Scope of DA reaction between 2-furoic acids and maleimides in aqueous solution.

Entry	R1	R2	3	Conv. 1a	exo-3, %	endo-3, %	Isolatedb, %
1	H	Me	3a	98	97	1	77(92)c
2	H	H	3b	95	95	Trace	68
3	H	nPr	3c	96	93	3	72
4d	H	Ph	3d	51	51	Trace	21
5d	H	Cy	3e	18	16	2	n.d.
6d,e	H	Cy	3e	56	53	3	31
7	Me	Me	3f	93	88	5	75
8	CH2OH	Me	3g	91	72	19	51f
9d	CH2OH	Ph	3h	28	28	Trace	11
10g	CHO	Me	3i	<10	∼5	Trace	n.d.
11g	COOH	Me	3j	20	20	0	n.d.
12g,h	COOH	Me	3j	56	56	0	n.d.

^aConversion determined from the ¹H-NMR ratios of product isomers and starting material in the crude mixture.

^bIsolated yield after acidification.

^c40 mmol scale.

^dPoor dissolution of 2.

^eMethanol was used as cosolvent.

^fAfter hydrogenation on Pd/C.

^gExtensive hydrolysis of 2a to maleic acid.

^hWith 2 equiv. of NaOH.