Btlrgi-Dunitz trajectory

Eschenmoser et al. have also postulated that the Btlrgi-Dunitz trajectory must be taken into account in discussions of the kinetic preference for formation of allose 2,4,6-triphosphate in the hexose series and of ribose 2,4-diphosphate in the pentose series, by aldomerization of glycolaldehyde phosphate in aqueous NaOH solution [37]. In this analysis, however, an additional interaction between the donor and acceptor substituents (b <-> f in Figure 6.40) is supposed to increase when the approach is non-perpendicular. 

Scheme 9.41. A representation of the acid-catalyzed aldol condensation reaction (using a proton [H ] as a typical acid) between two equivalents of ethanal (acetaldehyde [CH3CHO]). The steps, both protonation-deprotonation and carbon-carbon bond forming are reversible (the retroaldol reaction).The carbon-carbon bond forming step is shown as occurring on the re face of the aldehyde and with an approximate Btlrgi-Dunitz trajectory. Attack at the si face is equally likely and the product is racemic.

Now that we have decided which are the important conformations, how do we know which gives the product We need to decide which is the most reactive. All we need to do is to remember that any nucleophile attacking the carbonyl group will do so from the Btlrgi-Dunitz angle—about 107° from the C=0 bond. The attack can be from either side of C=0, and the following diagrams show the possible trajectories superimposed on the two conformations we have selected, which are in equilibrium with one another. 

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