[(q5-C,H,)Re(NO)(PPh3)]+ and analogons unsaturated chiral complexes (see element symbols along the fingertips) can form two diastereomeric adducts with prochiral alkenes, aldehydes, and ketones. The ratio of these diastereomers is a measure of the chiral recognition, symbolized by the hands, of the reaction partners. The rhenium complex mentioned is depicted above the hands (Re: light gray, P: yellow, N: light blue, 0: dark blue, Ph, Cp: dark gray). The selectivity of alkene complexation can be analyzed (and predicted) by considering the sizes of the substituents a 4 on the alkene and correlating them with the steric conditions in the chiral complexes, which can be represented with three-dimensional bar graphs.

Das Kation [Li.(-)

7] The overall yield OF 6a from Keinp's tricarboxylic acid was 44%: m.p. 195-196"C, [a]" = 89.7" (c = 1.51, CH,CI,). For 6 b (43%) m.p. 107 -109 C, IllD = -252; (c = 1.04, CH,CI,). All the new compounds were characterized by a full complement of high-resolution spectra.[XI Further details of the crystal structure investigation are available on request from the Director of the

No abstract

Dedicated to Professor Wolfgang Liittke on the occasion of his 65th birthdayEven today, there are conspicuous gaps in the register of organic synthetic methods. Thus, numerous types of homoenolate reagents are listed; yet no generally applicable method for the homologue of the aldol addition, viz. the homoaldol addition, is to be found. The methods documented up to 1980 offer only singular solutions, and, moreover, are not stereoselective. The present article gives an overview of the existing possibilities and then describes how the gap can be closed by using a new class of homoenolate reagents: 2-alkenyl esters of N,N-diisopropylcarbamic acid are lithiated and the resulting allyllithium compounds tailored to high regio-and diastereoselectivity in addition reactions to the carbonyl group via metal exchange. Altogether, the homoaldol reactions now offer a general and reliable, highly stereoselective entry to y-hydroxycarbonyl compounds, whereby the C(P)-C(y) bond is formed. Furthermore, it is demonstrated how problems associated with the application of ally1 anions as synthetic reagents can be solved today.