Scandium Perchlorate as a Superior Lewis Acid for Regioselective Ring
Opening of Aziridine Carboxylate with Indoles

Abstract

In the synthesis of optically active tryptophan derivatives, Lewis acid-promoted coupling between indole and optically active serine-derived aziridine carboxylate is attractive because of the flexibility and convergence. Scandium perchlorate has been found to be a superior Lewis acid to the previously reported scandium triflate with respect to the yields as well as the regioselectivity of aziridine ring opening. The scope and limitation of this Lewis acid are also described.

Key words

tryptophan - indole - aziridine - scandium triflate - scandium perchlorate

References

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10

The similar byproducts to the regioisomer 3b were reported, see Ref. [1] [2b]

12

Kozikowski [1] and Bennani [3] had estimated the effects of the Lewis acids such as Zn(OTf)₂, Sc(OTf)₃, AlCl₃, EtAlCl₂, Me₂AlCl, TiCl₄, SnCl₄, Mg(OTf)₂, ZnBr₂ BF₃·OEt₂, BBr₃ and lanthanide triflates.

16

Sc(OTf)₃ purchased from Aldrich, TCI and Taiheiyo Kinzoku Corporation were examined.

23

The Yb(OTf)₃-mediated coupling of indole 4 (see Table [2] ) with aziridine 2 gave 48% yield of 10 as a single product after 20 h at r.t., while the coupling of 4-chloroindole(7) was very sluggish and gave a complex mixture.

27

Application of the Lewis acid Sc(ClO₄)₃ to synthesize the analogs of α-C-mannosyltryptophan such as galactosyl- and glucosyl-analogs will be reported elsewhere.

30

The optical rotation of 10a depends on the solvent used for measurement.