CONFORMATIONAL STRUCTURE AND DYNAMICS OF ARYLMETHOXYACETATES - DNMR SPECTROSCOPY AND AROMATIC SHIELDING EFFECT
CONFORMATIONAL STRUCTURE AND DYNAMICS OF ARYLMETHOXYACETATES - DNMR SPECTROSCOPY AND AROMATIC SHIELDING EFFECT
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DOI：
10.1021/jo00108a008
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发表时间：
1995-02-10
影响因子：
3.6
通讯作者：
RIGUERA, R
中科院分区：
文献类型：
--
作者：
LATYPOV, SK;SECO, JM;RIGUERA, R
利用低温H-1和C-13 NMR谱，并通过MM、AM 1和芳香屏蔽效应计算，计算了（R）-和（S）-芳甲氧基乙酸酯的基态构型和能量.在溶液中，标题化合物是由两个处于平衡状态的旋转异构体组成的。旋转异构体ap具有C-alpha-O和C=O键的反周面，旋转异构体sp（能量低0.4-1.1 kcal/mol）具有这些键的同周面。在ap和sp中，芳基环都是ca。垂直于C=O键并与CalphaH键共面。它们的相对种群和其温度依赖性的DNMR光谱评估是在协议的立体选择性报道的Diels-Alder反应的二烯基-AMAA酯和支持的“垂直模型”提出来解释立体选择性。对于构象中的芳环屏蔽的酯的醇部分，计算的环电流屏蔽增量比较以及从低温NMR光谱获得的实验位移值，并同意在Mislow-Dale-Mosher-Trost方法的NMR手性识别的构象模型。
The ground state conformational geometry and energy of esters of (R)- and (S)-arylmethoxyacetic acids were evaluated from low temperature H-1 and C-13 NMR spectra and by means of MM, AM1, and aromatic shielding effect calculations. In solution, the title compounds are constituted by two rotamers in equilibrium. Rotamer ap has the C-alpha-O and C=O bonds antiperiplanar and rotamer sp (lower in energy by 0.4-1.1 kcal/mol) has these bonds synperiplanar. In both the ap and the sp the aryl ring is ca. perpendicular to the C=O bond and coplanar with the CalphaH bond. Their relative populations and its temperature dependence evaluated by DNMR spectra are in agreement with the stereoselectivity reported for Diels-Alder reactions of dienyl-AMAA esters and support the ''perpendicular model'' proposed to account for that stereoselectivity. For conformations in which the aryl ring shields the alcohol moiety of the ester, calculated ring current shielding increments compare well with experimental shift values obtained from low-temperature NMR spectra and agree with the conformational model used in the Mislow-Dale-Mosher-Trost method for chiral recognition by NMR.