Aldehydes and Achiral Enolates

For purposes of illustration, consider the erythro selective reaction illustrated in eq. [69]. For aldehydes containing an adjacent asymmetric center (R, Rl = medium and large alkyl substituents), the bias for nucleophilic addition from a given diastereotopic face of the aldehyde is predicted empirically by Cram s rule (the open-chain 

One of the earliest studies to address the issue of aldehyde dia-stereoface selection in enolate condensation was performed by Matsumoto and coworkers (91). The Reformatsky reaction of methyl 2-bromopropionate and 2-phenylpropanal (98) afforded the four aldol adducts illustrated in eq. [70]. Although the aldol diastereo- 

Recently, the issue of aldehyde diastereoface selection has been examined for the enolates 99 to 103 (2,26,33,64). For these substituted enolates, aldol diastereoselection has been demonstrated to 

Remote steric effects have also been noted to play an unanticipated role in the sense of asymmetric induction. This is apparent from related condensations carried out on aldehydes 106 (26) and 107 (eqs. [76]-[78]) (26,92). Other examples illustrating the influence of remote structural perturbations on the carbonyl addition process have been observed in these laboratories. The addition of the lithio benzoxazole 110 to aldehyde 108 proceeded with good Cram diastereoface selection (95a), whereas the same nucleophilic addition to aldehyde 109 was stereorandom (95b). 

To date, detailed studies of the influence of metal ion on related aldol condensations have not appeared. One might project from 

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