Ring Expansions of 2‐Alkenylazetidinium Salts – a New Route to Pyrrolidines and Azepanes

Abstract

Series of enantiomerically pure 3-alkenylpyrrolidines, substituted azepanes and stereodefined aminoalkenes were synthesized from 2-alkenylazetidinium trifluoromethanesulfonate salts. The high chemoselectivity of these reactions was found to be strongly dependent both on the nature of the base involved in the process (PhLi or KHMDS) and on the relative cis or trans stereochemistry of the intermediate ammonium ylide. When the ylide and the adjacent alkene are

Ring Expansions of Nonactivated Aziridines and Azetidines

“…They unambiguously showed that it involves the formation of the ylide, which is the rate determining step, followed by the concerted rearrangement of the ylide involving a diradical intermediate stabilized within a solvent cage, and the migration with retention of configuration [140]. We also contributed to the knowledge of this reaction, demonstrating its regio-and diastereoselectivity [141][142][143]. Several examples are depicted in Scheme 79.…”

“…Azepanes, i.e., seven-membered azaheterocycles, can be produced in good yields from 2-alkenylazetidinium salts [142]. As mentioned earlier, a condition for the success of this reaction is a cis-relationship between the reacting nitrogen ylide and the adjacent alkene; otherwise, only Stevens rearrangement occurs.…”

Azetidines: New Tools for the Synthesis of Nitrogen Heterocycles

“…12 This last transformation requires quite harsh conditions, but, interestingly, no degradation is observed. This goes against another preconceived idea about azetidines: while azetidine itself is rather sensitive, other substituted azetidines are not that unstable at all.…”

Molecular Rearrangements: Part 1. Pericyclic Reactions

“…Allyloxyphosphonium ylids (32), generated in situ, have been shown to undergo [3,3]-sigmatropic rearrangements readily on heating to give phosphonate derivatives (33) in good yields. Diastereoselectivity has been found to be low; following DFT cal-de culations, this has been attributed to the small energy gap between axial and equatorial ylid substituents, because of the long P-C ylid bond.…”

“…The contrasting behaviour of these two groups of substrates has been attributed to the cis or trans relationship between the anion and the vinyl group in the ylids (35) and (37). 32 A DFT study of the [2,3]-sigmatropic rearrangements of some hydrogen and alkyl prop-3-enyl sulfoxides to the corresponding prop-3-enyl sulfenates has found the rearrangements to be concerted, with the competing exo and endo transition states very close in energy. 33 Enantiomerically pure branched allylic alkyl sulfides (39) ee have been shown to give N -allylic-N -Boc-sulfimides (40) via a [2,3]-sigmatropic rearrangement, with essentially complete transfer of chirality, on treatment with NBoc-3,3-di(ethoxycarbonyl)oxaziridine.…”