Reaction of dihydrolipoic acid with juglone and related naphthoquinones: unmasking of a spirocyclic 1,3-dithiane intermediate en route to naphtho[1,4]dithiepines

Article abstract of DOI:10.1016/j.tet.2010.03.103

The reaction of dihydrolipoic acid (DHLA) with 5-hydroxy-1,4-naphthoquinone (juglone) gives rise to the novel naphtho[1,4]dithiepine derivatives through ring expansion of an unstable spirocyclic 1,3-dithiane intermediate, which was isolated and completely characterized. Reported herein is also the characterization of novel reaction products of DHLA with other naphthoquinones and the extension of the study to the spirocyclic adduct formed by reaction with a representative 2-substituted naphthoquinone.

Full text of DOI:10.1016/j.tet.2010.03.103

Tetrahedron 66 (2010) 3912–3916
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Reaction of dihydrolipoic acid with juglone and related naphthoquinones:
unmasking of a spirocyclic 1,3-dithiane intermediate en route
to naphtho[1,4]dithiepines
*
Giorgia Greco, Lucia Panzella, Alessandro Pezzella, Alessandra Napolitano , Marco d’Ischia
Department of Organic Chemistry and Biochemistry, University of Naples ‘Federico II’, Via Cintia 4, I-80126 Naples, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of dihydrolipoic acid (DHLA) with 5-hydroxy-1,4-naphthoquinone (juglone) gives rise to the
novel naphtho[1,4]dithiepine derivatives through ring expansion of an unstable spirocyclic 1,3-dithiane
intermediate, which was isolated and completely characterized. Reported herein is also the character-
ization of novel reaction products of DHLA with other naphthoquinones and the extension of the study to
the spirocyclic adduct formed by reaction with a representative 2-substituted naphthoquinone.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 27 November 2009
Received in revised form 10 March 2010
Accepted 29 March 2010
Available online 2 April 2010
Keywords:
Juglone
Dihydrolipoic acid
Spiro 1,3-dithiane
Naphthodithiepine
1. Introduction
tyrosinase to form 5-S-lipoyl-DOPA.13 As part of our studies on
thiol/quinone coupling as a viable biomimetic access to novel
The synthetic and biomedical relevance of thiols and quinones
and the manifold chemical issues associated with their conjugation
reactions have elicited unabated interest over the past decades.1–7
Recently, studies of thiol/quinone coupling and the properties of
their products have gained increasing importance in the search for
new structural motifs, which can enhance the antioxidant and
antinitrosating activities of phenols.8 In this regard, the effect of
adjacent sulfur substituents on the H-atom donor properties of
phenolic groups has received considerable attention.9
bioactive compounds for practical applications in medicinal and
food chemistry and in cosmetics, we recently focused on the
reaction of DHLA with juglone (5-hydroxy-1,4-naphthoquinone),
a characteristic constituent of walnut husks, which has received
considerable attention as a useful scaffold for the synthesis of
bioactive compounds, e.g., antioxidants,14 cysteine protease
inhibitors,15 and antitubercular,16 and antimelanoma agents.17
Studies of this chemistry were further prompted by the growing
interest in sulfur-substituted naphthoquinones as bioactive
compounds.18 During this study we came across unexpected
mechanistic features of this reaction which led to the discovery of
a novel class of spirocyclic DHLA derivatives of potential chemical
and practical interest. Reported herein is the characterization of
the novel reaction products of DHLA with juglone and other
naphthoquinones, the expedient isolation of the spirocyclic
juglone/DHLA adduct, and the extension of the study to the
Despite the vast literature on thiol/quinone coupling, surpris-
ingly little is known about the reaction behavior of dihydrolipoic
acid ((R)-6,8-dithiooctanoic acid, DHLA). DHLA and its disulfide,
a-lipoic acid, provide a well-known prosthetic group in the a-keto
acid dehydrogenase complexes of the mitochondria.10 Lipoic acid is
also a strong endogenous antioxidant11 and is capable of inhibiting
the conjugation of nonenzymatically oxidized catecholamines with
cysteine and glutathione in vitro,12 suggesting formation of
adducts. So far, the chemical reactivity of DHLA with quinones has
remained virtually unexplored in spite of the many synthetic
opportunities offered by the 1,3-dithiopropane group. The only
insights into this issue stem from recent papers showing that DHLA,
spirocyclic adduct formed by reaction with
2-substituted naphthoquinone.
a representative
2. Results and discussion
but not the parent disulfide, reacts with L-DOPA in the presence of
a
-Lipoic acid possesses a stereocenter at C-6 and the R-isomer,
which is synthesized naturally, was used in this study. DHLA was
prepared fresh before use by NaBH4 reduction of the commercial
* Corresponding author. Tel.: þ39 81674133; fax: þ39 81674393; e-mail address:
alesnapo@unina.it (A. Napolitano).
a
-lipoic acid followed by extraction into an organic solvent.19
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.03.103

G. Greco et al. / Tetrahedron 66 (2010) 3912–3916
3913
Addition of DHLA to an ethanol solution of juglone in the
presence of triethylamine (TEA) as basic catalyst resulted in the fast
development of a purple red coloration due to the formation of
a main product ([MþH]þ m/z 379), which was isolated and sub-
jected to complete NMR spectroscopic characterization. The lack in
the proton spectrum of the expected resonances for the H-2 and H-
3 protons of juglone suggested a cyclic adduct in which both the SH
functionalities of lipoic acid were linked to the 1,4-naphthoquinone
unit forming a [1,4]dithiepine ring. Analysis of the 13C NMR spec-
trum showed, unexpectedly, that some of the signals were present
the proton and carbon spectrum revealed that most signals were
present as a close pair, indicating that the compound was in fact
a mixture of two diastereoisomers that differed in the configuration
of the spirocenter, and could not be separated under a variety of
chromatographic conditions. Exposure of 1c to TEA in ethanol or to
phosphate buffer, pH 7.4, resulted in quantitative conversion to an
equimolar mixture of 1a,b, confirming its intermediacy in the route
to naphthodithiepines.
On this basis, a plausible reaction mechanism for the reaction of
DHLA with juglone can be proposed (Scheme 1).
as a close pair, in particular those at
attributed to the C-2 and C-3 carbons of the naphthoquinone
moiety, and those at 34.7/34.8 and 45.3/45.5, assigned to the C-5
d 145.7/145.9 and d 143.0/143.2,
OH O
HS
d
and C-6 carbons of the lipoic acid unit. Based on this observation, it
was concluded that the isolated product was in fact an intimate 1:1
mixture of two closely related species. Accordingly, the products
were formulated as the two positional isomers 5-(2H-3,4-dihydro-
7-hydroxy-6,11-dioxonaphtho[3,2-b][1,4]dithiepin-2-yl)pentanoic
acid (1a) and 5-(2H-3,4-dihydro-7-hydroxy-6,11-dioxonaph-
tho[2,3-b][1,4]dithiepin-4-yl)pentanoic acid (1b) (overall isolated
yield 70%, formation yield >95% as determined by HPLC).
Unfortunately, all attempts at separating the compounds, e.g., by
TLC and reverse phase HPLC under a variety of elutographic
conditions, were defeated.
+
HS
R
R=(CH2)4COOH
b
O
[O]
OH O
S
OH O
O
S
fast
a
S
H
R
HS
a
H
R
O
1c
fast
OH O
I
b
Compounds 1a,b displayed a chromophore with lmax at 416 and
533 nm (EtOH), which shifted to 497 and 547 (sh) in alkali (Fig. S1)
and was stable both in the solid state and in solution. It was
insensitive to hydrogen peroxide or thiols. Upon reduction with
NaBH4 it was changed into a finelystructured chromophore (Fig. S2),
which rapidly recovered the original features by reoxidation in air.
Formation of regioisomeric 1a,b by the reaction of DHLA with
juglonewas ofmechanistic interest. Previous studiesof the reactions
of juglone with thiols (e.g., thioglycolic acid, arylthiols) indicated the
3-position as the most reactive site.20 This was attributed to electron
delocalization from the OH grouptothe carbonyl group at position 4,
slow
[O]
R
S
HS
O
II
OH O
O
S
S
slow
[O]
R
1a
OH O
R
S
S
O
1b
decreasing the electrophilic character of the corresponding
b posi-
Scheme 1. Proposed mechanism of reaction of juglone with DHLA.
tion at C-2 and, hence, favoring attack at C-3. On this basis, and
assuming a preferential reactivity of DHLA via the less hindered SH
groupat C-8,13 isomer 1awas predicted tobe the mainproduct via an
initial chemo- and regioselective coupling. Though somewhat dis-
appointing from a synthetic perspective, the unexpected lack of
selectivity raised mechanistic issues that prompted further insights
into this reaction. An explanation for this inconsistent result came
eventually from careful HPLC analysis of the reaction course under
higher dilution conditions. Kinetic monitoring showed the initial
formation of a species (lmax 356 nm) that gradually decayed con-
comitant to formation of the purple adducts. Initial attempts to
isolate this unstable intermediate were unsuccessful. However after
several trials it was found that conversion of this intermediate to
products was blocked by acidification of the reaction mixture. Thus,
acid quenching of a preparative scale mixture allowed the isolation
of the labile intermediate. NMR spectroscopic analysis of this latter
indicated a significant structural difference in the naphthoquinone
moiety relative to 1a,b. In particular, the resonances due to the C-2
In this scheme, selective addition of DHLA via the SH group at
C-8 to the 3-position of juglone leads to adduct I, which then
evolves via a fast intramolecular cyclization to give the spirocyclic
1,3-dithiane 1c. This latter would be engaged in fast equilibria with
the starting adduct I and the isomer II. Generation of products 1a,b
would then be the final outcome of slow, irreversible cyclization/
oxidation processes from the ring-opened adducts I and II. The
equimolar formation of 1a,b is therefore the signature of the spi-
rocyclic compound 1c, which partitions equally between the ring
opening routes a and b. Kinetic analysis of the reaction course
(Fig. 1) demonstrates indeed the fast accumulation of 1c and its
slow decay concomitant to product formation.
100
80
60
40
20
0
and C-3 carbons (
a quaternary carbon signal at
function, and a methylene resonance at
protons at 3.33). A down-field shift of the carbonyl resonances by
ca. 10 ppm was also apparent, indicating loss of the conjugated
double bond. Moreover, the proton signals at 3.33 showed long-
range correlations with the quaternary carbon resonances at
133.6 and 53.1, as well as with the carbonyl signals ( 191 and 196).
Likewise, correlations were apparent between one of the C-8 pro-
tons of the lipoic acid unit ( 2.8) and the carbon signal at 53.1.
d
145 and 143 in 1a,b) were apparently replaced by
53.1, suggestive of a dithioketal
50.0 (HSQC cross peak with
d
d
d
d
d
d
0
30
60 120
Time (min)
300
480
d
d
Based on these extensive 2D-NMR data, the compound was
identified as the spirocyclic 1,3-dithiane derivative 1c. Inspection of
Figure 1. Kinetic profile of juglone decay (starting concn 0.5 mM) and product for-
mation with DHLA; A: juglone, ,: 1c, :: 1a,b.

3914
G. Greco et al. / Tetrahedron 66 (2010) 3912–3916
The reversible formation of the spirocyclic adduct raised the
O
HS
HS
issue of what factors orient the initial ring closing attack of SH
toward the S-substituted 3-position instead of the adjacent
unsubstituted 2-position of I.
+
R
O
R=(CH2)4COOH
To address this issue, the reactions of DHLA with the parent
1,4-naphthoquinone, acetylated juglone and naphthazarin (5,8-
dihydroxy-1,4-naphthoquinone) were investigated. In all cases the
corresponding naphtho[1,4]dithiepines 2–4 were obtained in
25–44% isolated yields.
[O]
O
O
O
S
S
fast
S
R
HS
R
O
AcO
O
O
5b
fast
S
S
S
S
O
O
slow
R
R
R
R
O
2
O
3a
R
O
O
[O]
S
HS
AcO
OH O
O
S
S
S
S
slow
[O]
R
O
O
O
OH O
4
O
3b
S
S
R= (CH2)4COOH
O
5a
Scheme 2. Proposed mechanism of reaction of menadione with DHLA.
For these reactions, the mass balance was accounted for by side
products, possibly ring-opened diadducts and polymeric species
that were not investigated further. Notably, all attempts to identify
or isolate the first formed adducts of the type I met with failure, on
account of their marked tendency to cyclize or react further via the
free SH group.
catalysis.21 No report is available of fast, efficient, and mild, base-
induced conversion of 1,3-dithianes to 1,4-dithiepines.
3. Conclusions
The generation of an equimolar mixture of regioisomers from
acetylated juglone supported operation of the spirocyclic mecha-
nism, which, however, could not be demonstrated despite several
efforts. Nonetheless, the failure to isolate spirocyclic dithiane
precursors to 2–4 under various concentration, pH, and tempera-
ture conditions does not rule out their transient generation in the
mixtures. On this basis, it can be argued that the observed accu-
mulation of the spirocyclic intermediate in the case of juglone
reflects specifically remote electronic control exerted by the OH
substituent disfavoring fast attack at C-2.
To address in more detail the generation and fate of spi-
rocyclic dithianes, subsequent experiments were directed to
investigate the reaction of DHLA with menadione (2-methyl-1,4-
naphthoquinone). Product analysis showed the formation of the
S,S0-bisnaphthoquinone adduct 5a (16%) and of a spirocyclic 1,3-
dithiane intermediate, which was isolated and identified as 5b.
For this latter product the presence of the spirodithiane ring at
the 3-position of the menadione unit was deduced from distinct
We have disclosed herein the reaction of DHLA with represen-
tative 2,3-unsubstituted 1,4-naphthoquinones leading to an
attractive, chemically robust yet virtually unexplored heterocyclic
system of potential practical interest. Besides the novel naph-
tho[1,4]dithiepine derivatives, a significant outcome of this study is
the identification and chemical investigation of labile spirocyclic
1,3-dithiane derivatives of DHLA. To the best of our knowledge, this
is the first example of base-induced ring expansion of 1,3-dithianes
to 1,4-dithiepines under mild conditions. Overall, these results fill
a gap in the current knowledge of thiol/quinone reactions, and
shed new light on the synthetic and practical opportunities offered
by lipoic acid chemistry beyond the traditional boundaries of
biomedicine.
4. Experimental
4.1. General methods and materials
correlations of both the methyl protons at
C-8 protons of the lipoic acid unit ( 2.77) with the dithioketal
carbon resonating at 60 in the HMBC spectrum. Also in this
case, most of the proton and carbon signals appeared as a couple,
suggesting that this product, too, was mixture of dia-
stereoisomers. Thus, the presence of the methyl group on the
2-position inhibits formation of stable seven-membered intra-
molecular cyclization products, by preventing the irreversible
oxidation step leading to the thermodynamic stable 1,4-naph-
thoquinone system. The ring-opened isomeric adducts deriving
from the spiro compound can then evolve only by coupling of
the SH group with a second molecule of quinone to give 5a
(Scheme 2).
d 1.5 and one of the
d
1,4-Naphthoquinone, 5-hydroxy-1,4-naphthoquinone (juglone),
5,8-dihydroxy-1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone
(menadione), (R)-(þ)-1,2-dithiolane-3-pentanoic acid (lipoic acid),
sodium acetate, acetic anhydride, sodium borohydride, and tri-
ethylamine (TEA) were commercially available and were used as
obtained. DHLA was prepared according to a literature procedure.19
HR ESIþ/MS spectra were obtained in 0.5% TFA/methanol 1:1 v/v.
UV/vis spectra were obtained on a diode array spectrophotometer.
IR spectra were obtained on a FT spectrophotometer. 1H NMR
spectra were recorded at 200 or 400 MHz, 13C NMR spectra at 50 or
100 MHz. 1H, 1H COSY, 1H, 13C DEPT HSQC, and 1H, 13C HMBC
experiments were run at 400 MHz on an instrument equipped with
a 5 mm 1H/broadband gradient probe with inverse geometry using
standard pulse programs. The HMBC experiments used a 100 ms
d
a
The reaction behavior of the spirocyclic intermediates described
herein is worthy of further attention. 1,3-Dithianes are valuable
synthetic tools and have been employed in rearrangement
reactions leading to ring expanded derivatives, but these reactions
proceed usually in poor yields under conditions of acid or metal
long-range coupling delay. Chemical shifts are reported in
d values
(ppm) downfield from TMS. Analytical and preparative TLC were
carried out on silica gel plates (0.25 and 0.50 mm, respectively)

G. Greco et al. / Tetrahedron 66 (2010) 3912–3916
3915
using chloroform/methanol/acetic acid (90:10:1) (eluant A) or
cyclohexane/ethyl acetate/acetic acid (70:30:1) (eluant B) as
eluants. HPLC analysis of the reaction mixtures was carried out on
an apparatus equipped with a UV detector set at 254 nm using
(4H, m, 3-H, and 5-H), 1.54 (1H, m, 7-H), 1.58 (2H, m, 4-H), 2.24/
2.26 (1H, m, 7-H), 2.36 (2H, t, J¼7.2 Hz, 2-H), 2.79/2.82 (1H, m,
8-H), 3.38 (1H, m, 8-H), 3.54 (m, 1H, 6-H); juglone moiety dH 3.33
(2H, s, 2-H), 7.28 (1H, d, J¼9.2 Hz, 6-H), 7.50 (1H, d, J¼7.2 Hz, 8-H),
7.60 (1H, m, 7-H), 11.95 (1H, s, OH); DHLA moiety dC (100 MHz,
CDCl3) 24.4 (CH2, C-4), 25.4 (CH2, C-3), 29.1 (CH2, C-8), 31.0 (CH2,
C-7), 33.5 (CH2, C-2), 35.5 (CH2, C-5), 41.8 (CH, C-6), 178.2 (C, C-1);
juglone moiety dC 50.0 (CH2, C-2), 53.1 (C, C-3), 114.1 (C, C-4a),
117.7/117.8 (CH, C-8), 124.6/124.7 (CH, C-6), 133.6 (C, C-8a) 136.6/
136.7 (CH, C-7), 162.8/163.1 (C, C-5), 191.4 (C, C-1), 196.3/196.4
(C, C-4).
a Sphereclone ODS (5
m
m, 4.6ꢀ250 mm) column. 0.5% TFA (solvent
a), acetonitrile (solvent b) were used as follows: 0–45 min 5–90%
solvent b, 45–50 min 90% solvent b (gradient A), at a flow rate of
0.8 mL minꢁ1. Preparative HPLC was carried out on an instrument
coupled with a UV detector set at 254 nm, using an Econosil C18
column (10
m
m, 10ꢀ250 mm) (gradient A, flow rate 3 mL minꢁ1).
Purity of isolated compounds was estimated by 1H NMR analysis.
Signals due to isomeric compounds are reported as d/d.
4.2. General procedure for conjugation of 1,4-
naphthoquinones with DHLA
4.1.1. 5-Acetoxy-1,4-naphthoquinone. The title compound was pre-
pared by acetylation of juglone according to a literature procedure
with modifications.22 A solution of 5-hydroxy-1,4-naphthoquinone
(100 mg, 0.58 mmol) and sodium acetate (590 mg, 7.2 mmol) in
acetic anhydride (2 mL) was stirred at 120 ꢂC. After 30 min the
mixture was diluted in 0.1 M phosphate buffer (pH 7.4) (5 mL) and
extracted with chloroform (3ꢀ10 mL). The combined organic pha-
ses were dried over sodium sulfate, filtered, and taken to dryness in
vacuo. The compound was isolated as a brown powder (Rf 0.44
(eluant B), 110 mg, 88% yield, purity >98%); UV (EtOH): lmax
The reaction was performed as described for 1a,b. The reaction
mixture was evaporated to dryness, and the residue obtained was
dissolved in chloroform and fractionated by preparative TLC
(eluant A) to afford 2–4 (reaction time: 2 and 4, 10 min; 3, 60 min).
4.2.1. 5-(2H-3,4-Dihydro-6,11-dioxonaphtho[2,3-b][1,4]dithiepin-2-yl)-
pentanoic acid (2). Compound 2 was obtained as a purple red oil (Rf
0.62, 48 mg, 44% yield, purity >98%); UV (EtOH): lmax 236, 293,
340 nm (log
3
3.41); IR(CHCl3): nmax 1766,1671,1613,1596 cmꢁ1; HR
514 nm (log 3 4.07, 4.25, 3.32); IR(CHCl3): nmax 3522, 1710, 1659,
ESIþ/MS: found m/z 239.0332 ([MþNa]þ), calcd for C12H8O4Na m/z
239.0320; dH (200 MHz, CDCl3) 2.43 (3H, s), 6.83 (1H, d, J¼10.6 Hz),
6.93 (1H, d, J¼10.6 Hz), 7.37 (1H, d, J¼7.8 Hz), 7.74 (1H, t, J¼7.8 Hz),
8.02 (1H, d, J¼7.8 Hz); dC (50 MHz, CDCl3) 21.0 (CH3), 123.2 (C),
124.9 (CH), 129.7 (CH), 133.5 (C), 134.8 (CH), 137.3 (CH), 139.8 (CH),
149.4 (C), 169.3 (C), 183.6 (C), 184.1 (C).
1592 cmꢁ1; HR ESIþ/MS: found m/z 363.0710 ([MþH]þ), calcd for
C18H19O4S2 m/z 363.0725; dH (400 MHz, CDCl3) 1.51 (1H, m), 1.68
(5H, m), 1.91 (1H, m), 2.21 (1H, m), 2.36 (2H, t, J¼6.8 Hz), 2.95 (1H,
m), 4.18 (1H, m), 4.34 (1H, m), 7.66 (2H, m), 8.03 (2H, m); dC
(100 MHz, CDCl3) 24.3 (CH2), 26.3 (CH2), 29.8 (CH2), 33.6 (CH2), 33.7
(CH2), 34.7 (CH2), 45.3 (CH), 126.8 (CH), 126.9 (CH), 131.7 (C), 131.8
(C), 133.7 (2ꢀCH), 144.2 (C), 144.4 (C), 178.8 (C), 180.2 (2ꢀC).
4.1.2. 5-(2H-3,4-Dihydro-7-hydroxy-6,11-dioxonaphtho[3,2-b][1,4]di-
thiepin-2-yl)pentanoic acid (1a) and 5-(2H-3,4-dihydro-7-hydroxy-
6,11-dioxonaphtho[2,3-b][1,4]dithiepin-4-yl)pentanoic acid (1b). To
a solution of juglone (50 mg, 0.3 mmol) in absolute ethanol (6 mL),
4.2.2. 5-(7-Acetoxy-2H-3,4-dihydro-6,11-dioxonaphtho[3,2-b][1,4]di-
thiepin-2-yl)pentanoic acid (3a) and 5-(7-acetoxy-2H-3,4-dihydro-6,11-
dioxonaphtho[2,3-b][1,4]dithiepin-4-yl)pentanoic acid (3b). Compound
3 was obtained as a purple red oil (Rf 0.79, 48 mg, 38% yield, purity
TEA (42
mL, 0.3 mmol), and DHLA (62 mg, 0.3 mmol), previously
dissolved in ethanol (600
m
L), were added under vigorous stirring.
>95%); UV (EtOH): lmax 296, 520 nm (log 3 3.92, 2.97); IR(CHCl3):
After 10 min the reaction mixture was evaporated to dryness, and
the residue obtained was dissolved in chloroform and fractionated
by preparative TLC (eluant A) to afford 1a,b as a purple red oil (Rf
0.59, 79 mg, 70% yield, purity >98%); UV (EtOH): lmax 291, 416,
nmax 3527, 1769, 1712, 1655, 1509 cmꢁ1; HR ESIþ/MS: found m/z
421.0793 ([MþH]þ), calcd for C20H21O6S2 m/z 421.0779; dH
(400 MHz, CDCl3) 1.49 (1H, m), 1.66 (5H, m), 1.89 (1H, m), 2.19 (1H,
m), 2.35 (2H, m), 2.42/2.43 (3H, s), 2.93 (1H, m), 4.13 (1H, m), 4.31
(1H, m), 7.29 (1H, d, J¼8.0 Hz), 7.67 (1H, t, J¼8.0 Hz), 7.99/8.01 (1H,
d, J¼8.0 Hz); dC (100 MHz, CDCl3) 21.1 (CH3), 24.3 (CH2), 26.3 (CH2),
29.7/29.8 (CH2), 33.5 (2ꢀCH2), 34.7 (CH2), 45.2/45.3 (CH), 122.9/
123.0 (C), 125.3/125.4 (CH), 129.5 (CH), 133.4/133.5 (C), 134.6 (CH),
143.1/143.3 (C), 145.4/145.6 (C), 149.6 (C), 169.4/169.5 (C), 178.5 (C),
178.6 (C), 179.4 (C).
533 nm (log
3 3.92, 3.38, 3.15); IR(CHCl3): nmax 3688, 3522, 1713,
1626, 1457 cmꢁ1; HR ESIþ/MS: found m/z 379.0658 ([MþH]þ), calcd
for C18H19O5S2 m/z 379.0674; DHLA moiety dH (400 MHz, CDCl3) 1.53
(1H, m, 4-H), 1.68 (5H, m, 3-H, 4-H, and 5-H), 1.94 (1H, m, 7-H), 2.23
(1H, m, 7-H), 2.38 (2H, m, 2-H), 2.97/2.99 (1H, m, 8-H), 4.19 (1H, m,
8-H), 4.37 (1H, m, 6-H); juglone moiety dH 7.20 (1H, d, J¼8.0 Hz, 6-H),
7.55 (1H, t, J¼8.0 Hz, 7-H), 7.60 (1H, m, 8-H), 11.8 (1H, br s); DHLA
moiety dC (100 MHz, CDCl3) 24.3 (CH2, C-3), 26.3 (CH2, C-4), 29.8
(CH2, C-8), 33.5 (2ꢀCH2, C-2, and C-7), 34.7/34.8 (CH2, C-5), 45.3/45.5
(CH, C-6),178.5 (C, C-1); juglone moiety dC 114.2 (C, C-4a),119.7/119.8
(CH, C-8), 124.3 (CH, C-6), 131.7 (C, C-8a) 136.1 (CH, C-6), 143.0/143.2
(C, C-3), 145.7/145.9 (C, C-2), 161.4 (C, C-5), 179.5 (C, C-1), 185.1
(C, C-4).
4.2.3. 5-(2H-3,4-Dihydro-7,10-dihydroxy-6,11-dioxonaph-
tho[2,3-b][1,4]dithiepin-2-yl)pentanoic acid (4). Compound 4 was
obtained as a purple red oil (Rf 0.67, 48 mg, 25% yield, purity
>95%); UV (EtOH): lmax 284, 507, 548 nm (log
3 4.14, 3.91, 3.74);
IR(CHCl3): nmax 3690, 3601, 3503, 1713, 1597, 1456 cmꢁ1; HR ESIþ/
MS: found m/z 395.0606 ([MþH]þ), calcd for C18H19O6S2 m/z
395.0623; dH (400 MHz, CDCl3) 1.52 (1H, m), 1.68 (5H, m), 1.94 (1H,
m), 2.21 (1H, m), 2.35 (2H, m), 2.97 (1H, m), 4.18 (1H, m), 4.37 (1H,
m), 7.18 (2H, s), 12.17 (1H, br s); dC (100 MHz, CDCl3) 24.2 (CH2),
26.2 (CH2), 29.7 (CH2), 33.4 (CH2), 33.6 (CH2), 34.7 (CH2), 45.4
(CH), 110.8 (2ꢀC), 129.1 (2ꢀCH), 144.5 (C), 144.8 (C), 157.5(C), 157.6
(C), 178.9 (C), 183.1 (2ꢀC).
4.1.3. 5-(20,30-Dihydro-spiro[1,3-dithiane-2,30-[5]hydroxy[1,4]naph-
thoquinone-4-yl])pentanoic acid (1c). To a solution of juglone
(50 mg, 0.3 mmol) in absolute ethanol (600 mL), TEA (42
0.3 mmol), and DHLA (62 mg, 0.3 mmol), previously dissolved in
ethanol (600 L), were added under vigorous stirring. After
mL,
m
10 min the mixture was treated with TFA (6 mL) and concentrated
in vacuo. The residue was purified by preparative HPLC to provide
1c as a yellow oil (tR 33.4 min, 50 mg, 44% yield, purity >98%); UV
(EtOH): lmax 232, 356 nm; IR(CHCl3): nmax 3689, 3510, 1704, 1636,
1455 cmꢁ1; HR ESIþ/MS: found m/z 381.0845 ([MþH]þ), calcd for
C18H21O5S2 m/z 381.0830; DHLA moiety dH (400 MHz, CDCl3) 1.51
4.3. Procedure for conjugation of menadione with DHLA
To a solution of menadione (52 mg, 0.3 mmol) in absolute eth-
anol (6 mL), TEA (42
mL, 0.3 mmol), and DHLA (62 mg, 0.3 mmol),
previously dissolved in ethanol (600
mL), were added under

3916
G. Greco et al. / Tetrahedron 66 (2010) 3912–3916
vigorous stirring. After 2 h the reaction mixture was evaporated to
dryness, and the residue obtained was dissolved in CHCl3 and
fractionated by preparative TLC (eluant A) to afford 5a (yellow oil, Rf
0.79, 26 mg, 16% yield, purity >98%) and 5b (light yellow oil, Rf 0.70,
23 mg, 20% yield, purity >95%).
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.tet.2010.03.103.
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Acknowledgements
This work was supported in part by a grant from Regione
Campania (L5 2006 project). We thank the Centro Inter-
dipartimentale di Metodologie Chimico-fisiche (CIMCF) of Naples
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Supplementary data
NMR spectra of 1-5 and acetylated juglone and UV/vis spectra of
1a,b in EtOH, in alkali and after reduction with NaBH4 are provided.