Allylboronate glyceraldehyde

Figure 13. Reaction of D-glyceraldehyde acetonide with tartrate allylboronate 36.

Related Allylboronate Reagents. A stereoselective synthesis of anti 1,2-diols has been achieved by using a DIPT-modified ( )-y-[(cyclohexyloxy)dimethylsilyl]allylboronate reagent. This reagent is best applied in double asymmetric reactions with chiral aldehydes such as o-glyceraldehyde acetonide (eq 9). 

The biologically important 2-deoxypentoses can be prepared readily by the two-carbon chain elongation of 2,3- -isopropylidene-D-glyceraldehyde following Roush s allylation method (Scheme 13.54), relying on the highly diastereoselective additions of enantiomerically pure allylboronates derived from (R,R) and (5, 5 )-tartaric acid [100]. Similarly, the 2,6-dideoxyhexose derivative 155 was obtained by Roush and Straub (Scheme 13.54) [101]. 

The two-carbon chain elongation of protected D-glyceraldehyde (R)-24 and (R)-62 can be realized via ( -7-alkoxyallylboronate additions, followed by alkene ozonolysis. The allylboronate 162 generated in situ adds to (R)-62 giving a single stereoisomer 163, which can be converted to D-arabinose derivatives following ozonolysis (Scheme 13.57). Similarly, 164 added to (R)-24 gave a major adduct 165 that was then converted into 2-0-methyl-D-arabinose derivatives [105]. 

A more complete picture of relative diastereoselection in the reactions of allylboronates and chiral aldehydes is given in Table 5 structures of the products in the glyceraldehyde acetonide (151) series are given in Scheme The data for (150) and (151) reconfirm the conclusion of Table 4 that dia-... 

The first examples of highly diastereoselective double asymmetric reactions involving chiral allyl metal reagents were obtained in reactions with D-glyceraldehyde acetonide (151 Table 6). Aldehyde (151) displays an 80 20 preference for (154) in reactions with the achiral pinacol allylboronate (144 entry 4),25.ioi selectivity for (154) improves to 96-98% with reagents (-)-(215) and (RJi)-... 

Hoffmann et al have also examined the addition of allylboronates (74) and (75) to 2,3-isopropyl-idene-D-glyceraldehyde oximes (73 E Z = 3 7 equation 16 and Table 18). Cram selectivity is obtained. As is the case of the parent 2,3-isopropylidene-D-glyceraldehyde, - optically pure boronate (74 entry 1, Table 18) provides better diastereofacial selectivity than (75). The relationship, if any, between oxime geometry and Cram diastereoselectivity has not been established. 

Table 18 Cram Selectivity in the Addition of Allylboronate Esters to 2,3-0-Isopropylidine-D-glyceraldehyde...

A mixture of 0.66 mmol allylboronate with (/ ,R)-A,A -bis(cyclohexylmethyl)-A,A -ethylenetartramide as chiral auxiliary, 1.0 mL THE, and 600 mg 4A powdered molecular sieves was stirred at room temperature for 30 min and was cooled again to -78°C. A solution of 5.2 /u.L pivaldehyde (0.044 mmol) in 200 /xL dry THE was added dropwise. After the addition was complete, the solution was warmed to room temperature and stirred for 30 min. The mixture was cooled to -78°C again and then a solution of 0.069 g D-glyceraldehyde pentylidene ketal (0.44 mmol) in 0.2 mL dry THE was added dropwise down the side of the flask. After the addition was complete, the solution was maintained at -78°C for 36 h. Excess acetaldehyde was then added dropwise via syringe, the cooling bath was removed, and the solution was warmed to room temperature. The mixture was filtered... 

The allylation and crotylation of a-alkoxy aldehydes provide chiral 1,2-diol synthons which can be used in the synthesis of polyoj genated small molecules, for example, natural and unnatural sugars The tartrate-derived allylboronates I and 2 provide reagent-controlled selectivity in reaction with chiral glyceraldehyde acetonide 30. The intrinsic selectivity of the aldehyde is estimated by its reactions with pinacol allylboronates 33 and 34. The reagents 1 and 2 overcome the aldehyde s... 

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