H-, I3C-, and "0-NMR spectra for the 2-substituted enaminones MeC(O)C(Me)=CHNH(t-Bu) (l), EtC(O)C(Me)=CHNH(t-Bu) (2), PhC(O)C(Me)=CHNH(t-Bu) (3). and MeC(O)C(Me)=CHNH(f-Bu) (4) are reported. These data show that 3 exists mainly in the (E)-form, 4 in (Z)-form, and 1 and 2 as mixtures of both forms. Polar solvents favour the (E)-form. The (Z)-and (E)-forms exist in the 1,2-syn,3,N-anti and 1,2-anti,3,Nanti conformations A and B, respectively. The structures of the (E)and (Z)-form are confirmed by X-ray crystal-structure determinations of 3 and 4. The shielding of the carbonyl 0-atom in the "0-NMR spectrum by intramolecular H-bonding ranging from -28 to -41 ppm, depends on the substituents at C(l) and C(2). Crystals of 3 at 90 K are monoclinic, with a = 9.618(2) A, b = 15.792(3) A, c = 16.705(3) A, and p = 94.44(3)", and the space group is P2Jc with Z = 8 (refinement to R = 0.0701 on 3387 independent reflections). Crystals of 4 at 101 K are monoclinic, with a = 16.625(8) A, b = 8.637(6) A, c = 11.024(7)& and fi = 101.60(5)", and the space group is Cc with Z = 4 (refinement to R = 0.0595 on 2106 independent reflections). ') Part 6: [l]. The general name enaminone is used for compounds containing the O=C-C=C-N moiety.
') The descriptors syn and anti are short forms of syn-periplanar and anti-periplanar, respectively.