By Benjamin List
As nucleophiles, basic alkenes tend to be so unreactive that purely hugely lively electrophiles, resembling carbocations, peroxides, and halogens will react with them. For the new release of carbon-carbon bonds, milder tools will frequently be required. thankfully, it really is attainable to extend the reactivity of alkene-type p-nucleophiles through introducing electron-donating substituents. Substitution of 1 H with an OH or OR offers an enol or a vinyl ether, that are already far better nucleophiles. utilizing nitrogen rather than oxygen, one obtains even larger nucleophiles, enamines. Enamines are one of the so much reactive impartial carbon nucleophiles, displaying charges which are even equivalent to a few charged nucleophiles, reminiscent of enolates [1, 2]. such a lot enamines, regrettably, are delicate to hydrolysis. The mum or dad enamine, N, N-dimethylvinylamine, has in truth been ready , yet seems to be uns- ble. Enamines of cyclic ketones and lots of aldehydes can simply be remoted, notwithstanding [4-7]. The instability of enamines may well initially seem to curb the software of enamines as nucleophiles, yet truly this estate will be considered as an additional advantage: enamines may be with no trouble and swiftly generated catalytically through the use of an appropriate amine and a carbonyl compound. The condensation of aldehydes or ketones with amines at the beginning gives an imine or iminium ion, which then swiftly loses a proton to have enough money the corresponding enamine (Scheme 1).
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Extra info for Asymmetric Organocatalysis
L. Moore, T. Rovis The observed diastereoselectivity of the protonation event may be explained by Model C (Scheme 13). In Model C, an intermolecular proton transfer would yield the minor diastereomer. Alternatively, the proton transfer may be intramolecular and occur from the more sterically hindered face of the enolate, providing D. Bn H R EtO N O O N N Ph Bn EtO O H R H O O H O c N N Ph N d Scheme 13 Intramolecular protonation The mechanistic hypothesis was tested with experiments involving a pair of substrates differing only in olefin geometry about the a,b-unsaturated ester.
The reaction scope is quite general and tolerates both aromatic and aliphatic aldehydes (Table 9). Carbene Catalysts 97 O H O 20 mol% catalyst Me CO2Et Me PhMe, 23 oC, 24h CO2Et O H O 93 94 N Bn BF4 N N Ph Bn 76a 85-88% 90% ee 3:1 to 15:1 dr (8) N N Ph N 95 88% 90% ee 15:1 dr Table 9 Highly diastereoselective intramolecular Stetter reaction O H R O Bn EWG a O H CO2Et 95 O 20 mol% 98 Yield ee (%) (%) dr Entry 97 Product N N 92 O 97 Yield ee (%) (%) dr d O Et b O H CO2Et 80 84 20:1 5 e 95 c O 95 18:1 O O H H CO2Et O 94 99 50:1 80 88 15:1 H O O 80 O Bn 3 O H H O 2 EWG R Entry 97 Product 35:1 4 H CF3 PhMe, 23 oC, 24h 96 1 N 83 13:1 6 f O O H H N Ph O 98 J.
L. Moore, T. Rovis Nicolaou and co-workers recently published a formal synthesis of (±)-platensimycin utilizing Stetter methodology . Aldehyde 162 was treated with achiral N-pentafluorophenyl pre-catalyst 164 and readily underwent cyclization to yield 163 as a single diastereomer (Scheme 26). After an additional seven steps late stage intermediate 165 was formed to complete the formal synthesis. O F N N N F BF4 F F 100 mol% 164 H Br O O F O Br Et3N, CH2Cl2, 45 C 162 163 64% OH O O O 7 steps O H OH OH Me 165 Me O N H O (±)-platensimycin Me Scheme 26 Nicolaou et al.
Asymmetric Organocatalysis by Benjamin List