Reactions with Chiral a-Oxygenated Aldehydes

The presence of an a-alkoxy substituent on the substrate aldehyde leads to the formation of a differentially protected 1,2,3-triol upon addition of a y-oxygenated allylic stannane (Eq. 43). Such additions are of potential use for the synthesis of carbohydrates and extended polyols. The resident double bond in the adduct can be further functionalized by dihydroxylation or epoxidation to extend the polyol chain. 

When the aldehyde and alkoxystannane are enantioenriched the issue of matching and mismatching must be addressed. An early examination of this issue involved S)-2-benzyloxypropanal and the enantiomeric y-OMOM tributylstannanes derived from crotonaldehyde (Eq. 44) [64], Two sets of experiments were performed. In the first BF3 OEt2 was used as the Lewis acid promoter. Matching was observed with the R)-stannane and the anti, syn- E) adduct was formed as the major component of a separable 92 8 mixture. The mismatched (5)-stannane gave a 67 33 mixture of syn, syn-(E) and anti, anti- E) under these conditions. 

A second set of experiments involved the use of MgBr2 as the Lewis-acid promoter. In this case the (7 )-stannane was mismatched resulting in a 75 25 mixture of syn, anti-(E) and syn, syn- Z) isomers. The matched (5) stannane afforded a 93 7 mixture of syn, syn-(E) and syn, anti-(Z) adducts. 

These results can be understood on the basis of a Felkin-Ahn transition state arrangement for the BF3 experiments and a chelation transition state arrangement with MgBt2 as the promoter. The matched transition states are pictured in Fig. 12. The latter additions are strongly substrate-controlled, which accounts for the formation of (Z) products as the minor adducts. Evidently the methyl substituent is an efficient facially directing group in the chelated aldehyde substrate. 

With the previous experiments and the derived transition state models as guidelines it was possible to select matched pairings of protected threose and erythrose aldehydes with the foregoing stannanes to prepare potential hexose precursors (Eq. 45) [65], 

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