Skew-Chair-Boat type

In many cases a type of deterministic search is conducted, where all probable structures are entered individually and minimized separately. The possible structures are generated by drawing all possible combinations of the chair, boat and skew boat conformers for six-membered rings, and X and S conformers for five-mem-beredrings116 37 64-107 1081. 

Pyranose Conformations. Figure 3 shows the different conformations for 6-membered rings (adapted from a drawing by Jeffrey and Yates (27)). There is a 9 parameter besides Q and ( > because several types of puckering are possible for a given Q and < ). In addition to the E (envelope) notation used in Figure 3, six-membered rings with only one out-of-plane atom are also called sofas or half-boats. The E descriptor was selected here because S is already used to denote skewed pyranose conformations (which have two atoms on opposite sides of the plane, separated by one atom). The H label is already used for half-chairs, which have two adjacent atoms on opposite sides of the mean plane. Typically, the E and H forms are not iiqportant unless a double bond is present. 

For complexes with bidentate 1,2-diamines only one type of chelate ring conformation could be ascertained. The six-membered chelate rings of 1,3-diamines may exist in a chair 6a, skew boat 6b or boat 6c conformation. The least stable of the three types is 6c and was not taken into account so far. When the chelate ring carries... 

The seven-membered septanoid ring can adopt four conformations called chair (C), boat (B), twist-chair TC) and skew (S). Representatives of each type, in which the ring oxygen occupies the upper right-hand corner, are depicted in Figure 2.28. In the chair and boat conformations, the reference plane is formed by four adjacent coplanar atoms, which are separated by one and two atoms, respectively. These separating atoms are placed on the opposite sides of the plane in the chair... 

The conformations of rings A, B, and C in these ewt-kaurane-type diterpenoids were elucidated by X-ray analysis and use of NMR data. Thus, rings A, B, and C of shikoccidin (15) were shown to have chair conformations (73), while ring A of mebadonin (16) was shown to have a skew boat conformation because of the steric interaction between the 20-... 

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