Cycloalkanes from Annulated Pyranosides

Cycloadditions to these enones yield annulated pyranosides , a number of which have become available in recent years in the authors laboratory (23). These bicyclic molecules open avenues for transcription of stereochemical information from sugars to carbocyclic systems, generalized procedures for which are currently not available. 

Enone (83) has been our most widely used substrate. The annulation may be stereoselective, in which case only one diastereomer of (166) is obtained and the problem is to determine which. If the annulation is non-stereoselective, then diastereomeric forms of (166) are given and 

For the case of (166) where X=0, Ri = H, C-3 is a potentially epimerisable centre, if not on the bicycle itself, then on the acyclic form (167), derivable by opening of the pyranose ring. Similarly, the anomeric centre of (166) is a protected aldehyde hence for (167) where Y=0 and R2 = H, C-2 is also a potentially epimerisable centre. 

Oiu previous discussion indicates that for the case where C-4 is a tetrahedral centre [/. e. (166) X=Y, Z], it should be possible by nuclear magnetic resonance methods to assign the orientation and hence the chiralities of these groups. The same holds for the substituents A and B on the cycloalkyl moiety. 

Thus in either case, the annulated pyranoside has the capability for enantioselective transformation. 

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