Investigation of diastereoselective acyclic α-alkoxydithioacetal substitutions involving thiacarbenium intermediates.

Reported herein is an experimental and theoretical study that elucidates why silylated nucleobase additions to acyclic α-alkoxythiacarbenium intermediates proceed with high 1,2-syn stereocontrol (anti-Felkin-Anh), which is opposite to what would be expected with corresponding activated aldehydes. The acyclic thioaminals formed undergo intramolecular cyclizations to provide nucleoside analogues with anticancer and antiviral properties. The factors influencing the selectivity of the substitution reaction have been examined thoroughly.

Stereoselective quaternary center construction via atom-transfer radical cyclization using silicon tethers on acyclic precursors.

A novel strategy for the stereoselective construction of all-carbon quaternary centers on acyclic molecules using a two-step tandem process is reported. The first step involves an intramolecular and stereoselective atom transfer radical cyclization reaction from an allyl or vinyl subunit attached on a silyloxy, serving as a tether, to a tertiary radical alpha to an ester. A subsequent mild acidic elimination leads stereoselectively to a quaternary center bearing an allyl or a vinyl in high yield.