Far Away from Flatland. Synthesis and Molecular Structure of Dihetera[3.3.n]propellanes and Trihetera[3.3.n]propellanes: Advanced Analogues of Morpholine/Piperazine.

An approach to di- and trihetera[3.3.n]propellanes (n = 2-4 ), advanced morpholine and piperazine analogues, is developed. The key step of the reaction sequence included a [3 + 2] cycloaddition reaction of unsaturated vicinal dicarboxylic acid derivatives and in situ generated azomethine ylide resulting in the formation of the pyrrolidine ring. One more heteroaliphatic ring (i.e., pyrrolidine or tetrahydrofuran) was annelated by nucleophilic cyclization of the appropriate 1,4-dielectrophilic intermediates. There were 11 examples of the title products obtained in 3-5 steps on a multigram scale with 10-72% overall yields. Additionally, molecular structures of homologous dihetera[3.3.n]propellanes, analogues of morpholine, were obtained from X-ray diffraction studies and analyzed using exit vector plots (EVPs). It was shown that the scaffolds obtained are somewhat larger as compared to the parent morpholine and bicyclic 3-oxa-7-azabicyclo[3.3.0]octane. Moreover, despite very similar chemical structures, they provide a very distinct spatial position of heteroatoms, which is clearly seen from the conformation adopted by a formal eight-membered ring including both N and O atoms (i.e., crown, boat-chair, twist chair-chair, and boat-boat for the oxaza[3.3.2]-, -[3.3.3]-, -[4.3.3]propellanes, and 3-oxa-7-azabicyclo[3.3.0]octane, respectively).

Scalable Synthesis of Biologically Relevant Spirocyclic Pyrrolidines.

Synthetic approaches toward multigram preparation of spirocyclic α,α-disubstituted pyrrolidines from readily available starting materials are discussed. It was shown that although a number of synthetic methodologies have been known to date, many of the title compounds remain hardly accessible. The most appropriate literature method (which relied on reaction of imines and allyl magnesium halide, followed by bromocyclization) was identified and optimized. It was found that the method is most fruitful for simple non-functionalized substrates. Two novel approaches based on the Sakurai or Petasis reactions of cyclic ketones, followed by hydroboration-oxidation at the allyl moiety thus introduced, were elaborated. The latter method had the largest scope and was beneficial for the substrates containing organosulfur or protected amino functions. For the synthesis of 4-azaspiro[2.4]heptane, an alternative synthetic scheme commencing from tert-butyl cyclopropanecarboxylate (instead of the corresponding ketone) was developed. It was shown that the whole set of the methodologies developed can be used for the synthesis of various spirocyclic α,α-disubstituted pyrrolidines-advanced building blocks of potential importance to medicinal and agrochemistry-at up to a 100 g scale.