20452-67-9Relevant articles and documents
Forming All-Carbon Quaternary Stereocenters by Organocatalytic Aminomethylation: Concise Access to β2,2-Amino Acids
Shao, Ying,Sun, Jiangtao,Tang, Shengbiao,Wang, Kai,Yu, Jianliang
, p. 23516 - 23520 (2020/10/21)
The asymmetric synthesis of β2,2-amino acids remains a formidable challenge in organic synthesis. Here a novel organocatalytic enantioselective aminomethylation of ketenes with stable and readily available N,O-acetals is reported, providing β2,2-amino esters bearing an all-carbon quaternary stereogenic center in high enantiomeric ratios with a catalytic amount of chiral phosphoric acid. Typically, this transformation probably proceeds through an asymmetric counter-anion-directed catalysis. As a result, a concise, practical, and atom-economic protocol toward rapidly access to β2,2-amino acids has been developed.
Observable enols of anhydrides: Claimed literature systems, calculations, and predictions
Rappoport, Zvi,Lei, Yi Xiong,Yamataka, Hiroshi
, p. 1405 - 1431 (2007/10/03)
The literature describing the observation of enols of carboxylic anhydrides and mixed carboxylic-sulfuric anhydrides was examined. In the phenylbutyric anhydride system, the alleged enol was shown to be ethylphenylketene, and the monoenol EtC(Ph)=C(OH)OC(=O)CH(Ph)Et (5) and the dienol (6) should not be observed according to calculations. Calculations also show that the claimed enols H2C=C(OH)OSO2Y, Y= SO3-, Ac (15) and the enol of 2H-pyran-2,6(3H)-dione (7) are too unstable to be observed. The bulky enols of β,β-ditipylacetic formic (35a) or trifluoroacetic (35b) anhydride were calculated to be unstable with pKEnol=7.7 (6.2). The suggestion that compounds with the 3-acyl or 3-aroyl-2H-pyran-2,6(3H)-dione skeleton are enolic was examined. In the solid state, all the known structures show that enolization takes place on C(5)=O. However, B3LYP/6-31G** calculations show that, for 3-acetyl-4-methyl-2H-pyran-2,6(3H)-dione (10, R1=Me, R2=H), which is completely enolic, the enol on the acetyl group (cf. 12) is only 0.9 kcal/mol more stable than the enol on the anhydride (cf. 11). Calculations also revealed that 3-(trifluoroacetyl)-2H-pyran-2,6(3H)-dione (28) should exist in nearly equal amounts of the enol of anhydride (cf. 30) and the enol of the acyl group (cf. 29), whereas the enol of anhydride (cf. 32) is the only stable species for 3-(methoxycarbonyl)-2H-pyran-2,6(3H)-dione (31). Furan-2,5-diol (27) and 5-hydroxyfuran-2-one (26) are calculated not to give observable isomers of succinic anhydride (25) (pKEnol=30 and 18, resp.) in spite of the expected aromatic stabilization of 27. Surprisingly, the calculations reveal that the enol (NC)2C=C(OH)OCHO (38) is less stable than its tautomeric anhydride (37) (pKEnol=1.6). Comparison of calculated pKEnol values for (NC)2CHC(=O)X (41) and MeC(=O)X indicates that the assumption that substitution by two β-CN groups affects similarly all the systems regardless of X is incorrect. A pKEnol((NC)2CHC(=O)X) vs. pKEnol(MeC(=O)X) plot is linear for most substituents with severe and mild negative deviations, respectively, for X=NH2 and MeO. Appropriate isodesmic reactions have shown that the β,β-(CN)2 substitution increases the stabilization of the enol of amide (X=NH2) by 14.6 kcal/mol over that for the anhydride (X=OCHO), whereas the amide form is 7.1 kcal/mol less destabilized than for the anhydride. The pKEnol value for (MeOCO)2CHCOOCHO (43) is 3.6, i.e., stabilization by these βelectron-withdrawing groups is insufficient to make the enols observable.
