Print version ISSN 0103-5053
J. Braz. Chem. Soc. vol.9 no.4 São Paulo July/Aug. 1998
A Common Approach to the Synthesis of Monocyclofarnesyl Sesquiterpenes
Paola Ciceria,c, F.W. Joachim Demnitz*b, Márcia C.F. de Souzab, and Maik Lehmanna,d
aPreclinical Research, Novartis Pharma AG, CH-4002 Basel, Switzerland
bDepartamento de Química Fundamental, Universidade Federal de Pernambuco, Cidade Universitária, 50.670-901 Recife - PE, Brazil
cPresent address: Pharmaton S. A., Via Mulini, CH-6934 Bioggio, Switzerland.
dPraktikant from the University of Marburg, Germany
Received: August 10, 1998
3b-hidroxi-4,4,10b-trimetil-9-decalona e seu análogo deshidroxi são intermediários úteis cuja oxidação de Baeyer-Villiger e subsequente homologação por dois carbonos propicia a construção rápida do esqueleto monociclofarnesólico. Os estudos sintéticos visando a uma variedade de sesquiterpenos são discutidos.
3b-hydroxy-4,4,10b-trimethyl-9-decalone and its dehydroxy analogue are useful intermediates whose Baeyer-Villiger oxidation and subsequent two-carbon homologation provides a quick entry into the monocyclofarnesyl skeleton. The synthetic approaches to a variety of sesquiterpenes are discussed.
Keywords: Baeyer-Villiger reaction, monocyclofarnesyl, sesquiterpenes, aplysistatin, ambilol, 7-hydroxy-6,11-cyclofarnes-3(15)-en-2-one, ancistrofuran, synthesis
Sesquiterpenes having a monocyclofarnesyl skeleton 1 are ubiquitous in nature1.
In such natural products the trimethylsubstituted ring is often additionally functionalized in the C3-neopentyl position (e.g. 2 - 4) as well as at the methyl-bearing tertiary carbon atom (e.g. 2 - 7#a). In view of the placement of functionality in the well known trans-decalones 8a2 and 8b2a,3 it was quite obvious that they could serve a role as common starting materials for many monocyclofarnesyl-sesquiterpenes syntheses. More specifically, Baeyer-Villiger oxidation of the B-ring ketone would provide 9a/b containing the correctly functionalized carbocyclic ring with appropriate stereochemistry for a variety of natural products. Introduction of two carbons atoms a to the lactone-carbonyl (alternatively in the open chain form) would complete the construction of the monocyclofarnesyl skeleton, (Scheme 1).
We are currently engaged in using this strategy for the synthesis of several sesquiterpenes. Recently we disclosed the total synthesis4 of (±)-farnesiferol-C 2 which closely followed along these lines. Here we would like to present a review and update on our ongoing efforts to synthesize aplysistatin 45, ambilol-A 5#b6, 7-hydroxy-6,11-cyclofarnes-3(15)-en-2-one 67 and ancistrofuran 78.
Results and Discussion
At the outset, the abovementioned Baeyer-Villiger oxidation of the 9-decalone system deserves some comment. The apparently straightforward B-ring ketone oxidation to the e-lactones 9a/b bore some surprises. Thus, performing the oxidation on the hydroxy ketone 8b provided the unconventional product 10 arising through participation of the 3b-hydroxy group. Substrates bearing an acid labile OH protecting group which is removed under the Baeyer-Villiger reaction conditions also resulted in 10 whereas the stable 3b-acyloxy derivatives 8c/d gave the expected lactones 9c/d4.
The mechanism of this rearrangement is currently the subject of investigation. Thus, the question of whether transanular oxa-ring formation occurs stepwise (8b ® 9b ® 12 ® 10) or in a concerted fashion with direct participation of the 3b-hydroxyl (8b ® 13 ® 10) is being addressed by us by means of a computational analysis of the thermodynamic and kinetic aspects of the possible reaction paths9 (see Scheme 2).
The 3-methylene analogue 8a gave an equally surprising product upon Baeyer-Villiger oxidation in acidic media (CF3CO3H/CF3CO2H/CH2Cl2), namely the hydroxy spirolactone 1610. Controlled buffered conditions (CF3CO3H/ CF3CO2H/Na2HPO4/CH2Cl2/0 °C, 72% or m-CPBA/NaHCO3/DCE/RT-reflux, 74%) were necessary to avoid the formation of 16 and secure the desired lactone 9a. We propose, that in acidic media 9a proceeds via olefin 14 and epoxide 15 to 16 (see Scheme 3).
Whilst the interesting rearrangement in the Baeyer-Villiger reaction of 8b giving 10 allowed the synthesis of farnesiferol-C 24, the other (lactone) products in hand (9a and 9c/d) open the way for our synthetic approaches to the natural products 4 - 7.
The decalones 8a/b are prepared2,3 from the well known Wieland-Miescher ketone 1711 which has served as starting material in many syntheses2a,2b,3a,4,12. The advent of the availability of the Wieland-Miescher ketone in enantiomerically pure form13 has widened its scope so as to permit the synthesis of natural products in their optically pure state14. We have recently found an improved and very efficient method for a rapid, selective and high-yielding preparation of the monoketal 18 of this diketone, uncontaminated by bisketal (which is extremely tedious to separate!)15. In summary, these developments allowed an expedient preparation of the common decalone intermediates 8a-d in appreciable quantity along established lines3b and in optically pure form.
The decalone 8d was oxidised using trifluoroperoxyacetic acid / trifluoroacetic acid in dichloromethane providing exclusively the lactone 9d in 75% yield. The final two carbons of the monocyclofarnesyl skeleton were then incorporated by alkylation of the lithium enolate (LDA/TMEDA/THF/-78 °C) with 1,2-dibromoethyl ethyl ether, cleanly providing the ethoxyethylated product 19d (79%) as a 2:1 mixture of diastereoisomers. Although the stereochemistry at the alkylated carbon in 19d was of no consequence for subsequent synthetic manipulations, the alkylation was shown to have occurred exclusively from the a-face (opposite to the angular methyl substituent) by means of an X-ray crystal structure of the minor diastereomer; (Fig. 1).
With a conclusive structure proof of the C-15 monocyclofarnesyl skeleton now in hand, the mixture of diastereomers 19d was treated with 5% aqueous K2CO3 in THF/acetone resulting in a one-pot conversion to the hydroxy butenolide 20d involving e-lactone hydrolysis, butyrolactone formation of the intermediate g-bromo acid and b-ethanol elimination thus completing the overall transformation of decal 8d to the advanced sesquiterpene intermediate 20d in just three steps and 47% overall yield. The hydroxybutenolides 20 serve as intermediates for aplysistatin 417 (from 20d) and ancistrofuran 7 (from 20a). The latter compound was prepared in a similar fashion from 8a in 40% overall yield (see Scheme 4).
Several routes from 20a to ancistrofuran can be envisaged. Our initial results indicate, that phenylselenation of the dienolate derived from the TBDMS-ether derivative gives, albeit in low yield (amongst other products) the deconjugated phenylseleno derivative 21. Clearly, oxidative elimination of this compound will provide the exocyclic a,b-unsaturated lactone 22, deprotection and ring closure of which would result in the butenolide 23 which has been previously converted to ancistrofuran8g. Therefore the transformation of 21 into 23 would complete a formal total synthesis of ancistrofuran. At the time of writing, our efforts in this regard are continuing and we will report the results at an appropriate opportunity. Owing to the fact, that the absolute configuration of ancistrofuran is not known, we are in a position to answer this question by virtue of the use of chiral material stemming from the (+)-Wieland-Miescher ketone.
If the introduction of the final two carbon atoms of the monocyclofarnesyl skeleton is delayed until a later point in the synthetic sequence, one may open the way to farnesiferol-C 24, ambilol-A 5 and 7-hydroxy-6,11-cyclofarnes-3(15)-en-2-one 617. In a procedure analogous to that used for the synthesis of farnesiferol-C,4 a standard Barbier-Wieland approach (PhMgBr / Et2O then p-TsOH / CHCl3) provided diphenylolefin 24 from lactone 9a. At the time of deadline for submission we are involved in the oxidative cleavage of this compound to the hydroxy acid 25. Treatment of this intermediate with methyllithium followed by TBDMS protection and vinylmagnesium bromide addition (final two monocyclofarnesyl carbons) should result in the allylic alcohol 264. It then remains to prepare the allylic bromide 274 and couple it with the 3-methylfuran Grignard reagent 2816 in order to complete the synthesis of ambilol-A.
We would like to thank Novartis AG / Basel / Switzerland for financial support. M.C.F. d. S. thanks the Departamento de Assuntos Estudantis of the University for a scholarship.
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