Abstract
A straightforward synthesis of the enantiomerically pure nitro derivatives 31 and epi‐32, which are particularly useful intermediates for the synthesis of highly functionalized γ‐lactams, is presented. (+)‐(R)‐3‐Hydroxy‐3‐phenylpropanoic acid (20) and its ethyl ester 25 were prepared from (+)‐L‐mandelic acid (21). Condensation of 20 with pivalaldehyde furnished the novel enantiomerically pure 1,3‐dioxan‐4‐one 17, the absolute configuration of which was established by X‐ray crystal‐structure analysis. Treating the lithium enolate of 17 with the nitro alkene 18 led, in a Michael‐type addition, to a 1 : 1 mixture of two diastereoisomeric products. The stereocontrol of the addition was limited to the novel stereogenic center next to the lactone function. When the lithium enolate of 25 was treated with 18, the same selectivity was observed but with a lower chemical yield. Very facile separation of the isomers was achieved later in the synthetic sequence, when one isomer cyclized selectively to the nitro lactone 31, while the other one was isolated as hydroxy ester epi‐32. The relative configuration of racemic epi‐32 could be established by X‐ray crystal‐structure analysis.