= Solution
The first <reagent> supplies a vinyl <nucleophile>, $\mathrm{CH_2{=}CH{-}}$, rather than an <allyl group>. \b[Two successive <conjugate additions> install vinyl at C2 and a 2-formylethyl chain at C1.] Number the original nitro-bearing <alkene> <carbon> C1. Vinylmagnesium <bromide> attacks the beta <carbon> C2 of the <nitroalkene>, shifting the original C1=C2 <electrons> into a <magnesium> <nitronate>. The new C2–vinyl bond remains intact during the second stage; its terminal double bond is not the <electrophilic> site in that stage.
The <carbon> end of the <nitronate> at C1 then attacks the terminal beta <carbon> of <acrolein>. Move the <acrolein> C=C <electrons> towards its <carbonyl> and the C=O <electrons> onto <oxygen>. This forms the C1–CH2CH2CHO connectivity through an <aldehyde> <enolate>; <protonation> by <methanol> and <tautomerization> restore the <aldehyde>. The <nitronate> returns to a neutral <nitro group> as it makes the second C–C bond. Thus the chain is attached through the <carbon> formerly beta to the <acrolein> <carbonyl>, rather than by a direct addition to its <aldehyde> <carbon>.
After the first addition, C2 bears the <vinyl group> while the <nitronate> <carbon> C1 remains approximately planar. Approach of the second <electrophile> to the less crowded face, opposite the adjacent vinyl <substituent>, gives the new formylethyl chain trans to vinyl. Consequently \b[nitro and vinyl are cis], as depicted: at C1 the chain and <nitro group> occupy opposite faces. This is a <substrate (chemistry)>-controlled <diastereoselective> <tandem Michael addition>. The <reagents> and starting material contain no <chiral> source, so the depicted <product (chemistry)> represents a racemic pair rather than an enantioselective synthesis. Steric facial selection explains the relative configuration without requiring a uniquely established <magnesium> aggregate or transition-state <conformation>.
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