Konstantinovic, S

Mihailovic, M Lj

Sebastijan, Z

Vukicevic, R D

NOPR

Indian Journal of ChemistryVol. 37B, November 1998, pp. 1126-1128Transformation of enals and enones into the glycoside-type derivatives byelectrochemical generation of phenyl seleny cationR D Vukicevic'", Z Sebastijan", S Konstantinovic" & M Lj Mihailovic'''Department of Chemistry, Faculty of Science, University of Kragujevac,R Domanovica 12, P.O. Box 60, Yu-34000 Kragujevac, FR YugoslaviabFaculty of Chemistry, University of Belgrade and Institute of Chemistry, Technology and Metallurgy,Belgrade, P.O. Box 550, Yu-lIOOI Belgrade, FR YugoslaviaReceived 14 May 1998; accepted 27 July 1998The synthesis oftetrahydrofuran- and tetrahydropyran-typecyclic acetals has been achieved by electro-chemical cyclization of A4-unsaturated carbonyl compounds. The reaction is performed by electrolysis ofthese substrates and diphenyl diselenide in a saturated solution of KBr in methanol, by using an undividedcell, whereas graphite and Cu are used as an anode and a cathode, respectively.The reaction of selenium containing electrophileswith. the double bond of organic compoundsfollowed by an attack of the suitable nucleophilesto u-arylselenocarbocations derived at the firststage of this reaction, is the well-known method forthe synthesis of 1,2-bifunctional seleniumcontaining organic compounds. This reactionbecame a powerful tool for organic synthesisbecause the selenium moiety can be easilyremoved from an organic molecule either byoxidative or by reductive processes, givingunsaturated or saturated products, respectively'.This technique is frequently used for the synthesisof naturally occurring compounds'. Further, manynaturally occurring organic compounds containcyclic acetal or ketal moieties, which facilitate thesyntheses of glycoside-type compounds. Thus,several articles describing reactions of unsaturatedcarbonyl compounds with phenylselenenyl halidesand corresponding alcohols giving cyclic acetalsand ketals (some of which, are naturally occurringproducts) have appeared in the literature',In continuation of our investigations ofelectrophilic reactions of functionalized4 or simplealkenes' we report herein the transformation ofunsaturated aldehydes and ketones into thecorresponding cyclic aceta Is and ketals. Accordingto above mentioned reports from the literature' andto our earlier experience gained from reactions ofthe electrochemically generated PhSeGl ion", wehave chosen this ion as the key intermediate forsuch type of transformation.We started our investigations by a constantcurrent electrolysis of A4-alkenals la-d (Scheme I,Table I) in methanol solution of diphenyldiselenide and potassium bromide. The electrolysiswas performed in an undivided cell by using agraphite stick as the anode, whereas the cathodewas a copper spiral, As expected, the majorreaction products were the corresponding selenatedglycoside type-compounds, i.e. cyclic acetalscontaining tetrahydrofuran- or tetrahydropyran-type rings (2a-d or 3a-d), yield 51-60%. Thesecompounds were isolated by columnchromatography and identified by IR and NMRspectral data. Besides these compounds, in somecases, small amounts of 2-methoxy-l-phenyl-seleno-5-alkanals or their actals were also isolated(up to 21 and 15%, respectively).. The data in Table I show that the nature of thedouble bond of the unsaturated aldehyde exhibit astrong influence on the size of the ring formed.Table l--Electrochemical cyclization of enals and enonesCompd RI R2 R3 Product Yield(%)la H H H 2a 56Ib CH3 H H 3b 52Ie H CH3 H 2e 54Id CsHl1 H H 3d 60Ie H H CH3 2e 59If CH3 CH3 CH3 2f+3f 51VUKICEVI<~ et al.: TRANSFORMA nON OF ENALS & ENONES INTO GLYCOSIDE-TYPE DERIVATIVES 1127R2, T>:y- c?'---R3 H~Rl Ph~e.J O-CH3la-f. R :~~ f· •electrolysis 0 RPhSeSePhi~Br2 or Br~~ ~ PhS!d B?~-ef'vR3- I-f + PhSe~O/"'OCH3R2 Rl2a-f 3a-f~ and R2 = H, CH3 or C5H11;W = H or CH3Scheme IThus, 4-pentenal Ia, alkenal with the unsubstituteddouble bond, and Z-4-hexenal Ic, givetetrahydrofuran-type products 2a and 2c, res-pectively whereas E-4-hexenal I band E-4-decenalLd by this reaction give the tetrahydropyran-typeproducts 3band 3d, respectively. We believe thatthis reaction is stereospecific because each of thefour products derived has only one singlet for themethoxyl group in the 'nNMR spectra.Two ~4-alkenones, namely 5-hexen-2-one Ieand 6-methyl-5-hepten-2-one If, also underwentthis reaction. The compound Ie gave thecorresponding tetrahydrofuran-product 2e, likealkenal Ia (in 58% yield). On the other hand, thecompound If afforded a mixture of products whichcould not be separated. But on the basis of spectraldata the major products obtained were five and sixmembered cyclic acetals (2f and 3f) accompaniedby some other products. Contrary to the cyclization,of alkenals, in the case of akenone Ie twodiastereoisomers (cis and trans) were formed,because there were two singlets for the methoxygroup and two for the methyl group in the lH NMRspectra. The ratio of two diasteroisomers was39:61. Which one of the two is predominant is yetnot known.We suppose that the reaction starts by theoxidation of bromide ions at the anode for somespecies (bromine or bromonium ions) which areable to oxidize diphenyl diselenide to thephenyl selenium cation. This cation,then, reactswith the double bond of the unsaturated carbonylcompound giving an a-phenylselenyl carbocation3PhSD, R3orland R2Rl 0 OCH3which undergo nucleophilic attack at the carbonyloxygen. This process is probably supported bysimultaneous nucleophilic attack of the methanolicoxygen to the carbonyl carbon (see Scheme I).Experimental SectionIn a typical procedure, 154 mg (1 mmole) of Idand 156 mg (0.5 mmole) of diphenyl diselenidewas electrolyzed in 10 mL of saturated solution ofKBr in methanol at a constant current (100 mA; 2F/mol). At the end of the reaction the yellowcolour of diphenyl diselenide disappeared and thesolution became colourless. Methanol wasevaporated and to the rest was added 20 mL ofwater. The mixture was extracted with ether anddried over anhydrous Na2S04. After evaporation ofthe solvent, the reaction mixture on columnchromatography (Si02/pet. ether-ethyl acetate; 9: 1)gave 205 mg of 3d (60%) and 60 mg of dimethylacetal of 4-methoxy-5-phenylselenyl-decanal(15%).The above method used for preparing glycoside-type compounds, which does not require expensiveand complex equipment, compares very favourablywith existing methods and can be very useful forthe synthesis of natural products.References1 Claude Paulmier. Selenium reagents and Intermediates inOrganic Synthesis. (Pergamon Press, Oxford), 1986.2 Nicolaou K C & Petasis N A, Selenium in NaturalProducts Synthesis. (CIS, Inc., Philadelphia), 1984.3 Current S & Sharpless K B, Tetrahedron Left, 1978, 5075;Stevens R V & Hrib N, Tetrahedron Left, 22, 1981,4791;Ley S V, Lygo B, Tetrahedron Left, 23, 1982, 4625; Ley S1128 INDIAN 1 CHEM, SEC B, NOVEMBER 1998V, Lygo B, Molines H & Morton 1 A, 1 Chem Soc ChemComm, 1982, 1251.4 Mihailovic M Lj, Konstantinovic S & Vukicevic R,Tetrahedron Lett, 28, 1987, 4343; Mihailovic M Lj,Vukicevic R & Konstantinovic S, Tetrahedron Lett, 28,1987, 6511; Vukicevic R, Konstantinovic S & MihailovicM Lj, Tetrahedron, 47, 1991, 859.5 Vukicevic R, Konstantimonic S, Joksovic Lj, PonticelliG & Mihailovic M Lj, Chem Lett, 1995, 275; Vukicevic RD, Joksovic Lj, Konstantinovic S, Markovic Z &Mihailovic M Lj, Bull Chem Sac Jpn, 71 (No.3), 1998, (inpress).

Transformation of enals and enones into the glycoside-type derivatives by electrochemical generation of phenyl seleny cation

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Transformation of enals and enones into the glycoside-type derivatives by electrochemical generation of phenyl seleny cation

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