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Red-Al®

ChemFiles Volume 5 Article 9

Red-Al®, or sodium bis(2-methoxyethoxy)aluminum dihydride (Vitride®, SMEAH), is a versatile reducing agent and a good substitute for LiAlH4 in many reactions. Red-Al® is nonpyrophoric, although still moisture-sensitive, and is available in solution, allowing for easier handling compared to LiAlH4.

Red-Al® is particularly effective at the reduction of epoxides. A recent example employs Red-Al® in a key step in the asymmetric synthesis of (R)-fluoxetine hydrochloride. The enantiomerically pure α,β-epoxyamide was reduced by Red-Al® in the presence of 15-crown-5 to produce the β-hydroxyamide in good yield and excellent selectivity (Scheme 23).1

Scheme 23

Cao and co-workers utilized Red-Al® to convert propargyl chlorides to allenes, which were subsequently used in a tandem Pauson-Khand reaction to create 3,7-diisopropylsilyldicyclopenta-[a,e]pentalene, a 14π annulene (Scheme 24).2

Scheme 24

The preparation of highly functionalized α,β-disubstituted alkenoates can be achieved under mild conditions by reducing an acetylenic ester with Red-Al® and quenching the reaction with iodine to yield the vinyl iodide. The vinyl iodide can be subsequently transformed via Sonogashira or Stille coupling to form the desired α,β-disubstituted alkenoates (Scheme 25).

Scheme 25

Red-Al® displays excellent (E)-selectivity in the reduction and does not reduce the methyl ester.3 Red-Al® reduction can selectively produce dialdehydes from aromatic diesters in the presence of N-methylpiperazine (Scheme 26). Although Red-Al® is regularly employed in the reduction of esters to aldehydes, reduction of the diester without the addition of the amine produced only the dicarbinol.4

Scheme 26

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Materials

     

References

  1. Kakei, H. et al. Angew. Chem., Int. Ed. Engl. 2004, 43, 317.
  2. Cao, H. et al. J. Am. Chem. Soc. 2003, 125, 3230.
  3. Meta, C. T.; Koide, K. Org. Lett. 2004, 6, 1785.
  4. Hagiya, K. et al. Synthesis 2003, 823.

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