Diastereoselectivity diastereotopic face selectivity

In the discussion of the stereochemistry of aldol and Mukaiyama reactions, the most important factors in determining the syn or anti diastereoselectivity were identified as the nature of the TS (cyclic, open, or chelated) and the configuration (E or Z) of the enolate. If either the aldehyde or enolate is chiral, an additional factor enters the picture. The aldehyde or enolate then has two nonidentical faces and the stereochemical outcome will depend on facial selectivity. In principle, this applies to any stereocenter in the molecule, but the strongest and most studied effects are those of a- and (3-substituents. If the aldehyde is chiral, particularly when the stereogenic center is adjacent to the carbonyl group, the competition between the two diastereotopic faces of the carbonyl group determines the stereochemical outcome of the reaction. 

The diastereoselectivity observed in simple systems led to investigation of enantiomerically pure aldehydes. It was found that the E- and Z-2-butenylboronates both exhibit high syn-anti diastereoselectivity with chiral a-substituted aldehydes. However, only the Z-isomer also exhibited high selectivity toward the diastereotopic faces of the aldehyde.38... 

A highly diastereoselective Paterno-Buchi reaction by a new approach was reported by Bach et The chiral auxiliary bound to m-hydroxybenzaldehyde 5 offers a binding site to which the reaction partner can be attached by two hydrogen bonds. In the complex, the two enantiotopic faces of the alkene 6 become diastereotopic and can be differentiated. The dihydropyridone 6 approaches the aldehyde from one face only (Scheme 4). Because the reaction within the complex is presumably faster than the reaction with unbound dihydropyridone, the face selection is enhanced by this rate acceleration. In toluene at -10 C, lactam 5 was converted with high diastereoselectivity to diastereoisomer 7 (>90% DE). The oxetano[2,3-fo]piperidone fragment could be cleaved from the auxiliary by transesterification (NaOMe in MeOH). 

In general, diastereoface selectivity in the allylic sulfoxide-to-sulfenate Mislow-Evans rearrangement is possible in cases such as 74 (Scheme 18.19). having the sulfoxide attached to a chiral center, or with substrates of type 75, where the alkene faces are rendered diastereotopic by a structural element distal from the allyl sulfoxide. These diastereoselective... 

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