Tropolone zeolites

Upon exposure to UV light, a-tropolone methyl ether (142), included within chirally modified Y zeolite, has been found to undergo 4 7r-electron disrotatory electrocyclic ring closure to afford " the bicyclic photo-isomer (143). 

There is also rapid development in the domain of standard silica-based zeolites. Their versatility can be extended by imprinting. For instance, Davis and Katz [15] recently successfully carried out imprinting and obtained a silica framework with pore walls anchoring three aminopropyl groups in cavities. Another achievement was reported by Ramamurthy, Schefer and coworkers [16]. The latter authors were able to obtain 90% diastereomeric excess of a product of the photochemical reaction in a commercially available zeolite containing chiral tropolone ether 433 in its pores. 

Figure 41 Photocyclization of tropolone alkyl ether within X- and Y-zeolites. The product configuration is arbitrarily assigned based on HPLC retention time. The nature of the favored enantiomer is reversed between wet and dry zeolites and between NaY and NaX. The extent of ee depends on water content, the nature of the cation, and the number of cations (NaY versus NaX).

Figure 45 Photocyclization of (S)-tropolone 2-methylbutyl ether within chirally modified NaY zeolites. The diastereomeric excess (%) and the isomer enhanced are shown on the HPLC traces. The first eluted isomer on the HPLC column is arbitrarily assigned 93.

Joy, A., Kaanumalle, L.S. and Ramamurthy, V. (2005) Role of cations and confinement in asymmetric photochemistry enantio- and diastereo-selective photocydization of tropolone derivatives within zeolites. Organic Biomolecular Chemistry, 3, 3045-3053. 

Joy, A., Scheffer, J.R. and Ramamurthy, V. (2000) Chirally modified zeolites as readion media photochemistry of an achiral tropolone ether. Organic Letters, 2, 119-121. 

Irradiation of (S )-tropolone 2-methyl butyl ether in solution yields a 4-electron electrocyclization product as a 1 1 diastereomeric mixture (Sch. 8) [106]. In solution the presence of the chiral auxiliary in proximity to the reactive center has no influence on the product stereochemistry. When irradiated within NaY zeolite, however, the same molecule affords the cyclized product in 53% diastereomeric excess. The restricted space of the zeolite supercage apparently forces communication between the chiral center and the reaction site. 

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

The most successful supramolecular photochirogenesis with chirally modifiea zeolites is the enantiodifferentiating photocyclization of tropolone derivatives in[ zeolites modified with (— )-ephedrine or (— )-norephedrine, giving bicyclo[3.2.0J heptadienes in high ees. It is interesting to compare this result with the highlj ... 

NaY zeolites modified with (— )-ephedrine or (— )-norephedrine showed even better performance in the photolysis of chiral tropolone ether 22c immobilized in the supercage, affording 23c in 90% de. In contrast, irradiation of 22c in solution or on a silica surface in the presence of enantiopure ephedrine or norephedrine gave 1 1 diastereomeric mixtures. 

Figure 1 An adsorption (top)-desorption (bottom) model for chiral induction on a zeolite surface, incorporating a reactant (tropolone alkyl ether, shown at the left), a chiral inductor (with four different substituents, at the right), and a cation (small ball on the surface). Tropolone s carbonyl and ether oxygens hydrogen-bond to chiral inductor, while its tt system interacts with zeolite s cation ion. ...

Irradiation of tropolone alkyl ether 22 (Scheme 14) led to a 4ir-disrotatory ring closure to yield bicyclo[3.2.0]heptadienone 23 with two chiral centers, while prolonged irradiations led to the formation of a secondary product 24 [76-78]. As the same photocyclization was performed in chirally modified zeolites, it is interesting to compare the asymmetric photochemical behavior of 22 in the distinctly different chiral confined media of zeolites and cyclodextrins. Even in the... 

Furthermore, tropolone methyl ether 25 included in NaY and RbY zeolites that had been modified with optically pure norephedrine smoothly underwent enantioselective [2 + 2] photocyclization to afford the primary photoproduct 26 in around 40% ee (Scheme 7) [53]. The best results obtained are up to 78% ee... 

In Scheme 19, results of photocyclization of twelve tropolones covalently linked with chiral auxiliaries via amide linkages are presented [289,290]. In no case is the diastereomeric excess in solution above 15%. Neither the variation of solvent-nor the concentration of the reactant improved the selectivity. On the other hand, within zeolites the des are well above 40%. The extent of de is dependent on the alkali ion present within Y zeolites and the best numbers are provided in the scheme. 

Scheme 22). Results of irradiation of a number of 6,6-dimethyl-2,4-cyclohexa-dienones and 2,2-dimethyl-1,2-dihydronaphthalenones are presented in Scheme 23 and 24. As with tropolones, in these systems the chiral auxiliaries, which have little influence in solution, are very effective within zeolites. 

In principle, enantiomerically pure cyclized products from tropolone ethers can be obtained by controlling the mode of ring closure. On the basis of this premise, the observed selectivity within zeolites could be rationalized. When tropolone alkyl ether is adsorbed on a surface, cyclization inward or outward is... 

Based on the observation that the best ee is obtained with bifunctional chiral agents (ephedrine, pseudoephedrine, norephedrine, and valinol see Scheme 43), we tentatively conclude that a multipoint interaction between the reactant molecule, the chiral inductor, and the zeolite interior is necessary to induce preferential adsorption of tropolone alkyl ether from a single enantiotopic face. The dependence of chiral induction (% ee) on the nature of cations (Scheme 45) suggests a crucial role of the cation present in the supercages in the chiral induction process. This is further strengthened by the results observed with wet and dry zeolites. The presence of water decreases chiral selectivity (Scheme 45). Water molecules... 

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