4802 J . Org. Chem., Vol. 44, No. 26, 1979
de Silva, Watanabe, and Snieckus
General Route to Anthraquinone Natural Products via Directed Metalation of N,N-Diethylbenzamides' S. 0. d e Silva, M. W a t a n a b e , and V. Snieckus* Gue!ph- Waterloo Centre f o r Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada,
N2L 3GI
Received August 9, 1979 Regiospecific, short, and efficient synthesis of the naturally occurring anthraquinones islandicin (la), digitopurpone lb), erythroglaucin (IC), catenarin (ld), cynodontin (le),and soranjidiol (If) have been achieved in 20-30% overall yields by using directed metalation of easily accessible methoxylated N,N-diethylbenzamides (3 and 10a--d)as the key step. In a representative example (Scheme IV), sequential treatment of N,N-diethyl-3,5-dimethoxybenzamide(3) with sec-BuLi and 2,5-dimethoxy-4-methylbenzaldehyde (4) gives the hydroxyamide 5 which, without isolation, is subjected to p-toluenesulfonic acid in refluxing toluene to provide the phthalide 6. Hydrogenolysis of 6 affords the benzylbenzoic acid 7 which upon Friedel-Crafts cyclization with trifluoroacetic anhydride yields the anthracenol8. Compound 8, when exposed to chromium trioxide, undergoes facile oxidation to the anthraquinone 9. Selective demethylation (BBrJ of 9 gives erythroglaucin (IC), while vigorous conditions (pyridine hydrochloride) provide catenarin (Id). Formal syntheses of islandicin (la) and digitopurpone (lb) and total syntheses of cynodontin (le) and soranjidiol (If) have been effected by analogous six-step sequences starting with the benzamides loa-d and, in three of the four cases (loa-c), the same p tolualdehyde (4). Methoxy and methyl substituents on the aromatic ring offer no complications in metalation ortho to the CONEt, function. The condensation of the resulting species with aromatic aldehydes is compatible with acidic methyl hydrogens in the aldehyde component. A n t h r a q u i n o n e s 1.1) have long-standing interest for chemists b o t h as plant2 a n d insect3 n a t u r a l products and as important synthetic substance^.^ T h e independent and a l m o s t simultaneous synthesis of t h e principal m a d d e r pigment alizarin by Perkin and by Graebe and Liebermann in 1868 provided t h e cornerstone for t h e E u r o p e a n dye industry5 a n d stimulated intense synthetic activity in this field of investigation. I n modern times, a multitude of new structural types have relegated t h e anthraquinone dyes t o a minor position, a n d as a result, t h e synthetic work has been correspondingly minimal.6 T h e recent renaissance in a n t h r a q u i n o n e synthesis7 is d u e , in large p a r t , t o t h e discovery of useful a n t i t u m o r activity in t h e related more complex anthracycline antibiotics (2).8 A number of new syntheses of anthracyclines are d e p e n d e n t o n t h e availability of unsymmetrically oxygenated a n t h r a q u i n o n e intermediate^.^ T h e classical s y n t h e s i s of a n t h r a q u i n o n e s involves double Friedel-Crafts condensation of phthalic acid derivatives or phthalic anhydrides with substituted phenols (1)Dedicated to Professor Virgil Boekelheide in the year of his 60th birthday. A preliminary account of a portion of this work has appeared: S.0. de Silva and V. Snieckus, Tetrahedron Lett., 5103 (1978). (2) R. H. Thomson, "Naturally Occurring Anthraquinones", 2nd ed., Academic Press, New York, 1971. (3) K. S.Brown, Jr., Chem. SOC.Rev., 4, 263 (1975); F. L. C. Baranyovits, Endeauor, 2, 85 (1978). (4) K. H. Schunderhutte, Chem. Synth. Dyes, 6, 211 (1972). (5) For a historical account, see L. F. Fieser, J . Chem. Educ., 7, 2609 (1930). (6) E. Siegel, Int. Reu. Sci.: Org. Chem., Ser. Two, 3, 259 (1976). (7) See, inter alia: (a) G. Roberge and P. Brassard, Synthesis, 148 (1979); (b) P. G. Sammes and D. J. Dodsworth, J . Chem. Soc., Chem. Commun., 33 (1979); (c) F. M. Hauser and R. P. Rhee, J . Am. Chem. SOC.,101,1628 (1979); (d) M. Braun, Angew. Chem., Int. Ed. Engl., 17, 945 (1978); (e) D. W. Cameron, G. I. Feutrill, P. G. Griffiths, and D. J. Hodder, J . Chem. SOC.,Chem. Commun., 688 (1978); (f) R. K. Boeckrnan, Jr., T. M. Dolak, and K. 0 . Culos, J . Am. Chem. SOC.,100, 7098 (1978); (9) D. Rutolo, S. Lee, R. Sheldon, and H. W. Moore, J . Org. Chem., 43, 2304 (1978); (h) M. J. Broadhurst, C. H. Hassall, and G. J. Thomas, J . Chem. SOC.,Perkin Trans. 1 , 2502 (1977). (8) (a) F. Arcemone, Lloydia, 40, 45 (1977); (b) T. R. Kelly, Annu. R e p . Med. Chem., in p r e w (9) See, for example: {a) F. Suzuki, S.Trenbeath, R. D. Gleim, and C. J. Sih, J . Org. Chem., 43, 4159 (1978); (b) L. M. Harwood, L. C. Hodgkinson, and ,J. K. Sutherland, J . Chem. Soc., Chem. Commun., 712 (1978); (c) K. Krohn and M. Radeloff, Chem. Ber., 111, 3823 (1978).
b\
bH
la, R ' b, R ' c, R' d, R ' e, R '
Hb
= M e ; R 2 = R 3 =R ' = H (islandicin) = R ' = R ' = H ; R 2 = M e (digitopurpone) = Me; R 2 = R 3 = H; R 4 = OMe (erythroglaucin) = Me; R 2= R 3= H; R ' = O H (catenarin) = Me; R 2 = R 4 = H:R ' = OH (cynodontin)
+ OH
8
If, soranjidiol
I R2
I
I
0
I HO
I L-a-daunosamine
2a, R' = OH; R 2 = OMe (adriarnycin) b, R' = H;R 2 = OMe (daunomycin) C , R' = H ; R 2 = OH (carminomycin)
or phenol ethers.'O For unsymmetrically oxygenated anthraquinones, this route (Scheme I ) lacks regiochemical control in t h e intermolecular Friedel-Crafts reaction"J2 and carries t h e potential of a Hayashi rearrangement1°J3 (10) R. H. Thomson in "The Chemistry of the Quinonoid Compounds", Part 1, S. Patai. Ed., Wiley, New York, 1974, p 136 ff. See also footnote 10 in ref 9a. (11)Recent examples: (a) R. J. Blade and P . Hodge, J . Chem. Soc., Chem. Commun., 85 (1979); (b) S. Terashima, S.-s. Jew, and K. Koda, Tetrahedron Lett., 4937 (1978); (c) D. G. Davies. R. Hodge, and P. Yates, J . Chem. SOC.,Perkin Trans. I , 2399 (1974): M) C. M. Wong, R. Schwenk, D. Popien, and T.-L. Ho, Can. J . Chem., 51, 466 (1973): (e) Z. Horii, H. Hakusai, and T. Mornose, Chem. Pharrn. Bull.. 16. 1262 (1968).
0022-3263/79/1944-4802$01.00/0C 1979 American Chemical Society
General Route to Anthraquinone Natural Products
J . Org. Chem., Vol. 44, No. 26, 1979 4803 Scheme 111
Scheme I n
CH3O
RO
OR
OR
0
o
OR
OR
R +
CH30
Rb
0
OR
RO
6
RH-
b:
OR
Scheme I1 Directed Metalation
Z = NMe,,(CH,),NEVle, ( n = 1, 2), CONHR, CSNHMe, CH,OH, OMe, OCH,OMe, SO,NMe,, SO,NHMe, CF,, F, CH=NR, and 2-oxazolino
in the intramolecular acylation involving equilibrating o-benzoylbenzoic acid intermediate^.'^ Consequently, syntheses of the related anthracyclines based on this approach are inefficient and ambiguous,12and efforts have been focused on developing new regiospecific routes to these substance^.'^ Discovered by Gilman and by Hauser, the directed metalation reactionlE has recently matured into a major synthetic method which allows the introduction of diverse substituents into aromatic substrates ortho to certain functional groups (Scheme 11). It provides useful methodology for the construction of 1,2-disubstituted benzene derivatives which are usually not accessible by classical electrophilic substitution chemistry. The observation of Beak and Brown" i;hat the N,N-diethylcarboxamide (12) For an exception, see the 3-bromophthalide route to anthraquinones [K. S. Kim, M. u'.Spatz, and F. Johnson, Tetrahedron Lett., 331 (1979)] and to anthracyclines [K. S. Kim, E. Vanotti, A. Suarato, and F. Johnson, J . Am. Chem. SOC.,101, 2483 (1979)l. (13) M. S. Newman, Acc. Chem. Res., 5 , 354 (1972). (14) Recent examples: (a) R. D. Gleim, S. Trenbeath, F. Suzuki, and C. J. Sih, J . Chem. Soc., Chem. Commun., 242 (1978); (b) A. S. Kende, J. L. Belletire, J. I,. Hermmann, R. F. Romanet, E. L. Hume, R. H. Schlessinger, J. Fayos, and J. C. Clardy, Synth. Commun., 3, 387 (1973). (15) Successful alternatives: photochemical Fries route [A. S. Kende, J. L. Belletire, T. J. Bentley, E. Hume, and J. Airey, J. Am. Chem. Soc., 97, 4425 (1975)], intramolecular base-catalyzed route (see ref 9), anionic quinone bisketal route [J. S Swenton and P. W. Reynolds, J. Am. Chem. Sac., 100, 6188 (1978);see also ref 7d], Diels-Alder route [T. R. Kelly, J. W. Gillard, R. N. Goerner,
