Ring Expansions of Cyclopropanated Carbohydrates

<p>The combination of cyclopropanes and carbohydrates functionality within a single molecule gives the synthetic chemist an interesting combination of reactivity and chirality to expand further upon. However, until recently relatively little work has been done in this field. Following the report of methodology to convert cyclopropanated carbohydrates into oxepines in high yields, this investigation set out to improve the selectivity of the rearrangement and also investigate the further utility of the subsequent products. Focused investigation of the starting substrates and reaction conditions led to a minor adaptation of the previously successful methodology for the ring expansion of cyclopropanated glycals. This resulted in a substantial increase in the selectivity of the reaction to generate oxepine rings in good to excellent yields with a range of nucleophiles. One of the oxepines was subsequently chosen for elaboration into a range of synthons for further investigations. These yielded a variety of oxepanes in good yields with well defined stereo- and regioselective outcomes. In the course of these reactions several unexpected products were isolated. These were further investigated with labelling experiments and a mechanism for their formation was proposed. Finally the methodology was applied to the total synthesis of a naturally occurring oxepine containing compound. Despite the apparent simplicity of the target, the effort was ultimately unsuccessful.</p>

Ring Expansions of Cyclopropanated Carbohydrates

<p>The combination of cyclopropanes and carbohydrates functionality within a single molecule gives the synthetic chemist an interesting combination of reactivity and chirality to expand further upon. However, until recently relatively little work has been done in this field. Following the report of methodology to convert cyclopropanated carbohydrates into oxepines in high yields, this investigation set out to improve the selectivity of the rearrangement and also investigate the further utility of the subsequent products. Focused investigation of the starting substrates and reaction conditions led to a minor adaptation of the previously successful methodology for the ring expansion of cyclopropanated glycals. This resulted in a substantial increase in the selectivity of the reaction to generate oxepine rings in good to excellent yields with a range of nucleophiles. One of the oxepines was subsequently chosen for elaboration into a range of synthons for further investigations. These yielded a variety of oxepanes in good yields with well defined stereo- and regioselective outcomes. In the course of these reactions several unexpected products were isolated. These were further investigated with labelling experiments and a mechanism for their formation was proposed. Finally the methodology was applied to the total synthesis of a naturally occurring oxepine containing compound. Despite the apparent simplicity of the target, the effort was ultimately unsuccessful.</p>

Synthetic Approaches to Non-Tropane, Bridged, Azapolycyclic Ring Systems Containing Seven-Membered Carbocycles

This Short Review highlights various synthetic approaches to bridged azabicyclic ring systems containing seven-membered carbocyclic rings. Such ring systems are common to a number of biologically active natural products. The seven-membered ring in such systems is generally formed in one of four ways: 1) cyclization of an acyclic precursor; 2) ring expansion or rearrangement of a different ring size; 3) cycloaddition; and 4) use of a synthetic building block with the seven-membered ring already present. Representative examples of each approach from both total synthesis and methodological studies are discussed, with an emphasis on work publishedin the last twenty years.1 Introduction2 Cyclization Reactions3 Ring Expansions and Rearrangements4 Cycloadditions5 Strategies Involving Seven-Membered Ring Building Blocks6 Conclusion

Recent synthesis of thietanes

Thietanes are important aliphatic four-membered thiaheterocycles that are found in the pharmaceutical core and structural motifs of some biological compounds. They are also useful intermediates in organic synthesis. Various synthetic methods of thietanes have been developed, including inter- and intramolecular nucleophilic thioetherifications, photochemical [2 + 2] cycloadditions, ring expansions and contractions, nucleophilic cyclizations, and some miscellaneous methods. The recently developed methods provide some new strategies for the efficient preparation of thietanes and their derivatives. This review focuses on the synthetic methods to construct thietane backbones developed during 1966 to 2019.