Studies towards the syntheses of rocaglate natural products and synthetic analogues via ESIPT photocycloaddition
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The total syntheses of isomeric aglain natural products (±)-foveoglin A and (±)-perviridisin B have been achieved via ESIPT (excited states intramolecular proton transfer)-mediated selective (3+2) photocycloaddition of 3-hydroxyflavone with trans,trans-1,4-diphenyl-1,3-butadiene (DPBD). Using TADDOLs or Pirkle’s alcohol as chiral hydrogen-bonding additives, enantioselective ESIPT photocycloaddition was performed providing access to (+)-foveoglin A which also enabled confirmation of its absolute configuration. Photophysical studies have been conducted for ESIPT photocycloadditions revealing the possibility for exergonic electron transfer from the excited triplet state of 3-hydroxyflavones to dipolarophiles with appropriate redox potentials, which also provided a rationale for the observed selectivity. Further application of ESIPT photocycloaddition using 1-alkyl-2-aryl-3-hydroxyquinolinones (3-HQ’s) to synthesize nitrogen-containing analogues of flavaglines, aza-rocaglates, will be described. Differential photoreactivity between 2-aryl-3-hydroxyquinolinones (N-H-3-HQ’s) and 1-alkyl-2-aryl-3-hydroxyquinolinones (N-alkyl-3-HQ’s) was observed. A rationale for this observation was also provided based on photophysical measurements. A novel method to synthesize N-alkyl-3-hydroxyquinolinones using sodium hydride as base was discovered to overcome limited access to photoreaction substrates. A recirculating photoflow reactor was applied to the ESIPT photocycloaddition to increase the efficiency of the reaction. Initial biological testing indicates that aza-rocaglates do not possess activity in comparison to related rocaglates which also provides further information on the SAR of the natural product scaffold. Computational studies were conducted in collaboration with Prof. David Coker’s group using Metadynamics simulation to study tetrakis-9-phenanthrenyl TADDOL-mediated asymmetric ESIPT photocycloadditions. With a choice of collective variables based on hydrogen-bonding interactions between TADDOL and 3-hydroxyflavone, the free energy surfaces associated with formation of the hydrogen-bonding complexes between TADDOL and the 3-hydroxyflavone/methyl cinnamate or 3-hydroxyflavone/stilebene pairs were obtained. The representative three-component model from the obtained free energy minimum indicate that in addition to hydrogen-bonding interactions, π-π stacking between the phenanthren-9-yl groups of TADDOL and the 3-hydroxyflavone substrate also facilitate the asymmetric photocycloaddition which has provided information for future asymmetric catalyst designs.