Mechanistic Studies on the 1,2-Spin-Center Shift in Carbohydrate Systems with a Fluorenylcyclopropyl Radical Clock
The mechanism of the 1,2-spin-center shift in carbohydrate systems was studied with a fluorenylcyclopropyl radical clock. The 1,2-rearrangement of the acyl fluorenylcyclopropane group without opening of the cyclopropane ring provides the strongest evidence that the 1,2-spin-center shift in carbohydrate systems occurs through a concerted transition state without the intermediacy of a 1,3-dioxolanyl radical.
Diastereoselective Alkylations of Chiral Tetrazolo[1,5-]azepines via Heterobenzylic Anion Intermediates
The alkylations of chiral seven-membered rings fused to tetrazoles are highly diastereoselective. The diastereoselectivity depended on the placement and the size of the substituent on the ring and on the electrophile. Subsequent alkylations occurred with high stereoselectivity, allowing for the construction of quaternary stereocenters. Computational studies revealed that torsional effects are responsible for the observed diastereoselectivities. Substituted products can be reduced to the corresponding secondary amines, thus providing an approach for synthesizing diastereomerically enriched azepanes.
Origin of High Diastereoselectivity in Reactions of Seven-Membered-Ring Enolates
Unlike many reactions of their six-membered-ring counterparts, the reactions of chiral seven-membered-ring enolates are highly diastereoselective. Diastereoselectivity was observed for a range of substrates, including lactam, lactone, and cyclic ketone derivatives. The stereoselectivity arises from torsional and steric interactions that develop when electrophiles approach the diastereotopic π-faces of the enolates, which are distinguished by subtle differences in the orientation of nearby atoms of the ring.