[Radu]

http://icho2014.hus.edu.vn/Preparatory-Problems/0001/01/Preparatory-Problems-224.html

    I have a question, which I thought it was a mistake, but they didn't correct it in preparatory problems' review.
    We are told that compound F, which resulted from nitrene-alkene 1,3 dipolar cycliazation has S configuration at C atoms at the common bond, and it has R configuration at the initial chiral centre, given we started from L-cysteine. But if it so, then write F and you'll notice the three substituents of tetrahydrothiophene are not all-cis, as they are in the final product. No modification in these carbons' configurations  is brought about by the preceeding reactions
     So, have I done something stupid like misprioritising the substituents? Or what?
     My opinion is that the mistake came from not considering the fact that reduction of L changed R into S, without changing the configuration, but simply changing the substituents(my work is below).