Table 1. Optimization of substrates and reaction conditions for the synthesis of 1A.

Entry a	DBU (equiv.)	DBAD (equiv.)	Bu3P (equiv.)	Yield of 1A b (%)
1	0.1	2	2	0
2	0.1	3.8	3.8	0
3	0.2	3.8	3.8	0
4	0.05	1	1	0
5	0.05	2	2	46 ± 8 c
6	0.05	3.8	3.8	44 d
7	0.05	7.2	7.2	0

^aReaction conditions: CO₂ was bubbled in a solution of 1 (138.6 μmol, 1.0 equiv.), DBU (0.05–0.2 equiv.) in MeCN (1 mL), r.t. for 40 min. Mitsunobu reagents (7.2–1 equiv.) in MeCN (0.5 mL) were added and the solution stirred for 10 min. A (7.2 equiv. of a 0.5 M solution in THF) was added and quenched after 30 min.

^bYield of isolated 1A calculated from compound 1.

^c N = 4.

^d N = 1.