Models Dreiding

Fig. 1. Stereostmeture of teicoplanin aglycone [89139-42-4] obtained from a Dreiding model (83) where is hydrogen, is nitrogen, O is carbon, is...

The absolute eonfiguration, (25, 45, 55 as shown or 2R,4R,5R), eannot be dedueed by NMR. For larger structures the insertion of the shift values and the coupling constants in the stereo projection of the struetural formula, from whieh one ean eonstruet a Dreiding model, proves useful in providing an overview of the stereoehemieal relationships. 

EDTA, leading to a postulate that more than one equivalent of Ca2+ can be captured by X (e.g. one Ca2+ sequestered by the three amines and the three carboxylates and another Ca2 + by the remaining half the donor groups), as the Dreiding model suggests. The fact that there was no interaction at neutral pH of X with phosphate or oxalate anions was separately confirmed. Thus, the dissolution of Ca3(P04)2 and Ca(C204) is entirely due to the cation complexation mechanism. 

The proton assignments of H-17e, H-17a, H-6a, and H-6e are made by examination of the dihedral angles with Dreiding models and the J values. These and the other proton assignments are confirmed by 2D COSY experiments. 

Examination of Dreiding models showed that 177 is a highly strained and rigid molecule and that only the relative arrangement of groups as in structure 177 or its enantiomer is possible. 

R)-16 was difficult at this point, but it was readily achieved at the alcohol stage, despite the additional stereocenter at C(8). As described, both ketones, (S)-16 and (i )-16, were obtained in almost equal amounts. Acid or base-mediated equilibration of the stereocenter at C(8) produced a 7 3 mixture of (S)-16 and (R)-16 (Scheme 3). This experimental result was in accordance with Dreiding models of the cyclooctane diastereoisomers. 

To avoid these difficulties the concept of the stereomodel4 was introduced, i.e., a geometric figure, W hich contains only the information required in a particular context. Thus, the formulas above describe different models of dimethyl ei.s-l,2-cyclohexanedicarboxylate, formula 1 contains information about the conformation and configuration, while formula 2 contains information about the configuration of the molecule only. The term stereomodel should not be confused with mechanical model (i.c., a Dreiding model). However, mechanical models are very useful representations of stereomodels. 

If a box full of Dreiding models of the enantiomers of alanine had to be sorted into the two types, this simple problem could be solved by comparing models pairwise as to whether they are superposable or enantiomorphic. Lord Kelvin3 introduced terms for this type of relationship and described it thus ... 

In his constant search for better methods of teaching he made a 60-minute color-sound movie Techniques of Organic Chemistry, and developed a set of precise plastic molecular models, which are larger than, but have the same relative dimensions as, Dreiding models. Unlike the latter, however, the Fieser models have been so inexpensive to manufacture that even undergraduate students have been able to afford a set. 

An especially intriguing problem concerns the stereochemistry of the [7C4 + Tc J-adducts [e.g., 213 and 214]. Dreiding models reveal two stable conformations, chair and boat (Fig. 1). Whereas interconversion appears... 

The spatial arrangement (stereochemistry) of cyclohexane and other organic compounds are studied conveniently with the aid of Dreiding models, which are made with standard bond angles and scaled bond distances. The bonds... 

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