4665 configuration of 1. The bridgehead methyl a t C ( 1) has the /3 configuration, all other substituents are a. In general bond distances and angles agree with generally accepted values and there are no abnormally short intermolecular contacts. Work in progress indicates that we have isolated several compounds having the same new carbon skeleton, which we wish to name the dollabellane skeleton 4. Thus the acetate 1 is ( I S * , 2E, 4R*, 7E, lOS*, 11R*, 12R*)-lO-acetoxy-l8hydroxy-2,7-d011abelladiene.~ The dollabellane skeleton could be formed by cyclization of geranylgeraniol pyrophosphate as shown.
within this pair of reactions has been clearly indicated by recent s t ~ d i e s . ~Through -~ reactions of H C O [ P ( O C H ~ )and ~]~~ C H ~ C O [ P ( O C H with ~ ) ~ H+ ] ~ ~and CH3+, we have obtained a series of Y2Co[P(OCH3)3]4+ complexes which provide further information on intimate mechanism in reductive elimination. A valuable and general synthetic procedure has been developed from the elimination reactions. The cis complex6 H2Co[P(OCH3)3]4+ was readily generated by protonation of HCo[P(OCH3)3]4 with a nonmineral acid5.8 and isolated in high yields as H2Co[P(OCH3)3]4+PFg-.9.10In solution a t 25", the dihydride" slowly evolved hydrogen to form Co[P(OCH3)3]5+.12 Exposure of solut i o n ~ ' ~of, the ' ~ dihydride cation to deuterium gave an equilib r i ~ mmixture l ~ ~ of D2Co[P(OCH3)3]4+ and H2 in -24 h; no H D was detected.I5 Hence a conventional equilibrium (eq 1) ki
H2Co[P(OCH3)3]4+ +H2
I
k-
4
I
The compounds isolated from the digestive glands of mollusks have invariably been traced to a dietary source. This has not been possible in the present study, since Dollabella is a nocturnal feeder and there are no reports of its dietary preferences. Acknowledgments. We wish to thank J. R. Lance for identifying the preserved materials. This research was supported by a grant from the National Science Foundation (GB-37227). Supplementary Material Available: Fractional coordinates (Table I), important bond distances (Table 11),important bond angles (Table 111), and observed and calculated structure factors (Table IV) (1 1 pages). Ordering information is given on any current masthead page. References and Notes (1) F. M. MacFarland, Calif. Acad. Sci., Memoirs, 6, 546 (1966). (2) M. 0. Stallard and D . J. Faulkner, Comp. Biochem. Pbysiol. B, 49, 25 (1974). (3) T. K. Devon and A. I. Scott, "Handbook of Naturally Occurring Compounds, Vol II. Terpenes", Academic Press, New York. N.Y., 1972. (4) G. Germain, P. Main, and M. M. Woolfson, Acta Crystallogr., Sect. 6, 24, 274 (1970). (5) The following crystallographic programs were used: W. R. Busing, K. 0 . Martin, and H. L. Levy, "A F W a n Crystallographic Least Squares Program", USAEC Report ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, Tenn.. 1965; C. Johnson, "ORTEP, A Fortran Thermal-Ellipsoid Plot Program", USAEC Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge. Tenn., 1965. (6) Camille and Henry Dreyfus Teacher-Scholar Grant Awardee, 19721977. (7) We follow the IUPAC Tentative Rules for the Nomenclature of Organic Chemistry for designating chiral centers for which the relative but not absolute configuration is known (E-5.10). J. Org. Cbem., 35, 2849 (1970).
Chris Ireland, D. J. Faulkner* Scripps Institution of Oceanography La Jolla, California 92093 Ames Laboratory-USERDA
Janet Finer, Jon Clardy*6 and Department of Chemistry Iowa State University Ames, Iowa 5001 I Received April 16, 1976
Reductive Elimination Reactions Sir: Within the set of important class reactions in coordination and catalytic chemistry' is the symmetric pair of "oxidative addition" and "reductive elimination". Mechanistic complexity
+ Co[P(OCH3)3]4+
(I)
I
must be extant with the H2 elimination step strictly intramolecular. An analogous exchange was observed in acetonitrile although a small amount of H D was produced due to a deprotonation step (eq 2 ) that occurs slowly in donor solvents.I5
S
+ H ~ C O [ P ( O C H ~ )+ ~ ] ~SH+ + slow
+ HCo[P(OCH3)314
(2)
Kinetic studies of the reaction of the dihydride with trimethyl phosphite (eq 3) H2Co[P(OCH3)314+ + (P(OCH3)3
+
H2
+ CO[P(OCH3)315+
(3)
and the isolation of solid Co[P(OCH3)3]4+PF6- and of labile adducts of Co[P(OCH3)3] 4+ further delineate the solution chemistry of the dihydride and the clean character of this reversible reductive elimination step. Reaction rate for the phosphite reaction (eq 3) with the dihydride in dichloromethane solution was, under pseudo-first-order conditions, essentially insensitive to reactant ratio. Reaction order in phosphite was established by the initial rates method, and the data were fully consistent with a steady state condition for the intermediate complex formed in eq 1, followed by reaction 4 Co[P(OCH3)3]4+
+ P(OCH3)3 k4,Co[P(OCH3)3]5+
(4)
where the very stable Co[P(OCH3)3]5+ complex was formed with the condition of -dlH2Co[P(OCH3)3]4+)/dt = ~ ~ ~ ~ { H ~ C ~ [ P ( O C H ~ ) ~ I ~ + J [[H21 P ( O+Ck4b H~)~I/(~[P(OCH3)3]) at 30 "C with kl = 4.2 X s-l. Reaction rate was substantially higher in coordinating solvents indicating that the further equilibrium reaction (eq 5) Co[P(OCH3)3]4+
+ S + SCo[P(OCH3)3]4+
(5) must be considered in such solvents (vide infra). The slow decomposition of the dihydride in the absence of added phosphite results from reaction 4 with free phosphite derived from the dissociative reaction (eq 6). H ~ C O [ P ( O C H ~ )+ ~ ]H~~+C O [ P ( O C H 3~' ) ~ ] + P(OCH3)3 ( 6 ) An estimate of the ligand dissociation rate, for dihydride, based on IH D N M R spectral6 in the +20 to 70° range, is about I O 2 s-] a t 70". Methylation of HCo[P(OCH3)3]4 or protonationl' of C H ~ C O [ P ( O C H quantitatively ~)~]~ (eq 7) yielded methane; the major cobalt productsi2identified were C O [ P ( O C H ~ ) ~ ] ~ + and cobalt metal. Communications to the Editor
4666
C H ~ ( H ) C O [ P ( O C H ~ )4 ~ JCH4 ~ + + Co[P(OCH3)3]4+ (7)
nation of methane from (CH3)2Co[P(OCH3)3]4+ is the cationic carbene CH2Co[P(OCH3)3]4+.We have evidence, from Methane elimination was very fast even at -78’; the meththe decomposition of (CH3)2Co[P(OCH3)3]4+ in a CD2C12 ylhydrido cobalt complex was too unstable for isolation or of a possible carbene-solvent reaction. The major spectral characterization. Protonation of C H ~ C O [ P ( O C H ~ ) ~medium, ]~ hydrocarbon product was CH4 but small amounts of C2H6 and in a deuterated solvent gave only CH4 and protonation of CH2=CD2 were detected. Attempts to isolate the putative CD3Co[P(OCH3)3]4 gave only CD3H. Hence this elimination carbene complex from the reaction mixture have been unsucreaction is clean and fully intramolecular in character; there cessful and no characteristic “carbene” reactions with other can be no long-lived radical intermediates. substrates have been demonstrated. Intermediacy of Co[P(OCH3)3]4+ was demonstrated by trapping reactions as in the reaction of C H ~ C O [ P ( O C H ~ ) ~ ] ~Disparity in the rates for the two cases of methane elimination may reflect a multistep methane elimination step in the with NH4PF6 at 0’ which gave methane and Co[Pdimethylcobalt cation which could‘ consist of reactions (OCH3)3]4(NH3)+PF6-.’8,19 Similarly, protonation of the 9-1 1, methyl complex in the presence of olefins and acetylenes gave high yields of isolable Co[P(OCH3)3]4L+complexes.20Hence, ( C H ~ ) ~ C O [ P ( O C H ~+ ) ~( IC~H+~ ) ~ C O [ P ( O C H ~ ) ~ I ~ + the reductive elimination reaction in this CH3(H)Co+ complex -k P(OCH3)3 ( 9 ) has broad synthetic value which we have also extended to CoL3L2+ complexes by protonation of q3-C3H5Co(CH3)2Co [P(OCH3)313+ + [P(OCH3)3]35 in the presence of four-electron donors like (CH2)(H) (CH3)Co[P(OCH3)313+ (10) butadiene (butadiene produced the fluxional q4-C4H6Co[P( OCH3)3] 3+ caion2I). (CH~)(H)(CH~)CO[P(OCH~)~]~+ CH4 Isolation of Co[P(OCH3)3]4+ as a blue solid salt was achieved by an indirect procedure22(eq 8).
f“B
whereas the elimination step in the methylhydridocobalt cation is probably “concerted”. Equilibrium 9 probably lies largely to the right.28 25O We note the significanceof this general class30of compounds Co[P(OCH3)3]4+ PF6- (8) vacuum not only for mechanistic studies of the reductive elimination step but also for an extensive evaluation of the steric,3’ Direct reaction23of the blue salt with hydrogen and with donor charge,32 and electronic factors that influence the thermodyligands24was quantitative to give H ~ C O [ P ( O C H ~ ) ~ ] ~ + P F ~ namic and kinetic parameters of the elimination step. Thus, and Co[P(OCH3)3]4L+PF6-, respectively. On dissolution of in the Y2ML4= class, L may be varied extensively within the the blue salt in non- or weakly coordinating solvents,25a relcobalt and iron30groups. A systematic study of this class of atively rapid redox and redistribution reaction occurred with complexes is in progress as well as of allylmetal protonation the formation of Co[P(OCH3)3]5+, cobalt metal, and preand alkylation reactions that should yield important inforsumablyI2 Co [P( OCH3) 31 ,53+. mation about metal complexes often postulated as reaction In reductive elimination for cis dihydrido- and methylhyintermediates especially in olefin isomerization. dridometal complexes, it would appear that rates will invariably be higher for the latter type of complex. In our cobalt Acknowledgment. We gratefully acknowledge support of system, the methane elimination unlike the dihydrogen elimthis research by the National Science Foundation. ination was irreversible (reversibility in this type of system References and Notes would be an uncommon phenomenon) but this alone does not account for rate differences between the two cobalt complexes (a) J. Halpern, Adv. Chem. Ser., No. 70, 1 (1968): (b) R. Ugo, Chirn. /nd. (Man), 51, 1319 (1969), (c) J. P. Collman. Acc. Chem. Res., 1, 136 (1968): since k 1 (eq 1) for the dihydride is several orders of magnitude (d) C. A. Tolman, “Transition Metal Hydrides”, E. L. Muetterties, Ed., Marcel smaller than the rate of methane elimination in CH3(H)Dekker, New York, N.Y., 1972, p 271. Co[P(OCH3)3]4+. Interestingly, C ~ H ~ ( H ) C O [ P ( O C H ~ ) ~ ] ~ +(a) For oxidative-addition mechanistic information cf. A. V. Kramer, J. A. J. S. Bradley, and J. A. Osborn, J. Am. Chem. SOC., 96, 7145 Labinger, is more stable than CH3(H)Co[P(OCH)3)3]4+, a point of (1974). and references therein: (b) Complex reactions ensue in the intersignificance since C ~ H ~ ( H ) ~ C O [ P ( O C3 H ] is ~an) ~intermeaction of photochemically generated Re(C0)5with HP,8. H. Byers and T. L. Brown, J. Am. Chem. SOC., 97, 3260 (1975). whereas photochemically diate in the catalytic hydrogenation of arenes with C3H5Coactivated 16-electron metal carbonyl complexes ostensibly add Hz in a [P(OCH3)3]3.26’27 simple concerted fashion, M. Wrighton and M. A. Schroeder. J. Am. Chem. Alkylation of CH3Co[P(OCH3)3]4 has been effected with SOC.,95, 5764 (1973), and J. Nasielski, P. Kirsch, and Wilputte-Steinert, J. Organornet. Chem., 27, C13 (1971). (CH3)30+PF6- in a variety of solvents. The alkylation step (a) Elimination of hydrogen from IrH2(C0)2(ER3)2+(E = As. P) by reaction was slow; a precise determination of reaction rate has been with phosphites, phosphines, and CO goes at a rate independent of the concentration and nature of the ligand: M. J. Mays, R. N. F. Simpson, and precluded by the lack of a good oxonium salt solvent inert to F. P. Stefanini, J. Chem. SOC.A, 3000 (1970). A photochemical study of reactants and products in the alkylation reaction although hydrogen elimination from HzlrCi(cO)[P(C~H5)3]2 has also been reported: dimethyl carbonate seemed unreactive. The dimethylcobalt G. L. Geoffroy, G. S. Hammond, and H. B. Gray, J. Am. Chem. SOC., 97, (b) Reductive elimination of dihydrogen from HpO~(CO)4 ac3933 (1975). complex, (CH3)2Co[P(OCH3)3]4+, was too unstable for tually occurs from a dinuclear intermediate. J. Evans and J. R. Norton, ibid., conventional isolation, and reductive elimination occurred in 96 7577 (19741 this complex at a rate intermediate between the two examples (4) (a) A different ordering of reductive elimination rates for YpOs(CO)4complexes was reported by Evans and Norton3b but mechanistic features discussed above.28 Methane was irreversibly produced and of the cobalt and osmium systems differ quite sharply. (b) (CH3)2Felittle deuterium incorporation in the methane was observed [ ( C B H ~ ) ~ P C H ~ C H Z P ( decomposes C ~ H ~ ) ~ ~ ~ with liberation of CH4, CZH6. and C2H4 to form a postulated iron carbene complex. Substantial ethane when the alkylation reaction was effected in deuterated solformation differentiates the iron from our cobalt system. T. lkariya and A. vents. Reaction of (CH3)30+ with CD3Co[P(OCH3)3]4 in Yamamoto, J. Chem. SOC.,Chem. Commun., 720 (1974). (c) Concerted reductive elimination of phenyl groups (to give biphenyl) is presumed for dichloromethane produced CH3D, CD3H, CH4, and CD4; no (C6H5)2Pt(PR3)2complexes on solid state pyrolysis: P. S. Braterman, R. CHzD2 was detected.29CH4 and CD4 were produced probably J. Cross, and G. B. Young, ibid., 627 (1975). (d) Cis-reductive elimination because the alkylation step was reversible. Thus, as in the reof ethane is observed for (CH3)3AUP(C6H&: (A. Tamaki, S. A. Magennis. J. K. Kochi. J. Am. Chem. SOC.,96,6140 (1974))and for Pt(CH3)3L3+ and ductive elimination reaction in the H2Co+ and CH3(H)Co+ (H. C. Clark and L. E. Manzer, Inorg. Chem., 12, 362 (1973)). (e) More cations, the elimination step in the dimethyl cation appears to general discussions or reviews of reductive elimination reactions involving be intramolecular. The expected product in reductive elimiaikylmetal complexes are: M. C. Baird, J. Organomet. Chem., 64, 289
-
Journal of the American Chemical Society
/
98:15
/
July 21, 1976
\ . _ .
.I.
4667 (1974); P. S. Bratermanand R. J. Cross, J. Chem. Soc., Dalton TranS., 657 (1972): and P. S.Braterman and R. J. Cross, Chem. SOC.Rev., 2, 271 (1973). (5) E. L. Muetterties and F. J. Hirsekorn, J. Am. Chem. Soc., 96, 7920 (1974). (6) In solution, the complex yields a proton NMR spectrum fully consistent with
Mechanism of the Formation of Cyclopentadienone Derivatives from 4-Aryl-2,6-di- tert-butylphenols by Base-Catalyzed Oxygenation
a cis ~tereochemistry.~ However, since there is a very slight asymmetry in the Co-H mutliplet, a very small amount of the trans may be gresent. Cis-trans interconversionshould be very fast in the solution state. ,'Thus, although experiments show a cis dihydride to be formed in the reverse of reaction 1, this does not prove that a concerted cis oxidative addition prevails. (7) P. Meakin, E. L. Muetterties, and J. P. Jesson, J. Am. Chem. SOC.,95, 75
Sir: Recently it has been shown that 4-aryl-2,6-di-tert-butylphenols (1) are easily oxygenated in the presence of t-BuOK in t-BuOH to give antiaromatic 3-aryl-2,5-di-tert-butylcyclopentadienones ( 5 ) in good yields.' The reaction provides a (1973). (8) Mineral acids decompose the complex. novel and convenient method for the synthesis of such anti(9) An analogous hydride, HzCo[P(OCzH5)3]4+,was obtained from the HCoaromatic compounds.2 We now wish to report the mechanism [P(OCzH~)314. of this interesting reaction, clarified with the aim of isolation (IO) The dihydrides were fully characterized by NMR and elemental analysis. (11) In vacuum, thermolysis of the salt begins above -150' as signaled by a of intermediates employing 1 (R = 4-OMe) which quantitameasurable rate of hydrogen evolution. tively gives the cyclopentadienone 5 (R = 4-OMe). (12) Another cobalt species must be formed (stoichiometry considerations). It has been found that modification of the reaction condiThis species is probably Co[P(OCH3)31s3+but characterization data are not as yet unequivocal. The overall reaction would be: SCO[P(OCH&]~+ tions makes it possible to isolate the intermediates. Thus, 6Co[P(OCH3)3]5+ +2Coo + C O [ P ( O C H ~ ) ~ ]The ~ ~ +trigonal . bipyramidal oxygen was bubbled through a solution of 1 (R = 4-OMe) (1 5 Co[P(OCH3)3]5+ is in a virtual thermodynamic sink and invariably forms in these reductive eliminations. For this reason, extended studies to mmol) in t-BuOHlpetroleum ether (1:l) (100 ml) containing Y2CoL4+ systems with L = P(CH3)3or P(O-i-C3H7)3may simplify mechat-BuOK (75 mmol) at -25 OC for 40 min. Acidification of the nistic studies because of the expected stability of these COL4+ cations. (13) Reaction conditions were 25' and -700 mm DZ pressure in dichlororeaction mixture with aqueous NH4C1 solution and evaporamethane solution. tion of the solvent afforded hydroperoxide 2 in 85% yield: (14) (a) The H2-D2 exchange reaction is substantially faster than the slow yellow needles; mp 99-101'; ir (KBr) 3340 (OOH), 1665 "decomposition" of H2Co[P(OCH3)3]4+through the intermediate CoIP(OCHd314' to Co[P(OCH3)3]5+, CO[P(OCH3)3]63f, and Coo. (b) Coorcm-] (C=O); N M R (CDC13) 6 1.03 (9, H, s, t-Bu), 1.28 (9 dinated deuterium was favored. H, s, t-Bu), 3.84 (3 H, s, OMe), 6.61 (1 H, d, vinyl H, J = 2.8 (15) Proton exchange between HzCo[P(OCH&]4+ and HCo[P(OCH3)3]4 is moderately fast in solution especially if the solvent is capable of solvating Hz), 7.05 (1 H, d, vinyl H, J = 2.8 Hz), 6.7-7.6 (4 H, m, the proton. ArH), 9.00 (1 H, s, OOH, exchangeable with D z O ) . Treat~ The multiline lH spectrum of the CISdihydride broadens to a single resoment of this hydroperoxide with t-BuOK in t-BuOH at 75 OC nance as temperature increases and then the single resonance shar ens on further temperature increase to 70' (thermal decomposition at 808and quantitatively gave 5 (R = 4-OMe), while the treatment at above is of sufficient consequence that spectral studies above this temroom temperature resulted in the formation of 3 and 5 (R = perature were not feasible). There was no evidence of the expected fast exchange intramolecular limiting spectrum of a binominal quintet. Fast 4-OMe) in 75 and 25% yields, respectively. The ratio of 3 to dissociation of phosphite ligands in this upper temperature range is con5 (R = 4-OMe) depends on the reaction temperature: the sistent with the DNMR data. higher temperature causes the higher yield of 5 (R = 4-OMe).l Protonation of the methyl complex is a substantially faster step than the complimentary alkylation [typically (Cl-l&0+PFB- was used] of the hydride, As was shown by separate experiments,] the cyclopentadian observation consistent with the relatively congested cobalt coordination enones 5 are formed in high yields upon heating the epoxysphere in these complexes (close approach of H+ to cobalt should be less hindered than that of CH3+ in for example (CH3)30+J. o-quinols of type 3 with t-BuOK in t-BuOH at 75 OC. It is, Elemental analysis and NMR characterization. AB3 'P spectrum. therefore, obvious that the formation of 5 (R = 4-OMe) on the This ammonia complex reacted with H2 and with P(OCH3)3to form H2Cooxygenation of 1 (R = 4-OMe) proceeds by a mechanism in[P(OCH3)3]4+,and Co,[P(OCH3)3]5+, respectively. The first step in these reactions is dissociation of the ammonia ligand. volving these intermediates. All attempts to isolate any interThese included ethylene, propylene, I-hexene, and diphenylacetylene. mediate between 3 and 5 (R = 4-OMe) in this reagent system The acetylene complex was more stable than the ethylene complex: both these complexes showed the expected A2B2 31PNMR spectrum. were unsuccessful. However, adsorption of 3 on activated basic This complex gave a low temperature ABz 31Ppattern which is that exalumina (activity I) in CHzCl2 followed by elution with MeOH pected for a square pyramidal form with an apical phosphite ligand (analafforded isomeric ring contracted product 4 in 50% yield: ogous to that established for the isoelectronic ~ ~ - c ~ H ~ F e ( c o ) ~ ) . The yellow propylene complex was prepared and isolated as a PF6- salt colorless needles; mp 158-160O; ir (KBr) 3460 (OH), 1730 at -78'. Evacuation of this salt at 25' led to complete propylene removal (CHO), 1710 cm-' (ring C=O); N M R (CDC13): 6 0.63 (9 to give the blue CO[P(OCH3)3]4+PF6- salt. Reaction of the solid at 25' H, s, t-Bu), 1.33 (9 H, s, t-Bu), 3.39 (1 H, s, OH, exchangeable "3, P(OCH&, and CZH4. with D20), 3.85 (3 H, s, OMe, 7.24 (1 H, s, vinyl H), 6.9-7.4 In dichloromethane, the green solution of the complex rapidly preci itated Coo. Acetonitrile formed a red, weak complex with C O ( P ( O C H ~P).~This ] ~' (4 H, m, ArH), 9.17 (1 H, s, CHO); together with 5 (R = 4complexation stabilized such solutions: decomposition occurred over a OMe) (25%). I n addition, treatment of 3 with the same basic several hour period. alumina in t-BuOH at 75 OC quantitatively afforded 5 (R = F. J. Hirsekorn, M. C. Rakowski, and E. L. Muetterties, J. Am. Chem. SOC., 97. 237 11975). 4-OMe). The compound 4 was also obtained by treatment of C~H&OHZ[P(OCH~)~] 3 in the absence of a reducible substrate slowly 3 with silica gel or CF3COOH at room temperature. Taking orders of ~ . . . eliminates DroDvlene. The stabilitv of this intermediate is -1-2 ~ ~ account of the conclusions by Hart et aL8 for the acid-catalyzed magnitude greaier than that of C3H5(H)Co[P(OCH3)j]$_
-
(28) Methane formation was never quantitative. For example, in dichloromthane the yield of methane was about 70-75% with a 1:l stoichiometry. Reactivity of the cobalt intermediate complex towards the solvent may part1 account for this result. Extensive dissociation of (CH3)2C~[P(OCH3)3]4 to (CH3)2Co[P(OCH3)3]3+ is expected on steric grounds. Liberated phosphite ligand should then react rapidly with the oxonium salt. In fact, a higher yield of methane was obtained when the ratio of oxonium salt to CH3Co[P(OCH3)3]4 was raised to 2: 1. (29) Analysis based on high resolution mass spectral data. We are indebted to Dr. J. Henion for obtaining these data. (30) D. H. Gerlach, W. G. Peet. and E. L. Muetterties. J. Am. Chem. SOC., 94,
Y
4545 (1972). (31) Steric factors may also tend to favor the elimination step in cobalt vis a vis iron since the metal radius is smaller in the former.
(32) For evidence that charge plays an important role see R . Ettore, Inorg. Nucl. Chem. Left, 5, 45 (1969).
E. L. Muetterties,* Patricia L. Watson Spencer T. Olin Chemistry Laboratory Cornel1 University Ithaca. New York 14853 Received March 30, 1976
Scheme I
,
I B u . $ IB u
-'"$!&
0 2 , IBuOKI%$&,tBuOK O-ZO'C
-25'C \ '
R
OMe
1
I
-
\ / Me0
3
O2 ,tBuOK !BuOH, 75'C
" 6Me
x5
4 A,
dMe
6
A,
R: 4-OMe, 3-OMe, 2-OMe, 4-Me, 3-Me. 2-Me, 4-CI. H
Communications to the Editor
