Transition structure chair favoured

However this may be, it does not help to explain the course of the alkali-catalysed reaction of the lya-hydroxy isomer (2). The alkali-catalysed reactions are formulated [204] as rearrangements of the i7 alkoxy anions, and the stereochemistry found in the products corresponds to reactions in the side chain conformation with the C(i7) and C(20) oxygen functions ri s-orientated by the mutual interactions of the anionic charge and the electric dipole of the carbonyl function. Both isomers react by migration of C i3), leading to the lya-epimeric-17-keto derivatives. In the ly/if-hydroxy isomer (i), this mode of reaction is the one favoured by a conformational preference for the chair-like transition state (E), but as Wendler has emphasised, the lycc-hydroxy-ao-ketone (2) appears to react through a transition state (G) of boat-like structure. Indeed,... 

It must be emphasized that the ratio is independent of the value of the equilibrium constant for C5H11 + 02 C5Hn02. The relative amount of 1,2- to 1,4 isomer is, however, much larger than expected (Table 1.17) for alkyl radicals, k(l,4s)/k(l,6s) 0.07, compared with the value of [1,2 cyclohexane oxide]/[l,4 cyclohexane oxide] = 0.60 0.10. Walker and Gu-lati point out that although about 99.5% of the cyclohexane molecules adopt the chair form at 753 K, the O—O group at either an axial or an equatorial position in the chair form is not favourably structured for a low strain energy in the transition state, as shown by... 

In drawing this chair, we have one choice do we allow the aldehyde to place R equatorial or axial Both are possible but, as you should now expect, there are fewer steric interactions if R is equatorial. Note that the enoiate doesn t have the luxury of choice. If it is to have three atoms in the six-membered ring, as it must, it can do nothing but place the methyl group pseudoequatoriai. The aldol formed from the favoured transition state structure, with R pseudoequatoriai, is shown below— first in the conformation of the transition state, and then flattened out on to the page, and it is anti. 

---