Half-chair transition state

Treatment of amide 4 with formic acid triggers a nucleophilic addition of the amide NH2 to the protonated ketone, and an elimination of water, to give iminium ion 14. Reduction of 14 then occurs exclusively from the a face, probably via the half-chair transition state shown in Scheme 9.3,... 

Alternatively, the interaction may produce a flatter chair-conformation which is for that reason more similar to the relatively flat half-chair transition state conformation. Many examples of this effect have been encountered subsequently, and have already been discussed iu Section V. 

H-TS = half-chair transition state Scheme 9 In situ anomerization procedure... 

The same flattened half-chair transition state (Scheme 30c, Fig. 11) has been postulated in hydrolysis of the 4a a-methyl derivatives of [85] (Taira et ai, 1986). 

The postulate of a half-chair transition state may be valid if relative rates may be accounted for by least motion principles (Hine, 1977). The equatorial phosphate isomer, populating a twist-boat ground state, may require less... 

A stereoselective approach has been developed to enable chirality transfer from a chiral alcohol to a quaternary carbon center through a half-chair transition state as shown in Scheme 25 [21]. In that case, a mixture of alcohol 89 (64% ee) is converted into the dienone 91 (61% ee) with very good chemical yield as well as very good enantioselectivity (95% ee after correction). 

For explaining the dilemma of the diminished anti-selectivity of fraws-enolates and the striking observation of syw-selective traws-enolates, various alternative geometries of the transition state have been proposed, partly based on plausibility, partly on experimental results, and partly supported by calculations. Skewed transition state models, where the dihedral angle between the enolate double bond and the carbonyl group approaches 90 "C, were suggested by Dubois and Fellmaim [75] and Heathcock [67c], boat-like models were proposed by Evans and coworkers [67a], and twist-boat or half-chair transition states were discussed based upon calculations [76,77]. 

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