Et a ) L-Amino-acid ester, cat. TsOH, MeCN, 30-80". b ) HgCI,, MeCN/H,O 3:1, r.t. Pd(dba), (3 mol-Yo) and PPh, (12 mol-%) at 25-30° for 8-10 h gave the mono-ally1 derivatives 4a-e, 5a, b, and 6a, b in yields of 40-80%0 (Scheme 3 ) , the diastereoisomeric excess (de) being 1 H 5 %, as reported earlier [lo]. Subsequent allylation under essentially the same conditions gave the C I , ~ -disubstituted products 7a-e, 8a, b, and 9a, b in 40-75 YO yield. The products were fully characterised by 'H-and I3C-NMR spectroscopy. The signals in the I3C-NMR spectra were assigned to the individual C-atoms by their chemical-shift values and by the appropriate use of a DEPT experiment. The quaternary C-atom in all these compounds gave a characteristic signal at ca. 96 ppm. Interestingly, for the L-proline derivatives 7a+, only one set of peaks was observed in both 'Hand 13C-NMR spectra, indicating the absence of cisltrans rotamers [9-111, whereas the unsubstituted and monosubstituted precursors 3a, b and 4a+, respectively, showed two sets. In analogy with the earlier studies [l I] [12] of N-acylprolines bearing a quaternary C-atom, the trans-amide configuration was assigned to 7a-e.
Among the many methods known for the conversion of a NO, group to a NH, group, the most widely used one is catalytic hydrogenation using noble metals like Pd/C or PtO,.
Earlier, we were successful in converting ethyl N-(nitroacety1)-L-prolinate (3b) to the corresponding cyclic dipeptide by catalytic hydrogenation (20% Pd/C (w/w), EtOH, 50 psi) in a Purr apparatus . However, the tertiary NO, group in 7a and 7b was resistant to reduction under these conditions, the products being the nitroacylamino acids 10a, b obtained by hydrogenolysis of the benzyl ester and hydrogenation of the side-chain double bonds (Scheme 4 ; IR: NO, at 1550s and 1390m cm-'; I3C-NMR: quaternary C-atom adjacent to the NO, group at ca. 96 ppm).
Very recently, Vettiger and Seebach have reported the reductive acetylation of aliphatic nitro compounds to the corresponding acetylamino derivatives using Zn/ AcOH/Ac, O [14]. This procedure turned out to be successful with our substrates as well. E.g., stirring a suspension of 7a in AcOH/Ac,O with Zn dust at 40-60" for 6-12 h gave the N-acetyldipeptide benzyl ester lla in 75 % yield (I3C-NMR: quaternary C-atom at 67.72