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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Absolute configurations of Emycin D, E and F; mimicry of centrosymmetric space groups by mixtures of chiral stereoisomers
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Absolute configurations of Emycin D, E and F; mimicry of centrosymmetric space groups by mixtures of chiral stereoisomers

机译：Emycin D，E和F的绝对构型;手性立体异构体混合物模拟中心对称空间群

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摘要

The crystal structures of Emycin E (1), di-o-bromobenzoyl-Emycin F (2) and o-bromobenzoyl-Emycin D (3) have been determined by X-ray analysis at low temperature. Emycin E and o-bromobenzoyl-Emycin D both crystallize with two molecules in a triclinic unit cell. These two structures can be solved and refined either in the centrosymmetric space group P (1) over bar, with apparent disorder localized at or around the expected chiral centre, or in the non-centrosymmetric space group P1 as mixtures of two diastereomers without disorder. Only the latter interpretation is consistent with the chemical and spectroscopic evidence. Refinements in the centrosymmetric and non-centrosymmetric space groups are compared in this paper and are shown to favour the chemically correct interpretation, more decisively so in the case of the bromo derivative as a result of the anomalous dispersion of bromine. Structures (1) and (3) provide a dramatic warning of the dangers inherent in the conventional wisdom that if a structure can be refined satisfactorarily in both centrosymmetric and non-centrosymmetric space groups, the former should always be chosen. In these two cases, despite apparently acceptable intensity statistics and R factors (5.87 and 3.55%), the choice of the centrosymmetric space group leads to the serious chemical error that the triclinic unit cell contains a racemate rather than two chiral diastereomers! The weakest reflections are shown to be most sensitive to the correct choice of space group, underlining the importance of refining against all data rather than against intensities greater than a specified threshold. The use of similar-distance restraints is shown to be beneficial in both P1 refinements. Di-o-bromobenzoyl-Emycin F crystallizes in the monoclinic space group P2(1) with one molecule in the asymmetric unit and so does not give rise to these problems of interpretation. The absolute configuration of the two bromo derivatives, and hence the Emycins in general, was determined unambiguously as S at the chiral centre C3. [References: 19]
机译：通过低温X射线分析确定了霉素（1），二邻溴苯甲酰-霉素E（2）和邻溴苯甲酰基-霉素D（3）的晶体结构。霉素（Emycin E）和邻溴苯甲酰-霉素（Emycin D）都在三斜晶胞中以两个分子结晶。这两个结构可以在bar上的中心对称空间群P（1）中解决，并且可以将明显的无序分布在预期的手性中心处或周围，或者在非中心对称空间群P1中作为两种非对映异构体的混合物来求解和改进。仅后者的解释与化学和光谱学证据一致。本文比较了中心对称和非中心对称空间群的精炼结果，结果表明它们有利于化学上正确的解释，而对于溴衍生物的异常分布，则决定性地更有利于化学上正确的解释。结构（1）和（3）强烈警告了传统观点固有的危险，即如果可以在中心对称和非中心对称的空间组中令人满意地完善结构，则应始终选择前者。在这两种情况下，尽管强度统计数据和R因子（5.87和3.55％）显然是可以接受的，但选择中心对称的空间基团会导致严重的化学错误，即三斜晶系单位细胞包含外消旋体而不是两个手性非对映异构体！事实表明，最弱的反射对正确选择空间组最敏感，从而强调了根据所有数据而不是强度大于指定阈值的细化的重要性。在两个P1改进中，使用相似距离约束被证明是有益的。二邻溴苯甲酰-霉素E在单斜空间群P2（1）中结晶，其中一个分子位于不对称单元中，因此不会引起这些解释问题。两种溴衍生物的绝对构型，以及因此通常的霉素，在手性中心C3明确确定为S。 [参考：19]

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