Angiogenesis-Inhibitory Piperidine Alkaloids from the Leaves of Microcos paniculata

Abstract

Eight new 2,6-disubstituted piperidin-3-ol alkaloids (1−8), featuring a C 10 unsaturated alkyl side chain, together with three previously reported analogues (9−11) were isolated from the leaves of medicinal plant Microcos paniculata. Their structures and absolute configurations were elucidated unambiguously by means of 1D and 2D NMR spectroscopic data analysis, modified Mosher's method, Snatzke's method, and quantum chemical electronic circular dichroism (ECD) calculations, as well as single-crystal X-ray crystallography. The isolates were evaluated for their antiangiogenic effects on human umbilical vein endothelial cells (HUVECs). Compound 2 displayed an inhibitory effect on tube formation of HUVECs in a concentration-dependent manner.

Microgrewiapine C: Asymmetric Synthesis, Spectroscopic Data, and Configuration Assignment

“…Thus, all three would share the same absolute configuration of the three common stereogenic centers around the piperidine ring. 10 This assertion has very recently found credence in the results of the studies of Che, Ye, and Wang et al, 5 who reported the (re)isolation of microgrewiapine A (albeit from the leaves rather than the stem bark) of M. paniculata . They determined the structure and relative configuration of their sample of microgrewiapine A unambiguously by single-crystal X-ray diffraction analysis and assigned the (2 S ,3 R ,6 S ) absolute configuration to the alkaloid by comparison of its experimental ECD spectrum with the calculated ECD spectra [CAM-B3LYP/6-31+G(d)] of both enantiomers.…”

“…They determined the structure and relative configuration of their sample of microgrewiapine A unambiguously by single-crystal X-ray diffraction analysis and assigned the (2 S ,3 R ,6 S ) absolute configuration to the alkaloid by comparison of its experimental ECD spectrum with the calculated ECD spectra [CAM-B3LYP/6-31+G(d)] of both enantiomers. 5 The specific rotation value of their sample of microgrewiapine A {[α] D 25 −22.8 ( c 8.5, MeOH)} 5 was, however, found to be opposite in sign to the value originally reported by Kinghorn et al {[α] D 25 +15.4 ( c 0.1, MeOH)} 1 for their sample of the alkaloid [note that although the concentrations of these samples are very different, their relative magnitudes are consistent with our previous observation of increasing magnitude of the specific rotation value of our synthetic sample of microgrewiapine A with increasing concentration of the sample {[α] D 25 −16.0 ( c 0.1, MeOH); [α] D 25 −24.6 ( c 0.5, MeOH); [α] D 25 −26.2 ( c 1.0, MeOH)}]. 9 As Kinghorn et al had also assigned the (2 S ,3 R ,6 S ) absolute configuration to their sample of microgrewiapine A on the basis of the results of a Mosher’s ester analysis, 1 it now seems almost certain that a typographical error of sign of the specific rotation value was made by Kinghorn et al in the report of the original isolation study, 1 as this is the only outlier from the significant weight of evidence now consistent with natural microgrewiapine A possessing the (2 S ,3 R ,6 S ) absolute configuration; hence it seems that microcosamine A, microgrewiapine A, and microgrewiapine B are indeed “three homochiral alkaloids”.…”

“… These observations suggest that all four are genuine secondary metabolites produced by M. paniculata , i.e., that the N -oxides microgrewiapine B and microgrewiapine C are not merely artifacts of the extraction process (produced on aerial N -oxidation of the parent compounds). As such, they all belong to a growing family of alkaloids based on a 2-methyl-6-(deca-1′,3′,5′-trienyl)­piperidine-3-ol core that have been isolated from this organism. − Most of these alkaloids have been assigned the 2,3- trans -3,6- trans relative configuration, ,− as illustrated by the structures of microgrewiapine A, microgrewiapine B, and microcosamine A (Figure ), meaning that the 2,3- cis -3,6- cis relative configuration assigned to microgrewiapine C is somewhat unusual: in fact, only one other alkaloid originating from M. paniculata has thus far been found to possess the 2,3- cis -3,6- cis relative configuration, viz., microconine (Figure ). Although only the relative configurations of both microgrewiapine C and microconine were assigned in the original isolation studies (chiefly by 1 H NMR 3 J coupling constant analysis), our recent total synthesis of microconine resulted in the (2 R ,3 R ,6 R ) absolute configuration being proposed for this alkaloid upon comparison of the specific rotation of our synthetic sample of known (2 S ,3 S ,6 S ) absolute configuration {[α] D 22 −29.0 ( c 1.0, CHCl 3 )} with that of the natural sample {[α] D 22 +29.2 (CHCl 3 )}. , Meanwhile, Kinghorn et al performed a Mosher’s ester analysis on microgrewiapine A and on this basis assigned it the (2 S ,3 R ,6 S ) absolute configuration .…”

“…This assertion has very recently found credence in the results of the studies of Che, Ye, and Wang et al, who reported the (re)­isolation of microgrewiapine A (albeit from the leaves rather than the stem bark) of M. paniculata . They determined the structure and relative configuration of their sample of microgrewiapine A unambiguously by single-crystal X-ray diffraction analysis and assigned the (2 S ,3 R ,6 S ) absolute configuration to the alkaloid by comparison of its experimental ECD spectrum with the calculated ECD spectra [CAM-B3LYP/6-31+G­(d)] of both enantiomers . The specific rotation value of their sample of microgrewiapine A {[α] D 25 −22.8 ( c 8.5, MeOH)} was, however, found to be opposite in sign to the value originally reported by Kinghorn et al {[α] D 25 +15.4 ( c 0.1, MeOH)} for their sample of the alkaloid [note that although the concentrations of these samples are very different, their relative magnitudes are consistent with our previous observation of increasing magnitude of the specific rotation value of our synthetic sample of microgrewiapine A with increasing concentration of the sample {[α] D 25 −16.0 ( c 0.1, MeOH); [α] D 25 −24.6 ( c 0.5, MeOH); [α] D 25 −26.2 ( c 1.0, MeOH)}] .…”

“…Either way, the configurational assignments for microgrewiapine C and microconine made solely on the basis of comparison of reported specific rotation values, thus equating them to 10 and 14′ , respectively, seem very unlikely when the potential of such a common biosynthetic origin is considered. This again identifies a need for reisolation of these alkaloids from their natural source to confirm their specific rotation values, in particular the sign of both of these values, either of which may have been erroneously reported (as is seemingly the case with the sign of the specific rotation value reported by Kinghorn et al for microgrewiapine A). , It is also of note that the sign of the specific rotation value of our synthetic sample of 14 has been observed to change with the concentration of the sample {[α] D 22 −29.0 ( c 1.0, CHCl 3 ); [α] D 22 −20.9 ( c 0.1, CHCl 3 ); [α] D 22 −8.6 ( c 0.06, CHCl 3 ); [α] D 22 +7.2 ( c 0.04, CHCl 3 )}, and the concentration at which the value for the natural product was obtained was not given in the original report {[α] D 22 +29.2 (CHCl 3 )}. Given the various issues that have been noted during our studies into these alkaloids (inconsistent reports of specific rotations and variation of the sign and magnitude of the values of the specific rotations of these alkaloids), it is promulgated that ideally both specific rotation values and ECD spectra should be reported for any new compounds such that similar inconsistencies do not occur in the future; the reisolation of these alkaloids should thus also allow for a more detailed investigation of their absolute configuration by ECD analysis, which should resolve these discrepancies.…”