Nitroolefins, reaction with dienes

Enamines (e.g. 377) have been shown to react with conjugated nitroolefins 378 to give mainly dihydro-l,2-oxazine A -oxide derivatives 379 as products of kinetic control (sometimes a cyclobutane ring is formed in these reactions see Section II.B). The stability of these heterocycles is largely dependent on the parent enamine and the type of substituent used on the nitro olefin as has been extensively studied by Valentin and coworkers " . Usually they open into the corresponding nitroalkylated enamines 380 (equation 82), in particular in a solution of methanol or deuteriated chloro-and often an equilibrium between the two forms is established. Stable 1,2-oxazine A -oxides have been obtained in the reaction of 2-nitro-l,3-dienes with cyclic enamines . 

The first amine-catalyzed Diels-Alder reaction of a,(i-unsaturated carbonyl compounds as dienes involving in situ enamine activation with chiral amine 29 through raising the diene HOMO energy was accomplished by Barbas and coworkers in 2002 with moderate enantioselectivity (up to 38% ee) [17a]. In 2007, Cordova and coworkers described chiral amine 29 catalyzed enantioselective Diels-Alder reactions between a,P-unsaturated cyclic ketones and nitroolefins with up to 86% ee [17b]. Two years later, Xu and coworkers successfully disclosed a highly efficient enantioselective Diels-Alder reaction of cyclohexenones 31 with aromatic nitroolefins 32 using amine 30 as catalyst in seawater or brine to give products 32 with excellent enantioselectivities (Scheme 38.10) [18a]. Notably, sea water and brine... 

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