Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (12.VI.95)A concept of first choice for the synthesis of the title compounds had been proposed by Dane in the late 1930s. It was soon turned down, because the opening move -a chirogenic Diels-Alder reaction -did not work. With Lewis acids as mediators, however, a successful start has been achieved now. With Ti complexes of chelating ligands (Seebach 's TADDOLs ( = a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanols)), enantioselective formation of the desired adducts does occur. Efficient total syntheses of 2 and 3a have been accomplished.

The four photo-cleavable ligands 1 4 of the macrobicyclic-cryptand type have been synthesized by introducing a photo-sensitive 2-nitrobenzyl-or 4,5-dimethoxy-2-nitrobenzyl-ether bond into one of the bridges. These compounds are expected to retain the selective binding features of the parent cryptands and to allow the photolytically induced release of alkali ions in aqueous solution. The crystal structures of the ligand 5-(2-nitrophenyl)-4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane (3) and of its KSCN complex 13 have been determined. They are analogous to those of the corresponding parent species, confirming the macrobicyclic geometry and the cryptate nature of the complex. Spectroscopic properties are reported.

Natural products have been synthesized for billions of years in animals, plants, and microorganisms. As a rule they occur enantiomerically pure. Their chiral character corroborates their use in metabolism or as biologically active agents. Natural products may be insufficient in quality or quantity. They have recently begun to become accessible, either unchanged or modified, by biological synthesis; here, too, they are obtained enantiomerically pure. In the last twenty years chemical synthesis has become a major concern of organic chemists. Their target compounds are primarily enantiomerically pure natural products or biologically active variants thereof.

Bei -185" bewirkt UV-Licht eine quantitative Photo-Isomerisierung des cyclo-Isomeren 6, das durch Photo-Decarbonylierung von 5 bei -75" zuganglich ist, zum seco-Isomeren 12. Durch Bestrahlen des ersten in Substanz isolierten Isodihydroanthracen-Derivats 7, das durch thermische Isomerisierung bereits unterhalb von Raumtemperatur aus dem Benzocyclobuten-Derivat 6 entsteht, mit Licht der Wellenlangen um 404 nm bei -185" ist ebenfalls vollstandig 12 erhaltlich. Licht der Wellenlangen um 530 nm wandelt Tetraphenyl-o-chinodimethan bei -185" ausschlieBlich in 7 um. Je nach den verwendeten Wellenlangenbereichen sind photostationare Zustande mit unterschiedlicher Beteiligung von 12 und 7 erreichbar. Die Konstitutionen von 6 und 7 werden spektroskopisch und durch chemische Folgereaktionen gesichert; die Konstitution von 12, das bei -180" kein ESR-Signal zeigt, geht aus seinen Verknupfungen mit 6 und 7 hervor. Die unter sehr milden Bedingungen erfolgende thermische Isomerisierung von 12 zum kinetisch bevorzugten 6 sowie die bei -185" eintretende photochemische Isomerisierung von 12 zu 7 lassen sich als elektrocycliscbe Transformationen des zwangslaufig nichtplanaren seco-Isomeren verstehen.