Iridoids, XXI. Synthesis of [ 13 C]Secologanin

Acid‐catalyzed reaction of natural secologanin ( 1 ) with ethylene glycol to 5a , followed by acetylation gave the peracetylated acetal 5b in almost quantitative yield. Oxidation of 5b with equimolar or catalytic amounts of osmium tetraoxide afforded stereoselectively the diol 6 as main product (31% resp. 28%) besides recovered starting material. 5b was cleaved with lead tetraacetate to give 96% of the aldehyde 10 . Reaction of 10 with triphenylphosphonio‐[ 13 C]methanide led to the labelled secologanin derivative [10‐ 13 C]‐ 5b and the pyran 12 in a 27% and 52% yield, respectively. Acid‐catalyzed transacetalization of [10‐ 13 C]‐ 5b to [10‐ 13 C]‐ 5c , base‐catalyzed solvolysis of the acetate groups to [10‐ 13 C]‐ 5d and subsequent acid‐catalyzed cleavage of the acetal gave [10‐ 13 C]secologanin ([10‐ 13 C]‐ 1 ). To determine its configuration at C‐9, 6 was transformed via 8a to the tricyclic system 8d , which was independently synthesized via a photocycloaddition of the galactose derivative 13 and diformylacetate 14 .