Annulation of dihydroimidazoles: a 1,3-dipolar cycloaddition route to pyrrolo[1,2-a]imidazoles, pyrrolidines and pyrroles

Synthesis of some new 2-heterosubstituted 4,5-dihydroimidazoles

“…Similar treatment of the salt 22c afforded mono-deuteration. However, when the salts 19, 21 and 22a-c were subjected to the one-pot deprotonation-cycloaddition protocol 5 with methylacrylate, no cycloadducts were observed. These studies, whilst not to date affording 1,3-dipolar cycloaddition, have generated some new 2-heteroatom substituted 4,5-dihydroimidazoles 3 via cyclic ureas, and explored generation of the corresponding imidazolium salts.…”

“…Downfield shifts of product peaks were observed relative to the urea, although these putative salts proved too hygroscopic for further analysis. In any event, one-pot attempts by our usual protocol 5 to induce formation of salts 13 or 14, deprotonation to an ylide and cycloaddition were unsuccessful. The major isolated product in each case was urea 11a, presumably from quaternary salt hydrolysis.…”

“…Although this strategy was ultimately not fruitful, The sulfur substituent could also act as a leaving group to access O-and N-substituted compounds. When 1-benzyl-4,5-dihydroimidazole 5 was deprotonated (BuLi, THF, -78°C) and reacted with diphenyl or dibutyl disulfides, the sulfur-substituted dihydroimidazoles 5a,b were isolated (Scheme 3), however the major product (40%) was 1-benzyl-tetrahydroimidazol-2-one 6a, arising presumably from adventitious hydrolysis during isolation. …”

An Unexpected Stereochemical Course of Dihydroimidazolium Ylide Cycloadditions

“…[2,8] With a view to developing a traceless dipolarophile activation, [9] we have investigated potentially removable sulfur-containing activating groups, principally sulfone. We report herein the results of cycloadditions using a range of vinyl sulfones, the unexpected stereochemical outcomes observed, and a new secondary reaction of the cycloadducts.…”

“…[2] Thus the dihydroimidazole, alkylating agent, and dipolarophile (3 mol equiv of dipolarophile) were mixed in THF at reflux and DBU was added dropwise over 4 h. Using tert-butyl bromoacetate and phenyl vinyl sulfone, cycloadduct 3 a was isolated (45 %) after column chromatography and recrystallization (Scheme 2). In contrast to our previous reports with carbonyl-and nitrile-activated dipolarophiles, pyrroloA C H T U N G T R E N N U N G [1,2-a]imidazole 3 a results from the unexpected exo approach of the dipolarophile to the dipole.…”