aldol Reaction of Aldehydes with Ketones Utilizing 3-Methyl-2-phenyl-2-(2-oxoalkyl)benzothiazolines as an Enolate-Transferring Reagent
作者:Hidenori Chikashita、Shin-ichiro Tame、Seiji Yamada、Kazuyoshi Itoh
DOI:10.1246/bcsj.63.497
日期:1990.2
The treatment of a variety of 3-methyl-2-phenyl-2-(2-oxoalkyl)benzothiazolines with aldehydes in the presence of 2 equivalents of SnCl4 in dichloromethane at −78°C underwent a carbon–carbon bond cleavage at the 2-position of the benzothiazoline ring releasing an 2-oxoalkyl moiety followed by the concurrent aldol-type reaction with aldehydes to afford the corresponding β-hydroxy ketones. Among the Lewis acids examined, SnCl4 was found to be most effective, while a similar reaction employing trimethylsilyl trifluoromethanesulfonate as an electrophilic promoter mainly gave the corresponding aldol-condensation product. In the case of the reaction with 3-methyl-2-phenyl-2-(1-substituted 2-oxoalkyl)benzothiazolines, a diastereomeric mixture of α-substituted β-hydroxy ketones as cross aldol products was obtained and the anti preference was generally observed for reactions with the benzothiazolines possessing an aromatic ketone moiety in modest to poor selectivity ranging between 69:31 and 57:43. In contrast to this, the reaction of the benzothiazoline possessing an alkyl ketone moiety, 3-methyl-2-phenyl-2-(1-methyl-2-oxobutyl)benzothiazoline, with benzaldehyde showed syn preference in modest selectivity of 70:30.
将多种3-甲基-2-苯基-2-(2-氧烷基)苯并噻唑啉与醛在氯仿中于-78°C存在2当量的SnCl4的条件下反应时,发生了苯并噻唑啉环2位的碳-碳键断裂,释放出2-氧烷基基团,随后与醛发生了同时的醛醇型反应,从而生成相应的β-羟基酮。在考察的路易斯酸中,SnCl4被发现是最有效的,而采用三甲基硅基三氟甲磺酸盐作为电亲核促进剂的类似反应主要生成相应的醛缩合产物。在与3-甲基-2-苯基-2-(1-取代2-氧烷基)苯并噻唑啉反应的情况下,获得了作为交叉醛醇产物的α-取代β-羟基酮的对映体混合物,且在与具有芳香酮基团的苯并噻唑啉的反应中一般观察到了反式优先选择性,其选择性在69:31到57:43之间波动。与此相对,具有烷基酮基团的苯并噻唑啉3-甲基-2-苯基-2-(1-甲基-2-氧丁基)苯并噻唑啉与苯甲醛的反应则表现出顺式优先选择性,选择性为70:30。