Stereochemistry of Back-Biting Processes Stereoselectivity

Tsuruta observed in the cyclooligomerization of t-butyloxirane that both (R)- and (S)-t-birtyloxiranes predominantly yield cyclic tetramers however, all the other macrocycles (from dimer to octamer) have also been observed in smaller concentrations  

This stereoselectivity in cyclooligomerization has not been observed for (RS)-t-butyloxirane. The presence of two enanthiomers, (R) and (S), apparently decreases the probability of the formation of the preferred conformation of the linear chain which leads to the G G T sequence. 

Highly specific oligomerization was also observed for trans- and cis-1,2-dimethyl-thiirane. 

Trans-l,2-dimethylthiirane gives a cyclic product, 1,2,5-trithiepane, which is formed in a kinetically distinct back-biting step when monomer is already converted into a linear polymer  

The trans monomer has the configuration (SS or RR). In the Sn2 attack in the propagation step, inversion of configuration on the carbon atom being attacked 

---