Abstract
For the first time ent-labdanes have been synthetised starting from ent-halimic acid, following a route that is the reverse of the biosynthetic one leading to the former compounds.
Highlights
Introduction entHalimic acid (Figure 1) is a bicyclic diterpene that has been used as a starting material for the synthesis of different bioactive molecules such as the ent-halimanolides [1], chettaphanin I and chettaphanin II [2], (+)-agelasine C [3] and sesterterpenolides with an disydiolane skeleton [4].Due to its functionality, ent-halimic acid can be viewed as an excellent synthon for the synthesis of new natural products
The bicyclic system of ent-halimic acid can be visualized as the basis for the BC rings of tricyclic diterpenes or quassinoids with a picrasane skeleton (Scheme 1) whereby the C-4 quaternary carbon and the C-5 carbon of the starting material are incorporated with the correct configuration as C-10 and C-9, respectively, in the targets
In the present work we have explored the use of ent-halimic acid for the synthesis of biologically active compounds with different carbon skeletons
Summary
As can be observed in Scheme 1, the synthesis of the picrasane skeleton would require a rearrangement of the abeopicrasane skeleton, namely a 1,2 migration of the Me-18 group, in the same manner as seen in the biosynthetic pathway of euphane and tirucallane, biosynthetic precursors of the quassinoids [5]. Scheme 2 shows the 1,2-shift of the Me-20 of ent-halimanes that leads to ent-labdanes. A rearrangement of this kind would be required to access the picrasane skeleton quassinoids This rearrangement would be the opposite of the biosynthetic route, in which the ent-labdanes are the precursors of ent-halimanes [6]. In the present work we have explored the use of ent-halimic acid for the synthesis of biologically active compounds with different carbon skeletons. In particular we have studied the rearrangements with different Lewis acids of epoxides 3 and 4 (Figure 2), derived from ent-halimic acid
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