The conversion of turraeanthin and turraeanthin A into simple meliacins by a route involving an oxidative rearrangement of probable biogenetic importance

Abstract

Treatment of turraeanthin (VIII) and its 3ct-epimer, turraeanthin A (IX), with sodium periodate in aqueous dioxan containing a trace of perchloric acid, followed by dehydration of the products with toluene-p-sulphonic acid in benzene, has afforded tetranortirucallane triterpenoids, (XIX), (XX), and (XXIII), containing a p-substituted fury1 side-chain. Epoxidation of these tetranortriterpenoids followed by rearrangement by treatment with boron trifluoride-ether complex in benzene has provided a pathway to the simplest meliacins [e.g. (XXIX)].THE structures of the tetranortriterpenes of the meliacin (limonoid) group, such as gedunin (11) and meldenin (111) ,2 can all be formally derived from the tetracyclic system found in (XXIX), with oxygen a t C-3 and C-7a and the p-fury1 substituent a t C-17. It has been suggested3y4 that compound (XXIX) might be a key intermediate in their biosynthesis, and that this might arise from the tetranortirucallane derivative (XIX) by the formation of its 7a,8a-epoxide (XXIV) followed by rearrangement. Alternatively the epoxidation and re-.arrangement could occur at an earlier stage in the 7 The nomenclature used for compounds with the carbon skeleton found in ( 111) is based on the name meliacan for the parent compound (I) after discussion with the-Editor. The steroid rules of nomenclature which apply to furostan and spirostan are then used to derive names of derivatives such as (XXIX), which is 3cr,7a-dihydroxymeliaca-14,20,22-triene.

Ratio of products formed on photo-oxidation of the neem triterpenoids nimbin and salannin

“…A large number of the compounds belong to the class of limonoids, in which the side chain has been shortened and cyclized to a furan ring (a) (Scheme 1). 2 A further, smaller group, containing a butenolide ring neem seeds, 9 and the isomer 1cwas first found in the stem bark of the neem tree.…”