DUALISTIC REACTIVITY OF LITHIUM AND ZINC DIENOLATES WITH IMINES - EFFECTS OF COUNTERION, TEMPERATURE, AND SUBSTITUENTS ON ALPHA-COUPLNG ANDGAMMA-COUPLING

Lithium and zinc dienolates of 3-butenoic, 3-methyl-2-butenoic, 3-pent enoic, and 4-phenyl-3-butenoic methyl esters are reacted with imines R (1)C(H)=NR(2)[3a: R(1) = 2-pyridyl, R(2) = Ph; 3b: R(1) = Me(3)-SiC=C; R(2) = Ph; 3c: R(1) = 2-pyridyl, R(2) = (R)-PhCK(CH3); 3d: R(1) = Me( 3)SiC=C; R(2) = (R)-PhCH(CH3)]. Depending on metal counterion, tempera ture, and substituents, the C-C coupling occurs either at the 2-positi on (alpha-coupling) or at the C-position (gamma-coupling) of the dieno late, giving beta-amino esters and alpha,beta-unsaturated esters, resp ectively. The alpha-coupling occurs at -78 degrees C and is reversible , whereas gamma-coupling products are formed upon warming to -20 degre es C. The C-C coupled products may undergo irreversible ring closure t o beta-lactams (from beta-amino esters) or 5,6-dihydropyridin-2-ones ( from Z)-alpha,beta-unsaturated esters). Starting from enantiomerically pure imine 3c, high (71-92%) asymmetric inductions were realized in b oth beta-lactams and 5,6-dihydropyridin-2-ones; gamma-coupling of enol ate 2a and chiral imine 3d occurred with low diastereoselectivity (15% ). Four factors favor the formation of beta-lactams: (i) the use of zi nc dienolates, (ii) 4-substitution of the dienolates, (iii) nonaromati c imine N-substituents, and (iv) a low reaction temperature (-78 degre es C). The product formation is discussed in terms of six-membered cyc lic transition states, involving either zinc enolates (alpha-coupling) or isomeric allylzinc species (gamma-coupling).