Table 1 Screening the reaction conditions

From: Catalytic enantioselective nitrone cycloadditions enabling collective syntheses of indole alkaloids

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Entry

Cat.

Solvent

Additive

Dr

Yield (%)a

Ee (%)b

1

4a

DCM

–

5:1

37

50

2

4a

CHCl3

–

7:1

42

73

3

4a

DCE

–

4:1

47

9

4

4a

Toluene

–

6:1

80

5

5

4a

Et2O

–

1:1

21

9

6

4a

THF

–

1:1

30

15

7

4a

CHCl3

3 Å MS

8:1

42

81

8

4a

CHCl3

4 Å MS

6:1

39

77

9

4a

CHCl3

5 Å MS

6:1

35

75

10

4b

CHCl3

3 Å MS

8:1

71

81

11

4c

CHCl3

3 Å MS

3:1

52

29

12

4d

CHCl3

3 Å MS

2:1

57

70

13

4e

CHCl3

3 Å MS

2:1

15

25

14

4f

CHCl3

3 Å MS

2:1

54

66

15c

4b

CHCl3

3 Å MS

10:1

81

94

16d

4b

CHCl3

3 Å MS

>19:1

85

98

  1. Reaction conditions: 1a (0.20 mmol), 2a (0.40 mmol), and catalyst 4 (0.02 mmol) in solvent (2.0 mL) at −20 °C.
  2. aIsolated yield, dr were determined via 1H NMR of the crude products.
  3. bThe ee values were determined by chiral HPLC analysis.
  4. cThe reaction was conducted at −40 °C.
  5. dThe reaction was conducted at −60 °C.
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