Synthesis and Molecular Structure of Ethyl [n-tosyl-(r)-prolyloxy]-2(s)-[4-cyano-8,8-ethylenedioxy-5-oxo- 5,6,7,8-tetrahydroindolizin-3-yl] Acetate, a Key Intermediate in the Total Synthesis of (20s)-camptothecins

The synthesis of optically active


20(S)-Camptothecins [20(S)-CPTs] (Figure
) have attracted lasting interest in organic synthetic chemistry due to their unique chemical structures and potent antitumor activities, and considerable effort has been devoted over the last thirty years to the development of elegant methods for the total synthesis of 20(S)-CPTs [1].Among the numerous asymmetric synthesis approaches to these alkaloids described, the strategy utilizing the (S)-configuration tricyclic hydroxylactone 2 as a key chiral intermediate promises the most efficient industrial scale synthesis to 20(S)-CPTs.To date three synthetic methods for the preparation of compound 2 have been reported using a chiral auxiliary to induce asymmetry [2], a chemocatalyst to catalyze asymmetric synthesis [3] and optical resolution [4].We were especially interested in the asymmetric synthesis of compound 2 developed by Tagawa et al. [2] for its use of commercially available starting materials, mild reaction conditions and simple manipulations.

Results and Discussion
The synthesis of optically active hydroxyl lactone 2 is depicted in Scheme 1. Firstly, 2-bromoindolizine analogue 3, prepared by the reported methods [6], was condensed with N-tosyl-(R)-proline to give a diastereoisomeric mixture 4a and 4b.Subsequently, the mixture afforded the appropriate optical active compound 5a under the asymmetric induction of the chiral auxiliary reagent N-tosyl-(R)proline via the diastereoselective ethylation and fractional crystallization from isopropanol.Finally, compound 5a was treated by similar processes [2] to produce the key intermediate tricyclic hydroxyl lactone 2.
Diastereoisomers 4a and 4b were obtained as a low melting (49.6-61°C) 1:1 mixture inseparable by chromatographic purification.However, it is interesting to note that the crude 4a/4b mixture could be converted into isomer 4a with a high melting point of 172.6-174.3˚C in 70% yield by crystallization from methanol.This result was evident from the changes in their respective 1 H-NMR spectra.In the spectrum of the mixture there were two sets of signals (δ H 6.65, 6.63 and δ H 6.30, 6.19), representing the H-C(10) and H-C(2) protons of the two diastereoisomers 4a and 4b, respectively.After crystallization, the 1 H-NMR spectrum only showed the δ H 6.65 and 6.30 signals of one isomer.The complete proton and carbon chemical shift assignment for the structure of isomer 4a was established by the combination of 1D and 2D NMR experiments such as DEPT, H-H COSY, as well as HMQC and HMBC (see Table 1 and Figure 2).By comparing 1 H-NMR spectra data with those of similar compounds [4c], we elucidated the new chiral carbon atom C(2) to be (S)-configuration.To further confirm the absolute configuration of isomer 4a, we performed an X-ray crystal diffraction analysis.A perspective view the numbering scheme adopted for the 4a molecule is depicted in Figure 3  In order to explain the (2S, 17R)-configuration preference of the title compound crystallized from methanol, the total energies of compounds (2S, 17R)-4a and (2R, 17R)-4b were calculated using the HYPERCHEM program [7] at the semi-empirical PM3 computational level, where a root-mean-square gradient of the forces acting on each atom of 0.05 kcal/molÅ as the convergene criterion was employed.The final energies of 4a (-7332.73a.u.) was slightly lower than that of 4b (-7327.60a.u.).Thus, qualitatively, there is energy advantage for the indolizine analogue 4a.We conclude that there is a fast dynamic enol tautomerism equilibrium between compounds 4a and 4b (Figure 4).Upon changing the exterior condition such as heating, the equilibrium was destroyed and (2R, 17R)-4b was converted into the relatively stable (2S, 17R)-4a with lower energy.

Conclusions
For the first time we have prepared optically active [N-tosyl-(R)-prolyloxy]-2(S)-[4-cyano-8,8ethylenedioxy-5-oxo-5,6,7,8-tetrahydroindolizin-3-yl] acetate (4a) and characterized it by 2D-NMR spectroscopy and X-ray crystallography.Theoretical calculations have further confirmed the configurational analysis.From the point of view of the synthetic strategy and atomic economy, it is preferable if chiral centers are constructed as soon as possible in an asymmetric synthetic route.Use of optically active compound (2S, 17R)-4a as substrate instead of the usual mixture of diastereoisomers 4a and 4b, should help improve the stereoselectivity of the subsequent ethylation process.Further studies are under way to optimize the reaction conditions for this diastereoselective ethylation and to develop a more practical and scaled-up preparation of 20(S)-CPTs.

X-ray techniques
A block single crystal of the title compound 4a suitable for X-ray diffraction was grown by slow evaporation from a methanol solution.Crystal and experimental data are summarized in Table 2.The data were collected with a Bruker SMART CCD area-detector diffractometer equipped with a graphite-monochromatized Mo Kα radiation (λ = 0.71073Å) at 293(2) K.The crystal structure data have been deposited at the Cambridge Crystallographic Data Ceter [8].Absorption corrections were made with semi-empirical from equivalents.The crystal structure was resolved by direct methods using SHELXS-97 [9a] and refined by full-matrix least-squares methods on F 2 using SHELXL-97 [9b].All non-H atoms were refined anisotropically.

Table 2 .
Crystal and experimental data for compound 4a.