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1
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0002058336
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Cyclopropenes and methylenecylopropanes as multifunctional reagents in transition metal catalyzed reactions
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Reviews on [3 + 2] cycloadditions for five-membered carbocycle formation:
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(1987)
Top. Curr. Chem.
, vol.135
, pp. 77
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Binger1
Buch2
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5
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0001796781
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Donor-acceptor-substituted cyclopropanes: Versatile building blocks in organic synthesis
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Related studies on the synthetic use of vicinally donor-acceptor substituted cyclopropanes via Lewis acid mediated retro-aldol type ring opening:, A review
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(1988)
Top. Curr. Chem.
, vol.144
, pp. 73
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Reissig1
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9
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0001862125
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Highly diastereoselective synthesis of cis-3,4-disubstituted .GAMMA.-lactams by the reaction of ethyl 2,2-dialkoxycyclopropanecarboxylates with N-tosyl aldimines.
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(1990)
Chemistry Letters
, pp. 905
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Saigo1
Shimada2
Hasegawa3
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13
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0010408022
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Preparation of 1, The stereochemistry of 1 seems not to be essential for this reaction; see ref 2.
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(1982)
J. Org. Chem.
, vol.47
, pp. 748
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Trost1
Ornstein2
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14
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84918759339
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Enol trimethylsilyl ethers fail to react with 1 under the similar reaction conditions presumably due to rapid silicon-aluminum exchange. The aluminum enolate seems not to participate in the cycloaddition.
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15
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84918740419
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2, 17 (one of the epimers separated by silica gel column chromatography), and 21 derived from 19 (desulfurization with Raney-Ni). The diagnostic NOEs and coupling constant for the structure determination are shown below. For 17, cis relationship of the ring system was presumed on the basis of the general cis preference for such a five-membered ring annulation on a five-membered ring. The stereochemistry of the other major products was assigned on the analogy of these cases.
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16
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84918720099
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The cis relationship between the siloxy group and the vicinal methyl group in the major isomer suggests that the silyl oxonium ion is transoid to the forming five-membered ring in the transition state (cf. A) because the alternative cisoid silyl oxonium ion would strongly disfavor the vicinal cis methyl group. The β-orientation of the methyl group should be favored to prevent a 1,3-diaxial type repulsion with the enolate moiety.
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17
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84918721902
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The sharp contrast between entries 5 and 6 is striking. Though the stereochemistry of the products obtained in entry 5 has not been determined, the aldol moiety should be highly controlled as trans by analogy of other cases. Therefore, it is likely that trans fused isomers formed to some extent. The methyl group in 13 is supposed to play the following two roles for the high cis selectivity during the five-membered ring cyclization; 1, it sterically disfavors the trans cyclization, 2) it cancels the equatorial preference of the enolate side chain on the six-membered ring in the transition state in entry 5 (the enolate side chain must be necessarily equatorial for trans cyclization).
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