Rings, bicyclic, stereoselectivity conformation

A solvent effect has been observed in the DIBAH reduction of the bicyclic 1,3-dioxane 2.51 [IM3] (Figure 2.25). In CHCI3 or CHjClj, the irons isomer 2.53 is predominantly formed, while in THF, the cis isomer 2.52 is the major product. With AlHBrj in EtjO, the stereoselectivity is even higher [IM3] (Figure 2.25). These results have been interpreted in terms of conformational effects, emphasizing the importance of the size of the acetal ring (six-membered vs. five-membered). 

Zhang and Tu employed the intramolecular Schmidt reaction as a key step in their total synthesis of ( )-maistemonine (6) and ( )-stemonamide (7) [7]. Treatment of 2,2-disubstituted cyclohexadione 4 with TiC provided the desired bicyclic amide 5 containing the central perhydroazaazulene ring system. The remarkable stereoselectivity was accounted for by invoking the preferred conformation B over A, the latter suffering from repulsive steric interaction between the allyl and the alkynyl substituents (Scheme 4.2). 

Spiro compounds, which are bicyclic organic compounds with rings connected through just one atom, form a class of molecules with unique chemical and conformational features often associated with important chemical or biological properties. They have attracted considerable attention from the synthetic community, and their stereoselective synthesis still constitutes an enormous challenge for chemists because of the presence of a chiral quaternary carbon atom. More specifically, a-spiro-8-lactones and -lac-... 

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