Trans-decalin skeleton

Some heteropolycycloalkanes, e.g. indolizidine, quinolizidine, tropane, and decahydro-quinoline, are parent skeletons of alkaloids. Carbon-13 shifts of these and other parent polycycloalkanes [408] (Tables 4.64 and 4.65) are predominantly determined by heteroatom electronegativity, ring size, stereoisomerism, and dynamic effects. Aza- and thiade-calins are representative examples (Table 4.64). As described for cis- and trans-decalin (Section 4.1.3), carbon nuclei in the cis isomers of aza- and thiadecalins are shielded... 

Figure 3. Skeletons of some terpenoids that contain trans-decalin moieties.

Because steroids (even those with a cis A-B ring junction) are essentially substituted trans-decalins they can t ring flip. This means, for example, that the hydroxyl group in cholestanol is held equatorial on ring A while the hydroxyl group in coprostanol is held axial on ring A. The steroid skeleton really is remarkably stable—samples of sediment 1.5 x 109 years old have been found to contain steroids still with the same ring-junction stereochemistry. 

The desired cyclization leading to the six-membered ring was achieved by intramolecular photocycloaddition of dioxolenone 107 as shown in Scheme 26. Consecutive cleavage of the ketal followed by retroaldol reaction augmented the initial ring system by a C2 unit to trans-bicycle 108 (90). Attempts to apply this reaction principle to a trans-decalin system in order to reach a taxane skeleton failed (97). 

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