1.1- Nitroethylene derivatives aldehydes

The racemic polyzonimine (19) is prepared as shown in Scheme 33. The expoxide (314) is rearranged to the aldehyde (315) by refluxing with LiBr-HMPA in benzene. Morpholine enamine (316) derived from 315 is condensed with nitroethylene, generated in situ from 2-acetoxynitroethane, to afford the nitroaldehyde (317). Ethylene acetalization, reduction over Raney nickel, and subsequent deacetalization give ( )-polyzonimine (19) in 22% overall yield from the epoxide (314) 113). 

Nitrovinylation of aldehydes and ketones.1 Aldehydes and ketones containing a methylene group at the a-position react with l-(dimethylamino)-2-nitroethylene (2) to give 2-flri-nitroethylidene derivatives (3). Ketones with two active methylene groups can react with 1 or with 2 eq. of (2). Treatment of derivatives of 4-ac/ -nitrocrotonaldehyde with nitroacetaldehyde produces the corresponding derivative of 1,3-dinitrobenzene. 

Most reactions of tertiary enamines have been performed with (5)-prdine derivatives. The first examples of 1,4-additions to a, 3-unsaturated aldehydes and ketones exhibited mediocre selectivities (ee < 60%) [173]. However, the reactions of two enamines of cyclohexanone, 7.109 and 7.110, with methylacrylate lead to the expected adducts with a high enantiomeric excess but a poor chemical yield [173] (Figure 7.72). The addition of an (.S)-prolinol-derived enamine to an a-tri-methylsilyl-a,P-unsaturated ketone also gives useful selectivity [162]. Seebach and coworkers reacted enamine 7.110 (R = Me) with a,P-unsaturated gem-diesters or 2-aryl-l-nitroethylenes and obtained the expected 1,4-adducts with a high selectivity [162] (Figure 7.72). Martens and Lubben [294] proposed the use of enamine 7.111 for similar purposes, but the selectivity was not as high as with 7.110 (R = Me) (Figure 7.72). 

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