Silica gel 60

Isotopically unmodified reference compounds bis(methylsulfanyl)methane
and furan-2(5H)-one were purchased from Merck (Darmstadt,
Germany). (²H₈)Naphthalene was also from Merck.
(²H₈)Bis(methylsulfanyl)methane was synthesized
as detailed in the literature.^{29 (link)} (²H₂)Furan-2(5H)-one was synthesized
according to a procedure published previously^{30 (link)} but with some modifications. In brief, 3a,4,7,7a-tetrahydro-4,7-epoxy-2-benzofuran-1,3-dione
(abcr, Karlsruhe, Germany) was deuterated with sodium borodeuteride
(Cambridge Isotope Laboratories, Tewksbury, MA, USA) under an argon
atmosphere. Acidic workup led to the intermediate (3,3-²H₂)-3a,4,7,7a-tetrahydro-4,7-epoxy-2-benzofuran-1(3H)-one, which was extracted by dichloromethane instead of
chloroform and then purified by chromatography (3 cm column diameter)
on silica gel 60 (0.040–0.063 mm; VWR, Darmstadt, Germany;
60 g). After being washed with n-hexane/ethyl acetate
(50/50, v/v; 150 mL), the compound was eluted with n-hexane/ethyl acetate (25/75, v/v; 150 mL) and ethyl acetate (200
mL). The solvents were removed in vacuo, and the purified (3,3-²H₂)-3a,4,7,7a-tetrahydro-4,7-epoxy-2-benzofuran-1(3H)-one was heated to 150 °C at 15 mbar to distill off
the target product (5,5-²H₂)furan-2(5H)-one (96% purity by GC–flame ionization detector)
obtained in a retro-Diels–Alder reaction.

Unless otherwise stated, starting materials and reagents were obtained from Sigma Aldrich, Munich, Germany and were used without further purification. Melting points were determined on a Büchi 530 apparatus and are uncorrected. All ¹H-NMR and ¹³C-NMR spectra were recorded in CDCl₃ (if not otherwise indicated) solution at 399.94 MHz and 75.42 MHz, respectively, with a Varian VXR 5000 spectrometer Varian, (Palo Alto, CA, USA). Chemical shifts are given in ppm (δ); multiplicities are indicated by s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) or dd (doublet of doublets). Elemental analyses were performed using an elemental analyzer model 240 C (Perkin-Elmer, Walthan, MA, USA). Acetone was freshly distilled from CaCl₂. Flash chromatography was carried out with silica gel 60 (230–400 mesh) (VWR, Radnor, AF, USA) eluting with an appropriate solution in the stated v:v proportions. All reactions were monitored by analytical thin-layer chromatography (TLC) with 0.25 mm thick silica gel plates (60 F 254) (Sigma Aldrich, Munich, Germany). Melting points were determined on a 530 apparatus (Büchi, Flawil, Switzerland) and are uncorrected. The purity of all new compounds was judged to be >98% by ¹H-NMR spectral determination.

Ethyl acetimidate hydrochloride and 4-(dibutylamino)benzaldehyde (DBA) were purchased from Sigma-Aldrich and used without further purification. The solvents and other reagents were purchased from Sigma Aldrich, Fluka and Merck and were dried and purified by standard laboratory methods. Trimethylolpropane triacrylate (TTA, Genomer 1330) and ethoxylated-(20/3)-trimethylolpropane triacrylate (ETA, Sartomer 415) were received as a gift from Rahn and Sartomer, respectively. Thin layer chromatography analysis was performed on silica gel 60 F₂₅₄ aluminium sheets from Merck. Column chromatography was performed by preparative MPLC using a Buechi Sepacore Flash System (Buechi pump module C-605, Buechi control unit C-620, Buechi UV-Photometer C-635 with peak detection set at λ = 295 nm, Buechi fraction collector C-660). Glass and polyethylene columns were used, packed with Silicagel 60 on VWR silica gel 60 (0.040–0.063 mm particle size).
The preparation and analysis of the photosensitive compounds and formulations was conducted in an orange light lab. The windows and fluorescent lamps were covered in adhesive foils or coated so that light with a wavelength < 520 nm was cut off.