127852-67-9Relevant academic research and scientific papers
Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron-Catalyzed Chemoselective Asymmetric Aldol Reaction**
Fujita, Taiki,Kanai, Motomu,Mitsunuma, Harunobu,Sameera, W. M. C.,Yamane, Mina
supporting information, p. 24598 - 24604 (2021/10/14)
The catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late-stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre-conversion of carboxylic acids to siloxy esters facilitated the boron-catalyzed direct aldol reaction, leading to the development of carboxylic acid-selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α-proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.
Unusual conformational aspects of some novel chiral non-racemic pyridinyl-2-phosphonates
Dros,Zijlstra,Van Duijnen,Spek,Kooijman,Kellogg
, p. 7787 - 7812 (2007/10/03)
Reaction of pyridinyl-2-phosphonyl dichloride (6) with 1-phenyl-2,2- dimethylpropane-1,3-diol (9) leads to the two epimeric 2-oxo-2-(2-pyridinyl)- 4-phenyl-5,5-dimethyl-1,3,2-dioxaphosphorinanes (10a,b). These can be separated and the stereochemistry assigned on the basis of 31P NMR spectroscopy. For 10a the pyridinyl substituent is arranged axially at phosphorus. Arguments derived from 2D NMR experiments indicated that the nitrogen of pyridine is locked in a conformation whereby the pyridinyl nitrogen points over the six-membered ring; in other words it is locked between the two ting oxygen substituents. This conclusion is substantiated by an X-ray crystal determination. Oxidation of 10a with hydrogen peroxide leads to the N-oxide (12). The crystal structure of 12 reveals that despite serious steric overcrowding the N-O bond is also oriented over the six-membered ring. Methylation of 10a with methyl trifluoromethanesulfonate affords the N- methyl pyridinium salt (13). NMR experiments indicate that in this case the methylated nitrogen has turned 'outside' of the six-membered ring. The borane adduct of 10a appears on the basis of NMR data to have a conformation wherein the complexed borane is located just outside of the six-membered ring. Although crystal structures have not been obtained the pyridinyl-2- thiophosphonates (15a,b) obtained from treatment of 10a and 10b with [(4- MeOC6H4)2PS]2 appear to have the same conformational properties as 10a and 10b. Restricted HartreeFock geometry optimizations have been carried out to aid in clarifying this unexpected conformational behaviour. These calculational results are in excellent accord with the experimental observations, and provide insight into the reasons for the conformational behaviour.
Asymmetric Synthesis of Functionalized Secondary Alcohols by Catalytic Ring-Cleavage Reactions of Cyclic Acetals Derived from (R)-1,3-Butanediol
Kinugasa, Motoharu,Hanada, Toshiro,Egusa, Takayuki,Fujita, Katsuhiro,Oku, Akira
, p. 3639 - 3650 (2007/10/03)
In the presence of a catalytic amount (0.1-0.2 molar amount) of a 2-phenyl-1,3,2-oxazaborolidin-5-one, prepared by the reaction of dichlorophenylborane and N-(trifluoromethylsulfonyl)-L- phenylalanine, and enol silyl ethers ((R2)2C=C-(R3)OTMS; R3 = t-BuS, EtS, EtO, Ph) (1.1-1.5 molar amount), chiral cyclic acetals 6 derived from (R)-1,3-butanediol and aldehydes undergo an efficient ring-cleavage reaction with the inversion of the stereochemistry at the acetal carbon to give the anti isomer of the corresponding products with high stereoselectivity. The reaction is applicable to acetals derived from aromatic, aliphatic, and α,β-unsaturated aldehydes. Not only enol silyl ethers, but also methallyltrimethylsilane and allyltributyltin, can be employed as nucleophiles, leading to the selective formation of the anti isomer of the corresponding allylated ring-cleavage products. Removal of the chiral auxiliary from these ring-cleavage products by a two-step sequence ((i) PCC (ii) Bn2NH2(CF3CO2)) furnishes enantiomerically enriched β-hydroxy carbonyl compounds and homoallyl alcohols. A modest level of kinetic resolution is observed in the ring cleavage of a racemic acetal catalyzed by a phenylboron compound derived from N-mesyl-L-phenylalanine.
