Enantioselective Addition Reactions Catalyzed by Carbohydrate-Derived Organocatalysts - Carbohydrates: Tools for Stereoselective Synthesis - page 358

Table 16.10

Enantioselective radical-chain reactions catalyzed by carbohydrate-derived thiols.

Catalyst Solvent Product Yield (%) ee (%)

18a Hexane 19a 72 50

18a Hexane/dioxane (5 : 1) 19b 93 87

18b Hexane 19a 79 40

18b Hexane/dioxane (5 : 1) 19b 96 84

18c Hexane 19a 79 3

18c Hexane/dioxane (5 : 1) 19b 92 5

18d Hexane 19a 84 76

18d Hexane/dioxane (5 : 1) 19b 90 95

18e (R 1 = R 2 = Piv) Hexane 19a 77 44

18e (R 1 = R 2 = Piv) Hexane/dioxane (5 : 1) 19b 95 87

18f (R 1 H, R 2

Hexane/dioxane (7 : 1) 19a 67 25

= = Ac)

18f (R 1 ,R 2 = phthalimide) Dioxane 19a 95 5

18g Hexane/dioxane (4 : 1) 19b 82 53 (S)

18g Hexane/dioxane (4 : 1) 19b 74 58 (S)

R 3 SiCH 2 CHR 1 R 2

XS R 3 SiH

XSH XSH

R 3 SiCH 2 CR 1 R 2 R 3 Si

R 1

R 2

Figure 16.3

Propagation cycle of thiol-catalyzed radical-chain reactions.

Non-racemic products were also obtained in the thiol-catalyzed, silane-mediated
free-radical reductive alkylation of methylene-lactones. In this reaction the adduct
20 was obtained with 54% ee using β -glucose thiol 18a as catalyst and in 53% ee

with the β -mannose thiol 18d (Scheme 16.8) [35].

Ph
Ph

Ph
Ph

5 mol% thiol

Ph 3 SiH

+ + t BuC(O)CH 2 Br t Bu

O O

O O

5 mol% dilauroyl peroxide
benzene, 80 °C

O

20
thiol 18a : 70% yield, 54% ee
thiol 18d : 74% yield, 53% ee

Scheme 16.8

Carbohydrate-derived thiol-catalyzed reductive alkylation of methylene-lactone.