Synthesis of hallachrome

Article abstract of DOI:10.1071/CH99072

Hallachrome (1), the only known example of a naturally occurring 1,2-anthraquinone, has been synthesized by successive cycloadditions involving the dienophile (8) with the dienes (7) and (12). Synthetic intermediates (18) and (6) are known derivatives of natural hallachrome.

Full text of DOI:10.1071/CH99072

C S I R O P U B L I S H I N G
Australian Journal
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Volume 52, 1999
© CSIRO Australia 1999
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Aust. J. Chem., 1999, 52, 833–836
Synthesis of Hallachrome
Donald W. Cameron,A,B D. Ross Coller A and Clinton A. McDonaldA
A School of Chemistry, The University of Melbourne, Parkville, Vic. 3052.
B Author to whom correspondence should be addressed.
Hallachrome (1), the only known example of a naturally occurring 1,2-anthraquinone, has been synthesized by suc-
cessive cycloadditions involving the dienophile (8) with the dienes (7) and (12). Synthetic intermediates (18) and
(6) are known derivatives of natural hallachrome.
Cycloaddition of the 1,1,2-trioxy butadiene (7)6 to
trichloro benzoquinone (8) followed by aromatization of the
crude cycloadduct with sodium acetate in methanol gave the
naphthoquinone (9) efficiently. As has been observed here
previously,7 the quinone (8) undergoes addition exclusively
to its monochloro side, generating a dichloro naphtho-
quinonoid dienophile for subsequent reaction. The 1H n.m.r.
spectrum of (9) showed b- and a-hydroxy resonances at
d 6.33 and 11.78 respectively, and a new aromatic singlet at
d 7.63. Selective protection of the b-hydroxy group then
resulted from heating in acetic anhydride, readily giving the
monoacetate (10). A new acetate resonance (d 2.41) replaced
the b-hydroxy signal of (9) and an a-hydroxy resonance was
retained, at d 11.72. The acetate (10) was then methylated in
high yield, with methyl iodide/silver(I) oxide. The 1H n.m.r.
spectrum of the product (11) showed appropriate C-methyl,
acetate and methoxy resonances at d 2.32, 2.41 and 3.89
respectively.
Hallachrome (1), the only known example of a naturally
occurring 1,2-anthraquinone, has been isolated from the sea-
worms Halla parthenopeia1,2 and Lumbriconereis impa-
tiens.2,3 It poses several synthetic challenges. The specific
substitution patterns of the respective outer rings have to be
assembled regioselectively, despite being separated by an
unsubstituted central ring. Furthermore, any precursor based
on a parent 1,2,7,8-tetraoxy anthracene skeleton, such as (2),
requires not only that selective O-methylation should
involve an a rather than a b position but that it should
proceed regioselectively so as to affect only the ring con-
taining the C-methyl group.
While the position of the a-methoxy group of (11) has
obvious structural parallel with the methoxy group of hal-
lachrome (1), it also was hoped to exert a controlling elec-
tronic influence, in the desired regiochemical sense, on
attachment of the third ring. Thus reaction between (11) and
the neat diene (12)8 proceeded largely through the
cycloadduct (13) which, on being aromatized with nucle-
ophilic attack of sodium acetate, gave largely the desired
anthraquinone (14) (52%). Formation of the latter involved
the expected acetyl-group migration from an a- to a b-oxy
group, giving a chelated a-hydroxy substituent in the
In 1985, Comber and Sargent4 published the first synthe-
sis of a hallachrome derivative, the leucotriacetate (3). This
involved formation of the tricyclic skeleton by intramolecu-
lar Friedel–Crafts acylation of an appropriate benzylbenzoic
acid derivative, and reduction of an intermediate anthrone. In
1993 Krohn and Khanbabaee synthesized hallachrome (1)
itself, for the first time.5 Their approach involved molybde-
num-catalysed oxygenation of the anthracene (4), generated
by reduction of the anthraquinone (5). The latter system was
obtained by cycloaddition chemistry.
1
product.7 The H n.m.r. spectrum of the product, however,
showed two a-hydroxy signals at d 13.02 and 12.65 in an
intensity ratio of 9:1. This suggested that the desired product
(14) was contaminated with its regioisomer (15), consistent
with which the hydroxy proton of the major isomer (14) was
the more deshielded, being peri to a methoxy carbonyl
system.9
This paper describes alternative access to (1) via the cor-
responding anthraquinone (6), in which all of the sub-
stituents of the two outer rings are correctly positioned. It
was hoped that (6) could be deoxygenated to (2), thereby
giving the corresponding quinone (1) on final oxidation.
While the overall yield of this mixture was satisfactory,
the two isomers (14) and (15) proved chromatographically
inseparable. However, repeated recrystallization of a sample
enabled isolation of pure (14). The aromatic region of its 1H
n.m.r. spectrum was consistent with the assigned structure,
10.1071/CH99072
Manuscript received 15 June 1999
© CSIRO 1999
0004-9425/99/090833

834
D. W. Cameron, D. R. Coller and C. A. McDonald
showing two ortho-coupled aromatic doublets (d 7.43, 7.81,
J 8.3 Hz) and a singlet (d 8.04).
acetic anhydride/pyridine. Pleasingly, the regioisomeric
mixture of acetates obtained was chromatographically sepa-
rable, allowing isolation of the major component (18) (85%).
This triacetate is a reported transformation product of natural
hallachrome (1).2 Physical data for synthesized (18) were in
good agreement with those cited for the naturally derived
material. Treatment of (18) with methanolic sodium methox-
ide gave the triol (6), whose data also agreed with those for
hallachrome-derived material.2
Confirmation that the cycloaddition to (11) had indeed
involved major control by the a-methoxy group, without sig-
nificant perturbation from its two adjoining b-substituents,
was provided by parallel chemistry involving its a-hydroxy
parent (10). This was expected to afford regiocontrol in the
opposite sense. Reaction between (10) and the diene (12)
gave a single hydrolysed adduct (16) (65%). This was O-
methylated with methyl iodide/silver(I) oxide and the
product (17), without purification, was then treated with
sodium acetate in acetic acid. The resulting anthraquinone
(15) was thereby obtained efficiently as the only regioiso-
meric outcome, within the limits of spectroscopic detection.
Conversion of (6) into hallachrome (1) itself required
reduction to at least the anthracene level and reoxidation to
give the 1,2-quinone. Controlled reduction of (6) proved dif-
ficult. A conventional approach via the corresponding
anthrone was envisaged but no stable anthrone could be
obtained from (6) under several standard reducing condi-
tions. The most effective involved hydrogenating (6) in
ethanol over Adams catalyst, such as has been used previ-
ously to form an anthrone from the natural anthraquinone
catenarin.10 While this gave no coherent outcome from (6),
addition of hydrochloric acid to the hydrogenating mixture
was found to afford far more extensive reduction than for
analogous treatment of catenarin. The result was a nearly
colourless solution possessing very little chromophoric char-
acter and showing only a broad electronic absorption band at
260 nm. Attempts to isolate a major reduction product from
this mixture were unsuccessful, owing to considerable asso-
ciated darkening. However, deliberate exposure of a chloro-
form solution of the material to air resulted in formation of a
red colouring matter, whose spectroscopic characteristics
pleasingly proved to be indistinguishable from those for hal-
lachrome (1).2,3 It was thereby obtained from (6) in a limited
but acceptable yield of 34%.
1
The aromatic region of its H n.m.r. spectrum showed two
ortho-coupled doublets (d 7.45, 7.80, J 8.2 Hz) and a singlet
(d 8.04), indistinguishable from the spectrum of the minor
isomer obtained from cycloaddition of (11).
With confirmation thereby obtained, of the desired
anthraquinone (14) being the major isomer from the latter
cycloaddition, its mixture with (15) was acetylated with
While the hydrogenation product could not be character-
ized as such, owing to the onset of oxidation, its lack of chro-
mophoric character seems compatible with over-reduction of
(6) having proceeded, possibly as far as the 9,10-dihydroan-
thracene (19), which then oxidized to (1) in air. Carrying out
the reduction under milder conditions, with a view to defin-
ing the process more effectively and reducing the extent of
side-reaction, gave only mixtures of products, none of which
could be made to serve as a basis for conversion into hal-
lachrome.
Experimental
General
Melting points were determined on a Kofler hot stage and are uncor-
rected. Microanalyses were carried out by National Analytical
Laboratories, Melbourne, or by Chemical and Microanalytical
Services, Geelong. Electronic spectra were recorded in methanol con-
taining 1% formic acid unless otherwise stated; a Perkin–Elmer
Lambda 2 spectrophotometer was used. Infrared spectra were recorded
as potassium bromide disks, a Perkin–Elmer 983G grating spectropho-
tometer being used. Proton nuclear magnetic resonance (1H n.m.r.)
spectra were recorded on a Varian Unity 300 spectrometer. The solvent
was (D)chloroform unless otherwise stated. High- and low-resolution
mass spectra were recorded with either a V.G. Micromass 7070F or a
JEOLAX-505H instrument at 70 eV. The mass of each ion is given fol-
lowed by its relative intensity. In general, only peaks greater than 20%
are quoted. Flash chromatography was carried out by using Merck
silica No. 9385 containing 2% oxalic acid. Thin-layer chromatography

Synthesis of Hallachrome
835
–1
(t.l.c.) was carried out on glass plates coated with Merck silica gel con-
taining 2% oxalic acid. The separated compounds were recovered by
using ethyl acetate or dichloromethane, washing the extract twice with
water and then drying it before evaporation. All solvents used were of
analytical reagent (A.R.) grade or were redistilled prior to use. Dried
solvents were used for all cycloaddition reactions and manipulations of
cycloadducts. Petrol refers to the fraction boiling in the range 60–80o.
Organic extracts were generally dried over magnesium sulfate before
evaporation at reduced pressure.
(4.56, 4.05, 3.88). nmax 1765, 1752, 1666, 1636 cm . m/z 384 (M,
1.4%), 342 (55), 300 (100), 282 (93).
(4␣,4ab,9ab)-6-Acetoxy-4a,9a-dichloro-5-hydroxy-4-methoxy-7-
methyl-3,4,4a,9a-tetrahydroanthracene-2,9,10(1 H)-trione (16)
Diene (12) (320 mg) was added to a solution of quinone (10) (100
mg) in benzene (5 cm3) and the solution was heated at reflux for 2 days.
The solvent was evaporated and the oily residue was subjected to
preparative t.l.c. in toluene/ethyl acetate (9:1). The major pale yellow
band (RF 0.4; purple staining to ammonia) was isolated to yield the
anthracenetrione (16) (86 mg, 65%), which was recrystallized from
ethyl acetate/petrol as colourless needles, m.p. dec. >209o (Found: C,
52.4; H, 3.6. C18H16Cl2O7 requires C, 52.1; H, 3.9%). lmax (loge)
(CHCl3) 251, 292, 358 nm (4.30, 3.83, 3.76). nmax 1766, 1733, 1648
2,3-Dichloro-5,6-dihydroxy-7-methyl-1,4-naphthoquinone (9)
Diene (7)6 (6.00 g) was added to a stirred solution of quinone (8)
(3.90 g) in dichloromethane (15 cm3). After 1 h, acetic acid (5 cm3) was
added followed 5 min later by sodium acetate (500 mg) and methanol
(50 cm3). The solution was stirred for 1 h, then acidified with aqueous
hydrochloric acid (1 M, 250 cm3). The suspension was extracted with
ethyl acetate (2´250 cm3), and the combined extracts were washed well
with water, dried and evaported. Flash chromatography eluting with
dichloromethane afforded a major red band consisting of the naphtho-
quinone (9) (4.56 g, 91%). Recrystallization from ethyl acetate/petrol
gave (9) as red needles, m.p. 205–206o (Found: C, 48.2; H, 2.5.
C11H6Cl2O4 requires C, 48.4; H, 2.2%). lmax (loge) (CHCl3) 279, 303,
–1
cm . d 2.33, s, Me; 2.40, s, OAc; 2.79, ddd, J 16.1, 2.7, 2.0 Hz, H 3a;
2.97, s, OMe; 3.07, d, J 15.1 Hz, H 1b; 3.14, dd, J 16.1, 3.2 Hz, H 3b;
3.72, dd, J 15.1, 2.0 Hz, H 1a; 4.17, dd, J 3.2, 2.7 Hz, H 4; 7.51, s, H 8;
11.80, s, OH. m/z 418 (M[37Cl2], 0.2%), 416 (M[37Cl35Cl], 0.9), 414
(M[35Cl2], 1.3), 372 (29), 279 (48), 270 (27), 119 (35), 85 (100).
2,6-Diacetoxy-5-hydroxy-1-methoxy-3-methyl-
9,10-anthraquinone (15)
–1
453 nm (4.23, 4.23, 3.83). nmax 3418, 1633, 1564 cm . d 2.36, s, Me;
Trione (16) (20 mg) in dichloromethane was stirred with methyl
iodide (5 cm3) and silver(I) oxide (100 mg) for 24 h. The suspension was
filtered and the filtrate was evaporated. The residue was treated with
sodium acetate (50 mg) in acetic acid (10 cm3) at 100o for 1 h, and then
the solvent was evaporated. The crude solid was subjected to prepara-
tive t.l.c. eluting with toluene/ethyl acetate (9:1). Isolation of the major
yellow band (RF 0.6) gave the 2,6-diacetoxy-9,10-anthraquinone (15)
as a yellow solid. Recrystallization from ethyl acetate/petrol afforded
(15) as yellow needles, m.p. 211–214o (Found: C, 62.4; H, 4.2.
C20H16O8 requires C, 62.5; H, 4.2%). lmax (loge) 259, 286sh, 390 nm
6.33, s, 6-OH; 7.63, s, H 8; 11.78, s, 5-OH. m/z 276 (M[37Cl2], 10%),
274 (M[37Cl35Cl], 63), 272 (M[35Cl2], 96), 239 (35), 237 (100), 209
(30), 89 (21), 18 (71).
6-Acetoxy-2,3-dichloro-5-hydroxy-7-methyl-1,4-naphthoquinone (10)
Quinone (9) (150 mg) was dissolved in acetic anhydride (10 cm3)
and heated to 100o for 20 h. The solvent was removed and the residue
was subjected to flash chromatography eluting with dichloromethane.
The major orange band afforded the naphthoquinone (10) (170 mg,
98%) as an orange solid, which was recrystallized from ethanol as
orange needles, m.p. 168–170o (Found: C, 49.5; H, 2.5%; M+•
313.9749. C13H8Cl2O5 requires C, 49.6; H, 2.6%; M+• 313.9749). lmax
(loge) (CHCl3) 253, 297, 438 nm (4.20, 4.32, 4.13). nmax 1772, 1680,
–1
(4.64. 4.13, 3.96). nmax 1760, 1669, 1636 cm . d 2.35, s, Me; 2.40, 2.42,
s, s, 2´OAc; 3.93, s, OMe; 7.45, d, J 8.2 Hz, H 7; 7.80, d, J 8.2 Hz, H 8;
8.04, s, H 4; 12.65, s, OH. m/z 384 (M, 3%), 341 (31), 299 (74), 298
(24), 282 (25), 281 (100), 254 (33), 115 (32).
–1
1642, 1577 cm . d 2.33, s, Me; 2.41, s, OAc; 7.65, s, H 8; 11.72, s, OH.
m/z 316 (M[37Cl35Cl], 0.7%), 314 (M[35Cl2], 1.1), 274 (66), 272 (100),
2,7,8-Triacetoxy-1-methoxy-3-methyl-9,10-anthraquinone (18)
237 (43), 43 (67).
The crude mixture of quinones (14)/(15) (9:1, 30 mg), acetic anhy-
dride (10 cm3) and pyridine (2 cm3) were heated together at 80o for 4 h.
The solution was poured into dilute hydrochloric acid (1 M, 250 cm3)
and extracted with ethyl acetate (2´100 cm3). The combined extracts
were washed with dilute hydrochloric acid (3´100 cm3) and with water
(2´100 cm3), dried and evaporated. Preparative t.l.c. of the crude
residue using toluene/ethyl acetate (9:1) gave a major pale yellow band
(RF 0.5) which afforded the triacetoxy quinone (18) (28 mg, 85%).
Recrystallization from ethyl acetate/petrol gave (18) as yellow laths,
m.p. 203–205o (Found: C, 61.9; H, 4.2. Calc. for C22H18O9: C, 62.0; H,
6-Acetoxy-2,3-dichloro-5-methoxy-7-methyl-1,4-naphthoquinone (11)
Silver(I) oxide (200 mg) and methyl iodide (3 cm3) were added to a
solution of quinone (10) (100 mg) in dichloromethane (30 cm3) and the
suspension was stirred away from light for 24 h. The mixture was then
filtered and the solvent was removed from the filtrate to give the naph-
thoquinone (11) (102 mg, 98%) as a yellow solid. Recrystallization
from ethanol gave (11) as yellow needles, m.p. 160–162o (Found: C,
51.2; H, 3.1%; M+• 327.9905. C14H10Cl2O5 requires C, 51.1; H, 3.1%;
M+• 327.9905). lmax (loge) (CHCl3) 253, 261, 279sh, 290, 370 nm
–1
–1
4.3%). lmax (loge) 261, 340 nm (4.56, 3.75). nmax 1766, 1672 cm . d
(4.53, 4.54, 4.53, 4.63, 4.11). nmax 1760, 1684, 1580 cm . d 2.32, s, Me;
2.33, s, Me; 2.36, 2.40, 2.46, s, s, s, 3´OAc; 3.87, s, OMe; 7.57, d, J 8.4,
Hz, H 6; 7.97, s, H 4; 8.22, d, J 8.4 Hz, H 5. m/z 426 (M, 0.5%), 383 (21),
342 (35), 300 (100), 71 (24), 69 (30), 56 (41), 53 (30).
2.41, s, OAc; 3.89, s, OMe; 7.91, s, H 8. m/z 330 (M[37Cl35Cl], 1.3%),
328 (M[35Cl2], 1.4), 288 (66), 286 (100), 270 (34), 268 (51), 240 (26).
Reported2 for (18) derived from natural hallachrome, m.p.
206–208o. d 2.29, s, Me; 2.33, 2.37, 2.42, s, s, s, 3´OAc; 3.88, s, OMe;
7.55, d, J 8.6 Hz, H 6; 7.96, s, H 4; 8.22, d, J 8.6 Hz, H 5.
2,7-Diacetoxy-8-hydroxy-1-methoxy-3-methyl-
9,10-anthraquinone (14)
A solution of quinone (11) (50 mg) in diene (12) (2 cm3) was stirred
at room temperature for 2 days. The bulk of the residual diene was
removed under high vacuum and to the residue was added sodium
acetate (200 mg) in acetic acid (50 cm3). The solution was heated at
100o for 2 h, then the solvent was evaporated. Flash chromatography
eluting with dichloromethane gave a yellow solid (30 mg, 52%) con-
sisting of two isomers (9:1). d (major isomer) (14) 2.35, s, Me; 2.40,
2.42, s, s, 2´OAc; 3.93, s, OMe; 7.43, d, J 8.3 Hz, H 6; 7.81, d, J 8.3 Hz,
H 5; 8.04, s, H 4; 13.02, s, OH. d (minor isomer) (15) 2.35, s, Me; 2.40,
2.42, s, s, 2´OAc; 3.93, s, OMe; 7.45, d, J 8.3 Hz, H 7; 7.80, d, J 8.3 Hz,
H 8; 8.04, s, H 4; 12.65, s, OH. Repeated recrystallization of a small
sample of this mixture from ethyl acetate/petrol gave the 2,7-diacetoxy-
9,10-anthraquinone (14), m.p. 196–198o (Found: C, 62.2; H, 3.8.
C20H16O8 requires C, 62.5; H, 4.2%). lmax (loge) 261, 284, 390 nm
2,7,8-Trihydroxy-1-methoxy-3-methyl-9,10-anthraquinone (6)
Quinone (18) (50 mg) was dissolved in methanol (25 cm3) and
treated with sodium methoxide (200 mg) for 15 min. The solution was
acidified with dilute hydrochloric acid, poured into water and extracted
with ethyl acetate (2´100 cm3). The combined extracts were washed
with water, dried and evaporated to give the trihydroxy quinone (6) (32
mg, 97%) as an orange solid. Sublimation (180o/1´10–4 Torr) gave (6)
as orange microneedles, m.p. 259–261o (Found: M+• 300.0626. Calc.
for C16H12O6: M+• 300.0634). lmax (loge) 278, 358sh, 421 nm (4.49,
–1
3.68, 3.81). nmax 3378, 1630 cm . d [(CD3)2CO] 2.37, s, Me; 2.81, br s,
3´OH; 3.94, s, OMe; 7.23, d, J 8.2 Hz, H 6; 7.68, d, J 8.2 Hz, H 5; 7.89,
s, H 4. m/z 300 (M, 100%), 283 (20), 282 (92), 257 (23), 254 (51).

836
D. W. Cameron, D. R. Coller and C. A. McDonald
Reported2 for (6) derived from natural hallachrome, m.p. 256–258o.
m/z 300 (M, 100%), 283 (28), 282 (98), 271 (13), 257 (30), 254 (66),
Acknowledgments
We acknowledge the financial support of the Australian
Research Council and an Australian Postgraduate Research
Award (to D.R.C.). We are grateful to Dr P. G. Griffiths for
discussion. We thank the Research School of Chemistry,
Australian National University, for a Visiting Fellowship (to
D.W.C.), during which part of this paper was written.
229 (15).
7-Hydroxy-8-methoxy-6-methyl-1,2-anthraquinone (Hallachrome) (1)
Quinone (6) (10 mg) was dissolved in ethanol (5 cm3), and Adams
catalyst (10 mg) and concentrated hydrochloric acid (10 drops) were
added. The suspension was stirred under an atmosphere of hydrogen for
3 h. It was then filtered, the filtrate evaporated, and the residue dis-
solved in chloroform. The clear solution was stirred in air for 24 h and
then evaporated. Preparative t.l.c. eluting with toluene/ethyl acetate
(7:3) gave a major red band (RF 0.5) which afforded hallachrome (1) (3
mg, 34%) as a red solid. Recrystallization from chloroform/petrol gave
(1) as red microneedles, m.p. 221–223o (Found: M+• 268.0733. Calc. for
C16H12O4: M+• 268.0735). lmax (loge) (MeOH) 249, 312, 497 nm (4.61,
References
1
Prota, G., D’Agostino, M., and Misuraca, G., Experientia, 1971, 27,
15.
2
Prota, G., D’Agostino, M., and Misuraca, G., J. Chem. Soc., Perkin
Trans. 1, 1972, 1614.
Cimino, G., De Rosa, S., De Stefano, S., and Sodano, G., J. Nat.
3
Prod., 1985, 48, 828.
Comber, M. F., and Sargent, M. V., Aust. J. Chem., 1985, 38, 1481.
Krohn, K., and Khanbabaee, K., Liebigs Ann. Chem., 1993, 905.
Cameron, D. W., Feutrill, G. I., Gamble, G. B., and Stavrakis, J.,
4
–1
4.47, 4.30). nmax 3404, 1695, 1650, 1612, 1581 cm . d 2.47, s, Me;
5
4.01, s, OMe; 6.16, br s, 7-OH; 6.47, d, J 10.0 Hz, H 3; 7.46, s, H 5;
7.55, d, J 10.0 Hz, H 4; 7.64, s, H 10; 8.74, s, H 9. m/z 268 (M, 16%),
240 (48), 225 (60), 197 (27), 111 (21), 97 (35), 95 (27), 85 (30), 83 (43),
81 (30), 71 (54), 69 (59), 67 (24), 56 (100).
6
Tetrahedron Lett., 1986, 27, 4999.
Cameron, D. W., Feutrill, G. I., and Keep, P. L. C., Tetrahedron Lett.,
7
Reported2 for natural hallachrome, m.p. 224–226o (dec.). lmax
(loge) (MeOH) 250, 312, 500 nm (4.51, 4.52, 3.75). nmax 3300, 1695,
1989, 30, 5173.
8
Danishefsky, S., and Kitahara, T., J. Am. Chem. Soc., 1974, 96, 7807.
Cameron, D. W., Gan, C.-Y., Griffiths, P. G., and Pattermann, J. A.,
9
–1
1652, 1610, 1580 cm .
Reported3 for natural hallachrome, d (CDCl3) 2.47, s, Me; 4.01, s,
OMe; 6.19, br s, 7-OH; 6.47, d, J 10.0 Hz, H 3; 7.46, s, H 5; 7.55, d, J
10.0 Hz, H 4; 7.64, s, H 10; 8.73, s, H 9.
Aust. J. Chem., 1998, 51, 421.
Cameron, D. W., Edmonds, J. S., and Raverty, W. D., Aust. J.
10
Chem., 1976, 29, 1535.