Synthesis of two novel C-19 analogues of (±)-alstoscholarisine A

Two new analogues of alstoscholarisine A, containing a phenyl or butyl substituent at the C-19 position, have been prepared in racemic form from the known skatole derivative. The syntheses of these compounds were accomplished in 13 steps, with a late-stage formation of the C-19 stereocenter. These derivatives are expected to have significantly changed biological activity, compared to alstoscholarisine A – a potent neuroactive natural product.


INTRODUCTION
Over last decades, as medicine made substantial progress towards a better understanding of neurological disorders, neuroactive substances have emerged as a very interesting and challenging area of research for chemists.[8][9][10] Recently, five monoterpenoid indole alkaloids, alstoscholarisine A-E (1-5), were isolated from the leaves of Alstonia scholaris (Fig. 1). 10 Although all these compounds show noteworthy ability to enhance adult NSC proliferation, the most potent one, alstoscholarisine A (1), has attracted significant attention among synthetic organic chemists.This interest was provoked partially by its exciting bioactivity, but also by its complex molecular structure, with 6/5/6/6/6-fused bridge rings and five contiguous chiral centers.[13][14] Fig. 1.Structures of alstoscholarisine A-E (1-5).
However, neither of these synthetic efforts was oriented toward the synthesis of alstoscholarisine A analogues.The fact that the only structural difference between alstoscholarisine A (1) and alstoscholarisine E (5) (which exhibits slightly lower activity) is the absolute configuration of the carbon C-19, indicates that a substituent at C-19 might be important for the bioactivity.
We have published a racemic synthesis of alstoscholarisine A (1) in 2016, in which the stereocenter at C-19 was formed in the last steps of the synthesis. 11herefore, it was decided to modify these last steps and to synthesize compounds 6 and 7, two new C-19 analogues of alstoscholarisine A (Scheme 1), in order to test whether a nonpolar, longer alkyl or an aryl substituent could alter the bioactivity.These two analogs could be accessible from the common lactonic intermediate 8 by the addition of the corresponding alkyl-or aryl-lithium reagent, followed by a subsequent stereoselective reduction of the intermediary hemiketal.

General methods
All chromatographic separations were performed on silica gel, 10-18 mesh, 60 Å (dry flash) and Silica 60 (0.063-0.200 mm), Merck.Standard techniques were used for the purification of the reagents and solvents.Petroleum ether refers to the fraction boiling at 70-72 °C.NMR spectra were recorded with a Bruker Avance III 500 ( 1 H-NMR at 500 MHz, 13 C-NMR at 125 MHz).Chemical shifts are expressed in ppm (δ) using tetramethylsilane as the internal standard, coupling constants (J) are in Hz.IR spectra were recorded with a Nicolet 6700 FT ANALOGUES OF ALSTOSCHOLARISINE A 937 instrument and expressed in cm -1 .Mass spectra were obtained with an Agilent Technologies 6210 TOF LC-MS instrument (LC: series 1200).
The characterization data for the synthesized compounds are given in the Supplementary material to this paper.

RESULTS AND DISCUSSION
The synthesis of analogues 6 and 7 commenced with the aldol reaction of skatole derivative 9 11 and N-Alloc protected aldehyde 10, 15 which was partially followed by the spontaneous migration of the Boc-group from nitrogen to the newly formed hydroxyl group (Scheme 2).Treatment of the reaction mixture with NaH ensured complete migration and subsequent in situ β-elimination of intermediate 11, to afford stereoselectively (E)-12.Palladium-catalyzed removal of the Alloc-protecting group delivered free amine 13, the intermediate for the key domino reaction that allowed the C and D rings of the target molecule to be constructed in one-step.Treatment of amine 13 with aldehyde 14 16 triggered the sequence, followed by 6-exo-trig cyclization of the intermediary enamine and final intramolecular N,N-acetalization.Low diastereoselectivity was observed in the formation of the tetracyclic core and NOESY experiment showed that a mix-ture of C-16 epimers 15a and b was obtained in almost equimolar ratio (for the NOESY spectrum and correlations, see the literature data 11 ).For this reason, we wished to improve further the yield of the desired isomer 15a.Simple treatment of the crude mixture of diastereoisomers 15a, b with DBU resulted in ester isomerization and the thermodynamically more stable tetracycle 15a was isolated in 71 % yield.Furthermore, the above-mentioned NOESY experiment also revealed the incorrect, i.e., axial, position of the selenium-containing substituent.Therefore, epimerization of the C-20 stereocenter prior the construction of tetrahydropyrane E-ring was necessary.Compound 15a was oxidized with mCPBA and thus the obtained selenoxide was treated with DIPA to afford alkene 16 in good yield.Reduction of ester group with DIBAL, followed by dihydroxylation of the double bond in 16 cleanly furnished triol 18 in almost quantitative yield, as an inseparable mixture of isomers (Scheme 3).Glycol cleavage in 18 was affected by treatment with lead tetraacetate to afford aldehyde 19, a substrate suitable for the C-20 epimerization.It is worth mentioning that aldehyde 19 was prone to decomposition, so it was subjected to isomerization directly, without purification.Initial experiments showed that axial position of the aldehyde group in 19 is thermodynamically favored: when the vinyl group in ester 16 was cleaved under ________________________________________________________________________________________________________________________ Available on line at www.shd.org.rs/JSCS/(CC) 2019 SCS.
ANALOGUES OF ALSTOSCHOLARISINE A 939 the described conditions, the obtained aldehyde underwent epimerization in the presence of DBU.However, the equilibrated mixture contained only 10-15 % of the desired equatorial isomer, thus indicating that the axial isomer is the thermodynamically more stable one.The hypothesis was that the less stable equatorial isomer could be trapped instantly after epimerization by an intramolecular attack of the hydroxyl group to give the hemiacetal.Indeed, upon exposure to DBU in chloroform, axial aldehyde 19 was converted to a mixture of isomeric hemiacetals 20, which were immediately oxidized with DMP in a one-pot protocol to yield lactone 8. Finally, lactone 8 was treated with phenyllithium or n-butyllithium, to afford the corresponding hemiketals 21 or 22, respectively.Addition of organolithium reagent to 8 was completely stereoselective, from the less crowded, convex face of the molecule.These hemiketals 21 and 22 were treated with TMSOTf and thus the formed oxonium ions were stereoselectively reduced with Et 3 SiH from the less hindered face to yield the alstoscholarisine A analogues 6 and 7 in good yield.

CONCLUSIONS
To summarize, the first syntheses of the C-19 alstoscholarisine A analogues 6 and 7 have been presented.These compounds were prepared in 13 steps, including a late-stage incorporation of the C-19 substituent.As analogues 6 and 7 were obtained in racemic forms, development of a HPLC method for the separ-

Scheme 3 .
Scheme 3. E-ring formation and completion of the synthesis of 6 and 7.