The biosynthesis of hyoscyamine: the process by which littorine rearranges to hyoscyamine

Abstract

The incorporation of isotope from specifically-labelled 3-phenyllactic acid 4 or littorine 7 into 3α-phenylacetoxytropane 10, 3α-phenylacetoxy-6β,7β-epoxytropane and 3α-(2′-hydroxyacetoxy)-tropane 9 has been demonstrated. Transformed root cultures of Datura stramonium or Brugmansia (Datura) Candida x B. aurea incorporated fed (RS)-3-phenyl[1,3-¹³C₂]lactic acid 4 into 3α-phenylacetoxytropane 10 and 3α-phenylacetoxy-6β,7β-epoxytropane wild the efficient retention of both ¹³C nuclei. In contrast, no label was incorporated into these two compounds from (RS)-3-pheny[2-¹³C²-²H]lactate 4. From this evidence it can be deduced that 3-phenyllactic acid 4 is not incorporated into 3α-phenylacetoxytropane 10via free phenylacetic acid 6, a route which would result in the loss of the C-1 of 3-phenyllactic acid 4. Furthermore, (RS)-(3′-phenyl[1′,3′-¹³C₂]lactoyl)[methyl-²H₃]tropine (littorine 7) was incorporated into 3α-phenylacetoxytropane 10, at up to 4% specific incorporation, with the retention of all the ¹³C and ²H nuclei. Label was also incorporated into 3α-(2′-hydroxyacetoxy)tropane 9 from (RS)-3-phenyl[1,3-¹³C₂]lactic acid 4 and (RS)-(3′-phenyl[1′,3′-^l3C₂]lactoyl)[methyl-²H₃]tropine 7. We propose, on the basis of these observations, a putative process for the rearrangement of littorine 7 to hyoscyamine 8 and suggest that both 3α-phenylacetoxytropane 10 and 3α-(2′-hydroxyacetoxy)tropane 9 arise as by-products of the rearrangement process.