Bromoacetaldehyde allyl acetals

A reductive radical cyclization methodology, which is catalyzed by zirconocene complexes, furnished butyrolactols 91 (Fig. 27) [168, 169]. When 2-allyloxy-3-halotetrahydropyrans or bromoacetaldehyde allyl acetals 90 were treated with 20-30 mol% of zirconocene dichloride, 1 equiv. of BEt3 and an excess Red-Al as the stoichiometric reductant, tetrahydrofurotetrahydropyrans or butyrolactols 91... 

Giese and coworkers investigated radical 5-exo cyclization reactions of bromoacetaldehyde allyl acetals 264 (Y=0, R1=OBu) catalyzed by 251 and found that either product 265 or 266 results selectively, depending on the amount of catalyst (1-40 mol%) and the reducing conditions (entry 8) [304]. Mild reducing conditions, such as Zn in DMF or a potential of —0.8 V vs Ag/Ag+, and higher... 

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