{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,19]],"date-time":"2026-08-19T19:16:13Z","timestamp":1787166973473,"version":"build-2736575974"},"reference-count":30,"publisher":"National Academy of Sciences","issue":"26","content-domain":{"domain":["www.pnas.org"],"crossmark-restriction":true},"short-container-title":["Proc. Natl. Acad. Sci. U.S.A."],"published-print":{"date-parts":[[2009,6,30]]},"abstract":"<jats:p>\n                    Calix[4]pyrroles having extended aromatic cavities have been functionalized with 4 ureas in the\n                    <jats:italic>para<\/jats:italic>\n                    position of their\n                    <jats:italic>meso<\/jats:italic>\n                    phenyl substituents. This elaboration of the upper rim was completed in 2 synthetic steps starting from the \u03b1,\u03b1,\u03b1,\u03b1-tetranitro isomer of the calix[4]pyrrole obtained in the acid catalyzed condensation of\n                    <jats:italic>p<\/jats:italic>\n                    -nitrophenyl methyl ketone and pyrrole. In dichloromethane solution and in the presence of 4,4\u2032-bipyridine\n                    <jats:italic>N-N\u2032<\/jats:italic>\n                    -dioxide the tetraurea calix[4]pyrrole dimerizes reversibly forming a cyclic array of 16 hydrogen bonds and encapsulating 1 molecule of bis-\n                    <jats:italic>N<\/jats:italic>\n                    -oxide. The encapsulated guest is bound in the cavity by hydrogen bonding to the 2 endohedral calix[4]pyrrole centers. Further evidence for dimerization of the tetraurea calix[4]pyrroles is provided by\n                    <jats:sup>1<\/jats:sup>\n                    H-NMR experiments and by the formation of mixed capsules.\n                  <\/jats:p>","DOI":"10.1073\/pnas.0809612106","type":"journal-article","created":{"date-parts":[[2009,3,4]],"date-time":"2009-03-04T21:40:29Z","timestamp":1236202829000},"page":"10455-10459","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":97,"title":["Self-assembly of dimeric tetraurea calix[4]pyrrole capsules"],"prefix":"10.1073","volume":"106","author":[{"given":"Pablo","family":"Ballester","sequence":"first","affiliation":[{"name":"Catalan Institution for Research and Advanced Studies, Passeig Llu\u00eds Companys, 23 E-08010-Barcelona, Spain; and"},{"name":"Institute of Chemical Research of Catalonia, Avenida Pa\u00efsos Catalans, 16 E-43007 Tarragona, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Guzm\u00e1n","family":"Gil-Ram\u00edrez","sequence":"additional","affiliation":[{"name":"Institute of Chemical Research of Catalonia, Avenida Pa\u00efsos Catalans, 16 E-43007 Tarragona, 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