Fragmentations are controlled by stereochemistry

Stereoselective reactions of acyclic alkenes 865 Fragmentations are controlled by stereochemistry 962... 

On the other hand, a double asymmetric version where both radical fragments are chiral has been investigated (equation 12) . The stereochemistry of the rearrangement products 20a and 20b is not the result of chelation control, but is determined by the stereochemistry of the hthium-bearing terminus. The significant difference in the stereospecificity of 19a and 19b reveals that a substantial level of mutal recognition of the radical enantiomers takes place during the recombination process. 

In Rainier s synthesis of the hemibrevetoxin B ring system,diene 169, which is obviously related to Danishefsky s diene, reacted with O-benzyl 4-hydroxybenzaldehyde, in the presence of zinc chloride, to give 170 in 92% yield. Pyrones and pyrans are particularly useful for the preparation of acyclic precursors to macrocyclic antibiotics. The stereochemistry of the highly oxygenated acyclic fragment can be controlled by exploiting the selectivity inherent in the cyclic pyran (sec. 6.5). 478 xhe disconnection for this reaction is... 

There are two main approaches to the problem of the formation of centers of asymmetry. The first of them, which is connected with the building up of the molecule by the introduction of small fragments in each step has the advantage that only one center of asymmetry is formed in each reaction and its stereochemistry can easily be controlled. The disadvantage of this approach is the great increase in the number of stages and, as a consequence of this, the fall in over-all yields. The second approach to the... 

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