P-nitroalcohol derivatives

In the DCR protocol using lipase as selector, selective P-nitroalcohol acylation was subsequently performed, yielding the corresponding acetylated products. Two products from the DCL were resolved by the process, where the major product proved to be the ester produced from 3-nitrobenzaldehyde and 2-nitropropane. The nitroaldol-lipase DCR process could, however, not only ampHfy specific P-nitroalcohol derivatives, but also lead to their asymmetric discrimination. The enantioselectivity of the process proved very high, resulting in high enantiomeric purity of the products. The (R)-enantiomer of the ester was thus resolved to 99% enantiomeric excess. 

The Henry reaction is a base-catalyzed C-C bond-forming reaction between nitroalkanes and aldehydes or ketones. It is similar to the aldol addition, and is also referred to as the nitroaldol reaction. Since its discovery in 1895 [1] the Henry reaction has become one of the most useful reactions for the formation of C-C bonds, and most particularly for the synthesis of P-nitroalcohol derivatives [2]. The general features of this reaction are (i) the potential offered by the nitro and hydroxyl groups on the products for transformation into other compound families such as P-amino alcohols, P-amino acids, or nitroalkenes (ii) only a catalytic amount of base is required (iii) up to two contiguous stereogenic centers may be created in a single step concomitantly to the C-C bond formation. Several recent reviews with a focus on the asymmetric Henry reaction and its applications have appeared [3j. 

Pitchumani and coworkers have found that per-6-amino-P-cyclodextrin (per-6-ABCD) catalyzes efficiently the Henry reaction in aqueous media [12], Thus, a series of aldehydes react with nitromethane or nitroethane to afford the corresponding P-nitroalcohol derivatives in excellent yields (88-99%) and selectivities (dr > 15 1 and 79-99% ee) when using aromatic aldehydes, whereas aliphatic aldehydes afforded variable results (72-88% yield, 52-79% ee). 

A very attractive method for the preparation of nitroalkenes, which is based on the reaction with NO, has been reported. Treatment of alkenes at ambient pressure of nitrogen monoxide (NO) at room temperature gives the corresponding nitroalkenes in fairly good yields along with P-nitroalcohols in a ratio of about 8 to 2. The nitroalcohol by-products are converted into the desired nitroalkenes by dehydration with acidic alumina in high total yield. This simple and convenient nitration procedure is applied successfully to the preparation of nitroalkenes derived from various terminal alkenes or styrenes (Eq. 2.27).53 This process is modified by the use of HY-zeolites instead of alumina. The lack of corrosiveness and the ability to regenerate and reuse the catalyst make this an attractive system (Eq. 2.28).54... 

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