Fragmentation retro-Mannich

The same workers reported the application of this cyclization-fragmentation sequence to substrates 79 and 81, which led to the formation of retro-Mannich fragmentation products 80 and 82, respectively, in good yield (Scheme 19)40. 

Winkler and coworkers have coupled the photocycloaddition and retro-Mannich fragmentation of an acyclic vinylogous amide with a subsequent Mannich closure to produce perhydroindole structures as outlined in Scheme 2445. The acyclic secondary... 

Scheme 27)48. Irradiation of 111 led to the formation of 112, which was formed by a sequence involving photoaddition and retro-Mannich fragmentation to the intermediate ketoimine, followed by tautomerization of the imine to enamine, transannular closure and dehydration. 

Intramolecular [ 2 + 2] photoadditions often proceed very efficiently with good stereochemical and regiochemical control. The vinylogous amide (96) is converted in this way into the adduct (97) with a high level of asymmetric induction retro-Mannich fragmentation of this adduct affords the imine (98), a useful intermediate in a synthesis of vindorosine. Intramolecular [ 2 + 2] photoadditions have also been observed in the N -( -alkenyl)-pyrimidines (99) and afford the diazatricyclodiones (100) as the sole photoproducts. 

A [2+2] cycloaddition/retro-Mannich fragmentation/Mannich closure cascade, conceptualised in Scheme 2, has been applied to the vinylogous amide (113) as a key step in the synthesis of the pentacyclic ring system found in the antileukaemia marine alkaloid manzamine... 

An intramolecular vinylogous amide photocycloaddition, retro-Mannich fragmentation and final Mannich cyclization cascade provided a useful method for the synthesis of the complex ABCE ring system [91], The preparation of the requisite photosubstrate is outlined in Scheme 28. After cluomatographic separation, the cis alcohol was irradiated and led to the formation of aminals in a 2 1 ratio. Equilibration studies of the Swem oxidation products revealed that exposure of the minor diketone to sodium methoxide in methanol effected conversion to the manzamine-unlike... 

Retro-Mannich fragmentation of the cycloaddition product 40 (not isolated) resulted in the formation of the imine 41, which upon a new Mannich closure reaction produced the cyclohexanone derivative 42. Further manipulation of 42 resulted in the formation of 43, which had previously been reported as a precursor for vindorosine (44) [27, 28]. 

White JD, Ihle DC (2006) Tandem photocycloaddition-retro-mannich fragmentation of enaminones. A route to spiropyrrolines and the tetracyclic core of koumine. Org Lett 8 1081-1084... 

Keywords C-C bond activatiOTi Chelation-assistant strategy Cooperative catalysis Ketone Organic catalyst Retro-Mannich fragmentation Transition metal... 

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