*3.2. Synthesis of (3S,3ȝS)-Alloxanthin (1b) and meso-Alloxanthin (1c)*

In the same procedure [20] as preparation of (3*R*)-phosphonium salt **5a** and (3*R*,3<sup>ȝ</sup>*R*)-alloxanthin (**1a**), (3*S*)-**5b** and (3*S*,3<sup>ȝ</sup>*S*)-alloxanthin (**1b**) were prepared. Spectral data except for optical data of compounds **1b**, **3b**, **4b** and **5b** were identical with the corresponding previous reported [20] enantiomers **1a**, **3a**, **4a** and **5a**. 

(3*S*,3<sup>ȝ</sup>*S*)-Alloxanthin (**1b**): HRMS (ESI) *m/z* calcd for C40H53O2 [M + H]+ 565.4040, found 565.4038. 

Compound **3b**: [΅]D26 102.9 (*c* 1.03, MeOH); HRMS (ESI) *m/z* calcd for C11H17O [M + H]+ 165.1274, found 165.1277. 

Compound **4b**: [΅]D23 68.1 (*c* 1.00, MeOH); HRMS (ESI) *m/z* calcd for C17H31OSi [M + H]+ 279.2139, found 279.2139. 

Compound **5b**: HRMS (ESI) *m/z* calcd for C33H62OPSi [M ƺ Cl]+ 533.4302, found 533.4293. 

*meso*-Alloxanthin (**1c**) was synthesized via condensation between **5b** and (3*R*)-C25- acetylenic apocarotenal **<sup>8</sup>**, which was prepared by Wittig reaction of C10-dialdehyde **7** with **5a** as follows. 

(2*E*,4*E*,6*E*,8*E*,10*E*)-2,7,11-trimethyl-13-[(*R*)-2,6,6-trimethyl-4-

triethylsilyloxycyclohex-1-en-1-yl] 

trideca-2,4,6,8,10-pentaen-12-ynal (**8**). NaOMe (1 M in MeOH; 1.2 mL, 1.2 mmol) was added to a solution of the (3*R*)-phosphonium salt **5a** (409 mg, 0.73 mmol) and C10- dialdehyde **7** (100 mg, 

0.61 mmol) in CH2Cl2 (10 mL) at 0 °C. After being stirred at 0 °C for 15 min, the mixture was poured into saturated aq. NH4Cl and extracted with AcOEt. The extracts were washed with brine, dried over Na2SO4 and evaporated to afford a residue, which was purified by flash column chromatography (AcOEt–*n*-hexane, 1:4) to give the (3*R*)-C25-acetylenic apocarotenal **8** (165 mg, 57%) as an orange viscous oil: UV-VIS Ώmax (EtOH)/nm 420; IR Αmax (CHCl3)/cmƺ1 2170 (CǓC), 1663 (conj. CHO), 1610 and 1599 (split) (C=C), 1552 (C=C); 1H-NMR (CDCl3, 300 MHz) Έ 0.61 (6H, q, *J*  = 8 Hz, SiCH2 × 3), 0.97 (9H, t, *J* = 8 Hz, CH2CH3 × 3), 1.14 and 1.18 (each 3H, s, 1-gem-Me), 1.49 (1H, t, *J* = 12 Hz, 

2-HΆ), 1.74 (1H, ddd, *J* = 12, 3.5, 2 Hz, 2-H΅), 1.89 (3H), 1.91 (3H) and 2.03 (6H) (each s, 5-Me, 9-Me, 13-Me and 13ȝ-Me), 2.11 (1H, br dd, *J* = 17.5, 9.5 Hz, 4-HΆ), 2.30 (1H, br dd, *J* = 17.5, 5.5 Hz, 4-H΅), 3.94 (1H, m, 3-H), 6.32 (1H, br d, *J* = 12 Hz, 14-H), 6.37 (1H, d, *J* = 15 Hz, 12-H), 6.46 (1H, br d,

*J* = 11.5 Hz, 10-H), 6.66 (1H, dd, *J* = 15, 11.5 Hz, 11-H), 6.70 (1H, dd, *J* = 14.5, 11.5 Hz, 15ȝ-H), 6.96 (1H, br d, *J* = 11.5 Hz, 14ȝ-H), 7.03 (1H, dd, *J* = 14.5, 12 Hz, 15-H), 9.46 (1H, s, CHO); 13C-NMR (CDCl3, 75 MHz) Έ 4.82 (C × 3), 6.83 (C × 3), 9.59, 12.96, 18.17, 22.53, 28.61, 30.45, 36.53, 42.11, 47.04, 65.01, 90.10, 98.16, 121.15, 123.84, 126.60, 127.73, 131.75, 134.51, 137.02, 137.07, 137.47, 138.70, 141.26, 148.75, 194.45; HRMS (ESI) *m/z* calcd for C31H47O2Si (MH)+ 479.3340, found 479.3347. 

Preparation of *meso*-alloxanthin (**1c**). NaOMe (1 M in MeOH; 0.24 mL, 0.24 mmol) was added to a solution of the (3*S*)-phosphonium salt 5b (113 mg, 0.20 mmol) and (3*R*)-C25-acetylenic apocarotenal 8 (59 mg, 0.12 mmol) in CH2Cl2 (10 mL) at room temperature. After being stirred for further 15 min, the mixture was poured into saturated aq. NH4Cl and extracted with AcOEt. The extracts were washed with brine, dried over Na2SO4 and evaporated to afford a residue, which was purified by flash column chromatography (AcOEt–*n*-hexane, 1:4) to give the TES-protected condensed product. Subsequently, to a solution of this condensed product in dry THF (5 mL) were added AcOH (1 M in THF; 0.20 mL, 0.20 mmol) and then tetrabutylammonium fluoride (TBAF) (1 M in THF, 0.40 mL, 0.40 mmol). After being  stirred at room temperature for 2 h, the mixture was concentrated to give a residue, which was purified by flash column chromatography (AcOEt– *n*-hexane–MeOH, 50:45:5) to provide *meso*-alloxanthin (**1c**) (70 mg, quant.) as red solids. Its spectral data were identical with those of (3 *R*,3<sup>ȝ</sup>*R*)-alloxanthin (**1a**) [20]. HRMS (ESI) *m/z* calcd for C40 H53 O2 [M + H]+ 565.4040, found 565.4033.
