Biosynthesis of the indolizidine alkaloid cyclizidine: incorporation of singly and doubly labelled precursors

Abstract

Incorporation of CH313C18O2Na and CD3CH213CO2Na into the indolizidine alkaloid cyclizidine 1, produced by Streptomyces species NCIB 11649, shows that the oxygen attached to C-2 is derived intact from acetate and that the cyclopropyl ring is derived from a single intact propionate unit. However, the level and stereochemistry of the incorporation of deuteriated sodium propionate indicates that it undergoes unexpected modification during incorporation into the cyclopropyl ring.

Identification and Analysis of the Biosynthetic Gene Cluster for the Indolizidine Alkaloid Iminimycin in Streptomyces griseus

“…We think that indolizidine rings of iminimycin and cyclidizine are biosynthesized by different mechanisms although they have similar structures. The methylene of cyclopropane ring of iminimycin was expected to be derived from malonyl‐CoA while the methyl group at a similar position in cyclizidine is derived from methylmalonyl‐CoA, [9] and therefore how the polyketide chains are folded to form indolizidine rings seems to be different between iminimycin and cyclidizine (Figure S9).…”

“…NCIB 11649 is the only Streptomyces ‐derived indolizidine alkaloid whose biosynthetic pathway has been studied. The heptaketide precursor of cyclizidine was proposed to be biosynthesized by PKS(s) based on a feeding experiment using 13 C‐labelled precursors [9] . It was also proposed that olefin migrations and subsequent cyclopropane formation might occur after the biosynthesis of the heptaketide precursor.…”

Identification of the Polyketide Biosynthetic Machinery for the Indolizidine Alkaloid Cyclizidine

“…Although the timing of the hydroxylation of the propionyl unit is unknown, we propose that the hydroxylation proceeds at a relatively early stage on the PKS assembly line based on the production of CLD-3. Previously, Leeper et al proposed that sequential reactions catalyzed by an acyl-CoA dehydrogenase and an enoyl-CoA hydratase on propionyl-CoA could lead to the terminal hydroxylation, 11 and it is yet to be determined if CycK, an acyl-CoA dehydrogenase homolog encoded in the gene cluster, is involved in the proposed reaction on propionyl-CoA, or more likely, on propionyl-ACP L . Alternatively, our gene knockout studies suggest that CycN could be related to the hydroxylation at C1.…”

“…We propose that the six-membered ring of CLD is formed through further transamination, which is followed by the likely concerted five-membered and three-membered ring cyclizations that lead to the generation of both the indolizidine scaffold and the terminal cyclopropyl ring found in CLD (Figure 2). 11 …”

Efficient syntheses of 13C‐labelled erythromycin biosynthetic intermediates. 2: (2S,3S,4S,5R,6R,7R)‐3,6,7‐trihydroxy‐2,4,6‐trimethyl[1‐13C]nonan‐5‐olide and S‐2‐acetylaminoethyl (2R,3S,4S,5R,6S,7R)‐3,5,6,7‐tetrahydroxy‐2,4,6‐trimethyl[1‐13C]nonanethioate

“…4 Although esterification of benzyl alcohol with 4 was carried out by using benzyl alcohol and 4 (unlabelled) in the presence of nbutyllithium under an argon atmosphere at À781C for 40 min in the Evans method, 21 ester 14 (unlabelled) 22 was obtained in only a 62% yield from 4 (unlabelled). However, esterification of benzyl alcohol with 4 in the presence of Et 3 N under an argon atmosphere at room temperature for 30 min gave ester 14 in a 79% yield from 4.…”