Substrate Control in the Asymmetric Aminohydroxylation of ­Monosubstituted Alkenes: The Enantioselective Synthesis of GABOB and ­Homoserine Derivatives

Abstract

The 4-nitrophenyl ether is an efficient directing group in the asymmetric aminohydroxylation reaction of but-3-en-1-ol derivatives. Either regioisomeric product can be obtained with useful levels of enantioselectivity, allowing for the short enantioselective synthesis of GABOB and homoserine derivatives.

Key words

amino acids - asymmetric catalysis - regioselectivity - asymmetric aminohydroxylation

References

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13

Determined by integration of the 300-MHz ¹H NMR spectrum of the crude reaction mixture.

14

Determined by chiral HPLC using Chiralpak AD or Chiralcel OD-H columns (250 × 4 mm, Daicel), eluent
i-PrOH-hexane, flow rate 0.5 mL min^-1, UV detection at λ = 270 and 254 nm. In the case of the 4 (R¹ = 4-NO₂Ph); Chiralcel OD-H column, eluent i-PrOH-hexane (8:92); 4, t _R = 39.4 min, ent-4, t _R = 46.6 min, 96% ee.

20

The stereochemistry of (R)-5 (R¹ = 4-NO₂Ph) was confirmed by comparison of the optical rotation of the corresponding diol {95% ee, [α]_D +10.9 [c 1.3, CH₂Cl₂]}, formed by removal of the PNP group (ref. 18), with its enantiomer {[α]_D -10.8 [c 1.7, CH₂Cl₂]}, synthesised in two steps from (S)-glutamic acid.