Chemistry and occurrence of taxane derivatives. Part 16. Rearranged taxoids fromTaxustimes;mediaRehd. cv Hicksii. X-Ray molecular structure of 9-O-benzoyl-9,10-dide-O-acetyl-11(15 rarr; 1)abeo-baccatin VI

摘要

J. CHEM. soc. PERKIN TRANS. 1 1994 Chemistry and Occurrence of Taxane Derivatives. Part 16.' Rearranged Taxoids from Taxus x media Rehd. cv Hicksii. X-Ray Molecular Structure of 9-0-Benzoyl-9,lO-dide-0-acetyl-I 1(15 -1)abeo-baccatin VI Luciano Barboni," Pierluigi Gariboldi,'ea Elisabetta Torregiani.a Giovanni AppendinO? Giancarlo Cravotto,b Ezio Bombardelli,c Bruno GabettaC and Davide Viterbod a Dipartimento di Scienze Chimiche, Via S. Agostino 7, 62032 Camerino (MC), Italy Dipartimento di Scienza e Tecnologia del Farmaco, Via Giuria 9,70725 Torino, Italy lndena S.p.A., Via Ripamonti 99, 20747 Milano, Italy Dipartimento di Chimica lnorganica, Chimica Fisica e Chimica dei Materiali, Via Giuria 7, 70125 Torino, Italy The roots of Taxus x media Rehd. cv Hicksii gave four new taxoids of the abeo 11(1 5 -1 )-type (2-5).Structures were elucidated by spectroscopic techniques, including X-ray analysis for compound 4. Compound 2 is conformationally fixed, whereas compounds 3-5 exist in solution as a mixture of rotamers, whose interconversion was investigated by variable-temperature NMR spectroscopy. Compound 2 has a C-13, C-15 oxygen bridge, and is the first natural taxoid with a b-oxygen function at C-13. 1 1 (15 -1)abeo-Taxanes are an emerging major structural type of taxoids.' Brevifoliol 1 was the first compound of this type isolated,' but its rearranged structure was recognised only Compound 1 is available in large amounts from HO--@13~15 the needles of Taxus breuifolia Nutt.,' whereas the other compounds of this type isolated to date are only minor 'OH H iBZc) e o 20constituents of yew extracts.All 11( 15 +1)abeo-taxanes 17 HO l6 17 isolated to date bear a hydroxy group at C-15, whereas compounds of the A1'*16-type have been obtained only from 1 2 the acid rearrangement of 1-hydroxylated taxanes.6-8 In spite of their widespread occurrence, 11(1 5 --+ 1)abeo-taxanes escaped detection for a long time, probably because of (i) their low concentration and (ii) some ambiguities in their NMR AcO-analysis. Indeed, the NMR spectra of abeo-taxanes are often very broad at room temperature, and of limited utility for structure el~cidation.~.~ Furthermore, when sharp 'H NMR spectra are observed, chemical shifts, multiplicities and NOE effects are similar to those of their corresponding taxanes, and a 3 R1=R4=H, R2=R3=Ac clear-cut distinction cannot be made without 13CNMR data.2 4 R'=Ac, R2=Bz, R3=R4=H As a result, several abeo-taxanes were originally reported as 5 R'=R3=Ac, R2=amp; R4=H1-hydroxylated taxanes.' As part of an ongoing investigation into taxoids from various 6 R' = Fl'l= CbCCONHCO, R2= R3= AC yew species, we report the isolation of four new abeo-taxanes from the roots of Taxus x media Rehd.cv Hicksii. This 7 R' = R3 = Ac, R2= Bz. R4= CIamp;CONHCO ornamental yew is popular as landscape material, and is extensively cultivated in nurseries. Previous studies had shown the presence of taxol and some related taxanes in this cultivar. *1 Results and Discussion The crystalline compounds 2-5 were obtained after repeated chromatographic separation of fractions containing baccatin VI 913 as the major constituent.? Compound 2 (C35H42012) showed sharp NMR spectra at room temperature.The presence of acetoxy groups at C-7, C-9, C-10 and of a benzoyloxy group 9 at C-2 was evident from the presence of diagnostical 'H and 13C signals (Tables 1 and 2) and the detection of a long-range 'H-carbonyl. The signals of the 4-acetoxy-5(20)-oxetane moiety "C correlation between the C-2 methine and the benzoate were also evident from the 'H and 13C NMR spectra. A rearranged abeo-taxane structure (singlet at 6 67.1)' and two t Full spectroscopical data on baccatin VI are not available in the additional oxygenated carbons S 81.4 (d) and 78.6 (s), C-13 literature, and are reported in Tables 1 and 2.and C-15 respectivelyl were also present. Since compound 2 3234 J. CHEM. SOC. PERKIN TRANS. 1 1994 Table 1 'H NMR spectra of compounds 2,3,7,9 (300 MHz; CDCl,; 6, in ppm from internal SiMe, ~ ~~ H 2 3" 7 9 2 3 5 6a 6.62 d (7.6) 3.26 d (7.6) 4.93 dd (9.9, 5.2) 2.46 m 6.60 d (8.0) 2.38 d (8.0) 4.72 dd (8.0,2.7) 2.37 m 6.47 d (7.3) 3.12 d (7.3) 4.93 d (7.2) 2.68 m 5.85 d (6.2) 3.16 (d (6.2) 4.95 br d (8.9) 2.48 m 6P 2.00 m 1.97 m 1.80 m 1.84 m 7 9 10 13 14a 16 14P 5.32 dd (12.5, 5.5) 5.00 d (5.7) 6.01 d (5.7) 4.37 br d (1.8) 1.67 br d (8.2) 2.20 dd (8.2, 1.8) 0.94 s 5.26 br s 20 5.94 br s 40 6.18 d (9.3) 5.60 t (7.6) 2.29 dd (14.0, 7.0) 2.49 dd (14.0, 7.3) 1.13 s 5.55 dd (7.9, 7.2) 6.11 d (10.6) 6.48 d (10.6) 5.75 br t (7.3) 2.04 dd (14.7, 7.4) 2.54 dd (14.7, 7.2) 1.66 s 5.50 dd (9.7, 7.8) 5.98 d (1 1.5) 6.20 d (1 1.5) 6.14 dt (8.6, 1.4) 2.20 m 1.76 s 17 1.08 s 1.13 s 1.58 s 1.21 s 18 19 1.78 s 1.74 s 1.85 s 1.69 s 1.93 s 1.79 s 2.01 d (1.4) 1.58 s 20a MeCO 2OP 4.25 d (7.8) 4.56 d (7.8) 2.22, 2.17, 1.99, 1.98 s 3.96 d (7.8) 4.56 d (7.8) 2.15, 2.07, 2.03, 1.99 s 4.16 d (7.3) 4.38 d (7.3) 2.19, 2.18, 1.76, 1.62 s 4.31 d (8.3) 4.10 d (8.3) 2.27,2.17,2.09,2.08, 1.98 s 4-OH 2.70 s 15-OH 2.44 s 2',6' 3'3' 4' 8.09 d (7.3) 7.62 t (7.3) 7.49 t (7.3) 7.95 d (7.8) 7.46 t (7.8) 7.59 t (7.8) 8.01 d (7.8), 7.97 d (7.8) 7.64 t (7.Q 7.62 t (7.8) 7.49 t (7.8), 7.48 t (7.8) 8.09 d (7.7) 7.46 t (7.7) 7.59 t (7.7) a 50 "C.Values in square brackets are linewidths in Hz. Table 2 NMR spectra of compounds 2,3,7,9 (75.43 MHz; CDCl,; 6, in ppm from internal SiMe,) C 2 3" 7 9 1 67.1 s 67.9 s (24) 67.9 s 78.8 s 2 65.8 d 69.8 d (1 5) 67.6 d 73.2 d 3 44.9 d 49.4 d (1 6) 44.1 d 47.2 d 4 82.0 s 73.7 s (26) 83.6 s 81.5 s 5 84.0 d 86.4 d 84.6 d 83.8 d 6 31.8 t 34.0 t (15) 34.8 t 34.5 t 7 70.5 d 70.8 d 70.4 d 71.7 d 8 43.4 s 43.3 s 43.8 s 45.7 s 9 73.2 d 76.2 d (1 8) 77.8 d 75.0 d 10 65.8 d 68.3 d 67.1 d 70.3 d 11 133.6s 136.5s (15) 135.1 s 133.6 s 12 148.0 s 146.3s (65) 148.9 s 141.2 s 13 81.4 d 79.7 d 78.1 d 69.6 d 14 45.8 t 36.8 t 36.7 t 35.1 t 15 78.6 s 75.4 s 78.8 s 42.7 s 16 26.0 q 26.1 q (50) 22.5 q 22.2 q 17 28.6 q 28.0 q (12) 22.3 q 28.2 q 18 11.8 q 11.9q 12.1 q 14.9 q 19 13.1 q 12.8 q 13.6 q 12.7 q 20 77.5 t 78.8 t (28) 74.3 t 76.3 t MeCO 21.8 q, 21.2 q, 21.1 q, 21.1 q, 22.0 q, 21.7 q, 22.7 q, 21.3 q, 21.1 q, 20.6 q 20.6 q, 20.6 q 21.1 q, 20.6q 21.2 q, 20.9 q, 20.7 q MeCO 170.0 s, 169.9 s, 170.9 s, 169.9 s, 170.6 s, 170.0 s, 170.4, s, 170.1 s, 169.3s, 168.7 s 169.4 s, 167.9 s 168.9 s, 168.9 s 169.8 s, 169.1 s, 168.8s 1' 129.9 s 129.7 s 129.3, 128.7 s 129.2 s 2',6' 130.0 d 129.4 d 130.4, 129.7 d 130.1 d 3'3' 128.8 d 128.1 d 128.8, 128.6 d 128.6 d 4' 133.7 d 133.6 d 134.0, 134.0 d 133.6 d 7' 166.4 s 165.5 s 168.9, 165.9 s 166.9 s acylcarbamate 159.3 s, 148.8 s, 91.0 s " 50 "C.Values in parentheses are linewidths in Hz. an observation consistent had no hydroxy groups (IR spectroscopy) and one additional oxygen bridge, and thus 13-H is CC, unsaturated degree had yet to be accounted for, the presence of with its unusual splitting pattern (br d, J 1.8 Hz).Compound 2is an oxygen bridge between these two oxygenated carbons the first natural taxoid with a P-oxygenated function at C-13,I4 seemed likely. This was unambiguously confirmed by the and the oxygen bridge between C-13 and C-15 is presumably functiondetection of a long-range correlation between 13-H and C-15. formed by SN2-type displacement of a 13~~-oxygenated Stereochemical constraints require a p-orientation for the by the tertiary C-15 hydroxy group. Ring B has a twist-chair J. CHEM. SOC. PERKIN TRANS.I 1994 H9 RO OR OH A B Fig. 1 Newman projection along C,,-C, and endocyclic torsion- angle sequence (Bucourt notation) of ring B in abeo-taxanes. A: twist-boat conformation; B: twist-chair conformation (ref. 15). conformation in compound 2, with the oxygen function at C-9 and C-10 pseudoaxial and 9-H and 10-H pseudoequatorial (J,,,o 5.7 Hz) (Fig. 1). This arrangement is different from that found in taxanes, where ring B has a twist-boat conformation and 9-H and 10-H are pseudoaxial (J -10 Hz).~ In taxoids, values of J9,10 -5 Hz can be misleading as regards the configuration at C-9 and C-10. Both cis and trans protons can in fact display values of this type, depending on the conformation of ring B.* In compound 2, a rotating frame NOE (ROESY) cross-peak between the C-19 methyl and 9-H and between the C-16 (pro-R)i methyl and the acetate at C-10 confirmed the trans relationship between 9-H and 10-H.The splitting pattern of 5-H in compound 2 (dd, J 9.0 and 5.2 Hz) is also somewhat unusual, since in oxetane-type taxoids this proton resonates as a broad doublet or as a doublet of narrow doublets. The 'H NMR spectrum of compound 3 (C3amp;4013) was rather broad at room temperature, but heating to 50deg;C sharpened all signals with the exception of those for 7-H and 9-H. In the I3C NMR spectrum taken at this temperature, only one set of lines was present, although some of them were rather broad ( 10-65 Hz). Compound 3 was stable at 50 "C, and its NMR spectra could be fully assigned at this temperature by mono-and bi-a241 C(251 Fig. 2 Molecular structure of compound 4 f0.25 ppm, respectively) could be rationalised in terms of anisotropic deshielding from the newly introduced carbamate carbonyl at C-4, in the concave face of the molecule.Acetylation of the C-15 hydroxy group is instead responsible for the downfield shift of the gem-dimethyl groups (Ab +0.48 and +0.51 for 16-H and 17-H, respectively). J,, measured at 50 OC, under conditions of fast exchange, was 9.7 Hz, only slightly lower than normal (10-11 Hz). This indicates that the conformational equilibrium of compound 3 is highly biased, with a large predominance of the rotamer with ring B in a twist- boat conformation. The biscarbamate derivative 6 displayeddimensional techniques (COSY, ROESY, J and 233Jc-Hsharp NMR spectra at room temperature, suggesting that correlations).A singlet at 6 67.9 in the 13C NMR spectrum indicated an abeo-taxane structure,2 and multiplicity and coupling patterns of all protons and carbons were identical with those of abeo-baccatin VI 8.17 Comparison of the molecular formulae revealed that compounds 3 and 8 differ by the absence of one acetyl group in compound 3. All esters group of compound 3 were bound to methine carbons (chemical-shift considerations and diagnostic JC-Hcorrelations). Therefore C-15 and C-4 bear free hydroxy groups, and compound 3 is 4-0-deacetyl-aheo-baccatin VI. This was confirmed by the formation of a biscarbamate 6 by in situ reaction with trichloroacetyl isocyanate (TAI).* The downfield acylation shifts observed for 3-H, 5-H and 7-H (Ad +0.78, +0.30 and * A value of 5.5 Hz for J9,10was reported in a synthetic taxane having a cis 9,lO-diolsystem (ref. 16). t The numbering of the gem-dimethyls of abeo-taxanes is ambiguous; we suggest considering the methyl pointing toward ring B (pro-R methyl, when the hydroxy group is oriented toward the oxygen function at C-10) C-16, and the other, pointing toward ring A (pro3 methyl, when the hydroxyisopropyl side chain is oriented as above) C-17. This numbering maintains the topological relationship between the methyls and the rest of the diterpenoid core observed in taxanes, although prochirality descriptors are inverted, owing to changes in prochirality .intramolecular hydrogen bonding plays a role in the conformational equilibration of compound 3 (see below). Compound 4 (C4oH4amp;1 3) showed very broad NMR spectra at room temperature, where most signals were featureless humps. Upon cooling to -40 "C, a spectrum containing two sets of sharper signals was obtained, whereas heating to 120 "C was required to get fast exchange and one set of signals. The spectrum at low temperature was too complex, and compound 4 was not stable enough at 120deg;C to allow thorough spectroscopic characterisation to be made. Eventually, crystals suitable for an X-ray analysis were obtained. The results showed that compound 4 is an abeo-baccatin VI derivative lacking the C-10 0-acetyl and with a benzoate in place of the acetate at C-9.The asymmetric unit is made up of two independent molecules having similar conformations. Only one of them is shown in Fig. 2. In the solid state, ring B adopts a twist-boat conformation and ring C an almost ideal 1,2-diplanar (= sofa) conformation. Both hydroxy groups are involved in hydrogen bonds: the one at C-10 acts as a donor toward that at C-15, which in turn acts as a donor toward the C-9 benzoate carbonyl of another molecule. A packing analysis showed that these are the only short contacts found in the structure. Acetylation of compound 4 gave the monoacetyl derivative 5, identical with a compound isolated from the fractions J. CHEM. SOC. PERKIN TRANS.1 1994 I I I I I I I I 6.0 5.5 5.0 4.5 4.0 3.5 3.0 9+12 I I I I 6.5 6.0 5.5 5.0 10+2 1 I I I I6.5 6.0 5.5 5.0 Fig. 3 'H NMR spectra of compound 5: (a) at room temperature; (b) at (CDCI,; 300 MHz). * Minor rotamer. containing compounds 2-4. The 'H NMR spectrum of tetraacetate 5 showed two sets of broad signals at room temperature, which sharpened upon cooling. At -20 "C two sets of sharp signals in a -4:l ratio were observed (Fig. 3), and all signals could be assigned. The value of Jg,lowas 10.4 Hz in the major rotamer and 4.7 Hz in the minor one. Thus, ring B has a twist-boat conformation with diequatorial ester groups at C-9, C-10 in the major rotamer and a twist-chair conformation with these group diaxial in the minor rotamer.An in situ reaction of compound 5 and TAI gave the carbamate 7, whose NMR spectra were sharp at room temperature (Fig. 3) and could be fully assigned. J9,10(1 1.5 Hz) showed that ring B adopts a twist-boat conformation in carbamate 7. The conformational features of abeo-taxanes 2-5 are dif- ferent. Compound 2 is anancomeric (conformationally fixed) and adopts a ring B twist-chair conformation to avoid a severe steric interaction between the pro-R methyl at C- 15 (1 6-H) and 9-H. Compounds 3-5 are instead each a mixture of rotamers, whose equilibration is governed by several factors, the major one being the formation of intramolecular hydrogen bonding between the hydroxy group at C-15 and the oxygen function at C-10.The a-hydroxyisopropyl group of abeo-taxane adopts a conformation that mimics the gem-dimethyl bridge of taxane: the pro-S methyl points towards ring A, and the other methyl towards ring B, allowing the formation of hydrogen bonding between the hydroxy group at C-15 and the C-10 oxygen. This arrangement is found in the solid-state conformation of both compound 4 and all the abeo-taxanes investigated by X-ray crystallography.2*20.t' Depending on the acylation state of the oxygen function at C-10, the hydroxy group at C-15 can act as a donor (acyloxy group at C-10) or as an acceptor (hydroxy at C-10) of hydrogen bonding. The existence of this arrangement also in solution is consistent with the ROE pattern of the geminal methyls and with the presence of an ROE correlation between 9-H and the 15-hydroxy proton.2 The C-15 hydroxy I I I I 4.5 4.0 3.5 3.0 I I I I4.5 4.0 3.5 3.0 6, -20 "C;(c)after addition of TAI (ie.,during formation of compound 7) group and the oxygen at C-10 are much closer when ring B adopts a twist-chair conformation, to the point that an oxygen bridge can form between C-15 and C-lO.'** Therefore, intra- molecular hydrogen bonding between the oxygen functions at C-15 and C-10 is expected to stabilise the twist-chair conformation more than the twist-boat.Acylation of the C-15 hydroxy group gives anancomeric compounds, since this hydrogen bonding is not possible any more, and the twist-boat conformation, having diequatorial oxygen functions at C-9 and C-10, is the only detectable conformation in solution (cf: carbamates 6 and 7).Since the abeo-taxane skeleton has a crowded topology, functionalities can strongly interact with each other, and the conformational equilibration of ring B is also affected by the formation of additional intramolecular hydrogen bonds as well as by the nature of the ester groups. Free hydroxy groups at C-4 and C-10, as in compounds 3 and 4, can act as hydrogen- bonding donors toward ester carbonyls at C-13 and C-9 respectively, and the phenyl ring of benzoate esters can be involved in lipophobic interactions. This subtle interplay of functionalities is responsible for the wide range of temperatures over which fast exchange is observed in abeo-taxanes (e.g., 50 "C for 3, 120 "C for 4).In oxetane-type taxoids, ring C is conformationally biased on account of the presence of the oxetane ring. In brevifoliol-type abeo-taxanes, where a C-4-C-20 double bond is present, ring C is instead flexible and the conformational analysis of these compounds is therefore more complex. Our results suggest that the broad NMR spectra sometimes observed in abeo-taxanes are related to stabilisation of the 'unusual' twist-chair conformation of ring B by intramolecular hydrogen bonding between the tertiary hydroxy group at C-15 and the oxygen function at C-10. Acylation of the tertiary hydroxy group at C- 15 gives anancomeric compounds, whose spectra are amenable to routine structure elucidation. The isolation of baccatin VI 9 and the abeo-taxanes 24from J.CHEM. soc. PERKIN TRANS. 1 1994 T. x media confirms the trend, already observed in other yew species, that taxanes and abeo-taxanes generally show different acylation patterns. Benzoylation at C-9, as in compounds 4 and 5, and deacylation at C-4, as in compound 3, are in fact unprecedented within baccatin VI derivatives. Baccatin VI has been obtained from at least four different species of yew. 13*21,22 abeo-Baccatin VI is still unknown as a natural product, since only partially deacylated or more benzoylated derivatives have been isolated. Migration of ester groups from 0-9 to 0-10 is documented in taxoids under conditions similar to the isolation procedure,23 and compound 4 may thus be an extraction artefact derived from the corresponding C-10 benzoate, a hitherto unknown compound.Experimental M.p.s were determined on a Biichi SMP 20 apparatus; optical rotations were measured on a Perkin-Elmer 24 1 polarimeter; a,-values are given in units of lo-' deg cm2 g-'; UV spectra were recorded on a Beckmann DB-GT spectrophotometer, IR spectra on a Perkin-Elmer model 127 spectrophotometer, EIMS spectra on a VG EQ apparatus, and 'H and 13C NMR spectra on a Varian UNITY 300 spectrometer operating at 299.94 MHz for 'H and 75.43 MHz for 13C; J-values are given in Hz. Silica gel LiChroprep Si 60 (1 5-25 pm, Merck) was used for medium-pressure (5-1 5 bar) column chromatography and silica gel 60 (70-230 mesh, Merck) for atmospheric-pressure column chromatography.HPLC purifications were carried out using a Hewlett-Packard series 1050 instrument equipped with an HP 1047 A refractive index detector; a Waters microporasil column (0.8 x 30 cm) was used for compounds 2 and 3, and a Waters Delta Pack C,, column (0.78 x 30 cm) for compounds 4 and 5. Commercially available roots of Taxus x media Rehd. cv Hicksii were identified by Dr. U. Boni (Indena S.p.A); a voucher specimen is kept at the Indena labs, Milano, Italy. Isolation of Compounds.-Finely ground roots (1 200 kg) were stirred at room temp. with MeOH-water (1 :1) and filtered. Counter-current extraction of the percolates with CH2C12 provided, after evaporation to dryness, a residue (28 kg), which was chromatographed on silica gel and eluted with toluene- Me2C0 (9:1),Three fractions A (12 kg), B (3.9 kg) and C (8 kg) were collected.Fraction B was chromatographed on silica gel and eluted with cyclohexane-Me2C0 (6 :4); fractions D (0.5 kg) and E (0.4 kg) were obtained. A portion (20 g) of fraction E was chromatographed on silica gel (hexane containing increasing amounts of EtOAc as eluent). Fractions eluted with hexane-EtOAc (1 : 1) gave a mixture of taxoids (2.7g), which was further separated by column chromatography to give baccatin VI 9 (290 mg) and a mixture of compounds 2 and 3, which were separated by HPLC hexane-EtOAc (6 :4) to give compounds 2 (42 mg) and 3(36 mg). 13-Deacetoxy-13,15-epoxy-1 1(15 ---+ 1)abeo- 13-epi-baccatin VI 2 (42 mg, 7 x lo-' ), powder, m.p.150deg;C (decomp.); a? +23.9 (c 0.58, CHCl,) (Found: C, 64.4; H, 6.4. C35H42012 requires C, 64.2; H, 6.5); A,,,(EtOH)/nm 230 and 265; v,,,(KBr)/cm-' 1745, 1720, 1370, 1240, 1040,995 and 710; m/z (negative ions) 654 (M-, 100). 4-De-0-acetyl-1 1 (15 -1)abeo-baccatin V1 3 (36 mg, 6 x lo-' ), powder, m.p. 222 "C (decomp.); aj$' -73.1 (c 0.33, CHC1,) (Found: C, 62.4; H, 6.7. Camp;4amp;13 requires C, 62.5; H, 6.6); A,,,(EtOH)/nm 230 and 268; v,,,(KBr)/cm-' 3550, 1730, 1380, 1240, 1200, 980 and 720; m/z (negative ions) 672 (M-, 100). Baccatin VI 9 (290 mg, 4.8 x lo4 ), m.p. 236-237 "C (from MeCN-water) (lit.,', 244-245 "C); a;' -8.7 (c 0.92, 3237 CHCl,) (lit.,I3 -5) (Found: C, 62.3; H, 6.7. Calc.for C37H46014: C, 62.2; H, 6.5). A portion (6 g) fraction D was chromatographed on a silica gel column (medium pressure) and eluted with CHC1,-MeOH (97: 3). The first twenty fractions were collected (2.7 g) and chromatographed at medium pressure with hexane-Me,CO (7 :3) as eluent; a crude mixture (580mg) containing compounds 4 and 5 was obtained. This mixture was chromatographed by HPLC MeCN-water-MeOH (75 :25 :4) to afford crude compounds 4 and 5. After final purification by HPLC MeCN- water-MeOH (70: 30 :4) and MeCN-water (7 :3) for com-pounds 4 and 5, respectively the dibenzoates 4 (35 mg) and 5 (40mg) were obtained. 9-0-Benzoyl-9,lO-dide-0-acetyl-1 1 (15 -+ 1)abeo-baccatin V14 (35 mg, 2.4 x lo4 ), m.p. 238 "C(from MeOAc-hexane); a;' -30.5 (c 0.91, CHCl,) (Found: C, 65.3; H, 6.2.C40H46013 requires C, 65.4; H, 6.3); A,,,(EtOH)/nm 232 and 265; v,,,(Nujol)/cm-' 3300, 1720, 1240, 1020 and 715; m/z (negative ions) 735 (M-, 100). 9-0-Benzoyl-9-de-O-acetyl-11(15-1)abeo-baccatin V1 5 (40 mg, 2.8 x lo4 ), oil; aj$O -32.5 (c 0.92, CHCl,) (Found: C, 64.9; H, 6.3. C42H4amp;14 requires C, 64.9; H, 6.2); A,,,(EtOH)/nm 230 and 265; v,,,(Nujol)/cm-' 3290, 1720, 1240, 1025 and 720; m/z (negative ions) 777 (M-, 100). Reaction of Compounds 3 and 5 with TAL-An excess TAI was added to an NMR tube containing a solution of substrate 3 or 5. The formation of the carbamate derivatives was monitored by 'H NMR spectroscopy, and the reaction was complete in 4 h. The solution of these carbamates was stable for several days.X-Ray Analysis.-Diffraction data were collected with a Siemens P4 diffractometer equipped with a graphite mono- chromator. The intensities were measured by o-scan with variable speed; the cell parameters were obtained and refined from 35 reflections. The intensities were corrected for background and Lorentz-polarisation effects, but no correction was applied for absorption. The structure was solved by direct methods using the SIR92 All subsequent calcul- ations were carried out by the SHELXTL IRIS ~ystern.~' The structure was refined by full-matrix least-squares techniques. Not all non-hydrogen atoms could be assigned anisotropic thermal parameters because of the limited number of observed reflections, due to the poor quality of the crystals; the phenyl and methyl groups were thus treated as fixed groups with hydrogen atoms in calculated positions.Atomic co-ordinates, bond lengths, angles, torsion angles and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre. Crystal data. C4oH4amp;13, M = 734.8, monoclinic, space group P2,, a = 9.233(3), b = 19.819(7), c = 21.098(6) A; = 99.01(2)"; Z = 4, D, = 1.282 (Mg rn-,); Mo-Ka radi- ation, A = 0.710 69 A, p = 0.096 mm-'; R = 0.104 for 3273 observed reflections having F 4.0a(F). References 1 Part 15, G. Appendino, M. Varese, P. Gariboldi and B. Gabetta, Tetrahedron Lett., 1994,35,22 17. 2 G.Appendino, L, Barboni, P. Gariboldi, E. Bombardelli, B.Gabetta and D. Viterbo, J. Chem. SOC.,Chem. Commun., 1993, 1587. 3 F. Blaza, S. Tachibana, H. Barrios and G. N. H. Towers, Phytochemistry, 1991,30, 1613. 4 G. I. Georg, S. R. Gollapudi, G. L. Grunewald, C. W. Gunn, R. H. Himes, B. K.Rao, X.Z. Liang, Y.W. Mirhom, L. A. Mitscher, D. G. Vander Velde and 0.M. Ye, Biorg.Med. Chem. Lett., 1993,3,1345. 5 A. Chu, J.Zajicek, G. H. N.Towers,C. M. Coucy-Breau, N. G. Lewis and R. Croteau, Phytochemistry, 1993,34,269. 6 G. Samaranayake, N. F. Magri, C. Jitrangsri and D. G. 1. Kingston, J. Org. Chem., 1991,56,5114. 7 A. Wahl, F. Gueritte-Voegelein, D. Guenard, M. T. Le Goff and P. Potier, Tetrahedron, 1992,48, 6966. 8 G. Appendino, H. C. Ozen, P. Gariboldi, E. Torregiani, B. Gabetta, R.Nizzola and E. Bombardelli, J. Chem. Soc., Perkin Trans. I, 1993, 1563. 9 G. Appendino, S. Tagliapietra, H. C. Ozen, P. Gariboldi, B. Gabetta and E. Bombardelli, J. Nat. Prod., 1993, 56, 514. 10 G. Kriissmann, Handbuch der Nadelgeholze, Verlang Paul Parey, Berlin, 1983, pp. 317-333. 11 L. Barboni, P. Gariboldi, E. Torregiani, G. Appendino, B. Gabetta and E. 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