Rearranged taxanes from Taxus baccata

Pergnmon Phymchmiwy. Vol. 36. No. 2, pp. 407411. IS94 Copynghl e) 1994 Elscvicr Scicna Ltd

Pnnkd in Great Britam. All rights mewed 0031.9422/w 9700+0.00

REARRANGED TAXANES FROM TAXUS BACCATA*

GIOVANNI APPENDINO,~ GIANCARLO CRAVOTTO, RENATA ENRI~~,JASMIN JAKuPovIc,t~ PIERLUIGI GARIBOLDI,~ B. GABE'ITA and E. BOMBARDELLIII

Dipartimento di Scienza e Tecnologia del Farmaco, Via Giuria 9, 10125 Torino, Italy; zlnstitut fiir Organische Chemie, Technische UniversitHt Berlin, Strak des 17. Juni, 135, loo0 Berlin. Germany; $Dipartimento di Scienze Chimiche, Via S. Agostino 1, 62032

Camerino (MC), Italy; lllndena S.p.A., Via Ripdmonti 99, 20141 Milano, Italy

(Receic;ed in revised form 30 November 1993)

Key Word Index-Taxus baccata; Taxaceae; taxanes.

Abstract-The needles of Taxus baccata gave two rearranged taxanes of the 11 (15+1)- and 2 (3+20) abeo-type. A taxicin I triester was also isolated.

INTRODUCTION

Very little is known on the biosynthesis of taxanes. An isoprenoid derivation is shown by the incorporation of mevalonic acid [I], but the various steps involved in the assembling of the bridgeheaded skeleton still await clari- fication. The early suggestion that taxanes are degraded triterpenoids related to quassinoids [2] has been aban- doned, and the current view is that taxanes are diterpen- oids from both the structural and biosynthetic point of view. The transannular cyclization of a cyclic 1,5-diene system is a common leitmotif in the biosynthesis of terpenoids, and the relationship between taxanes and verticillenes is the same as exists in the field of sesquiter- penoids between germacradienes and eudesmanes. How- ever, all attempts to cyclize verticillene derivatives to taxanes have failed [3], suggesting a more subtle bio- genetic sequence. Clues might come from the isolation of compounds related to the alleged intermediates of the verticillene pathway, and from a better knowledge of the structural variation within the natural taxanes. More than 100 diterpenes of the taxane group have in fact been isolated [4], but only a few basic structural types exist, and variation is mostly found in the acylation pattern. The isolation of-compounds based on a modified skeleton is therefore significant. We report here the isolation of two diterpenoids with a rearranged taxane skeleton, present in a minute amount in the needles of the European Yew (Taxus baccata L.).

REHJLTSANDDISCUSSION

The tetraol la was isolated as an amorphous powder. Its ‘H NMR spectrum was similar to that of baccatin VI

*Part 11 in the series ‘The Chemistry and Occurrence of Taxane Derivatives’. Part 10: Appendino, G., Cravotto, G., Garibaldi, P.. Gabetta, B. and Bombardelli, E. (1994) Gazz. Chim. Ital. (in press).

tAuthors to whom correspondence should be addressed.

(2) [S], but only three acyl groups were present, a benzoate and two acetates. The oxymethine protons at C- 2 and C-13 showed a three-bond coupling with the benzoate carbonyl and with one of the acetate carbonyls, respectively, thus locating two of the ester groups. The last acetyl was at the C-4 hydroxyl, as shown by the typical ‘H and 13C resonances of the oxetane moiety (Tables 1 and 2) and by experiments of stepwise hydroly- sis (see infra). The ‘%NMR spectrum of la showed a downfield signal (667.8) for one of the quaternary ali- phatic non-oxygenated carbons, suggesting that la has a rearranged abeotaxane skeleton [a]. Acetylation of la afforded a penta-acetylated taxane different from bac- catin VI (2) and formulated as lb on the basis of diag- nostic TAI (trichloroacetyl isocyanate)-acylation shifts (upfield shift of H-9, marked downfield shift of H-16 and H-17) and ROE effects (OH-H-9 ROE) [6]. Treatment of la with K&O3 in methanol hydrolysed the acetates at C- 13 and C-4, but left the benzoate unaffected. As a result, a mixture of lc and Id was obtained, in a ratio depending on the conversion (see Experimental). In baccatin VI derivatives, hydrolysis of the benzoate at C-2 is easier [7, 83, possibly on account of the presence of hydroxyl at C-l, which can provide anchimeric assistance to the hydrolysis of an ester group at C-2 via the formation of a hemi- orthoester. Similar effects, but involving Lewis acid chel- ation, have been observed in baccatin III derivatives between the hydroxyl at C-l and the benzoate at C-2 [9], and between the hydroxyl at C-7 and the acetyl at C-10 [lo]. A stable C-l, C-2 orthoester has also been prepared in a compound of the taxicin I series [ 111. Abeobaccatin VI derivatives isolated from the Himalayan [6] and the European Yew have a different acylation pattern, and this might be taxonomically relevant.

Compound 3a was obtained as a gum and showed spectral features different from those generally observed in taxanes. Indeed, besides a ketone carbonyl, five oxy- genated methines and a tetrasubstituted double bond, the 1 % NMR spectrum showed the presence of an additional

407

408 G. APPENDINO et al.

R’ R2 R’ la AC H AC lb AC AC AC lc AC H H Xd II fI It

R

OH

H3’ NMe2

ROOC Hi

A

trisubstituted double bond and three aliphatic methyl- enes. The ‘H NMR spectrum was similar to that of taxine A (3b) [ 121, formally a 2-(3-+20) abeotaxane derivative. However, the signals of the phenylisoscrine ester group were missing, and H-5 had moved upfeld. Furthermore, the chemical shift of H-62 was normal (6 1.78 ppm). These observations showed that 3a is deaminoacyltaxine A, and suggested a ratjonalization for the very unusual and remarkable chemical shift of H-61 in taxine A (6 -0.13 ppm!) [ 123. This upiield resonance is presumabfy the result of a conformation of type A of the phenylisose- rine amino acidic chain. As a result of this, the phenyl ring is oriented below H-65(, which lies in the shielding zone of the aromatic ring current. The presence of a conforma- tion of this type is in accordance with the resuh of an X- ray analysis of taxine A [i2], as well as with the large J-

0 % 8 OCinn

HO

R

49 H

4b AC

II

value (10 Hz) between H-2’ and H-3’ [ 12). The phenyiis- oserine chain of taxol adopts instead a conformation of type B, with a much smaller J value (ca 3 Hz) between H- 2’ and H-3’ f133. These differences might be retated in part to the presence of a different set of hydrogen bondings in N-acyl and N,N-dialkyl phenylisoserine de- rivatives, with nitrogen acting as a donor when acylated, and as an acceptor when dialkylated. Compound 3a has a marked caged conformation, with an intramolecular hydrogen bonding between the hydroxyl at C-5 and the acetate at C- 13.

Taxanes and 1 I( 15-r lkrheotaxanes are related by a Wagner-Meerwein rearrangement of ring A [63; taxanes and 2(3-+20) aheotaxanes, however, are apparently de- rived from isomeric verticilladiene precursors (AZ‘ ’ and A4’20’* ‘, respectively) (Scheme 1). The intramolecular

Rearranged taxanes from Taxus hoccata 411

11.

12.

13.

14.

Table 2. 13CNMR data for In-d, 3a (CDCI,, TMS as internal standard)

C ISI lb 1c* Idt 3n

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 Bnz

AC

67.8 s 68.2 s 68.7 d 68.2 d 44.9 d 44.9 d 79.8 s 19.4 s 84.9 d 84.5 d 37.2 r 34.8 t: 72.5 d 75.3 d 42.4 s 43.6 s 80.6 d 77.2 d$ 68.3 d 70.6 d

139.0 s 136.1 s 143.7 s 147.2 s 79.4 d 78.7 d§ 36.4 r 36.8 r$ 76.4 s 16.6 s 27.9 q 27.6 q 24.7 q 25.2 q 11.3q 11.8q 12.2 q 12.5 q 74.4 I 74.5 I

166.2 s 165.8 s 133.6 d 133.5 d 129.7 s 129.9 s 129.6 d 129.7 d 128.6 d 128.6 d 170.8 s 170.7, 169.7 22.1 q 169.6, 169.0

169.1 s 167.7 s 21.2q 21.9. 21.4. 21.1, 20.8 q

67.8 s 68.9 d 44.7 d 80.5 s 85.0 d 37.3 t 72.5 d 42.6 s 80.9 d 68.9 d

137.2 s 147.0 s 77.6 d 39.6 I 76.5 s 27.8 q 24.9 q 11.3 q 12.2 q 74.8 I

166.1 s 133.4 d 130.1 s 129.6 d 128.6 d 170.9 s 22.3 q

67.3 s 69.0 d 49.7 d 72.6 s 88.1 d 37.3 t 72.7 d 42.0 s 80.6 d 68.7 d

133.5 s 146.5 s 77.0 d 39.8 I 76.0 s 27.7 q 24.7 q 11.4q 12.oq 77.2 r

166.1 s 133.0d 130.2 s 129.6 d 128.4 d

_

44.8 d 70.3 d 26.3 I

139.1 s 69.1 d 30.9 t 67.1 d 53.2 s

214.1 s 77.0 d

132.8 s 135.9 s 69.7 d 35.1 r 37.2 s 35.1 q 23.9 q 17.9 q!? 18.6 qs:

123.3 d _

_

170.0 s 21.34 q

169.9 s 21.oq

*Taken at SO”. tCDCI, + DMSO-I,. :$Interchangeable signals.

Dukes, M., Eyre, D. H., Harrison, J. and Lythgoe, B. (1965) Tetrahedron Letters 4765. Graf, E., Kirfel, A., Wolff, G. J. and Breitmaier, E. (1982) Liebigs Ann. Chem. 376. Chmurny, G. N., Hilton, B. D., Brobst, S., Look, S. A., Witherup, K. M. and Beutler, J. A. (1992) J. Nut. Prod. 55, 414. Chiari, G., Appendino, G. and Nano, G. M. (1986)

J. Chem. Sot. Perkin Trans I1 263. 15. Barton, D. H. R. and de Mayo, P. (1957) Quart. Reo.

11, 189. 16. Zhang, Z., Jia, Z., Zhu, Z., Cui, Y., Cheng, J. and

Wang, Q. (1990) Planta Med. 56, 293. 17. Appendino, G., Gariboldi, P., Pisetta, A., Bombar-

delli, E. and Gabetta, B. (1992) Phytochemistry 31, 4253.

PHYTO 36:2-K