Asymmetric synthesis of hexapropionate synthons by sequential enantiotopic group selective enolization of meso diketones

Organic Letters

Volumn 8, Issue 12, 2006, Pages 2631-2634

  Ward, Dale E ^a   Gillis, H Martin ^a   Akinnusi, Olukayode T ^a   Rasheed, M Abdul ^a   Saravanan, K ^a   Sasmal, Pradip K ^a  

^a UNIVERSITY OF SASKATCHEWAN   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 33745564992     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol060802k     Document Type: Article

References (49)

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26. In additon to meso-12 (50%), this reaction gave a chiral bisaldol diastereomer (8%) and the known (ref 4e) monaldol adduct (10%). Similar aldol reactions of 11 with the Li enolate of 2 or with 7 (in the presence of Lewis acids) gave only the monoaldol adduct. The monoaldol adduct exists as stable cyclic hemiacetal (from cyclization of the δ-hydroxyaldehyde) and is resistant to a second aldol reaction (for related examples, see: De Brabander, J.; Oppolzer, W. Tetrahedron 1997, 53, 9169-9202). The formation of bisaldol adducts from 11 using the boron enolate of 2 presumably results because the initially formed aldol borinate adduct is sufficiently stable to undergo a second aldol in preference to cyclization.
27. Review: O'Brien, P. J. Chem. Soc., Perkin Trans. 1 1998, 1439-1457.
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29. Reversibility facilitates recycling which improves both the efficiency and efficacy of these processes and is especially important when enantioselectivity is modest. For a discussion, see ref 7.
30. Examples of enantioselective deprotonation where enantiotopic hydrogens are activated by different (but not independent) carbonyl groups are known. Imides: (a) Adams, D. J.; Simpkins, N. S. Chem. Commun. 1998, 1605-1606.
31. (b) Adams, D. J.; Blake, A. J.; Cooke, P. A.; Gill, C. D.; Simpkins, N. S. Tetrahedron 2002, 58, 4603-4615.
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34. cis-Piperidine-2,6-dicarboxylic acid diesters: (e) Goldspink, N. J.; Simpkins, N. S.; Beckmann, M. Synlett 1999, 1292-1294.
35. (f) Clive, D. L. J.; Wang, J.; Yu, M. Tetrahedron Lett. 2005, 46, 2853-2855. See ref 13b for an example of a meso-bicyclo[3.1.0]hexane-2,4-dione and a meso-1,2-cyclopropane-1,2-dicarboxylic acid diester.
36. Kinetic resolution is equivalent to an enantiotopic group selective reaction; i.e., groups on enantiomeric substrates are enantiotopic by external comparison. See: Mislow, K.; Raban, M. Top. Stereochem. 1969, 1, 1-38.
37. (a) Kim, H. D.; Kawasaki, H.; Nakajima, M.; Koga, K. Tetrahedron Lett. 1989, 30, 6537-6540.
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40. 2, -78 °C). Enolborination of 10a was possible with chlorodicyclohexylborane and sparteine but the enantioselectivity was very low (<3:1).
41. A THF solution of 14 (0.5-2 equiv) at -78 °C was rapidly cannulated to a THF solution of 10a (0.2-0.3 mmol) and TMSCI (10 equiv) at -100 °C. Reactions conducted at -78 °C were very capricious and much less enantioselective. Slow addition of 14 was less enantioselective. External quench (i.e., addition of TMSCI after addition of 14) gave considerably lower conversions.
42. The yield and ee of 15a are dependent on the reaction enantioselectivity and the conversion (see ref 7). Conversion of 10a was modulated by varying the amount of 14 added. The conversion with respect to amount of 14 added varied from 0.5 to 0.9 and was scale dependent.
43. Generated in situ from reaction of the corresponding amine hydrochloride with 2 equiv of BuLi. (a) Hall, P. L.; Gilchrist, J. H.; Collum, D. B. J. Am. Chem. Soc. 1991, 113, 9571-9574.
44. (b) Mair, F. S.; Clegg, W.; O'Neil, P. A. J. Am. Chem. Soc. 1993, 115, 3388-3389.
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46. The product distributions in entries 1 and 4 are similar to expecations for a nonselective reaction (e.g., using LDA as base). By contrast, a reaction with 9:1 group selectvity would not be expected to give 3% of the bis-product until ca. 50% conversion (ref 7).
47. 1H NMR of the crude reaction mixture after workup. Conversion is 1 minus the mole fraction of 10a.
48. See the Supporting Information for details.
49. s symmetrical ketones. If the enantioselectivity in the coordination step is lower than that in the deprotonation step (as expected) then dissociation of the complex must be much faster than deprotonation to prevent attenuation of the selectivity.