Catalytic enantioselective addition of organometallics to unprotected carboxylic acids
Their findings open new avenues to understand how unprotected forks, occurring in breast cancer cell lines mutated for BRCA2, are key determinants of the efficacy of chemotherapies. Temporarily paused forks are stabilized by the intra-S phase checkpoint and protected by the association of Rad51, which prevents Mredependent resection. However, if a fork becomes dysfunctional and cannot resume, this terminally arrested fork is rescued by a converging fork to avoid unreplicated parental DNA during mitosis.
Alternatively, dysfunctional forks are restarted by homologous recombination. Using fission yeast, we report that Rad52 and the DNA binding activity of Rad51, but not its strand-exchange activity, act to protect terminally arrested forks from unrestrained Exo1-nucleolytic activity.
To our delight, our catalytic system solves this long-standing problem, as it works with similar efficiency for both aliphatic and aromatic substrates, leading to final methylated carboxylic acids with excellent yields and ees 3a , 3b , and 3c. All alkyl Grignard reagents afforded addition products with excellent results, independent of the chain length and branching. Grignard reagents bearing olefinic substituents also function well, affording the corresponding product 3 g with excellent ee and yield. Products 3 h and 3i , derived from additions of the linear Grignard reagent n HexMgBr to 1a and crotonic acid, respectively, were obtained with high enantiopurities and yields as well.
The first applications that illustrate utility pertain to the use of our chiral products in stereoselective decarboxylative cross-coupling reactions.
Decarboxylative cross-coupling reactions are developing very rapidly, and various catalytic systems utilizing aliphatic carboxylic acids mainly achiral leading to diverse structural motives have been established over the past decade 14 , 15 , 16 , 17 , 18 , Here, we demonstrate how useful chiral analogues of those structural motives can be obtained by combining our methodology and decarboxylative couplings Fig.
Although some similar molecules can be obtained via other catalytic asymmetric methodologies, these are often limited to specific structures and feature varying levels of enantioselectivities 33 , However, these methods are only effective when an aryl group or directing carbonyl groups are present in the substrate. Synthetic utility of the process. Our methodology is also sufficiently mild and robust to be applied in more complex molecules.
Another synthetically useful transformation available with our methodology is the trapping of enolate intermediates formed upon conjugate addition Fig. Recently, chiral indole derivative 7a , synthesized in four steps from the commercially available S -citronellal, was reported Fig. However, as such chiral aldehydes or carboxylic acids are rarely commercially available, synthesis of similar chiral compounds with variations of the alkyl chain is difficult, thus limiting the number of molecules available for bioactivity screening.
Our strategy is based on activation of carboxylic acids via formation of transient silyl or boron intermediates, and it is crucial to overcoming the fundamental problem of carboxylate salt formation during the conjugate addition of organometallics to unsaturated carboxylic acid. In a flame-dried Schlenk tube equipped with septum and magnetic stirring bar, the carboxylic acid substrate 0.
Upon reaction completion, the mixture was quenched with HCl aqueous solution 2. The resulting mixture was extracted with CH 2 Cl 2 The combined organic phase was dried over MgSO 4 , filtered, and the solvent was evaporated on a rotary evaporator. Pentane 1.http://pierreducalvet.ca/20897.php
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The crude was purified by flash chromatography on the silica gel to yield the final conjugate addition product after solvent removal. Upon reaction completion, the mixture was quenched with saturated NaHCO 3 aqueous solution 2. The organic phase was further extracted with saturated NaHCO 3 aqueous solution 2. The combined aqueous phase was acidified with HCl aqueous solution 1. The combined organic phase was dried over MgSO 4 , filtered, and the solvent was evaporated on a rotary evaporator to yield the final conjugate addition product.
In a flame-dried three-neck round-bottom flask equipped with the septum and mechanistic stirring bar, the substrate 1a 1. The reaction was quenched with water The aqueous phase was discarded, and the organic phase was extracted with saturated NaHCO 3 aqueous solution The organic phase was washed with HCl aqueous solution The combined aqueous phase was acidified with HCl aqueous solution Supplementary Information and chemical compound information are available in the online version of the paper.
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