Stereoselective Synthesis of Bicyclic Lactones Via Annelation Protocol

A successful two-step annelation protocol of diesters and methyl bromoacetate with 2chlorocyclopentanone derivatives was efficiently pursued, which gave suitably substituted bicyclic lactones in high overall yields and with complete streoselectivity mediated by K-Selectride and Wilkinsons catalyst, is reported. As part of a program aimed at rapid synthesis of bicyclic lactones which inherently occurs in many active natural p roducts, this paper has shown a novel and rare methods for the synthesis of these important compounds based on alkylation of cyclopentanone derivatives and further demonstrate efficient reduction protocol of these compounds to the bicyclci lactones


Introduction
Bicyclic lactones systems are among Nature's preferred building blocks for the construction of tricyclic lactones of varied biological activ ities. In particu lar, the γ-butyrolactone mo iety is a recurrent feature of many naturally occurring substances examp les are brasoside and littoralissone. 1,2 The synthetic approaches to th is b icyclic lactones have been imaginative and numerous, [3][4][5][6][7][8] there is a continued demand fo r efficien t methods fo r the assemb ling o f these key co mp o u n d s wh ich g iv e es p ec ia lly h ig h lev e ls o f stereoselectivity. In this report, it is reported some of our preliminary resu lts in the estab lish ment o f an effect ive met h odo logy fo r th e const ruct io n o f t hes e impo rtant compounds. It was envisaged that reduction of substituted cyclopent anone derivat ives using methods o f alky lat ion d escrib ed by Fu mih in ko 9 wou ld g iv e th e ester (276). Stereocontrolled reduction with Select ride® reagents and subsequent lactonizat ion would furn ish the cis-fused bicyclic ring system (278). It therefore remained to investigate the annelation of a d iketone with methyl bro moacetate and later exp lo re the cy clization p roto co l. Th e su ccess o f th is synthetic methodology would requ ire the v iability of the condensation between the cyclopentanone derivatives with the diester and methyl bro moacetate. The most important step wou ld no w be the select ive Select rid e® med iated reduction of the ketone in co mpounds (276) and (283) to give the syn-alcohols which could readily ring closed into the Follo wing a report by Fu mih inko 9 that 2-chlorocyclopen tanone (275) can be converted into dimethyl 2-(2-o xocyclopentyl) malonate (276) we obtained the corresponding diester (276) in 53% yield. The 1H NMR spectrum fully supported the proposed structure (276), displaying a doublet centred at δ 3.84 ppm (J 5.8 Hz) corresponding to a methine proton bonded to the diester, a pair of three p roton singlets at δ 3.78 pp m and at δ 3.73 ppm for methyl esters and a one proton resonance at δ 2.97-2.88 ppm for the meth ine adjacent to the ketone. The presence of the ester groups and the ketone was manifested in the IR spectrum, with characteristic absorptions at, 1738 cm -1 and 1749 cm-1 respectively. The mass spectrum fu lly supported this with a mo lecular ion of m/ z 214 Scheme 1.1. The reduction of the ketone in (276) by treatment with 1.2 equiv of sodium borohydride at 0℃ furnished a mixture of two products which were chro matographically separable.
The major co mpound turned out to be (277) with the ketone group having been reduced, and having a distinct broad absorption in the IR spectrum at 3504 cm -1 corresponding to the hydroxyl group. The 1H NMR spectrum showed a one proton resonance at δ 3.96-3.91 pp m due the methine proton adjacent to the hydroxyl bearing carbon and two three proton singlets at δ 3.68 pp m and δ 3.67 pp m fo r the two metho xy groups. The C-2 proton appeared at δ 3.32 pp m with a coupling constant of 7.8 Hz, which lent support to the cis configuration of this predominant product at C-6 and C-2. The minor product (278) turned out to be the desired lactone, obtained in a total y ield of 26%. The IR spectrum displayed a characteristic band at 1742 cm -1 , that strongly supported the presence of a γ-lactone and absorption at 1733 cm -1 for the acyclic ester. The 1H NM R spectrum had a triplet at δ 5.06 ppm due to the C-7 methine proton with a coupling constant of (5.3 Hz) and a singlet at δ 3.73 pp m associated with the metho xy ester protons. The cis configuration between the C-2 and C-6 was fully confirmed due to a doublet at δ 3.68 ppm indicat ive of the proton adjacent to the ester with a characteristic cis coupling of (3.3 Hz). The mo lecular ion of m/ z 184 for the co mpound was also in accord with structure (278) Scheme 1.2.
In our attempt to imp rove the selectivity syn-alcohol which readily cyclizes to (278), sterically bulky reagents were considered as suitable reagents. It was decided to examine the reduction L and K-Selectrides®. [10][11][12] It was envisaged that reduction with these reagents, which are very bulky and have low coordinating ability, could follow the model of Felkin and Anh. 13 This plan finally rewarded us with success because as it was hoped, the Selectride reagents produced syn-alcohol selectivity under all conditions studied, the steroselectivity was found to increased with decreasing temperature to produce the diastereoisomers (277) and (278). The highest selectivity was achieved with K-Selectride® at -78 o C (100% syn-selectivity) and L-Selectride® (99:1 syn-selectivity) Table 1.1.
When (278) was subjected to further purificat ion on column chro matography eluting with hexane : ethyl acetate, a number of spots began to show in the TLC analysis, indicating decomposition of the lactone. Two products could be separated and identified fro m the comp lex mixture. The major product was the desired lactone, (278), but the decarboxylated co mpound (279) was also produced. The decarboxylated product (279) exhib ited an absorption in the IR spectrum at 1769 cm -1 wh ich is expected of the γ-lactone (279). The 1 H NMR spectrum included a one proton triplet at δ 5.02 pp m (J 5.0 Hz) accounting for methine adjacent to the oxygen in the γ-lactone , and a co mbination of a mu ltip let and a doublet at δ 2.97-2.80 pp m and at δ 2.79 ppm accounting for the diastereotopic CH 2 adjacent to the lactone carbonyl group. The absence of the exocyclic methyl ester protons was evident. These data, combined with a mo lecular ion of m/ z 126, led us to conclude that the decarboxylation had occurred to produce the lactone (279) Scheme 1.3.  Attempts to introduce functionality at C-2 of co mpound (280) by direct halogenation with either NBS or NCS were to no avail, and unchanged starting material was recovered, in some cases the reaction yielded co mpounds which could not be characterised. Attempted direct iodination o f co mpound (280) by treat ment with KIO 3 was likewise unsuccessful Scheme 1.4.
The approach at this juncture was to capitalise on the propensity of methyl bro macetate to react cyclopentene-1,3dione (282). Fortunately, ester (283) could be derived fro m cyclopentene-1,3-dione (282) in excellent yield. A modification of Fu mihin ko's conditions (CH 3 CN, THF and methyl boro moacetate) with cyclopententene-1,3-dione (282), afforded ester (283) in an isolable y ield of 78%. The 1H NM R spectrum of the ester (283) was diagonistic, with the methylene protons adjacent to the ester appearing as a singlet at δ 4.61 pp m, the three methyl ester protons appearing as a singlet at δ 3.38 pp m and a two proton trip let centred at δ 2.72 pp m. For methylene bonded to the ketone and the methylene adjacent to the enol hydroxyl, two protons were shown as a triplet centred at δ 2.49 ppm. Furthermore, the presence of the enol hydroxyl was manifested by a characteristic absorption at 3389 cm -1 in the IR spectrum. When the ester (283) was subjected to the conditions developed for cyclisation, namely, acidify ing the mixture to pH 2 and stirring at roo m temperature, TLC analysis indicated a new product had been formed, Ho wever, after aqueous work up, 1 H NM R analysis showed no reaction has taken place and only starting material had been recovered Scheme 1.5. Efforts to saturate the ring using PtO 2 or Pd-C, however, only met with failure. One may suppose, based on the structure of (283), that the steric hindrance in the vicinity of the olefinic bond is responsible for the lack of success with the hydrogenation protocol. However attempt to reduce the ketone in (283) using Wilkinson catalyst in catecholborane proved to be successful 14 , follo wed by attempts to cyclize the product using the conditions developed for cyclizat ion, namely, acidifying the mixture to pH 2 and stirring at room temperature, produced the bicyclic lactone (284) Scheme 1.

Experimental Techniques
Co mmercial reagents were obtained fro m A ldrich and Lancaster chemical suppliers and were used directly as supplied or purified prior to use following the guidelines of Perrin and A marego.15 Dichloro methane and acetonitrile were reflu xed over and distilled fro m CaH 2 prior to use. Diethyl ether and ethanol were obtained dry fro m A ldrich. THF was dried by distillation fro m the sodium benzophenone ketyl rad ical under nitrogen. Light petroleum is the fraction of petroleu m ether boiling in the range 30-40 o C, and it was fract ionally distilled through a 36 cm Vigreu x colu mn before use. Non-aqueous reagents were transferred under argon via syringe. Organic solutions were concentrated under reduced pressure on a Büchi rotary evaporator using a water bath. Thin-layer chro matography (TLC) was performed on Merck alu min iu m-backed p lates coated with 0.2 mm silica gel 60-F plates. Visualizat ion of the developed chromatogram was performed by UV fluorescence quenching at 254 n m, or by staining with a KMnO4 solution. 1H and 13C NM R spectra were recorded on a Bruker DPX250 (250 M Hz for protons) and a Brüker AMX400 (400 MHz for protons). Data for 1 H NM R are reported as follows: chemical shift (δ-pp m), mu ltiplicity (s = singlet, d = doublet, t = trip let, q = quartet, m = mult iplet), integration, coupling constant in (Hz). Data for 13C NMR spectra are reported in terms of chemical shift (pp m) down field fro m TMS. IR spectra were recorded on a Perkin Elmer Paragon 1000 or a Perkin Elmer 881 spectrometer as a thin film between sodiu m ch loride plates or as a KBr disk. A ll absorptions are reported in terms of frequency of absorption (cm -1 ). Mass spectrometric data were recorded on VG Autospec, under conditions of chemical ion isation (C.I) using ammonia as the ionising source. Peaks are quoted in the form (m/ z) (relative intensity).

-hydroxycyclopentyl)malonate (277) Methyl-2-oxo-hexahydro-2 H-cyclopenta[b]furan-3-ca rboxyl ate (278)
To a stirred solution of d imethyl 2-(2-o xocyclopentyl)ma lonate (276) (80 mg, 0.37 mmol, and 1.00 equiv) in ethanol (7 mL) in an Erlen meyer flask was added NaBH 4 at room temperature (16.8 mg, 0.44 mmol, 1.20 equiv) in small portions over a period of 15 min The reaction mixture was stirred for further 30 minutes and then poured into water (5 mL). (5%) dilute hydrochloric acid (4 d rops) was added and the organic layer was extracted with ether (2 x 10 mL), dried over MgSO 4  To a stirred solution of 1,3-cyclopentanedione (279) (5 g, 0.05 mmo l, 1.00 equiv) in acetonitrile (80 mL) at 0 o C was added triethylamine (7.6 g, 0.07 mmo l, 1.30 equiv), and the solution was stirred for 10 min at roo m temperature. Methyl bromoacetate (9.9 g, 0.07 mmol, 1.30 equiv) was then added and stirring was continued over night by which time TLC revealed a new spot. Saturated aqueous NH4Cl solution (15 mL) was added to the mixture and the organic layer was extracted with ethyl acetate (3 x 20 mL). The co mbined organic layers were washed with sat. NaHCO 3 solution (2 x15 mL), brine (2 x 15 mL), dried over MgSO 4 and concentrated in vacuo. The residue was purified by colu mn chromatography on silica gel using a solvent gradient hexane : ethyl acetate fro m (4:1) to afford the title compound as a white powder (6.6 g, 78%); m.p. (148-151  To a stirred solution of Wilkinsons catalyst (RhPPh 3 Cl) (236.00 mg, 0.589 mmo l, 0.5 equiv) in anhydrous THF (7 mL) under N2 cooled to -20 o C was added to methyl 2-(2-hydroxy -5-o xocyclopent-1-enyl)acetate (283) (200.0 mg, 1.18 mmol, 1.00 equiv ), followed by the addition of catecholborane (424.45 mg, 3.54 mmo l, 3.0 equiv). The reaction was allo wed to stir for 24 hr, by wh ich time TLC analysis revealed a new product been formed. The reaction was quenched by the addition phosphate buffer pH 7.00, (4 mL). The p roduct was extracted with ether (4 x 10 mL), washed with 10% aqueous sodium sulfite (2 x 5 mL) and saturated sodium carbonate (4 x 5mL). The ether layer was followed by the addit ion of d ilute hydrochloric acid (5%) at 0 o C until the pH 2 was reached, by which t ime, TLC analysis showed a new product has been formed. The aqueous layer was extracted with ether (4 x 10 mL), the extract dried over