Loïc Planas

Joëlle Pérard-Viret

Jacques Royer
email: royer@pharmacie.univ-paris5.fr

Mohamed Selkti

Alain Thomas

Highly Efficient Asymmetric Access to 1-Azaspiro[4.4]nonane Skeleton

Keywords: spiro compounds, asymmetric synthesis, ring expansion, rearrangement, pyrroles

Vinylogous Mukaiyama aldol type reaction of chiral non-racemic silyloxypyrroles followed by acidic treatment affords an efficient asymmetric access to 1-azaspiro[4.4]nonanes in high diastereoisomeric excess (up to 79%).

1-Azaspiro [4.4]nonanes are building blocks difficult to access since the formation of quaternary spiro centers has always been a tricky exercise in organic synthesis. Nevertheless, methods to form a spiro center a to the nitrogen of a pyrrolidine have been developed, and they are based upon a wide variety of different methodologies. 1 Among them, the use of radicals, 1a cycloadditions, 1b ring closure reaction, 1c palladium, 1d N-acyliminiums, 1e phenol oxidation 1f and more recently the use of aluminium amide in domino reactions, 1g have been described. It is noteworthy that only few of these methods are stereoselective 1b,e and can be used for asymmetric synthesis of natural products containing this moiety.

We planned to develop a new asymmetric entry to 1-azaspiro [4.4]nonane, like the lactam 1, as a key intermediate in the synthesis of (-)-cephalotaxine 2 (Scheme 1).

Scheme 1

We anticipated that this could be possible by the use of chiral non racemic a,b-unsaturated g-lactams 3 that we have been developing for some years. 2 The possibility to diastereoselectively substitute g-lactams 3 at the C-5 position with either an electrophile [via the silyloxypyrrole, path (a)] or a nucleophile [via a N-acyliminium, path (b), Scheme 2], permits to construct a quaternary spiro center at this carbon. As a matter of fact, the application of path (a) maintains the a,b-unsaturation, allowing further use of path (b).

Scheme 2

The ring expansion of cyclobutanol derivatives giving rise to spiro compound via a semi-pinacol type reaction is known for a decade. It was well described by Paquette with dihydrofuran and tetrahydropyran derivatives, 3 and recently, an example with piperidine compounds has been published. 4 The key step for this reaction is the formation of a carbocation a to the carbinol of cyclobutanol, may it be an oxonium or a N-acyliminium.

This method appeared very attractive for us, since we anticipated that the cyclobutanol derivative required for the semi-pinacolic rearrangement should be the a,b-unsaturated g-lactam 6 (Scheme 3). It could be quite readily accessible by a vinylogous Mukaiyama aldol condensation of the silyloxypyrrole 5 with cyclobutanone. In order to check this two step sequence we prepared silyloxypyrrole 5a derived from a-methylbenzylamine 4a using the methodology already reported, namely condensation of 4a with dimethoxydihydrofuran to give 3a followed by TBDMSOTf treatment in presence of base. The expected compound (6a) was obtained in 85% yield and 83% diastereoisomeric excess when silyloxypyrrole 5a was reacted with cyclobutanone (Scheme 3). [START_REF]Typical Procedure for Silyloxypyrroles 5. To a solution of 2,5-dimethoxy-2,5-dihydrofuran (1 equiv) and chiral amine 4a,c-e (1 equiv)[END_REF] The major isomer could be crystallized and allowed us to determine the absolute configuration of the new chiral center by X-ray analysis to be S (Figure 1).

Figure 1

Crystal structure of 6a, determined by X-ray. [START_REF]D 25 -13.8 ([END_REF] Then, 6a was placed under acidic conditions to achieve the ring expansion. Depending on the reaction conditions (Table 1), we observed either degradation or the elimination product or the expected product as a mixture of two diastereoisomers. The reaction could be carried out under different conditions but the best result was obtained by the use of concentrated hydrochloric acid in dichloromethane (Table 1, entry 7), as described for N-tosyliminiums by Dake's group. 4 We can suggest that water was needed for this reaction, to ensure the spiro center formation instead of elimination (Scheme 4).

Unfortunately, despite the excellent chemical yield, the diastereoisomeric excess was modest (20%), and we then decided to investigate other chiral auxiliaries in order to improve this result.

Various silyloxypyrroles 5a-e were prepared using our standard methodology from amines 4a-e in 41-73% overall yields. These silyloxypyrroles were engaged in the vinylogous Mukaiyama reaction followed by acidic treatment to give rise to the spiro compounds (Table 2).

In all cases, this two steps sequence occurred in very high yield. Using phenylglycinol derivatives allowed us to improve the diastereoisomeric excess of the ring expansion reaction, but a real hit was obtained with (S)-1-(1-naphthyl)ethylamine. This chiral amine has already been used in asymmetric syntheses with good stereochemical results 7 and is a suitable source of nitrogen and chirality for our target alkaloid. The diastereoisomeric excess could be further enhance from 74% to 79%, operating at 0 °C instead of 20 °C, while time to reaction completion was raised from 2 hours to 9 hours. Nevertheless, at -20 °C, no reaction occurred after 24 hours.

Because of the improved de observed with O-substituted phenylglycinols, we envisioned the use of O-methylnaphthylglycinol (Table 2, entry 5). Different asymmetric syntheses of naphthylglycinol exist, 9 but we preferred to prepare the more accessible racemic one. Thus, 4e was prepared in four steps from 1-napthaldehyde as shown in Scheme 5. The amino acid was first obtained from the corresponding aldehyde through a modified Strecker synthesis as described. 10 The reduction of the carboxylic acid function by LiAlH 4 furnished the aminoalcohol which was regioselectively methylated 11 (KH, THF, MeI) to give 4e in 67% for the two steps.

Unfortunenaly this new chiral auxiliary did not allow any improvement in the diastereoselectivity of the semi-pinacol rearrangement.

As suggested by the mechanism of the rearrangement and by the discrepancy between diastereoisomeric excesses of compounds 6 and 7, the stereochemical information of compound 6 was lost in the semi-pinacolic rearrangement. This was definitively proved by placing each diastereoisomer of 5d, easily separable by crystallization, under the same experimental conditions (Scheme 6). We did not notice any measurable difference between the two crude products, proving that there was no kind of survival of the initial configuration. [START_REF] Fuji | For a review about the memory of chirality[END_REF] So, the chirality of the new center was only controlled by the chiral auxiliary, due to the free rotation in the intermediate N-acyliminium.

Scheme 6

Major diastereoisomer 7d could be easily separated through crystallization by triturating the crude product in ether (68% of the diastereopure product could be obtained). We then derivatised it, on the way to determine the absolute configuration of the newly created spiro center (Scheme 7). First, the ketone was protected as the cyclic ketal 9d with ethylene glycol (77% yield), and the latter was treated with lithium in liquid ammonia, 13 to remove the chiral auxiliary. The ketal 10 (69% yield) could be deprotected to obtain the simple spiro amide 11 in 86% yield.

This compound was described in the Nagasaka's cephalotaxine synthesis and the comparison of its optical rotation ([a] D -64, lit. 14 -68) allowed us to assign the R-configuration to the spiro center of 7d. Following identical process, we also attributed the R-configuration to the spiro center of 7c. Since the ring enlargement of cyclopentanol derivatives has been described with dihydrofuran compounds 3 we envisaged this reaction starting with our g-lactam.

The aldol product 12 was readily obtained from the silyloxypyrrole 5a under the same conditions as above, in good yield (78%) and an excellent diastereoisomeric excess (90%).

However, this compound did not undergo rearrangement reaction. By varying the conditions, we obtained either the elimination product or the product of hydrolysis of the Nacyliminium moiety 4 (Scheme 8). To summarize, we developed a highly efficient method for the diastereoselective formation of 1-azaspiro [4.4]nonanes, allowing us to construct the spiro bicyclic system of (-)-cephalotaxine. Work is on progress to complete a new asymmetric synthesis of (-)-cephalotaxine based on this methodology.

Scheme 3

 3 Scheme 3Reagents and conditions: (a) 2,5-Dihydro-2,5-dimethoxyfuran, HCl, H 2 O, r.t.; (b) TBSOTf, Et 3 N, CH 2 Cl 2 , r.t.; (c) cyclobutanone, BF 3 ×OEt 2 , CH 2 Cl 2 , -78 °C; (d) aq conc. HCl, CH 2 Cl 2 , r.t.

a 4 Scheme 5

 45 Scheme 4

Scheme 8

 8 Scheme 8 Reagents and conditions: (a) Amberlyst 15, CH 2 Cl 2 , r.t.; (b) concd HCl, THF, r.t.

Table 1

 1 Semi-pinacolic Rearrangement of Cyclobutanol Derivative 6a Under Various Acidic Conditions

	Entry Acid	Solvent Temp 7a a	8a
	1C S A	C H 2 Cl 2 r.t.	degradation
	2	PTSA	toluene reflux	degradation
	3T f O H	C H 2 Cl 2 r.t.	degradation
	4	Amberlyst 15	CH 2 Cl 2 r.t.	70%	30%
	5	Amberlyst 15	toluene r.t.	75%	25%
	6	Dowex 50X8-100 CH 2 Cl 2 r.t.	90%	10%
	7c o n c d H C l	C H 2 Cl 2 r.t.	>99%	<1%

a Product ratios are determined by integration of 1 H NMR spectra.

Table 2

 2 Diastereoselectivity

	1	85 (83)	99 (20)
	2	84 (18)	99 (40)
	3	75 (48)	99 (63)
	4	77 (4)	99 (74-79) c

in the Rearrangement Reaction to 1-Azaspiro[4.4]nonane Skeleton 8 Entry Starting Amine 6, a,b Yield (de) % 7, b Yield (de) %

  Reagents and conditions: (a) Ethyleneglycol, PTSA, toluene, reflux; (b) Li/NH 3 , ethanol, -78 °C; (c) HOAc/H 2 O 50/50, reflux.

Scheme 7

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Synlett 2002, No. 10, 1629-1632 ISSN 0936-5214 © Thieme Stuttgart • New York

Acknowledgement

This work was supported by a grant from the M.E.N.R.T.


Loïc Planas

Joelle Pérard-Viret

Jacques Royer
email: jacques.royer@univ-paris5.fr

Loı ¨c Planas

Joe ¨lle Pe ´rard-Viret

Stereoselective Synthesis of (-)-Cephalotaxine and C-7 Alkylated Analogues

come

Introduction

(-)-Cephalotaxine 1 was isolated in 1963 from the evergreen plum yews of the genus Cephalotaxus as the most abundant alkaloid. 1 Its greatest interest is the powerful activity of its ester derivatives, harringtonine 2a and homoharringtonine 2b, against chronic myelogenous leukaemia (Figure 1). Homoharringtonine has thus been extensively studied and is now in phase III clinical trials. 2 However, the natural source of this alkaloid remains very limited, and some species of the Asian shrub, from which it is extracted, are endangered. 3 Furthermore, it should be mentioned that this material is only ca. 90% ee pure. 4 Recently, an efficient hemisynthesis of enantiopure homoharringtonine from (-)-cephalotaxine has been described, 5 which has increased the interest in the synthesis of this parent pentacyclic natural product. Thus, an efficient and scalable synthesis of enantiopure (-)-cephalotaxine should be quite interesting.

During the 1970s and the 1980s, many teams were engaged in this challenging synthesis, the main problem being the construction of the spiro quaternary center at rings C and D. This led to more than 10 racemic syntheses and many approaches. [START_REF] Jilal Miah | The Alkaloids[END_REF] There is still a pressing interest for an efficient synthesis of this structurally original alkaloid, and approaches continue to be published. [7][8][9] In the past decade, as new developments in asymmetric organic chemistry were growing, four asymmetric syntheses were proposed. The first one, published by Mori in 1995, was based on the use of unnatural D-proline. 10 Mori underlined and partially resolved the problem of racemization in the last steps since (-)-cephalotaxine was obtained with an 88% ee according to the optical rotation, while the ee of the next to last product was carefully examined and found to be 88% (chiral HPLC). In 1997, Nagasaka achieved the syntheses of both (-)-and (+)cephalotaxine by resolution of an intermediate following separation of diastereoisomeric ketal derivatives by 10.1021/jo049884l CCC: $27.50 © 2004 American Chemical Society HPLC. 9 Though no details were given in this report, the optical rotation of the target product corresponds to a high enantiomeric purity. In 1999, Ikeda published a synthesis from unnatural D-proline, 12 while Tietze performed an elegant and very efficient asymmetric synthesis based on palladium chemistry. 13 These syntheses were formal syntheses, and the ee of the last described intermediate was limited to 85-90%.

In this context, we wish to report herein a new scalable and highly efficient asymmetric synthesis of (-)-cephalotaxine in 98.7% ee. This synthesis is based on the stereoselective spirocyclization we reported in a previous communication. 14 We will describe herein the overall synthesis and a careful determination of the enantiomeric purity of (-)-cephalotaxine together with some C-7alkylated cephalotaxine derivatives.

Results and Discussion

The strategy we chose was focused on the achievement of a method for the construction of the C/D ring moiety of 1 and more precisely to develop an efficient access to the 1-azaspiro [4.4]nonane skeleton in an asymmetric fashion. This project was based upon the chemistry of R,β-unsaturated γ-lactams 3 we have developed for some years in the laboratory. These compounds were thought to be of particular interest in the asymmetric construction of a quaternary center R to the nitrogen of the pyrrolidine ring and hence a spiro derivative. Indeed, depending upon the conditions, these R,β-unsaturated γ-lactams 3 are equivalent to either the carbanion synthon I or the carbocation synthon II as depicted in the Scheme 1.

Silyloxypyrrole 4, equivalent to carbanion synthon I, reacts with electrophiles at C-5 to furnish 6. This last compound possesses the R,β-unsaturated lactam function from which a new reaction at the same center is possible through the anionic or cationic species.

We turned our attention to the semipinacolic rearrangement investigated by Paquette 15 through R-hydroxyoxonium and applied to R-hydroxyiminium by Dake. 16 We described in a previous communication the use of this rearrangement in the R,β-unsaturated γ-lactam series. 14 We showed that stereocontrolled rearrange-ment could be attained by the use of chiral auxiliary on the nitrogen atom, and we selected (S)-1-(1-naphthyl)ethylamine as the most efficient chiral appendage and starting material.

Thus, starting from (S)-1-(1-naphthyl)ethylamine, we prepared the silyloxypyrrole 4a in 54% yield (after purification) over two steps as described in Scheme 2. The vinylogous Mukaiyama aldol reaction of 4a with cyclobutanone affords the R,β-unsaturated γ-lactam 6a with 77% yield but only 4% diastereoisomeric excess. Acidic treatment of 6a (concentrated HCl in CH 2 Cl 2 at 0 °C) furnished the desired spiro compound 7 in quantitative yield with a good diastereoisomeric excess (80%).

Besides this good de, it was found that the two diastereoisomers can be very easily separated. Indeed, the major diastereoisomer was crystallized from ethanol furnishing 69% yield of pure compound as shown from 1 H NMR analyses. Additional product could be obtained after flash chromatography on silica gel of the mother liquor, giving almost the totality of the major diastereoisomer. The spiro compound 7 exhibiting the required configuration was then obtained in 86% yield.

Cephalotaxine Total Synthesis Achievement. To perform the cleavage of the chiral appendage, the ketone function of 7 was protected as the cyclic ketal 8, using ethylene glycol under standard conditions (Scheme 3). Reductive cleavage of the N-benzylic bond of 8 was then performed with lithium in liquid ammonia, and the bispiro lactam 9 was isolated in quantitative yield over the two steps. This material was used without purification through the next step, which consisted of N-alkylation with the tosylate of 3,4-methylenedioxypheny-2ethanol 11 to give 10 in 80% yield after purification. 1997, 8, 191-194. (12) Ikeda, M.; el Bialy, S. A.; Hirose, K.; Kotake, M.; Sato, T.; Bayomi, S. M.; Shehata, I. A.; Abdelal, A. M.; Gad, L. M.; Yakura, T. Chem. Pharm. Bull. 1999, 47, 983-7. (13) Tietze, L. F.; Schirok, H. J. Am. Chem. Soc. 1999, 121, 10264-10269.

( Deprotection of 10 in acidic medium finally provides 11 in a quantitative yield (Scheme 3). Eventually, enantiopure 11 was transformed to (-)-cephalotaxine following the procedure used by Kuehne for his racemic synthesis and summarized in Scheme 4. It consists of eight steps with an overall yield of 69%.

In our hands, the synthesis was achieved with an overall yield of 32%. Only two minor changes were done. First, we found that there is no need, in the oxidation reaction of 12 to 13, to have benzoquinone in addition with palladium acetate since the latter is used in a stoichiometric amount. 17 The second change was concerned with the last step, which we have to perform very carefully due to the sensitivity of cephalotaxine to racemization. Though no clear racemization mechanism was proposed, cephalotaxine is known to be sensitive to acidic conditions. That could explain why natural cephalotaxine was not isolated as an enantiomerically pure compound. It was reported that the methyl iodide derivative 19 (Y ) CH 3 , Scheme 5), prepared from natural cephalotaxine, was obtained in a racemic form. 18 The equilibrium between 19 and an open form like 20, having lost all asymmetric carbons, might be responsible for this racemization (Scheme 5). Such a reaction may arise upon treatment of cephalotaxine with acid (Bro ¨nsted or Lewis acid).

Thus, we decided to change the conditions described by Kuehne (LiAlH 4 in refluxing THF), and instead, we used the more reactive aluminum hydride that allows us achieve the reaction at 0 °C. In addition, the reduction of γ-lactams is described to be cleaner with this reagent. 19 These conditions furnished a 75% yield of (-

)-cephalo- taxine [[R] 20 D -182 (CHCl 3 , c ) 0.21) (lit. 20 [R] 20 D -188 (CHCl 3 ))].
We then determined the enantiomeric purity of this synthetic material. It could be easily analyzed by means of chiral HPLC, but for doing so we needed both enantiomers. Thus, we first studied a cephalotaxine sample obtained from a natural source which classically contained 5-10% (+)-cephalotaxine. 4 This analysis (Chiral HPLC/MS spectrometry) proved that the natural sample has an enantiomeric purity of 83%. Using the same HPLC conditions, we could determine that our synthetic sample contains 0.65% of (+)-cephalotaxine and hence (-)-cephalotaxine was prepared in 98.7% enantiomeric purity. It is noteworthy that the enantiomeric purity of this material corresponds to that of (S)-1-(1-naphthyl)ethylamine, which was used as chiral inducer and source of nitrogen.

C-7-Alkylated Analogues. The asymmetric synthesis we performed allowed us to achieve the synthesis of some analogues through the alkylation of the pyrrolidine ring of the alkaloid. A few approaches to the preparation of analogues of cephalotaxine were reported. They were mainly concerned with modification of the aromatic A ring (dimethoxyphenyl 21 and indole 22 in place of methylenedioxyphenyl group) or change of the size of the B, C,orD ring to a six-membered ring as reported by Tietze showing the versatility of his method. 23 But, to the best of our knowledge, only one analogue of cephalotaxine was reported: it consists of the change to dimethoxyphenyl instead of the methylenedioxyphenyl group. 21 Cephalotaxine analogues might be useful for accessing homoharringtonine analogues whose antitumoral activity should be evaluated. We planned to introduce alkyl substituents on the C ring in order to investigate what spatial surrounding is compatible with the biological activity. To attain several analogues, it appears necessary to be able to introduce a substituent quite late in the synthesis. We thus thought that alkylation of lactam 16 after protection of the diol moiety would be suitable.

The diol 16 was protected as an acetonide using standard conditions to give 21 in 86% yield (Scheme 6). Alkylation of 21 was achieved through deprotonation using s-BuLi in THF at -78 °C and addition of MeI, BuI, and PhCH 2 Br to provide diastereomeric alkylated lactams 22 and 23 in 88-97% yield and in a 4:1 ratio. The (S)-configuration of the new chiral center of the major isomer 22 was determined through 1 H NMR NOE experiments and could be easily explained by the geometry of the tetracyclic compound. In each series, the diastereoisomers could be separated by flash chromatography. Major diastereoisomers 22a-c were deprotected to the corresponding diol, and then the alkylated cepha-lotaxines 30a-c were obtained by application of the last three steps of the synthesis of cephalotaxine with the same yields as for the parent compound. Minor epimer 23a was also transformed to 31a.

Conclusion

By using an original and efficient stereoselective access to 1-azaspiro [4.4]nonane compounds, and following the Kuehne synthesis for the end of the sequence, we reported an asymmetric synthesis of (-)-cephalotaxine in 98.7% ee with an overall yield of 9.8% over a 16-step sequence. It thus proved the relevance of the chemistry of R,β-unsaturated γ-lactams for the synthesis of complex alkaloids. This synthetic sequence is adaptable for access to some alkylated analogues.

Experimental Section (1′S)-1-(1′-(1-Naphthyl)ethyl)-2-tert-butyldimethylsilyloxy-1H-pyrrole (4a).

To a solution of (S)-1-(1-naphthyl)ethylamine (3.42 g, 26.28 mmol) and 2,5-dimethoxy-2,5dihydrofuran (3.00 g, 17.52 mmol) in water (70 mL, 0.25 M) was added a concentrated aqueous solution of HCl (2.32 mL, 26.28 mmol). The reaction mixture was stirred at rt for 1 h, neutralized with solid NaHCO 3, and extracted with CH2Cl2. The combined organic layers were dried over Na2SO4 and concentrated under vacuum. A red oily mixture of the R,β-and β,γ-unsaturated lactams was then obtained. To a solution of this crude product (ca. 17.52 mmol) and NEt3 (3.50 g, 35 mmol) in CH2Cl2 (35 mL, 0.5 M) was slowly added tert-butyldimethylsilyl triflate (4.86 g, 18.40 mmol). The reaction mixture was stirred at rt for 1 h, and the solvent was then evaporated. The residue was purified by filtration on a pad of alumina (heptane/ AcOEt 88/12), providing 4a (3.50 g, 57% for two steps) as a slightly colored solid. Mp ) 97 °C (heptane-AcOEt). 1 -4.6, 18.2, 21.4, 25.7, 49.5, 87.7, 105.2, 109.8, 123.2, 125.8, 126.5, 128.1, 129.1, 131.0, 134.1, 139.7, 142.2

(1′S)-5-(1-Hydroxycyclobutyl)-1-(1′-(1-naphthyl)ethyl)-1,5-dihydropyrrol-2-one (6a).

To a solution of 4a (4.17 g, 11.86 mmol) in anhydrous CH2Cl2 (80 mL, 0.15 M) containing 3 Å MS was added, under argon, cyclobutanone (1.43 mL, 18.98 mmol). After 15 min at rt, the solution was cooled at -78 °C, and BF 3'Et2O (2.25 mL, 17.79 mmol) was added over the course of 15 min. The solution was stirred at -78 °C for 2 h, filtered, and allowed to warm to 0 °C. The reaction was then quenched by addition of water. An insoluble compound could be then filtered giving 0.94 g of the pure minor diastereoisomer. The aqueous phase was separated and extracted with CH 2Cl2. The crude mixture obtained after drying and evaporation of the solvent was triturated with ether. A 1.48 g crop of crystals consisting of the major diastereoisomer was thus obtained. From the residue, 0.40 g of a diastereoisomeric mixture was obtained after flash chromatography (CH 2Cl2/ MeOH, 96/4). Finally, compound 6a was obtained in a 77% combined yield. Major Diastereoisomer. Mp ) 196-198 °C (Et2O). 1 H NMR (300 MHz, CDCl3): δ ) 1.36 (m, 1H), 1.8-2.1 (m, 5H), 1.75 (s, 1H), 1.86 (d, J ) 7.0 Hz, 3H), 3.51 (t, J ) 1.5 Hz, 1H), 6.07 (q, J ) 7.0 Hz, 1H), 6.28 (dd, J ) 6.0, 1.6 Hz, 1H), 6.81 (dd, J ) 6.0, 1.8 Hz, 1H), 7.47 (m, 3H), 7.63 (m, 2H), 7.84 (m, 2H). 13 C NMR (75 MHz, CDCl3): δ ) 13. 6, 19.1, 31.9, 36. 2,49.0,69.6,75.9,123.2,125.2,125.8,126.1,127.3,128.5

SCHEME 6

129. 1, 129.2, 132.0, 134.0, 136.2, 145.7, 173.3. IR (KBr): ν ) 3382, 2977, 2934, 1658, 1394 cm -1 . Anal. Calcd for C20H21-NO2: C, 78.15;H, 6.89;N, 4.56. Found: C, 78.03;H, 7.06;N, 4.60. MS (CI, NH3): m/z ) 308 (MH + ), 238 (MH + -C4H6O).

[R] 25 D -13.8 (c ) 0.97, CHCl3). Minor Diastereoisomer. Mp ) 245 °C (CH2Cl2). 1 H NMR (300 MHz, CDCl3): δ ) 0.35-0.5 (m, 1H), 1.01 (s, 1H, D2O exch), 1.35-1.55 (m, 2H), 1.55-1.7 (m, 2H), 1.78 (d, J ) 7.1 Hz, 3H), 1.85-2.0 (m, 1H), 4.25 (t, J ) 1.6 Hz, 1H), 6.30 (m, 2H), 6.94 (dd, J ) 7.0, 6.0 Hz, 1H), 7.4-7.6 (m, 4H), 7.83 (d, J ) 8.1 Hz, 1H), 7.90 (d, J ) 8.3 Hz, 1H), 8.23 (d, J ) 8.4 Hz, 1H). 13 C NMR (75 MHz, DMSO): δ ) 17. 7, 24.4, 36.2, 40.6, 55.1, 74.8, 81.3, 128.5, 130.3, 130.8, 131.7, 132.9, 134.3, 134.5, 136.1, 139.0, 143.5, 151.3, 177.6. IR (KBr): ν ) 3388, 2976, 2937, 1660 cm -1 . MS(CI, NH3): m/z ) 308 (MH + ), 238 (MH + -C4H6O). [R] 25 D: -264.1 (c ) 0.52, CHCl3).

(5R)-(1′S)-1-(1′-(1-Naphthyl)ethyl)-1-azaspiro[4.4]nonane-2,6-dione (7).

To a solution of 6a (13.30 g, 43.27 mmol) in CH2Cl2 (930 mL, 0.05 M) at 0 °C was added concentrated aqueous HCl (6.14 mL, 64.90 mmol). After 24 h at 0 °C, the solution was allowed to warm to rt, and the solvent was removed under vacuum. The crude product was redissolved in CH2Cl2 and reevaporated twice to eliminate trace amount of acid and provide 7 (13.30 g, quant) as an amorphous solid. The pure major diastereomer of 7 (9.17 g, 86%) was obtained by crystallization in 40 mL of 95% EtOH and by flash chromatography of the mother liquor (silica gel; AcOEt/ cyclohexane 50/50). Mp ) 140 °C (Et2O). 1 H NMR (400 MHz, CDCl3): δ ) 1.07-1.17 (m, 2H), 1.34-1.55 (m, 2H), 1.62 (ddd, J ) 12.3, 8.5, 6.5 Hz, 1H), 1.67 (d, J ) 7.0 Hz, 3H), 1.87 (ddd, J ) 18.9, 11.6, 9.5 Hz, 1H), 1.93 (ddd, J ) 12.3, 8.9, 6.8 Hz, 1H), 2.23 (dd, J ) 18.9, 6.1 Hz, 1H), 2.43 (ddd, J ) 16.8, 8.5, 6.8 Hz, 1H), 2.53 (ddd, J ) 16.8, 8.9, 6.5 Hz, 1H), 6.25 (q, J ) 7.1 Hz, 1H), 7.3-7.6 (m, 4H), 7.83 (d, J ) 9.7 Hz, 1H), 7.79 (d, J ) 7.6 Hz, 1H), 8.00 (d, J ) 8.3 Hz, 1H). 13 C NMR (75 MHz, CDCl3): δ ) 7. 8, 18.9, 29.0, 31.7, 32.3, 35.7, 47.5, 72.3, 124.0, 125.1, 125.9, 126.3, 127.2, 129.1, 129.3, 132.7, 133.9, 136.1, 175.7, 217.0. IR (KBr): ν ) 2968, 1750, 1679, 1380, 1347, 780 cm -1 . MS(CI, NH3): m/z ) 308 (MH + ). Anal. Calcd for C20H21-NO2: C, 78.15; H, 6.89; N, 4.56. Found: C, 77.96; H, 7.15; N, 4.41. [R] (10.63 g, 34.58 mmol) in toluene (120 mL, 0.2 M) were added ethylene glycol (19,33 mL, 346 mmol) and PTSA (0.66 g, 3.46 mmol). The reaction mixture was refluxed in a Dean-Stark apparatus overnight, cooled to rt, poured in a saturated aqueous solution of NaHCO 3, and extracted with CH2Cl2. The organic phase was dried over Na2SO4 and concentrated under vacuum, providing 8 (12.15 g, quant) as a beige solid. An analytical sample was obtained by recrystallization in i-Pr2O. Mp ) 99 °C (i-Pr2O). 1 H NMR (300 MHz, CDCl3): δ ) 1. 5-1.65 (m, 2H), 1.65-1.85 (m, 4H), 1.90 (d, J ) 7.3 Hz, 3H), 2.13 (m, 1H), 2.35-2.5 (m, 3H), 2.67 (ddd, J ) 16. 3, 10.7, 8.7 Hz, 1H), 3.0-3.2 (m, 2H), 3.48 (m, 1H), 5.70 (q, J ) 7.3 Hz, 1H), 7.35-7.5 (m, 2H), 7.52 (dt, J ) 1.3, 8.2 Hz, 1H), 7.69 (d, 1H), 7.83 (d, 1H), 7.95 (t, J ) 8.5 H, 1 Hz). 13 C NMR (100 MHz, CDCl3): δ ) 16. 4, 20.4, 30.6, 30.8, 31.9, 32.2, 51.5, 63.6, 65.3, 72.7, 116.9, 123.2, 125.2, 125.3, 125.9, 126.2, 127.2, 129.2, 130.5, 133.8, 140.5, 178.5. IR (KBr): ν ) 3057, 2973, 2864, 1690, 1361 cm -1 . MS (CI, NH3): m/z ) 352 (MH + ). [R] H, 7.17; N, 3.99. Found: C, 74.44; H, 7.14; N, 3. To a cooled (-78 °C) solution of 8 (2.30 g, 6.51 mmol) in THF (32 mL, 0.2 M), in a flask equipped with a cold trap (-78 °C), was condensed ammonia (about 60 mL). To the mixture was added lithium (0.60 g, 0.086 mol) portionwise until persistent dark blue colorization of the solution occurred. The reaction mixture was stirred for 30 min and allowed to warm to rt (evaporating thus ammonia). It was poured into an aqueous saturated solution of NH 4Cl and extracted with CH2Cl2. The organic phase was dried over Na2SO4 and concentrated under vacuum. The crude product was purified by filtration through a pad of silica gel (CH 2Cl2/Et3N 99/1 and then CH2Cl2/MeOH/ Et3N 95/5/1), providing 9 (1.28 g, quant) as a white solid. An analytical sample was obtained by recrystallization in EtOH. Mp ) 75-76 °C (EtOH). 1 H NMR (300 MHz, CDCl3): δ ) 1. 6-1.95 (m, 7H), 2.25-2.55 (m, 3H), 3.8-4.0 (m, 4H), 6.13 (bs, 1H). 13 C NMR (75 MHz, CDCl3): δ ) 16. 6, 29.9, 30.8, 32.0, 35.7, 65.3, 66.2, 68.1, 116.6, 178.2 To a solution of 9 (0.31 g, 1.57 mmol) in anhydrous benzene (5 mL, 0.3 M) was added NaH (60 mg, 2.50 mmol) portionwise. The mixture was refluxed for 5 min and then cooled to rt. Solid toluene-4sulfonic acid 2-benzo [1,3]dioxol-5-ylethyl ester 11 (0.90 g, 2.81 mmol) was added, and the mixture was heated at reflux for 16 h. The reaction mixture was poured into an aqueous saturated solution of NH4Cl and extracted with CH2Cl2. The organic phase was dried over Na2SO4 and concentrated under vacuum. The crude product was purified by flash chromatography on silica gel (CH 2Cl2/MeOH/Et3N 95/5/1), providing 10 (449 mg, 80%) as a white solid. Mp ) 88 °C (i-Pr2O). 1 H NMR (300 MHz, CDCl3): δ ) 1.27 (m, 3H), 1.55-1.75 (m, 4H), 1.89 (m, 1H), 2.08 (m, 1H), 2.24 (dd, J ) 12.6, 9.7 Hz, 1H), 2.32 (dd, J ) 12.2, 10.0 Hz, 1H), 2.59 (m, 1H), 2.67 (m, 1H), 2.83 (ddd, J ) 13.0, 9.9, 4.7 Hz, 1H), 3.34 (ddd, J ) 13.7, 10.0, 7.0 Hz, 1H), 3.67 (m, 1H), 3.7-3.85 (m, 4H), 5.89 (s, 2H), 6.66 (d, J ) 7.9 H, 1 Hz), 6.72 (s, 1H), 6.73 (d, J ) 7.9 Hz, 1H). 13 C NMR (75 MHz, CDCl3): δ ) 16. 9, 30.5, 31.7, 32.9, 33.8, 35.1, 44.1, 64.9, 65.9, 71.2, 101.1, 108.4, 109.7, 118.6, 122.1, 133.7, 146.2, 147.8, 176.7 A solution of 10 (3.80 g, 11.00 mmol) in a water/acetic acid 9/1 mixture (56 mL, 0.2 M) was refluxed for 16 h and then cooled to rt. The mixture was poured into an aqueous saturated solution of NaHCO3 and extracted with CH2Cl2. The organic phase was washed with brine, dried over Na2SO4, and concentrated under vacuum, providing 11 (3.45 g, quant) as a yellow oil. 1 H NMR (300 MHz, CDCl3): δ ) 1.67-1.85 (m, 3H), 1.92-2.17 (m, 4H), 2.30-2.45 (m, 3H), 2.57-2.74 (m, 2H), 3.03 (ddd, J ) 13.4, 10.3, 6.5 Hz, 1H), 3.38 (ddd, J ) 13.5, 10.0, 5.1 Hz, 1H), 5.91 (s, 2H), 6.62 (d, J ) 7.6 Hz, 1H), 6.67 (s, 1H), 6.71 (d, J ) 7.6 Hz, 1H). 13 C NMR (75 MHz, CDCl3): δ ) 17. 6, 29.2, 29.9, 33.6, 34.9, 35.2, 43.8, 72.5, 101.1, 108.6, 109.6, 122.0, 133.1, 146.4, 148.0, 176.2, 217.1. IR (film): ν ) 2963, 2888, 1745, 1685, 1503, 1491, 1443, 1403, 1247, 1037 cm -1 . MS(CI, NH3): m/z ) 302 ([MH] + ). [R] 25 D: -13.6 (c ) 0.72, CHCl3). Anal. Calcd for C17H19NO4' 1 /2H2O: C, 65.79; H, 6.50; N, 4.51. Found: C, 65.63; H, 6.24; N, 4.53. Cephalotaxine (1). To a cooled (0 °C) solution of 18 (62 mg, 0.19 mmol) in anhydrous THF (1 mL) was added a cooled (0 °C) 1 M solution of AlH3 in THF (3 mL, 3 mmol, prepared from solid AlCl3 and 1 M LiAlH4 in THF). The reaction mixture was stirred for 2.5 h, treated at 0 °C by wet Na2SO4, and filtered after loss of color. The solids were thoroughly washed with distilled THF, and the filtrate was concentrated under vacuum. The crude product was purified by flash chromatography on silica gel (CH 2Cl2/MeOH/NH4OH 95/5/0.5), providing 1 (45 mg, 75%) as a white solid. 1 H NMR (400 MHz, CDCl3): δ ) 1.70 (sl, 1H), 1.72-1.80 (m, 2H), 1.88 (ddd, J ) 11.5, 8.0, 4.0 Hz, 1H), 2.01 (dd, J ) 11.5, 9.6 Hz, 1H), 2.36 (dd, J ) 14.4, 6.9 Hz, 1H), 2.53-2.66 (m, 2H), 2.92 (ddd, J ) 11. 8, 11.3, 7.1 Hz, 1H), 3.07 (m, 1H), 3.36 (ddd, J ) 14.1, 12.3, 7.8 Hz, 1H), 3.68 (d, J ) 9.4 Hz, 1H), 3.73 (s, 3H), 4.76 (d, J ) 9.1 Hz, 1H), 4.93 (s, 1H), 5.91 (s, 2H), 6.65 (s, 1H), 6.68 (s, 1H). 13 C NMR Stereoselective Synthesis of (-)-Cephalotaxine Org. Chem, Vol. 69, No. 9, 2004 3091

FIGURE 1 .

 1 FIGURE 1. Cephalotaxus alkaloids.

( 11 )

 11 Nagasaka, T.; Sato, H.; Saeki, S.-I. Tetrahedron: Asymmetry

  ) Planas, L.; Pe ´rard-Viret, J.; Royer, J.; Selkti, M.; Tomas, A. Synlett 2002, 1629-1632. (15) (a) Paquette L. A.; Negri J. T.; Rogers R. D. J. Org. Chem. 1992, 57, 3947-3956. (b) Paquette L. A.; Lanter J. C.; Jonhston J. N. J. Org. Chem. 1997, 62, 1702-1712. (c) Paquette L. A.; Kiney M. J.; Dullweber U. J. Org. Chem. 1997, 62, 1713-1722. (16) (a) Fenster M. D. E.; Patrick B. O.; Dake G. R. Org. Lett. 2001, 3, 2109-2112. (b) Fenster M. D. E.; Dake G. R. Org. Lett. 2003, 5, 4313-4316.

  SCHEME 1SCHEME 2

  SCHEME 4

  H NMR (300 MHz, CDCl3): δ )-0.05 (s, 3H), 0.23 (s, 3H), 0.80 (s, 9H), 1.89 (d, J ) 7.0 Hz, 3H), 5.31 (dd, J ) 3.6, 2.0 Hz, 1H), 6.01 (t, J ) 3.5 Hz, 1H), 6.16 (q, J ) 7.0 Hz, 1H), 6.35 (dd, J ) 3.4, 1.9 Hz, 1H), 7.10 (d, J ) 7.2 Hz, 1H), 7.42 (t, J ) 7.9 Hz, 1H), 7.50-7.55 (m, 2H), 7.77 (d, J ) 8.3 Hz, 1H), 7.88 (d, J ) 7.4 Hz, 1H), 8.08 (d, J ) 8.0 Hz, 1H). 13 C NMR (75 MHz, CDCl3): δ )-5.0,

  . IR (KBr): ν ) 2929, 2858, 1560, 1492, 1438 cm -1 .[R] 25 D: +11.7 (c ) 0.69, CH2Cl2). HRMS (CI, NH3, [MH] + ): m/z calcd 352.2097, found 352.2101.

  25 D: -48.4 (c ) 1.04, CHCl3). To a solution of 7

  25 D +163.0 (c ) 1.08, CHCl3). Anal. Calcd for C22H25NO3: C, 75.19;

  . IR (KBr): ν ) 2974, 1667, 1153 cm -1 . MS(CI, NH3): m/z ) 198 ([MH] + ), 215 ([M + NH4] + ). [R] 25 D: -49.4 (c ) 1.0, CHCl3). Anal. Calcd for C10H15-NO3: C, 60.90; H, 7.67; N, 7.10. Found: C, 60.37; H, 7

  . IR (KBr): ν ) 2945, 2884, 1682, 1491, 1444 cm -1 . MS (CI, NH3): m/z ) 346 ([MH] + ). [R] 25 D:-41.9 (c ) 1.27, CHCl3). Anal. Calcd for C19H23NO5: C, 66.07; H,6.71; N, 4.06. Found: C, 66.35; H, 6.68; N, 3.85.

J.Org. Chem, Vol. 69, No. 9, 2004 3089

Acknowledgment. This research was supported by the CNRS and the MENRT. We thank Prof. B. Bodo and Dr. A. Jossang from the "Museum National d'Histoire Naturelle" (Paris) for a gift of natural (-)cephalotaxine. 3092 J. Org. Chem., Vol. 69, No. 9, 2004 Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org) Powered by TCPDF (www.tcpdf.org)

Supporting Information Available: General experimental methods, detailed procedures, and analytical data for compounds 12-18 and 22-29 and selected 1 H NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

JO049884L

Planas et al.