To a solution of endo-NRB (1.0 g, 2.0 mmol) in DMF (6 ml) was added NaH (160 mg, 4.0 mmol, 60% w/w in oil), and the resulting mixture stirred at room temperature for 15 min, then at 80°C for a further 15 min A solution of 4-[(tert-butoxycarbonyl)amino]butyl methanesulfonate (Cao et al., 2016) (0.59 g, 2.2 mmol) in DMF (2 ml) was added, and the resulting mixture stirred at 100°C for 4 h. The reaction mixture was then allowed to cool, diluted with water and extracted with EtOAc. The separated organic phase was washed with brine, dried over anhydrous Na2SO4, filtered and the solvent removed in vacuo. Purification by column chromatography (hexane/EtOAc, 1:1) afforded N-4′-[(tert-butoxycarbonyl)amino] butyl-5-(α-hydroxy-α-2-pyridylbenzyl)-7-(α-2-pyridylbenzylidene)-5-norbornene-2,3-dicarboximide as an endo-exclusive mixture of four stereoisomers (white solid; 1.09 g, 1.60 mmol, 80%). 1H NMR (400 MHz, CDCl3) δ 1.30-1.68 (13H, m, C(CH3)3 and NCH2CH2CH2CH2NH), 3.05-3.20 (2H, m, NCH2CH2CH2CH2NH), 3.24-3.70 (4.2H, m, H-2, H-3, W/H-4 and NCH2CH2CH2CH2NH), 3.85–3.90 (0.4H, m, U/H-1 and V/H-1), 3.91-3.95 (0.4H, m, Y/H-4), 4.10–4.13 (0.1H, m, U/H-4), 4.26–4.30 (0.3H, m, V/H-4), 4.42–4.48 (0.6H, m, W/H-1 and Y/H-1), 5.55–5.67 (1.7H, m, OH and V/H-6 and Y/H-6), 6.02–6.04 (0.1H, m, U/H-6), 6.05–6.07 (0.2H, m, W/H-6), 6.70–7.62 (16H, m, ArH), 8.66–8.39 (2H, m, αPyr). To a solution of N-4′-[(tert-butoxycarbonyl)amino]butyl-5-(α-hydroxy-α-2-pyridylbenzyl)-7-(α-2-pyridylbenzylidene)-5-norbornene-2,3-dicarboximide (0.34 g, 0.5 mmol) in DCM (10 ml) was added TFA (2 ml), and the mixture stirred at room temperature for 3 h. The solvent was then removed in vacuo to afford N-4′-aminobutyl-5-(α-hydroxy-α-2-pyridylbenzyl)-7-(α-2-pyridylbenzylidene)-5-norbornene-2,3-dicarboximide trifluoroacetate as an endo-exclusive mixture of four stereoisomers [colorless gum (quant.)], which was used without further purification. To a solution of N-4′-aminobutyl-5-(α-hydroxy-α-2-pyridylbenzyl)-7-(α-2-pyridylbenzylidene)-5-norbornene-2,3-dicarboximide trifluoroacetate (100 mg, 0.14 mmol) in DCM (4 ml) was added DIPEA (73 μl, 0.42 mmol), and the mixture stirred at room temperature for 30 min. NBD-Cl (4-chloro-7-nitrobenzofurazan) (28 mg, 0.14 mmol) was added and the resulting mixture stirred at room temperature for a further 6 h. The mixture was then diluted with DCM, washed with saturated aq. NaHCO3, then brine, and the separated organic phase dried over anhydrous Na2SO4, filtered and the solvent removed in vacuo. Purification by column chromatography (DCM) afforded 5-(α-hydroxy-α-2-pyridylbenzyl)-N-4′-[(7″-nitro-2″,1″,3″-benzoxadiazol-4”-yl)amino]butyl-7- (α-2- pyridylbenzylidene)-5-norbornene-2,3-dicarboximide (NRB-AF12) as an endo-exclusive mixture of four stereoisomers (orange solid; 50 mg, 0.067 mmol, 48%). M.p. 108-112°C; 1H NMR (400 MHz, CDCl3/MeOD 50:1 v/v) δ 1.61–1.84 (4H, m, NCH2CH2CH2CH2N), 3.34–3.73 (6.2H, m, NCH2CH2CH2CH2N, H-2, H-3 and W/H-4), 3.89–3.98 (0.7H, m, V/H-1 and Y/H-4), 3.99–4.02 (0.1H, m, U/H-1), 4.18–4.24 (0.4H, m, V/H-4 and U/H-4), 4.28–4.38 (0.6H, m, Y/H-1 and W/H-1), 5.56–5.62 (0.7H, m, V/H-6 and Y/H-6), 6.07–6.09 (0.1H, m, U/H-6), 6.12–6.14 (0.2H, m, W/H-6), 6.14–6.24 (1H, m, H-5″), 6.79–7.75 (16H, m, ArH), 8.37–8.65 (3H, m, αPyr and H-6″).