502
of 2.650 8, is reasonable, the small angle of 113” suggests that a hydrogen bond does not exist. However, the N . . - o distance of 2.65 A is shorter than that f O ~ din molecule A, lending Cmknce to the idea of an attractive force between NB and OB-3. All the intermolecular distances less than 3.9 8,
were computed and surveyed but no unusually short contacts (other than hydrogen bonds) were found. Acknowledgments, We wish to thank the u. s. Public Health Service, Institute for Allergies and Infectious Diseases, for financial support through Grant No. AI-08201.
Photochemistry of Electron-Transport Quinones. 11.’ Model Studies with Plastoquinone-1 [2,3-Dimethyl-5-( 3-methylbut-2-enyl)-1,4-benzoquinone] David Creed,” Harold Werbin,*
and E. Thomas StromZb
Contribution f r o m the Division of Biology, University of Texas at Dallas, Dallas, Texas 75230, and Mobil Research and Development Corporation, Field Research Laboratory, Dallas, Texas 75221. Received July 2, I970 Abstract: A number of photoproducts have been characterized from near-uv irradiation of plastoquinone-1 (1, n = 1) under several conditions. In benzene or isopropyl alcohol under oxygen the naphthoquinone 2, the
novel tricyclic peroxide 3, and the benzofuranone 7 were formed. In the same solvents but under nitrogen, irradiation afforded 2, the chromenol(9a, n = O), and the benzoxepin 10. Two additional photoproducts isolated from irradiation in isopropyl alcohol under nitrogen were the isomeric spiroenones 13 (A and B). These oxidized dimers were not only interconvertible upon treatment with triethylamine but also gave rise to the pyranoxanthene 18. Irradiation in methanol or aqueous acetonitrile under nitrogen resulted in formation of the dihydrobenzofurans 4a and c in addition to 10 and, in methanol, 2. The characterization of these photoproducts will facilitate future studies on the photochemistry of the photosynthetic electron-transport quinones.
E
lectron-transport quinones located in membranous cellular substructures such as mitochondria and chloroplasts which perform, respectively, respiratory and photosynthetic functions, are redox components of electron-transport chains. The loss or diminution of some biological functions, such as growth, oxidative phosphorylation, and photosynthesis following ultraviolet irradiation of cells, has been attributed3to photochemical disruption of electron transport, the photolabile electron-transport quinones being the most likely targets for the radiation. Considerable evidence4 has accumulated that plastoquinone-9 (PQ-9, 1, n = 9) is an essential transport quinone in photosynthetic processes. Far-uv irradiation of autotrophic cells leads to destruction of PQ-9 and the concomitant loss of several partial photosynthetic reaction^.^ The more energetic radiations in the 300-nm region of sunlight reaching the earth can also (1) Part I: H.Werbin and E. T. Strom, J . Amer. Chem. SOC.,90, 7296 (1968). (2) (a) University of Texas at Dallas; (b) Mobil Research and Development Corporation. (3) (a) A. F. Brodie in “Biochemistry of Quinones,” R. A. Morton, Ed., Academic Press, London, 1965,p 384; (b) J. Jagger in “Research Progress in Organic-Biological and Medicinal Chemistry,” U. Gallo and L. Santamaria, Ed., North-Holland Publishing Co., Amsterdam, in press. (4) Summarized by F. L. Crane in “Biological Oxidations,” T. P. Singer, Ed., Interscience, New York, N. Y.,1968,p 563. (5) (a) K.E. Mantai and N. I. Bishop, Biochim. Biophys. Acta, 131, 350 (1967); (b) N. Shavit and M. Avron, ibid., 66, 187 (1963); (c) A. Trebst and E. Pistorious, Z.Nufurforsch. B, 20,885(1965); (d) K.E. Mantai, J. Wong, and N. I . Bishop, Biochim. Biophys. Acta, 197, 257 (1970).
modify plastoquinone-9. Therefore, autotrophic cells must have evolved mechanisms for protecting plastoquinones from the harmful radiations, or alternatively, if the quinones are destroyed, evolved processes for their restoration. These considerations prompted our initiation of studies to unravel the in vitro and in vivo photochemistry of PQ-9. There appears to be only one report on photodamage to PQ-9 in vitro, that of Eck and Trebst,6 who prepared a PQ-9 dimer by irradiating the quinone as a thin film. They isolated the same dimer-one in which the quinone ring of one molecule has added to one of the nine double bonds of a second molecule-from horse chestnut leaves but were uncertain whether it occurred naturally or was an artifact of the isolation procedure. Because of the limited availability of PQ-9 and other naturally occurring plastoquinones, we chose to study a model quinone PQ-17 (1, n = l), which has the main structural features of the plastoquinones except for the presence of only one isoprene unit in the side chain. We expected that characterization of PQ- 1 photoproducts, the subject of this report, would greatly facilitate our projected studies of the naturally occurring plastoquinones. Results Near-Uv Irradiation of Plastoquinone-1 under Oxygen. Following irradiation of PQ-1 (1, n = 1) in dry benzene (6) V. H. Eck and A. Trebst, Z. Nufurforsch. B, 18, 446 (1963). (7) P. M. Scott, J . B i d . Chem., 240, 1374 (1965).
Journal of the American Chemical Society / 93:2 / January 27, 1971
503
and complex absorption (3 H) between 6 7.42 and 8.00 or dry isopropyl alcohol, five photoproducts were deto the aromatic protons. tected by thin-layer chromatography (tlc). Three of The second photoproduct, Rf 0.29, the novel trithese have been separated by preparative tlc and identicyclic peroxide 3, has been the subject of a preliminary fied as 2,3,6-trimethyl-1,4-naphthoquinone(2), Rf communication. l 2 Its structure was assigned on the 0.66, 4,5-dihydro-3,3,8,9-tetramethyl-4,9a-epoxy-9aHbasis of its spectroscopic properties and its reduction 1,2-benzodioxepin-7(3H)-one (3), Rf 0.29, and 5to the dihydrobenzofuran 4a which was characterized hydroxy-6,7-dimethylbenzofuran-3(2H)-one (7), Rf 0.06. as the acetate 4b. Elemental analysis and M+ at mje 3 Ne 236 in the mass spectrum corresponded to the molecular CIZACr12Cn=C-CH2,,H formula CI3Hl60,and indicated photoaddition of one M A 2 Me+Me oxygen molecule to PQ-1. A peak at mje 204 (M+ G 32,62 % of the base peak) supported this notion. These I 2 data, its positive reaction to a starch-iodide test, and the The photoproduct with Rf 0.66 was a yellow crystalabsence of an OH group in its ir and nmr spectra proved line compound and had an ultraviolet spectrum (A, that the photoproduct was a peroxide. A more rigid 334,273,264,255 nm; sh, 250 nm) typical8 of 2,3-dialkylconformation of the side .chain of the photoproduct
/AMe
J
Y
Y
Table I. Nuclear Magnetic Resonance Assignments” for Plastoquinone-1 (1, N = 1) and 2,3-Dimethyl-5-( 3-hydroxymethylbut-3-enyl)-1,4-benzoquinone(11) Compd
Solvent
Me
HA
-CH*O
HX
H X
d, 7.5, 2
5.18,* t, J = 7.5, 1
6.47,” t, J = 1.5, 1
4.17, s, 2
3 . 19,bd, J = 7.7, 2
5.24,b t , J = 7.7, 1
6.51,c t, J = 1.5, 1
3.94,f d, J=5.3,2 (3.95, s, 2)
3.12,* d, J=7.7,2
5 . 20,b t,
6.49,c t, J=1.5,1
Me
I HAI.--CH\=C--M~
Me$
Me
CDCl3
HM
1.63, s, 3 1.79, s, 3 2.02, s, 6
3.
J
=
0 I,n=l
CH.014 C( H.4 ,.-CH \=C
Me
0
I
CDC13
-Me (CD3)SO
H\I l l d
1.83,e d, J = 1.3,3 2.01, s, 6 1 ,74, d,
J=l.l,3 1.95, s, 6
J=7.7,1
6 (relative to internal TMS, 6 = 0), followed by signal multiplicities, coupling constants ( J , hertz), and relative intensities. b Broadened by unresolved allylic and homoallylic coupling. Allylic coupling to Ha. The stereochemistry about the side-chain double bond is not known. e Allylic coupling to Hx. Hydroxyl proton gives rise to a triplet, J = 5.3 Hz at 6 4.60, disappearing on addition of D 2 0 . y
1,4-naphthoquinones, Its infrared spectrum (v,,, 1664, 1620, 1596 cm-l) was almost identical in the 15501700-cm- region with that of 2,3-dimethyl-l,4-naphthoquinone. These data and the molecular ion at m / e 200 (also the base peak) observed in the mass spectrum were indicative of a trimethyl-l,4-naphthoquinone,A strong peak at m,’e 118 (36 % of the base peak) was attributed to loss of (CH,)-C=C-(CHJ-C=O (m/e 82) from M-. This mode of fragmentation is typicalg of 2,3-dialkyl- 1,4-naphthoquinones and allowed assignment of two of the methyl groups to the 2 and 3 positions of the naphthoquinone ring. Thus the photoproduct was either 2,3,5- or 2,3,6-trimethyl-1,4-naphthoquinone. That it was the latter was shown by its melting point,l0 loo”, and nmr spectrum. A singlet (6 H) at 6 2.16 was attributed to the 2- and 3-methyl groups, a singlet (3 H) at 6 2.47 to the 6-methyl group,:’ (8) 2,3-Dimethyl-1,4-naphthoquinonehas, , ,A 330, 269, 260, 249, and 243 nm; menaquinone-9 has Xmax 325, 270, 260, 249, and 243 nm. Taken from ref 3a, p 49. (9) (a) J. H. Bowie, D. W. Cameron, and D. H. Williams, J . Amer. Chem. Soc., 87, 5094 (1965); (b) S. J.Di Mari, J. H. Supple, and H. Rapoport, ibid., 88, 1226 (1966). (10) 2,3,6-Trimethyl-1,4-naphthoquinone, mp loo”, E. Bergmann and F. Bergmann, J . Org. Chem., 3, 125 (1938); 2,3,5-trimethyl-1,4-naphthoquinone, mp 128”, 0. Kruber, Ber. Derrt. Chem. Ges. B, 73, 1174 (1940). (11) Methyl substituents in the 5 and 8 positions of 1,4-naphthoquinones and in the 1, 4, 5, and 8 positions of 9,lO-anthraquinones are cleihielded by the quinonoid carbonyl group. Thus in the nmr spectrum of 1,3-dime:hyl-9,10-anthraquinone,singlets at 6 2.78 and 2.45 are artributed to the 1- and 3-methyl substituents, respectively. Spectrum h o . 650 in “High Resolution N.M.R. Spectra Catalogue,” Vol. 2,
than that of PQ-1 (see Table I for the nmr spectrum of PQ-1) and probable saturation of the olefinic double bond were inferred by analysis of its nmr spectrum. It consisted of singlets (each 3 H) at 6 1.12 and 1.70 (gemdimethyls), quartets (each 3 H) at 6 1.87 and 2.07 (vi= 1.3 Hz), an octet (1 H) and nylic methyls, Jhomoallylic an ill-defined octet (1 H) centered at 6 2.77 and 3.10, respectively (HA, HB portion of an ABMX spectrum with J A B = 18 Hz, J A x = 6 Hz, J B X = 1.2 Hz, JBR.1 = 2 Hz), a quartet (1 H) at 6 4.30 (HX, J X A = 6 Hz, JXB = 1.2 Hz), and a triplet (1 H) at 6 6.12 ( H ~ T J ?, v I .= ~ JLIB = 2 Hz). These data supported either 3 or 5 as the struc-
Varian,Associates, Palo Alto, Calif., 1963. See also J. E. Bowie, D. W . Cameron, P. E. Schiitz, D. H. Williams, and N. S . Bhacca, Tetrahedron, 22, 1771 (1966), and references therein. (12) D. Creed, H. Werbin, and E. T. Strom, Chem. Cornmun., 47 (1970).
Creed, Werbin, Strom J Photochemistry of Electron-Transport Quinones
504
ture of the peroxide; the uv (234 nm, E 10,250) and ir (1685, 1640 cm-’) data were also consistent with either one but did not distinguish between them. Examination of molecular models of 3 and 5 revealed that on the basis of the relative magnitudes of the dihedral angles between HA, HB, and Hx, that for 3, JAx should be larger than JBX, whereas for 5, JAX and JBX should be about equal.13 The finding that J A x = 6 Hz and J B X = 1.2 Hz favored structure 3. Identification of the sodium borohydride reduction product of the peroxide as the dihydrobenzofuran 4a rather than the chromanol 6a proved that the peroxide was 3. The reduction, carried out at pH 7-8 owing to the lability of the peroxide to alkali, yielded a phenolic product that had a uv maximum at 300 nm which shifted to 315 nm on addition of alkali and reverted to 300 nm on subsequent acidification. Since this product was somewhat unstable and difficult to purify, it was acetylated under mild conditions (dry pyridine and acetic anhydride for 18 hr at room temperature or for 30 min at 100”). The acetylated material, a colorless oil (A, 283 nm, E 3000; 289 nm, E 3100), showed hydroxyl and ester carbonyl stretching frequencies (3450 and 1760 cm-I, respectively) in the infrared, indicating that monorather than diacetylation had occurred. Had the chromanol6a been the reduction product of the photoproduct it would have been diacetylated under these conditions, which normally acetylate secondary alcohols having a similar gem-dimethyl substitution pattern. l 4 Confirmation of 4b as the structure for the acetylated reduction product was provided by analysis of its nmr and mass spectra. The nmr spectrum consisted of singlets (each 3 H) at 6 1.20 and 1.33 (gem-diMe), 6 2.01 and 2.14 (ArMe), and 6 2.29 (COMe), a doublet (2 H) at 6 3.12 (3 protons,’: J = 9 Hz), a triplet (1 H) at 6 4.57 (2 proton, J = 9 Hz), and a singlet (1 H) at 6 6.66 (ArH). The 2 and 3 protons ga\e rise to a splitting pattern and coupling constants comparable to those observed for the similarly substituted dihydrobenzofuran columbianetin l 6 and the dihydrobenzofuran obtained by mild aerobic oxidation of colupulone. Furthermore, in the nmr spectrum run in (CD&SO, the hydroxyl proton gave rise to the singlet at 6 4.52 that would be expectedlb for the tertiary alcohol 4b but not for the secondary alcohol 6b. The mass spectrum of the acetylated reduction product had a strong peak at mje 59, that was attributed to the ion (CH&C=OH+, resulting from the simple fragmentation of the molecular ion (m/e 264) that would be expected for the tertiary alcohol 4b but not for the secondary alcohol 6b. (13) S. Sternhell, Quart. Reo., Chem. Soc., 23, 236 (1969), and references cited therein. (14) R. B. Woodward, A. A. Parchett, D. H . R. Barton, D. A . J. Ives, and R. B. Kelly, J. Chem. SOC.,1131 (1957). (15) The equivalence of these two protons is unexpected in view of the bulky 2 substituent but also is observed in the nmr spectra of columbianetin’6 and the oxidation product of colupulone, 17 benzofurans haveing the same 2 substituent. The expected ABX splitting pattern for the 2 and 3 protons is, however, observed in the nmr spectrum of 2-methyl2,3-dihydrobenzofuran, where JAS = Jt-8 =8.6 Hz, comparable to J2-3 = 9 Hz observed for 4b; J. Gripenberg and T.Hase, Acta Chem. Scand., 20, 1561 (1966). (16) Spectrum No. 310 in “High Resolution N.M.R. Spectra Catalogue,” Vol. 1, Varian Associates, Palo Alto, Calif., 1962. (17) D. N. Cahill and P. V. R. Shannon, J . Chem. SOC.C,936 (1969). (18) 0.L. Chapman and R. W. King, J. Amer. Chem. SOC.,86, 1256 (1964).
The third photoproduct, Rf0.06, a crystalline fluorescent compound, is probably not a primary photoproduct since it was not detectable after aerobic irradiations of PQ-1 for short intervals. Its molecular formula, C10H1003,was established by mass spectrometry. Its uv spectrum with A, ( E ) 267 (12,800), and 366 nm (6150) was similar to that of 5-hydroxybenzofuran-3(2H)-one. l 9 The nmr spectrum, run in (CD&SO, consisted of four sharp singlets at 6 2.17, 4.67, 6.76, and 9.48 with relative intensities of 6 :2 : 1 : 1, respectively. The singlet at 6 9.48 disappeared on addition of D 2 0and was attributed to a phenolic hydroxyl proton. These data left little doubt that the photoproduct was the benzofuran-3(2H)-one (7), and this was confirmed by its conversion to the enol acetate 8, 3,5-diacetoxy-6,7-dimethylbenzofuran, 2o 3
7
3
Ne& W
d
Me
y
CAc
Oh 7
v
s_
The acetylation of 7 was performed at 100” in acetic anhydride with anhydrous sodium acetate as catalyst. The white crystalline product thus obtained had M+ at mje 262 in the mass spectrum that corresponded to the addition of two functions to 7 ; these were also detected in the ir spectrum (1768 and 1758 cm-l). The aryl-substituted carbonyl streching frequency observed at 1672 cm-‘ in the ir spectrum of 7 was no longer present. The nmr spectrum of the diacetate consisted of singlets (each 3 H) at 6 2.17, 2.34, 2.36, and 2.43 (ArMe, -COMe), a singlet (1 H) at 6 7.07 (aromatic 4 proton), and a singlet (1 H) at 6 7.80 (2 proton). The 2 proton was deshielded by the adjacent acetoxy group, since it was found downfield with respect to the 2-proton signal at 6 7.52 in the nmr spectrum of benzofuran.2’ These data allowed unequivocal assignment of 8 to the diacetate. It reverted to the photoproduct 7 upon acid hydrolysis. Near-Uv Irradiation of Plastoquinone-1 under Nitrogen. Six photoproducts were detected by tlc in solutions of PQ-1 in benzene irradiated for 15 hr. Three of these were isolated by preparative tlc and identified as 2, plastochromenol-0 (9a, n = 0), Rf 0.34, and 3,8,9trimethylbenzoxepin-7-(2,5H)-ol(lo), Rf 0.3 1. The uv spectra of the unstable photoproduct with Rf 0.34 (X,, 332,266,232, sh 214 nm) and its acetate ,,A(, 314, 266, 228, sh 276, 234 nm) were almost identical with thosez2of plastochromenol-8 (sa, n = 8) and its acetate (9b, n = 8) and indicated that the photoproduct was plastochromenol-0 (9a, n = 0). The mass and nmr spectra of the acetylated photoproduct confirmed the structure (9b, n = 0). In its mass spectrum MT appeared at mje 246. The nmr spectrum consisted of a singlet (6 H) at 6 2.28 (-COMe), doublets (1 H) centered at 6 5.57 and 6.22 (3 and 4 protons, respectively, Js = 10 Hz), and a singlet (1 H) at 6 6.51 (ArH). The acetylated photoproduct was also identical (Rf, ir and uv spectra) (19)
252 and 366 nm G. Schenck, M Huke, and I
