Fig 1.
Proposal for the synthesis of new cyclohexadienal building blocks using different catalysts.
Fig 2.
FDA approved UV-filters and bemotrizinol, approved only in Europe.
The main UV filters structurally related to cinnamate esters, benzophenone, p-aminobenzoic acid (PABA) and salicilate derivatives, and two additional structures that can be found in ensulizole and bemotrizinol.
Fig 3.
General reaction to obtain chiral cyclohexadienals.
Table 1.
Experimental optimization of synthesis of chiral cyclohexadienals (4a, 4b) from citral (1) and α,β-unsaturated aldehyde 2.
Fig 4.
Synthesis of bicycle 12 from cyclohexadienal 4a.
Reagents: a) NaH2PO4.H2O (2.2 equiv.), NaClO2 (5%, 2.2 equiv.), 2-methyl-2-butene, tBuOH, r.t., 2h, 99%; b) p-TsOH, MeOH, r.t., 30%.
Fig 5.
Synthesis of different chiral cyclohexadienals aromatic and non-aromatic compounds.
Table 2.
Synthesis of chiral cyclohexadienals (20a-26) from other β-disubstituted-α,β-unsaturated aldehydes (13–19)a.
Fig 6.
X-ray crystal structure of 24a.
Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are shown as spheres of arbitrary radius (S3 File).
Table 3.
The area of regions UVA (315–400 nm) and UVB (280–315 nm) and molar extinction coefficient of some cyclohexadienals (4a, 20b, 21b, 22b, 23a, 23b) dissolved in iPrOH.
Fig 7.
UV-Vis absorbance spectra at different λ of 4a, 20b, 21b, 22b, 23a, 23b.
Amplification of the 200–450 nm region and the delimited UVA and UVB regions (ISO-21348).