Synthesis of (-)-cubebol by face-selective platinum-, gold-, or copper-catalyzed cycloisomerization: Evidence for chirality transfer

Angewandte Chemie - International Edition

Volumn 45, Issue 18, 2006, Pages 2901-2904

  Fehr, Charles ^a   Galindo, José ^a  

^a Analysis and Perception Department   (Switzerland)

Author keywords

Cycloisomerization; Cyclopropanes; Diastereoselectivity; Homogeneous catalysis; Natural products

Indexed keywords

CATALYSIS; COPPER COMPOUNDS; ISOMERIZATION; STEREOCHEMISTRY; SYNTHESIS (CHEMICAL);

CYCLOISOMERIZATION; CYCLOPROPANES; DIASTEREOSELECTIVITY; NATURAL PRODUCTS;

PLATINUM COMPOUNDS;

EID: 33746216354     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200504543     Document Type: Article

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28. Despite the slow addition of 15, it accumulates during the reaction. Diminishing the amount of reagents leads to a slower reaction and to the formation of by-products.
29. Synthesis of 20: a) 2,2-dimethylcyclohexanone, triethyl phosphonoacetate (1.2 equiv), NaOEt (1.1 equiv), pentane, reflux, 24 h (100 %);
30. b) lithium diisopropylamide (LDA, 1.05 equiv), THF, -25°C to RT, 30 min (80 %);
31. 2O, reflux, 45 min (93 %);
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34. The indicated absolute configuration of 23 is based on the stereoselectivity observed in the cycloisomerization of 8 a (cubebol synthesis).
35. The lower ee values of the rearrangement products when compared to the substrates reflect an imperfect stereocontrol and not a racemization, as the ee values of 21 and 22 remained constant during the reaction.
36. This mechanism has recently been proposed on the basis of a DFT computational study: see Ref. [3].
37. For the related cycloisomerization of propargylic alcohols or ethers, see E. Soriano, P. Ballesteros, J. Marco-Contelles, Organometallics 2005, 24, 3172 and Refs. [2a, 2c, 3b, 4, and 5].
38. Incidentally, this mechanism is closely related to the mechanism proposed for the Rautenstrauch rearrangement: X. Shi, D. J. Gorin, F. D. Toste, J. Am. Chem. Soc. 2005, 127, 5802; we cannot exclude that, depending on the substrate, the reaction pathways (a) or (b) compete with pathway (c).
39. -1. Calculations were performed at the DFT level (B3LYP/LACVP**), using Jaguar 5.5; Schrödinger, Inc., Portland, OR, 1991-2005. Most probably the same diastereoface selectivity is observed in the cyclization 21→22. The lower selectivity is certainly caused by the missing (Figure Presented) isopropyl group. Surprisingly, the cycloisomerization 25→26 is much less selective. This may be due to steric interactions between the methyl group and the metal. The cyclization 25→27 is highly selective. Probably the reaction conformer A (Scheme 6) in which the acetylene group is above the cyclohexene ring minimizes the steric interactions between the pivalate and the cyclohexene ring.
40. Chiral capillary column: CP-Chirasil-DEX CB (25 m x 0.25 mm) (Chrompack).