Trangmolins A–F with an Unprecedented Structural Plasticity of the Rings A and B: New Insight into Limonoid Biosynthesis

Abstract

The absolute stereostructures of trangmolins A-F (1-6), limonoids with three new and one known topologies of the rings A and B, were unambiguously determined by NMR spectroscopic investigations, single-crystal XRD analysis, and quantum-chemical electronic circular dichroism calculations. Compounds 1-3 contain a hexahydro-1H-inden-4-one motif, compound 4 comprises a hexahydro-2,6-methanobenzofuran-7-one cage, and compound 5 consists of a hexahydro-2H-2,8-epoxychromene scaffold. The C1-C30 linkage in 1-3 and the C3-C30 connection in 4 form two unprecedented types of ring A/B-fused carbobicyclic cores: viii and ix. The oxidative cleavage of the C2-C3 bond in 5 and heterocyclization in 4 and 5 constitute the unprecedented tricyclic 6/6/5 ring A/B(1) /B(2) - and 6/5/6 ring A(1) A(2) /B-fused topologies, respectively, which are uncovered, for the first time, in the construction of limonoid architectures. The diverse cyclization patterns of 1-6 reveal an unparalleled structural plasticity of rings A and B in limonoid biosynthesis.

Twenty-Nine New Limonoids with Skeletal Diversity from the Mangrove Plant, Xylocarpus moluccensis

“…for [M + H] + , 541.2074). The similarities between the NMR spectroscopic data of 22 ( Table 7 and Table 8 ) and those of trangmolin F [ 16 ], containing a ( Z )-bicyclo[5.2.1]dec-3-en-8-one substructure, revealed their close structural resemblance. However, the 3- O -isobutyryl function in trangmolin F was replaced by an acetoxy group (δ H 2.17 s 3H; δ C 170.4 qC, 20.6 CH 3 ) in 22 , being unambiguously confirmed by HMBC cross-peaks between H-3/C-32 and H 3 -33/C-32 ( Figure 7 a).…”

“…X. moluccensis , a true mangrove tree, is mainly distributed in Bangladesh, India, Indochina, Malesia, and tropical Australia. Previous chemical investigations of X. moluccensis resulted in the isolation of more than 100 limonoids with diverse carbon skeletons, such as mexicanolide, phragmalin, gedunin, andirobin, and khayanolide compounds [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Limonoids with diverse skeletons from X. moluccensis revealed structural plasticity in the limonoid biosynthesis of this mangrove species.…”

Guianolactones A and B, Two Rearranged Pentacyclic Limonoids from the Seeds of Carapa guianensis

“…Furthermore, C‐2 was bound to C‐3, and C‐3 was bound to C‐30. Bringmann et al., reported trangmolin D, which was the first example having C‐3‐C‐30 connection in limonoids . These results indicated that 2 was an unprecedented limonoid with a 6,6,5,6,6 ring system.…”

“…Li et al reported A‐ and B‐ rings in limonoids are mutable, while C‐ and d ‐ rings are relatively conservative . There have been many limonoids with cleavage between C‐16 and C‐17 in the d ‐ring.…”

“…Li et al reported A-and B-rings in limonoids are mutable, while C-and d-r ings are relatively conservative. [25] There have been many limonoids with cleavage between C-16 and C-17 in the d-ring. However,there is no precedent in which C-30 transposes to the d-ring and C-30 constitutes D 1 and D 2 rings, similar to GuianolactoneA( 1).…”