Table 1. Diels–Alder reaction catalyzed by human telomeric G-quadruplex DNA metalloenzyme.

Entry	DNA sequence (5′ → 3′)	M+	Conv. a (%)	endo/exo b	ee b (%, endo)
1 c	—	—	42	71/29	0
2	HT21: (GGGTTA)3GGG	—	92	97/3	90
Conformation stabilized by monovalent cations
3	HT21: (GGGTTA)3GGG	Na+	54	89/11	54
4		K+	48	88/12	49
5		NH4+	99	99/1	94
Flanking base variants
6	A-HT21: A(GGGTTA)3GGG	NH4+	97	98/2	90
7	TA-HT21: TA(GGGTTA)3GGG	NH4+	96	98/2	94
8	TTA-HT21: TTA(GGGTTA)3GGG	NH4+	98	98/2	93
9	HT21-T: (GGGTTA)3GGGT	NH4+	97	97/3	88
10	HT21-TT: (GGGTTA)3GGGTT	NH4+	95	96/4	81
11	HT21-TTA: (GGGTTA)3GGGTTA	NH4+	95	96/4	73

^aDetermined for the crude product by HPLC analysis on a chiral stationary phase (ESI note 5), reproducible within ±2%.

^bDetermined by chiral-phase HPLC. Reproducible within ±2%.

^cCu(L1)(NO₃)₂ alone as catalyst. Reaction conditions: 1a (1 mM), 2 (10 μL, 260 mM), human telomeric G-quadruplex DNA (50 μM), Cu(L1)(NO₃)₂ (100 μM), NaCl (50 mM) or KCl (150 mM) or NH₄Cl (30 mM), MOPS buffer (0.5 mL, 20 mM, pH 6.5), 4 °C, 24 h.