An Approach to the Construction of the Carbon Skeleton of Marine Nor-sesquiterpenes . Total Synthesis of ( ± )-Dehalo-Napalilactone

Nesse trabalho descrevemos uma abordagem sintética para a preparação de um esqueleto carbônico que tem dois centros quaternários vizinhos, um dos quais apresenta uma unidade espiro -butirolactona. Esse arranjo molecular é encontrado em nor-sesquiterpenos isolados de corais marinhos. A estratégia sintética utilizada se baseou no uso de uma reação de adição 1,4 do dimetilcuprato de lítio sobre a 2-metilcicloexenona, seguida da interceptação do enolato intermediário com brometo de alila, para obter a trans-2-alil-2,3-dimetilcicloexanona com moderada diastereosseletividade. Essa última já tem incorporada em sua estrutura um dos centros quaternários do esqueleto. O segundo centro quaternário, que porta a unidade espiro -butirolactona, foi preparado através de uma reação de adição de um reagente organolítio, seguido da separação dos isômeros e de etapas de oxidação. Essa estratégia permitiu obter o esqueleto carbônico dos sesquiterpenos e ao mesmo tempo relatar a síntese total de um derivado nor-sesquiterpênico não natural, em 6 etapas com um rendimento global de 16%, a partir da 2-metilcicloexenona.


Introduction
Chemical studies of the constituents of terrestrial organisms, particularly those of microorganisms and plants have long been carried out, and the development of this field has been remarkable due to the progress made in chemical instrumentation after World War II.Much work on the constituents of animals such as vitamins, hormones and pheromones has been reported 1 .
However, the search for new compounds from the sea is a relatively recent undertaking.Early studies on the chemistry of marine organisms were the domain of organic chemists, most of whom were concerned with the isolation, chemical characterization and phylogenetic variants of specific substances, for example, types of steroids present in diverse marine animals.A symposium held in 1960 on the biochemistry and pharmacology of compounds derived from marine organisms brought researchers together for the first time and gave cohesion and direction to this field 2 .
The living environments of marine organisms differ from that of terrestrial organisms, for example, in the seawater the concentration of halides is very high.Due to these differences the constituents in the marine organisms differ considerably from those of the terrestrial organisms.
The presence of halides in seawater has readily allowed marine organisms to incorporate bromine, chlorine, and iodine, in that order, into covalent organic structures.Marine organisms contain abundantly halogenated organic compounds, in particular brominated and chlorinated compounds 3 .In 1992, P. J. Scheuer et al. 4 reported the isolation and the structure of a new sesquiterpenoid, Napalilactone (1, Figure 1) from the soft coral Lemnalia africana.This compound was the first example of an halogenated norsesquiterpene to be isolated from a marine organism.This nor-sesquiterpene is biogenetically derivable from an aristolene carbon skeleton.It presents an unusual structure with two contiguous quaternary centers.One of the quaternary centers bears a spiro g-butyrolactone unity.Apparently, this halogenated nor-sesquiterpene is part of the coral's chemical defense system 4 .
As part of a current research program directed towards the total synthesis of some marine natural products, we disclose herein our results concerning a strategy for the preparation of the carbon skeleton of these nor-sesquiterpenoids.Our interest was focused on the development of a simple and direct methodology, which allowed us to control the relative configuration of the contiguous quaternary centers.Additional modifications in this methodology should permit us to synthesize 1 and 3 (Figure 1), in their racemic forms.In this study we describe the synthesis of (±)-dehalo-napalilactone (2), a non-natural nor-sesquiterpenoid derivative.

Results and Discussion
From our point of view, the carbon skeleton of the norsesquiterpenes 1 and 3 could be prepared from the a-allyl cyclohexanone 6, through the addition of a suitably functionalized organolithium reagent to furnish the diol 5 (Scheme 1).The preparation of the spiro-g-butyrolactone moiety could be secured by the oxidative cyclization of Scheme 1. Retrosynthetic analysis the diol 5, with the correct configuration at C10 (for napalilactone numbering, see Scheme 1).The required ketone 6 could be stereoselectively prepared through a conjugate addition of lithium dimethylcuprate to the double bond of 2-methylcyclohexenone (7), followed by the trapping of the copper enolate intermediate with allyl bromide.The control of the relative stereochemistry of the methyl groups at C4 and C5 (napalilactone numeration) should be secured in this step by this simple sequence (Scheme 1).
The ketone 7 could be easily prepared from 2methylcyclohexanol using a standard procedure 6 .Depending on the sucess attained with this strategy, some additional modifications should allow us to synthetize in the future the nor-sesquiterpenes 2 and 3 in their racemic forms.

Preparation of ketone 6
The 2-methylcyclohexenone (7) was prepared using a standard procedure 6 in three steps and 73% overall yield from commercial 2-methyl-cyclohexanol.To obtain the carbonyl compound 6 we decided to take advantage of the greater stereoselectivity and generally greater yields of 1,4addition products obtained using organocopper reagents.Boeckman 7 has described a methodology based on a stereo-and regioselective double alkylation of a,b-unsaturated ketones.The 1,4-addition of lithium dimethylcuprate to 2methylcyclohexenone 7 , followed by the regioselective alkylation of the copper enolate intermediate with allyl bromide, gave the allyl ketone 6a/b as a diastereoisomeric mixture (GC analysis, cis:trans 20:80) (Scheme 2).
The selectivity obtained in the preparation of ketone 6 can be rationalised by the conformation of the cuprate enolate intermediates A and B (Figure 2).Due to a A 1(2) strain 8,9 conformation B is preferred and the electrophilic attack takes place from the less hindered face of the double bond, thus leading preferentially to the methyl groups in a cis relationship (Figure 2).
The diastereoisomeric mixture was readily separated by column chromatography on silica gel to furnish ketone trans-6b, as a pure isomer in 62% yield.Boeckman reported a diastereoselection ratio of 10:90 (cis:trans).Unfortunately others 10 as well as ourselves were unable to reproduce this result.

Preparation of the spiro g-butyrolactone unity
To prepare the spiro g-butyrolactone moiety at C10, it was necessary to add a suitably functionalized C3 residue to the ketone 6b.In our view the most direct way to do this was through the 1,2-addition of an organometallic reagent.The C3 residue was readily obtained from 1,3-propanodiol by using the methodology recently described by Forsyth 12 and Chen 13 .Treatment of 1,3-propanodiol with sodium hydride in THF at 0°C, followed by the addition of pmethoxy-benzyl chloride furnished PMB-ether alcohol intermediate, in 84% yield.The mesylation of the alcohol, followed by substitution with NaI, provided the iodide 11, in 98% yield for the two steps (see experimental section).
The ketone trans-6b was treated at -23°C with the organolithium compound derived from the iodide 11 (generated by in situ treatment with an ethereal solution of t-butyllithium) to furnish the tertiary alcohol 8, as a mixture of diastereoisomers (ratio 8a/8b 1:1) (Scheme 2).
Unfortunately no stereoselectivity was observed in this step, however the diastereomeric alcohols 8a/8b were easily separated by flash column chromatography.
In order to proceed with our planned synthetic strategy it was necessary to determine the relative stereochemistry of the new stereogenic center.All attempts to do this by 1 H NMR (nOe) failed.In fact the results obtained with the nOe experiments were not conclusive.The problem of the relative stereochemistry of the diastereoisomers 8a/8b was solved by the ozonolysis of the separated diastereoisomers at -78°C which after treatment with dimethyl sulfide gave the hemiacetal 12 and the aldehyde 13 (Scheme 4).From our point of view the formation of the hemiacetal 12 from the alcohol 8a is an unambiguous proof that the hydroxyl group and the aldehyde are syn.These results confirmed that the hydroxyl group of alcohol 8a was a oriented and b oriented on alcohol 8b (Scheme 3).
To prepare the spiro-g-butyrolactone it was necessary to remove the p-methoxybenzyl group.All attempts to cleave this protection group using 2,3-dichloro-5,6dicyano-benzoquinone (DDQ) in the presence of the free tertiary hydroxyl group lead to a mixture, where it was impossible to detect the expected product.To avoid this problem the tertiary alcohol 8a was first transformed to the trimethylsilyl ether 9 (Scheme 2).Then the silylether 9 was treated with DDQ in dichloromethane/water, followed by the addition of tetrabutylammonium fluoride (n-Bu 4 NF) to remove the silyl group.This simple protocol provided the diol 5 in 92% yield for the two steps (Scheme 2).

Scheme 3. Chemical proof for the stereochemistry of 8a/8b
butyrolactone 10 by treatment with tetraisopropylammoniumperruthenate (TPAP) in the presence of molecular sieves (4Å) and morpholine N-oxide (NMO), in accordance with the methodology described by Mehta and Karra 14 .Under these conditions the primary hydroxyl group was oxidized to an aldehyde which was transformed in situ into a hemiacetal intermediate 14 , which was oxidized to the lactone 10 (Scheme 2).At this stage we had incorporated almost all the functionality of the nor-sesquiterpene structure with the suitable relative configuration.To complete our reaction sequence the product 10 was submitted to a modified Wacker reaction 15 .The lactone 10 was treated with PdCl 2 and Cu(OAc) 2 in a mixture of N, N-dimethylacetamide and H 2 O (7:1) to furnish the (±)-dehalo-napalilactone (2), as a white solid (Scheme 2).

General
The 1 H and 13 C NMR spectra were recorded on a Varian GEMINI BB-300 at 300MHz and 75.1 MHz respectively.The 1 H spectra were also recorded in an AW-80 Bruker at 80MHz and Inova 500MHz.The mass spectra were recorded using a CG/MS HP model 5988A and an Autospec-Micromass -EBE -High Resolution.The melting points were measured in open capilary tubes using an Electrothermal apparatus model 9100, and are uncorrected.Purification and separations by column chromatography were performed on silica gel, using normal or flash chromatography.Ether and THF were distilled from benzophenone ketyl under nitrogen.Dichloromethane was distilled from CaH 2 .TLC visualization was achieved by spraying with 5% ethanolic phosphomolybdic acid and heating.All the organolithium reagents were purchased from Aldrich Chemical Company.

Synthesis of (±)-2-allyl-2,3-dimethylcyclohexan-1-one (6a/6b)
To a suspension of CuI (7.79 g, 41.0 mmol) in anhydrous ether (90 cm 3 ) was added an ethereal solution of methyllithium (65 cm 3 , 82.0 mmol, ca.1.25 mol dm 3 ), at 0°C, under an inert atmosphere of N 2 .After 15 min at 0°C, a solution of 7 (3.0 g, 27.27 mmol) in anhydrous ether (30 cm 3 ) was added to the ethereal solution of lithium dimethylcuprate.After 60 min, at 0°C the solvent was removed under reduced pressure (CAUTION: Avoid drying the reaction media completely as it is well known in the literature that some dry RCu compounds can explode) 16 .To the resulting yellow wet solid was added DME (65 cm 3 ), under a N 2 atmosphere giving rise to a greenish black solution, to which allyl bromide (19.0 cm 3 , 218 mmol) was added, at 0 o C. The final solution was stirred for 15 min.After that, the reaction was quenched with a saturated solution of NaHCO 3 (200 cm 3 ), followed by the addition of a 10% solution of NH 4 OH (45 cm 3 ).The blue aqueous phase was extracted with pentane (3 x 200 cm 3 ).The combined organic layers were washed with a 10% solution of NH 4 OH (50 cm 3 ) and distilled water (100 cm 3 ).The organic phase was dried over anhydrous MgSO 4 and the solvent was removed under reduced pressure.The oily residue was purified by flash column chromatography (silica gel 230-400 mesh, eluting with hexane-ethyl acetate 99:1 v/v) to furnish ketone cis-6a (0.68g, 15%) and ketone trans-6b (2.74g, 62%), as colorless oils.
IR n max /cm To a solution of alcohol obtained above (3.0g,13.3 mmol) in dry dichloromethane (105 cm 3 ) was slowly added, at 0°C, triethylamine (2.3 cm 3 , 16.8 mmol) and mesyl chloride (1.55 cm 3 , 20 mmol).To the resulting mixture was added a solution of 4-dimetylaminopyridine (DMAP) in dry dichloromethane (5 cm 3 ).The final solution was stirred for 2h at room temperature.After which time cold water (100 cm 3 ) was added to the reaction and the mixture was extracted with ethyl acetate (2 x 100 cm 3 ).The combined organic layers were washed with a solution of HCl (0.1 mol dm 3 , 50 cm 3 ), NaHCO 3 5% (2 x 50cm 3 ) and brine (50 cm 3 ).The organic layer was dried over anhydrous Na 2 SO 4 and the solvent evaporated under reduced pressure.The residue (4.18g, quantitative yield) was sufficiently pure (by t.l.c) to be used in the next step without purification.
To a solution of the mesylate (4.10 g, 15.0 mmol) in acetone (156 cm 3 ) was added sodium iodide (11.25 g, 75 mmol).The resulting mixture was refluxed, under argon, for 6h.After cooling to room temperature distilled water (260 cm 3 ) was added and the mixture was extracted with ethyl ether (2 x 200 cm 3 ).The combined organic layers were washed with brine (2 x 130 cm 3 ).The aqueous phases were combined and extracted with more ethyl ether (2 x 200 cm 3 ).The organic layers were combined and dried over anhydrous Na 2 SO 4 .After evaporation of the solvent, the residue was purified by column chromatography to furnish iodide 11 (4.5 g, 98%).
IR n max / cm
Alcohol 8a: IR n max /cm
IR n max /cm
IV (n max /cm

Conclusion
In conclusion, this simple and direct strategy has permitted us to describe the first racemic total synthesis of dehalo-napalilactone (2), a non-natural sesquiterpene.Dehalo-napalilactone (2) was prepared in 6 steps from methylcyclohexenone with an overall yield of 16%.Additional modifications to this strategy are ongoing in our laboratory, our objective being the total synthesis of (±)-napalilactone ( 1) and (±)-pathylactone (3).

Figure 2 .
Figure 2. Rationalisation for the diasterereoselectivity obtained in the preparation of 6.