Synthesis of revised structure of klaivanolide (acetylmelodorinol)

Klaivanolide (acetylmelodorinol) is an anti-leishmanial/anti-trypanosomal agent isolated from Uvaria klaineana and the structure was originally assigned as a 7-membered lactone with extended conjugation and S -configuration at the lactone junction. Very recently, the structure of klaivanolide was revised as previously known compound acetylmelodorinol. Now, we have synthesized the revised structure in racemic form using a short route.


Introduction
2][3][4][5][6][7] The available treatment for most parasitic diseases with the existing drugs suffer from several limitations such as toxicity, parenteral administration, emergence and spread of drug resistance.There is a need for the discovery of new drugs which address some of these issues.Along these lines, klaivanolide (1), a natural product isolated from the Uvaria klaineana by Lauren's group in 2002, 8 showed potent anti-leishmanial activity.Compound 1 showed an IC 50 value of 1.75 µM against the sensitive strain and an IC 50 value of 3.12 µM against the amphotericin B-resistant strain of L. donovani, which is three fold better than the marketed drug pentamidine (IC 50 = 8.52 µM).The structure of 1 was elucidated by spectroscopic methods and the absolute configuration was assigned using the VCD spectroscopy by Figadère's group in 2009. 9Very recently, structural revision of natural product klaivanolide was published by same group. 10According to this report, the structure of klaivanolide was assigned to a previously known compound acetylmelodorinol (2) [11][12][13] (Figure 1).This natural product attracted our attention because of its potent biological activity and unique structure.As a part of our research group activity towards finding novel chemotypes against different diseases, [14][15][16] we have initiated a medicinal chemistry programme to develop a lead compound against leishmaniasis based on klaivanolide (acetylmelodorinol).While we were struggling to synthesize the target compound 1 with original structure, structural revision of natural product klaivanolide was published.Accordingly, we have stopped all our efforts to synthesize the previously assigned structure and directed our efforts to the revised structure of klaivanolide.Our efforts on this project are described here.

Results and Discussion
Retrosynthetic analysis of the target molecule is shown in scheme 1.The natural product 2 can be visualized from the exocyclic olefin 3 via allylic oxidation with selenium dioxide.The olefin 3 could be synthesised by aldol reaction between the known aldehyde (4) and 2-furanone.Scheme 1. Retrosynthetic analysis of acetylmelodorinol (revised structure of klaivanolide).
Although synthesis of acetylmelodorinol was known in the literature by four different routes, [17][18][19][20][21] we wanted to synthesize the same target using a short sequence which is amenable to analogues synthesis.Our planned synthesis began with a known aldehyde 4 prepared from corresponding olefin through ozonolysis, [22][23] which was subjected to aldol condensation with 2furanone 24 followed by exposure to triethylamine/acetic anhydride produced a mixture of E/Zisomers of 3 (~1:3 ratio). 25Allylic oxidation of exocyclic olefin present in 3 using selenium dioxide in 1,4-dioxane furnished two alcohols 5 and 6 in 69% yield.Both the compounds were cleanly separated on silica gel column.17][18][19][20][21] The compounds cismelodorinol ( 5) and trans-melodorinol (6) were subjected to acetylation to give corresponding acetylmelodorinol 2 and its trans-isomer 7, respectively.All the spectral data of the synthetic racemic acetylmelodorinol 2 is in complete agreement with the literature reports.

Conclusions
In summary, we have accomplished the total synthesis of acetylmelodorinol (revised structure of klaivanolide) and its related compounds in racemic form using a short sequence.The present route is useful for the generation of variety of analogues which ultimately can be useful for the identification of anti-leishmanial agents with novel chemotypes.

Experimental Section
General.All reactions were carried out in oven-dried glassware under argon or nitrogen unless otherwise specified, with magnetic stirring.Air sensitive reagents and solutions were transferred via syringe or cannula and were introduced to the apparatus via rubber septa.All reagents, starting materials, and solvents were obtained from commercial suppliers and used without further purification.Reactions were monitored by thin layer chromatography (TLC) with 0.25 mm precoated silica gel plates (60 F 254 ).Visualization was accomplished with either UV light, iodine vapours, or by immersion in ethanolic solutions of phosphomolybdic acid, paraanisaldehyde, or KMnO 4 followed by heating with a heat gun for ~15 sec.Column chromatography was performed on silica gel (100-200 or 230-400 mesh size).High resolution mass spectra (HRMS, ESI) were recorded with an ORBITRAP mass analyser (Q Exactive).Mass spectra were measured with electro spray ionization with an MSQ LCMS mass spectrometer.Infrared (IR) spectra were recorded on an FT-IR spectrometer as thin films.Optical rotations were recorded on a P-2000 polarimeter at 589 nm.Chemical nomenclature was generated using Chem Bio Draw Ultra 13.0.

3-Oxopropyl benzoate (4).
A solution of 3-butenyl benzoate (2.2 g, 12.5 mmol) in dry DCM (20 mL) was cooled to -78 ºC and stream of O 3 passed through it till reaction mixture becomes blue and excess of ozone was removed by passing a stream of oxygen for 3 minutes.Dimethyl sulfide (1.37 mL, 18.75 mmol) was added and reaction mixture was warmed slowly over the period of 3 h to room temperature and stirred for another 3 h.Solvent was removed under reduced pressure and crude aldehyde 4 (2.11 g, 95%) which was used as such without further purification for next step.