Approaches to the synthesis of some tyrosine-derived marine sponge metabolites: Synthesis of verongamine and purealidin N

Article abstract of DOI:10.1021/jo010015v

The oxidation of tyrosine ethyl ester (7) with Na₂WO₄/H₂O₂ in ethanol, dimethyldioxirane in acetone, or methyltrioxorhenium/H₂O₂ in EtOH gave the corresponding tyrosine oxime (8) in high yi

Full text of DOI:10.1021/jo010015v

J . Org. Chem. 2001, 66, 3111-3118
3111
Ap p r oa ch es to th e Syn th esis of Som e Tyr osin e-Der ived Ma r in e
Sp on ge Meta bolites: Syn th esis of Ver on ga m in e a n d P u r ea lid in N
Todd R. Boehlow, J . J onathan Harburn, and Christopher D. Spilling*
Department of Chemistry, University of Missouri-St. Louis, 8001 Natural Bridge Road,
St. Louis, Missouri 63121-4499
cspill@umsl.edu
Received J anuary 4, 2001
The oxidation of tyrosine ethyl ester (7) with Na2WO4/H2O2 in ethanol, dimethyldioxirane in acetone,
or methyltrioxorhenium/H2O2 in EtOH gave the corresponding tyrosine oxime (8) in high yield.
Controlled bromination of the aromatic ring gave the monobromo oxime (9), the dibromo oxime
(10), or the spiroisoxazoline (11) depending upon reaction conditions. Synthesis of the known
metabolite verongamine (15) was achieved by oxidation of O-methyl bromotyrosine methyl ester
and amidation of the resulting oxime ester (14) with histamine. The mono- and di-bromotyrosine
oxime derivatives (9 and 10) were further transformed into the naturally occurring nitriles (16
and 17) by base hydrolysis of the ester and acid-catalyzed decarboxylation. Wadsworth-Emmons
olefination of the dibromobenzaldehyde (20b) with phosphonate (18) gave the pyruvate silylenolether
(21b). Deprotection and in situ oxime formation gave the oxime ester (23b). Attempted purification
of the pyruvate ester resulted in a homoaldol condensation yielding butenolide (22). Amidation of
the oxime ester (23b) with histamine, followed by deprotection of the MOM ether gave the first
synthesis of purealidin N (28). Oxidative spirocyclization of the phenolic oxime ester (23d ) with a
polymer-bound iodosyl diacetate gave the spiroisoxazoline (24) and represents a formal synthesis
of aerothionin (26a ), homoaerothionin (26b), and aerophobin-1 (25).
In tr od u ction
have concentrated on developing chemistry for the latter
steps in the synthesis and rely on an Erlenmeyer
condensation for the key carbon-carbon bond forming
reaction for the formation of the oximes.9-11 In particular,
Over the past 30 years an ever increasing number of
tyrosine (1)-derived secondary metabolites have been
isolated from marine sponges of the order Verongida.1,2
It was reported that the metabolic pathway employed by
the sponge involves bromination of the tyrosine aromatic
ring and oxidation of the amine to an oxime (Scheme 1).3
The tyrosine oximes4 (2), exemplified by verongamine
(15),4a are then further metabolized (via an arene oxide),5
to form spirocyclic isoxazolines (3)5 such as aerophobin-1
(25),5a aerothionin (26a ),5b and homoaerothionin (26b).5c
The isoxazolines (3) can rearrange to give more stable
phenolic oximes (4), for example, purealidin N (28).6
Decarboxylation of the oximes leads to the formation of
nitriles (5),7 which can also undergo further oxidation to
give quinols (6).8 Many of these metabolites exhibit wide
ranging biological activity and as a result have become
targets for synthesis.9-13
(5) (a) Cimino, G.; De Rosa, S.; De Stefano, S.; Self, R.; Sodano, G.
Tetrahedron Lett. 1983, 24, 3029. (b) Fattorusso, E.; Minale, L.; Sodano,
G.; Moody, K.; Thompson, R. H J . Chem. Soc., Chem. Commun. 1970,
752. (c) Fattorusso, E.; Minale, L.; Moody, K.; Sodano, G.; Thompson,
R. H. Gazz. Chem. Ital. 1971, 101, 61. (d) McMillan, J . A.; Paul, I. C.;
Goo, Y. M.; Rinehart, J r., K. L. Tetrahedron Lett. 1981, 22, 39. (f)
Rotem, M.; Carmely, S.; Kashman, Y.; Loya, Y. Tetrahedron 1983, 39,
667. (g) Ciminiello P.; Fattorusso, E.; Forino, M.; Magno, S.; Pansini,
M. Tetrahedron 1997, 53, 6565. (h) Ciminiello P.; Constantino, V.;
Fattorusso, M.; Magno, S.; Mangoni, A. J . Nat. Prod. 1994, 57, 705.
(i) Acosta, A. L. Rodriguez, A. D. J . Nat. Prod. 1992, 55, 1007. (j)
Gunasekera, M.; Gunasekera, S. P. J . Nat Prod. 1989, 52, 753. (k)
Kernan, M. R.; Cambie, R. C. J . Nat. Prod. 1990, 53, 615. (l) Mancini,
I.; Guella, G.; Laboute, P.; Debitus, C.; Pietra, F. J . Chem. Soc., Perkin
Trans. 1 1993, 3121. (m) Longeon, A.; Guyot, M.; Vacelet, J . Experi-
mentia 1990, 46, 548. (n) Compagnone, R. S.; Avila, R.; Sua´rez, A. I.;
Abrams, O. V.; Rangel, H. R.; Arvelo, F.; Pin˜a, I. C.; Merentes, E. J .
Nat. Prod. 1999, 62, 1443. (o) Ciminiello P.; Dell’Aversano, C.;
Fattorusso, E.; Magno, S.; Pansini, M. J . Nat. Prod. 1999, 62, 590.
(6) Kobayashi, J .; Honma, K.; Sasaki, T.; Tsuda, M. Chem. Pharm.
Bull. 1995, 43, 403.
Many of the synthetic approaches to these metabolites
(1) Berquist, P. R.; Wells, R. J . Chemotaxonomy of the Porifera: The
Development and Current Status of the Field. In Marine Natural
Products: Chemical and Biological Perspectives; P. J . Scheuer, Ed.;
Academic Press: New York, 1993; Vol. 5, pp 1-50.
(2) Faulkner, D. J . Nat. Prod. Rep. 1998, 113. Faulkner, D. J . Nat.
Prod. Rep. 1997, 259 and previous reports in this series.
(7) (a) J ime´nez, C.; Crews P. Tetrahedron 1991, 47, 2097. (b) Quin˜oa`,
E.; Crews P. Tetrahedron Lett. 1987, 28, 3229.
(8) (a) Fattorusso, E.; Minale, L.; Sodano, G. J . Chem. Soc., Perkin
Trans. 1 1972, 16. (b) Fattorusso, E.; Minale, L.; Sodano, G. J . Chem.
Soc., Chem. Commun. 1970, 751. (c) Mazzarella, L.; Puliti, R. Gazz.
Chim. Ital. 1972, 102, 391.
(3) (a) Tymiak, A. A.; Rinehart J r., K. L. J . Am. Chem. Soc. 1981,
103, 6763. (b) De Rosa, M.; Minale, L.; Sodano, G. Comp. Biochem.
Physiol. 1973, 45B, 883. (c) Carney, J . R.; Rinehart, K. L. J . Nat. Prod.
1995, 58, 971.
(4) (a) Mierzwa, R.; King, A.; Conover, M. A.; Tozzi, S.; Puar, M. S.;
Patel, M.; Coval, S. J .; Pomponi, S. A. J . Nat. Prod. 1994, 57, 175. (b)
Arabshahi, L.; Schmitz, F. J . J . Org. Chem. 1987, 52, 3584. (c) Xynas,
R. Capon, R. J . Aust. J . Chem. 1989, 42, 1427. (d) J ames D. M.; Kunze,
H. B.; Faulkner, D. J . J . Nat. Prod. 1991, 54, 1137. (e) Rodriguez, A.
D.; Akee, R. K.; Scheuer, P. J . Tetrahedron Lett. 1987, 28, 4989. (f)
Pakrashi, S. C.; Achari, B.; Dutta, P. K.; Chakrabarti, A. K.; Giri, C.;
Saha, S.; Basa, S. Tetrahedron 1994, 50, 12009.
(9) Moody, K.; Thomson, R. H.; Fattorusso, E.; Minale, L.; Sodano,
G. J . Chem. Soc., Perkin Trans. 1 1972, 18.
(10) (a) Murakata, M.; Yamada, K.; Hoshino, O. Heterocycles 1998,
47, 921. (b) Murakata, M.; Yamada, K.; Hoshino, O. Tetrahedron 1996,
52, 14713. (c) Murakata, M.; Yamada, K.; Hoshino, O. J . Chem. Soc.,
Chem. Commun. 1994, 443. (d) Kacan, M.; Koyuncu, D.; McKillop, A.
J . Chem. Soc., Perkin Trans. 1 1993, 1771. (e) Noda, H.; Niwa, M.;
Yamamura S. Tetrahedron Lett. 1981, 22, 3247. (f) Forrester, A. R.;
Thomson, R. H.; Woo S.-O. J . Chem. Soc., Perkin Trans. 1 1975, 2340.
(g) Forrester, A. R.; Thomson, R. H.; Woo, S.-O. J . Chem. Soc., Perkin
Trans. 1 1975, 2348. (h) Okamoto, K. T.; Clardy, J . Tetrahedron Lett.
1987, 28, 4969.
10.1021/jo010015v CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/30/2001

3112 J . Org. Chem., Vol. 66, No. 9, 2001
Boehlow et al.
Sch em e 1
Sch em e 2a
new methods for the spirocyclization of phenolic oximes
have been reported,10 leading to syntheses of aerothionin
(26a ), homaerothionin (26b), and aerophobin-1 (25).11
More recently, Wasserman et al. showed that cyano
ylides could be employed in a very efficient preparation
of the oximino amides leading to syntheses of veronga-
mine (15) and aerothionin (26a ).12 In addition, total
synthesis of phenolic oxime psammaplin A and the nitrile
aeroplysinin-1 (6) have been published.13
In 1995 we communicated the results of our initial
studies on the oxidation, bromination, and decarboxyla-
tion of tyrosine esters.14 We report herein the application
of this sequence to the synthesis of the marine sponge
metabolite verongamine4 and a new method of pyruvate
oxime preparation using a Wadsworth-Emmons olefi-
nation leading to the first synthesis of purealidin N.6
a
Reagents and conditions: (a) Na2WO4, EtOH, 30% H2O2, 74-
83%; (b) dimethyldioxirane, acetone, 91%; (c) MeReO3, 30% H2O2,
EtOH, 87%; (d) CH3CONHBr, THF, -78 °C, 60%; (e) NBS, DMF,
0 °C, 69%; (f) NBS, DMF, 25 °C, 74%; (g) MeI, Cs2CO3 or K2CO3,
acetone.
suitable laboratory mimic (Scheme 2). Treatment of an
ethanolic solution of tyrosine ethyl ester (7) with sodium
tungstate (1 equiv) and 30% aqueous hydrogen peroxide
(10 equiv) at room temperature for 1 h gave the corre-
sponding oxime (8) in 83% yield as a single geometric
isomer.19 Oxidation of (7) using sodium vanadate in place
of the sodium tungstate was much slower (30 h) and gave
several products from which the oxime (8) was isolated
in 19% yield. However, oxidation of (7) with methyltri-
oxorhenium (MeReO3) in EtOH/H2O2, which has rapidly
emerged as a versatile oxidizing reagent,20 afforded the
corresponding oxime (8) in a high yield (87%).
Oxidation of the amine with dimethyldioxirane was
also investigated. Dimethyldioxirane is a powerful oxi-
dant and has been compared to the monoxygenase
enzymes in its oxidation chemistry.21 Oxidation of pri-
mary amines with excess dimethyldioxirane was reported
to give nitroalkanes in good yield.21b Attempts to limit
oxidation by employing 1 equiv of dimethyldioxirane has
met with some success for hydroxylamine formation;21d
however, 1-2 equiv of dimethyldioxirane generally re-
sulted in the formation of several products including
hydroxylamines, oximes, nitroso dimers, nitroalkanes,
nitrones, and oxaziridines.21f Dimethyldioxirane oxidation
of amines probably proceeds via the hydroxylamine to
the nitroso derivative, which then undergoes a rear-
rangement to give the oxime.21f It was anticipated that
the enhanced acidity of the ester R-proton would facilitate
Resu lts a n d Discu ssion
The oxidation of amines to oximes has been achieved
with several oxidizing agents; however, the yields are
often variable as a result of side reactions, in particular,
overoxidation and nitroso dimer formation.15 It has been
suggested that bromoperoxidase and peroxidase enzymes,
which have been isolated from Verongida,16 may be
responsible for bromination and amine oxidation in the
metabolism of tyrosine. The peroxidase enzymes often
contain transition metal oxides (Fe or V) in the active
site. Indeed, Di Furia et al.17 demonstrated that a model
system consisting of a combination of NH4VO3, H2O2, and
KBr in a two-phase solvent system would brominate
aromatic and olefinic substrates.
We focused our attention on the known18 oxidation of
primary amines with tungstate peroxo complexes as a
(11) (a) Forrester, A. R.; Thomson, R. H.; Woo, S.-O. Liebigs Ann.
1978, 66. (b) Nishiyama, S.; Yamamura, S. Tetrahedron Lett. 1983,
24, 3351 (c) Nishiyama, S.; Yamamura, S. Bull. Chem. Soc. J pn. 1985,
58, 3453. (d) Murakata, M.; Masafumi, T.; Hoshino, O. J . Org. Chem.
1997, 62, 4428.
(12) Wasserman, H. H.; Wang, J . J . Org. Chem. 1998, 63, 5581.
(13) (a) Hoshino, O.; Murakata, M.; Yamada, K. Bioorg. Med. Chem.
Lett. 1992, 2, 1561. (b) Andersen, R. J .; Faulkner, D. J . J . Am. Chem.
Soc. 1975, 97, 936.
(14) Boehlow, T. R.; Spilling, C. D. Nat. Prod. Lett. 1995, 7, 1.
(15) (a) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley
and Sons: New York, 1992; pp 1198-1199. (b) Challis, B. C.; Butler,
A. R. The Chemistry of the Amino Groups; Patai, S., Ed.; Wiley: New
York, 1968; pp 320-338 and references therein.
(19) Boehlow, T. R.; Spilling, C. D. Acta Crystallogr. 1995, C51, 2654.
(20) For a review see: Roma˜o, C. C.; Ku¨hn, F. E.; Hermann, W. A.
Chem. Rev. 1997, 97, 3197.
(21) (a) Murray, R. W. Chem. Rev. 1989, 89, 1187. (b) Murray, R.
W.; Rajadhyaksha, S. N.; Mohan, L. J . Org. Chem. 1989, 54, 5783. (c)
Murray, R. W.; Singh, M.; J eyaraman, R. J . Am. Chem. Soc. 1992,
114, 1346. (d) Murray, R. W.; J eyaraman, R.; Mohan, L. Tetrahedron
Lett. 1986, 27, 2335. (e) Wittman, M. D.; Halcomb, R. L.; Danishefsky,
S. J . J . Org. Chem. 1990, 55, 1981. (f) Crandall, J . K.; Reix, T. J . Org.
Chem. 1992, 57, 6759.
(16) Baden, D. G.; Corbett, M. D. Comp. Biochem. Physiol. 1979,
64B, 279.
(17) Conte, V.; Di Furia, F.; Moro S. Tetrahedron Lett. 1994, 35,
7429.
(18) Kahr, K.; Berther, C. Chem. Ber. 1960, 93, 132.

Synthesis of Verongamine and Purealidin N
J . Org. Chem., Vol. 66, No. 9, 2001 3113
Sch em e 3a
a rapid rearrangement and thus avoid some of the
competing reaction pathways of the nitroso compounds.
Gratifyingly, it was observed that tyrosine ethyl ester (7)
was smoothly oxidized with 2 equiv of dimethyldioxirane
to give the oxime (8) as a single isomer in excellent yield
(91%).
It is conceivable that in the biosynthetic pathway
bromination of the aromatic ring could take place on
either a tyrosine derivative or a corresponding oxime.
Bromination of tyrosine is well precedented;22 however,
the effect of the oxime on the stepwise electrophilic
bromination of the aromatic ring was uncertain and
posed questions about the relative reactivity of these
functional groups. However, it was observed that bromi-
nation could be controlled by judicious choice of solvent,
temperatures and reagent. Addition of 1 equiv of N-
bromoacetamide to (8) in THF at -60 °C gave predomi-
nantly the monobromide (9), 2 equiv of NBS (in DMF or
THF) at 0 °C gave the dibromide (10), and 3 equiv at (0
°C to room temperature) cleanly gave the spiroisoxazoline
(11) (Scheme 2). Interestingly, treatment of oxime (8)
with Di Furia’s bromoperoxidase enzyme mimic (NH4-
VO3, H2O2, and KBr in water/chloroform) resulted in the
formation of both the monobromo (9, 39%) and dibromo
(10, 27%) oximes. However, none of the spiroisoxazoline
(11) was observed showing this to be a mild and selective
brominating system.
Verongamine (15), a histamine H3 receptor antagonist,
was first isolated from the marine sponge Verongula
gigantea4 and has been previously synthesized.12 It
appeared that verongamine (15) could be synthesized by
selective O-methylation of the phenol functionality of
monobromo oxime (9), followed by amidation with his-
tamine. However, methylation of the oximes proved to
be troublesome (Scheme 2). Reaction of the tyrosine
oxime (8) with methyl iodide and cesium carbonate in
refluxing acetone gave a mixture of the two mono
methylated (12a and 12c) and the dimethylated (12b)
products. The oxime methyl ether (12a ) and dimethyl
ether (12b) were the major components, with only trace
amounts of the desired phenyl methyl ether derivative
(12c). Unpredictable results occurred using the mono- (9)
or dibrominated (10) oximes, leading to variable mixtures
of both mono- and dimethylated products. In each case
the products were easily differentiated by IR spectroscopy
with the phenol OH (12a ) stretching frequency appearing
at 3410 cm-1, the oxime OH (12c) stretching frequency
at 3290 cm-1, and the dimethylated compound (12b)
lacking a peak in the OH region.
a
Reagents and conditions: (a) TFA, CH2Cl2 then Na2WO4, 30%
H2O2, EtOH/H2O, 65% (over two steps); (b) histamine, MeOH, 60
°C, 72 h, 98%.
Sch em e 4a
a
Reagents and conditions: (a) KOH, THF; (b) HCl, isolate
(>90%); (c) CF3CO2H, CH2Cl2.
lished 1H and 13C spectra with natural and synthetic
verongamine (15).4,12
Rinehart and Tymiak demonstrated3 that the nitrile
nitrogen in aeroplysinin-1 is derived from the tyrosine
amine nitrogen. It was proposed that nitriles were formed
by the decarboxylation of oximino acids. R-Oximino
carboxylic acids have been reported to undergo decar-
boxylation in aqueous solution at elevated tempera-
tures.25 We believed that these reactions would be
accelerated by acids in nonaqueous solution. Hydrolysis
(KOH, 10% aq THF) of the ester oximes (9) and (10) gave
the corresponding acids in >90% yield (Scheme 4).
Trifluoroacetic acid catalyzed decarboxylation of the acids
in CH2Cl2 at room temperature gave the naturally
occurring nitriles (16) and (17) in 79% and 89% yield,
respectively.7
Synthesis of purealidin N (28) and the spiroisoxazo-
lines such aerothionin (26a ) could not be achieved from
tyrosine since they required a more elaborate oxime with
additional hydroxyl on aromatic ring. As alternative to
the Erlenmeyer condensation, the application of the a
Wadsworth-Emmons olefination using methyl 2-(tert-
butyldimethylsilyloxy)-2-(dimethylphosphono) acetate
(18)26a was investigated. The phosphonate (18), which can
be prepared in large quantities using a two-step pro-
cedure,26a has previously been used in the synthesis of
enol lactams, carbohydrates, and 3-hydroxy pyruvates.26
Wadsworth-Emmons reaction between phosphonate (18)
and an aldehyde results in the formation of an silyleno-
lether, which it appeared would yield a pyruvate on
hydrolysis and thus an oxime upon further reaction with
hydroxylamine.
An alternate route to verongamine (15) was examined.
The known Boc-protected O-methyl bromotyrosine meth-
yl ester (13)23 was deprotected by treatment with CF3-
CO2H in CH2Cl2, and the crude amine was oxidized with
Na2WO4/H2O2 in ethanol (Scheme 3) to the oxime ester
(14). Amidation of oxime ester (14) with histamine at 60
°C in MeOH for 72 h was successful in yielding veron-
gamine (15). When the reaction was conducted in CD3-
OD, partial deuterium exchange occurred at the C-2
position of the imidazole ring.24 The identity of veronga-
mine (15) was established through comparison of pub-
Reaction of 2-hydroxy-4-methoxybenzaldehyde (19a )
with MOMCl and EtNiPr2 in CH2Cl2 gave the protected
(22) Zeynek, R. Physiol. Chem. 1925, 144, 246 and 275.
(23) J ung, M. E.; Rohloff, J . C. J . Org. Chem. 1985, 50, 4909.
(24) (a) Vaughan, J . D.; Mughrabi, Z.; Chung Wu, E. J . Org. Chem.
1970, 35, 1141. (b) Harris, T. M.; Randall, J . C. Chem. Ind. (London)
1965, 1728. (c) Staab, H. A.; Wu, M.-Th.; Mannschreck, A.; Schwalbach,
G. Tetrahedron Lett. 1964, 15, 845.
(25) Ahmad, A.; Spenser, I. D. Can. J . Chem. 1961, 39, 1340.
(26) (a) Nakamura, E. Tetrahedron Lett. 1981, 22, 663. (b) Plainter-
Royon, R.; Anker, D.; Robert-Baudoy, J . J . Carbohydr. Chem. 1991,
10, 239. (c) Pujol, B.; Sabatier, R.; Driguez, P.-A.; Doutheau, A.
Tetrahedron Lett. 1992, 33, 1447.

3114 J . Org. Chem., Vol. 66, No. 9, 2001
Boehlow et al.
Sch em e 5a
a
Reagents and conditions: (a) NBS, DMF, 0 °C; (b) MOMCl, EtNiPr2, THF; (c) 18, LDA, THF; (d) HF‚Et3N, MeOH, silica.
Sch em e 6a
a
Reagents and conditions: (a) HF‚Et3N, MeOH then NH2OH‚HCl; (b) NBS, DMF, 0 °C; (c) TsOH, MeOH, 100%; (d) PSDIB, MeCN,
99%; (e) histamine, 60 °C, MeOH, 96%; (f) TsOH, MeOH, 87%.
aldehyde (20a ) in excellent yield (Scheme 5). Reaction
of the protected aldehyde with methyl 2-(tert-butyldi-
methylsilyloxy)-2-(dimethylphosphono) acetate (18) gave
the silylenolether (21a ) in quantitative yield. Deprotec-
tion of the silylenolether (21a ) with Et3N‚HF in MeOH
proceeded smoothly. However, attempted workup and
purification by SiO2 chromatography gave the butenolide
(22). It is probable that the butenolide is formed by a
homo-aldol condensation of the pyruvate, followed by a
lactonization.27 The structure of the butenolide was
ultimately proved by X-ray crystallography.28
dibromination of (23a ) with NBS in DMF only resulted
in mono-bromination to give (23c) in 87% yield. Under
more forcing conditions multiple compounds were formed.
It was decided to introduce the bromine substituents at
the earliest stage of the synthesis. Bromination of 2-hy-
droxy-4-methoxy-benzaldehyde (19a ) with NBS in DMF
produced the dibromoaldehyde (19b) in a 97% yield.
Again reaction with MOMCl yielded the protected alde-
hyde (20b) in high yield. Reaction of the protected
aldehyde (20b) with phosphonate (18), deprotection of the
silylenolether (21b), and in situ oxime formation pro-
ceeded as before to yield the oxime (23b). The synthesis
of the key oxime (23b) is efficient and scalable (performed
on a 40-g scale) and proceeds with high overall yield (83%
in four steps).
Fortunately, deprotection of the silylenolether (21a )
with Et3N‚HF in MeOH followed by immediate addition
of NH2OH‚HCl and Et3N yielded the required oxime
(23a ) in excellent yield (Scheme 6). However, attempted
The MOM group was removed using a catalytic amount
of TsOH in methanol to give the known phenolic oxime
(23d ) in high overall yield.11 The oxime ester (23d ) was
cyclized with NBS in DMF to give the spiro isoxazoline
(27) Brown, R. F.; J ackson, W. R.; McCarthy, T. D.; Fallon, G. D.
Aust. J . Chem. 1992, 45, 1833.
(28) Boehlow, T. R.; Rath, N. P.; Spilling, C. D. Acta Crystallogr.
1997, C53, 92.

Synthesis of Verongamine and Purealidin N
J . Org. Chem., Vol. 66, No. 9, 2001 3115
formed upon J EOL Mstation (J MS-700) mass spectrometer.
The polymer-supported (diacetoxyiodo)benzene (PSDIB) was
prepared in accordance with Togo et al.29a
(24), which displayed spectral characteristics that com-
pared well with those reported previously.11b In our hands
the isoxazoline (24) was sensitive to chromatography with
any attempted purification resulting in degradation.
Oxidative cyclization using the polymer-supported (di-
acetoxyiodo)benzene (PSDIB) reagent29 in MeCN at room
temperature gave the isoxazoline (24) in excellent yield
and purity. The spirocyclization occurred rapidly, and
workup involved only filtration, thus removing the need
for chromatography. The synthesis of the known spiroisox-
azoline (24) represents a formal synthesis of aerophobin-1
(25), aerothionin (26a ), and homoaerothionin (26b )
through routes developed by Nishiyama et al.11 and
Wasserman.12
Eth yl 3-(4-Hyd r oxyp h en yl)-2(E)-(h yd r oxyim in o)p r o-
p a n oa te (8). Meth od A. Tyrosine ethyl ester (7) (0.250 g, 1.19
mmol) was dissolved in a solution of DMD (35 mL, 0.69 M,
23.8 mmol) in acetone, and the resulting mixture was stirred
until the reaction was complete as indicated by TLC (SiO2,
100% Et2O). The solution was dried over Na2SO4 and filtered,
and the solvent was removed in vacuo. The resulting solid was
recrystallized from toluene to give the title compound (8) (0.24
g, 91%).
Meth od B. To a solution of tyrosine ethyl ester (7) (4.64 g,
22 mmol) in absolute EtOH (50 mL) at 0 °C were added Na2-
WO4 (7.32 g, 22 mmol), H2O2 (30%, 22 mL), and H2O (38 mL).
The resulting mixture was stirred until the reaction was
complete as indicated by TLC (SiO2, hexanes/Et2O/CH2Cl2,
5:5:1). The solution was extracted with EtOAc (3 × 50 mL)
and washed with aqueous NaHSO3 (2 × 50 mL) and H2O (2 ×
50 mL), dried over Na2SO4, and filtered. The solvent was
removed in vacuo to yield a pale yellow solid (8) (3.61 g, 74%)
Meth od C. A solution of MeReO3 (0.02 g, 0.08 mmol) in
H2O2 (1 mL, 30% H2O2) and absolute EtOH (1 mL) was added
to a stirred solution of tyrosine ethyl ester (7) (0.211 g, 1 mmol)
in EtOH (5 mL) until everything dissolved. The resulting
solution was stirred for an additional 15 min, and then the
reaction was diluted with Et2O (50 mL) and washed with H2O
(50 mL) and saturated Na2S2O3 (50 mL). The organic layer
was tested for peroxides (KI paper), dried over Na2SO4, and
filtered, and then the solvent was removed in vacuo to yield
(8) (0.195 g, 87%) as a light yellow solid: mp 140.5-141.5 °C
(toluene); IR (KBr) 3466, 3293, 1720 cm-1; 1H NMR (300 MHz,
(CD3)2CO) δ 7.10 (d, J ) 8.7 Hz, 1H), 6.72 (d, J ) 8.5 Hz, 1H),
4.17 (q, J ) 7.1 Hz, 2H), 3.82 (s, 2H), 1.22 (t, J ) 7.1 Hz, 3H);
13C NMR (75 MHz, (CD3)2CO) 164.5, 156.6, 151.9, 130.8, 128.0,
115.9, 61.7, 30.18, 14.5. Anal. Calcd for C11H13NO4: C, 59.19;
H, 5.83; N, 6.28. Found: C, 59.16; H, 5.88; N, 6.25.
Purealdin N (28) was first isolated from the marine
sponge Psammaplysilla purea6 and was reported to
possess potent antitumor activity (murine lymphoma
L1210 cells IC50 ) 0.07 µg/mL and human epidermoid
carcinoma KB cells IC50 ) 0.074 µg/mL). Purealidin N
(28) was synthesized in a manner analogous to that used
for verongamine (15). The MOM-protected oxime (23b)
was reacted with histamine in MeOH at 60 °C for 72 h
to give the corresponding amide (27) in an excellent yield
of 96%. Simple deprotection of the MOM group with
TsOH in MeOH gave purealidin N (28). This represents
the first total synthesis of purealidin N (28), and was
achieved in a 67% overall yield in six steps from the
commercially available 2-hydroxy-4-methoxybenzalde-
1
hyde (19a ). The structure of (28) was confirmed by H
and 13C NMR spectroscopy (in d6-DMSO and d4-MeOH)
1
and HR-FAB-MS. The H NMR shifts of the imidazole
protons for 28 (δ 7.40 and 8.89 ppm) in DMSO with added
TFA are in agreement with those reported for natural
purealidin N (28).6 Therefore, natural purealidin N (28),
chromatographed with a H2O/CH3CN/CF3CO2H eluent,
is most likely the TFA salt.
Synthetic purealidin N (28) and related oximes (23a ,
23c, and 23d ) were tested against three sensitive cancer
cell lines, lung NCI-H460, breast MCF7, and CNS SF-
268.30 At concentrations of 10-4 M, all of the compounds
were essentially inactive, showing inhibition of cell
growth of between 30% and 93% of the control. This is
in contrast to the high activity previously reported for
purealidin N isolated from natural sources.6
Eth yl 3-(3-Br om o-4-h yd r oxyp h en yl)-2(E)-(h yd r oxyim i-
n o)p r op a n oa te (9). NBA (0.165 g, 11.9 mmol) in THF (5 mL)
was added dropwise to a solution of oxime (8) (0.250 g, 11.9
mmol) in THF (2 mL) over 5 min at -78 °C. The resulting
mixture was stirred until the reaction was complete as
indicated by TLC (SiO2, hexanes/Et2O/CH2Cl2, 5:5:1). The
reaction mixture was diluted with Et2O (50 mL), washed with
H2O (2 × 50 mL) and saturated Na2S2O3 (50 mL), dried over
Na2SO4, and filtered. The solvent was evaporated in vacuo to
yield a yellow solid residue (0.317 g). The residue was
separated by chromatography (SiO2, hexanes/EtOAc, 7:3) and
then recrystallized from toluene to produce a colorless crystal-
line solid (9) (0.197 g, 57%): mp 135.5-136.5 °C; IR (KBr)
1
3435, 3280, 1723 cm-1; H NMR (300 MHz, CDCl3) δ 7.44 (d,
J ) 2.0 Hz, 1H), 7.19 (dd, J ) 8.0, 2.0 Hz, 1H), 6.93 (d, J )
8.0 Hz, 1H), 4.31 (q, J ) 7.0 Hz, 2H), 3.89 (s, 2H), 1.35 (t, J )
7.0 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 163.0, 150.9, 132.5,
130.1, 129.1, 115.9, 110.0, 62.1, 29.4, 14.2. Anal.Calcd for
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions were carried out under
an atmosphere of dry argon in oven-dried glassware unless
otherwise noted. Reaction temperatures were recorded at bath
temperatures. Elemental analyses were determined by Atlan-
tic Microlab, Inc., Norcross, Georgia. Column chromatography
was performed on E. Merck silica gel 60, 230-400 mesh
ASTM. Analytical thin-layer chromatography (TLC) was per-
formed on precoated glass plates (Merck Kieselgel 60 F254 0.25
mm), and visualization was effected by UV irradiation and
vanillin/H2SO4 dip followed by charring. Infrared spectra were
recorded on a Perkin-Elmer 1600 series Fourier Transform
spectrophotometer, and only the principal bands are listed;
nuclear magnetic resonance (NMR) spectra were obtained on
a Varian XL-300 or Bruker ARX-500. Melting points were
determined on a Electrothermal melting point apparatus and
are uncorrected. High-resolution mass spectometry was per-
C
11H12BrNO4: C, 43.73; H, 4.00. Found: C, 43.81; H, 4.04.
Eth yl 3-(3,5-Dibr om o-4-h ydr oxyph en yl)-2(E)-(h ydr oxy-
im in o)p r op a n oa te (10). NBS (0.16 g, 0.92 mmol) was added
portionwise to the oxime (8) (0.104 g, 0.46 mmol) in THF (4
mL) at 0 °C until the reaction was complete as indicated by
TLC (SiO2, hexanes/Et2O/CH2Cl2, 5:5:1). The reaction mixture
was diluted with Et2O (50 mL) and washed with H2O (50 mL)
and saturated Na2S2O3 (2 × 50 mL). The organic layer was
dried over Na2SO4 and filtered, and the solvent was removed
in vacuo. The resulting red solid was purified by chromatog-
raphy (SiO2, hexanes/Et2O 9:1) to yield a colorless solid (10)
(0.123 g, 70%): mp 194.0-194.7 °C; IR (KBr) 3405, 3246, 1726
1
cm-1; H NMR (300 MHz, (CD3)2CO) δ 11.61 (s, 1H), 8.44 (s,
1H), 7.46 (s, 1H), 4.21 (q, J ) 7.1 Hz, 2H), 3.85 (s, 2H), 1.25 (t,
J ) 7.1 Hz, 3H); 13C NMR (75 MHz, (CD3)2CO) δ 162.2, 150.7,
149.9, 133.5, 131.9, 111.1, 61.8, 30.6, 14.4. Anal. Calcd for
(29) (a) Togo, H.; Nogami, G.; Yokoyama, M. Synlett 1998, 534. (b)
Ley, S. V.; Thomas, A. W.; Finch, H. J . Chem. Soc., Perkin Trans. 1
1999, 669.
(30) These compounds were screened through the National Cancer
Institute developmental therapeutics program (www.dtp.nci.nih.gov).
C
10H9Br2NO4: C, 34.68; H, 2.91. Found: C, 34.94; H, 2.95.
Br om in a tion of Tyr osin e Oxim e (8) w ith Di F u r ia ’s
Br om op er oxid a se En zym e Mim ic. A solution of NH4VO3
(0.072 g, 0.61 mmol) and KBr (0.362 g, 3.0 mmol) in H2O (3

3116 J . Org. Chem., Vol. 66, No. 9, 2001
Boehlow et al.
mL) was then added to a solution of (8) (0.138 g, 0.61 mmol)
in CHCl3 (5 mL). Next, 30% aqueous H2O2 (0.122 mL, 1.22
mmol) was added, and the resulting biphasic mixture was
stirred until the reaction was complete as indicated by TLC
(SiO2, hexanes/Et2O/CH2Cl2, 5:5:1). The reaction was diluted
with CHCl3 (25 mL), washed with H2O (2 × 25 mL) and Na2S2-
O4 (2 × 25 mL), dried over Na2SO4, and filtered; the solvent
removed in vacuo to give a yellow solid (0.180 g). Chromatog-
raphy (SiO2, hexanes/Et2O/CH2Cl2, 8:2:1) yielded the mono-
bromotyrosine oxime (9) (0.064 g, 28%) and the dibromo-
tyrosine oxime (10) (0.073 g, 40%).
E t h yl 7,9-Dib r om o-8-oxo-1-a za sp ir o[4,5]-d eca -2,6,9-
tr ien e-3-ca r boxyla te (11). NBS (2.68 g, 15 mmol) in DMF
(30 mL) was added dropwise to oxime (8) (1.05 g, 4.7 mmol)
in DMF (10 mL) at 0 °C over a 15 min period. The reaction
mixture was diluted with Et2O (50 mL), washed with H2O (3
× 50 mL) and saturated Na2S2O3 (2 × 50 mL), dried over Na2-
SO4, and filtered; the solvent was removed in vacuo to yield a
purple solid (3.03 g). Chromatography (SiO2, hexanes/Et2O/
CH2Cl2, 8:2:1) gave a white crystalline solid (11) (1.38 g,
MHz, CDCl3) δ 163.7, 154.8, 150.9, 134.1, 129.4, 129.2, 112.1,
111.7, 56.5, 53.1, 29.6, 14.3; HRMS (EI) calcd for C11H12BrNO4
300.9950, found (M+) 300.9947.
Ver on ga m in e (15).4,12 A solution of oxime (14) (0.045 g,
0.15 mmol) and histamine (0.049 g, 0.45 mmol) in MeOH (1
mL) was heated for 72 h at 60 °C. The solvent was removed,
and the yellow gum obtained was purified by chromatography
(SiO2, CH2Cl2/MeOH, 5:1) to give a colorless solid (0.055 g,
97%): mp 121.5-123.0 °C (CHCl3); IR (KBr) 3408, 2927 cm-1
;
1H NMR (500 MHz, CD3OD) δ 7.62 (s, 1H), 7.48 (d, J ) 1.2
Hz, 1H), 7.25 (dd, J ) 8.4, 1.2 Hz, 1H), 6.95 (d, J ) 8.4 Hz,
1H), 6.85 (s,1H), 3.86 (s, 5H), 3.54 (t, J ) 7.1 Hz, 2H), 2.84 (t,
J ) 7.1 Hz, 2H); 13C NMR (125 MHz, CD3OD) δ 165.8, 156.1,
153.2, 136.2, 134.9, 132.0, 130.6, 118.1, 113.1, 112.3, 56.8, 40.4,
28.8, 27.8; HRMS (FAB, m-nitro benzyl alcohol) calcd for
C
15H18BrN4O3 (M + H)+ 381.0562, found 381.0563.
2-Br om o-4-cya n om eth ylp h en ol (16).7 KOH (0.05 g, 1.46
mmol) was added to a solution of oxime (9) (0.147 g, 0.48 mmol)
in THF/H2O (2 mL, 1:1) at room temperature, and the mixture
was stirred overnight. The solution was acidified with 1 M HCl
(15 mL) and extracted with EtOAc (30 mL). The organic
extract was successively washed with H2O (1 × 25 mL) and
HCl (1 M, 25 mL), dried over Na2SO4, and filtered, and the
solvent was removed in vacuo to yield a white solid (0.131 g,
99%).
78%): mp 170.0-170.1 °C; IR (KBr) 1735, 1680, 1607 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.37 (s, 1H), 4.40 (q, J ) 7.1 Hz,
2H), 3.52 (s, 2H), 1.41 (t, J ) 7.1 Hz, 3H); 13C NMR (75 MHz,-
CDCl3) δ 171.1, 158.9, 151.3, 144.1, 123.6, 86.1, 62.8, 43.0, 14.1.
Anal. Calcd for C11H9Br2NO4: C, 34.83; H, 2.37; N, 3.69.
Found: C, 34.85; H, 2.43; N, 3.65.
The crude solid (0.081 g, 0.295 mmol) was dissolved in CH2-
Cl2 (5 mL), TFA (3 mL) was added, and the resulting solution
was stirred overnight. The reaction mixture was diluted with
H2O (20 mL), neutralized with saturated NaHCO3 (20 mL),
and extracted with EtOAc (2 × 50 mL). The combined organic
extracts were dried over Na2SO4 and filtered, and the solvent
was removed in vacuo to yield a yellow solid (0.058 g).
Chromatography (SiO2, hexanes/Et2O, 8:2) yielded a pale
yellow solid (16) (0.056 g, 89%): mp 118-119 °C (lit.7 117-
118 °C); 1H NMR (300 MHz, (CD3)2CO) δ 7.54 (d, J ) 2.2 Hz,
1H), 7.24 (dd, J ) 8.4, 2.2 Hz, 1H), 7.02 (d, J ) 8.4 Hz, 1H),
3.88 (s, 2H); 13C NMR (75 MHz, (CD3)2CO) δ 154.5, 133.2,
129.1, 124.4, 119.0, 117.5, 110.4, 22.1.
2,6-Dibr om o-4-cya n om eth ylp h en ol (17). KOH (0.372 g,
9.3 mmol) was added to a solution of oxime (10) (0.515 g, 2.30
mmol) in THF/H2O (4 mL, 1:1) at room temperature, and the
resulting mixture was stirred overnight. The solution was
acidified with 1 M HCl (15 mL) and extracted with EtOAc (100
mL). The organic extract was successively washed with H2O
(1 × 75 mL) and HCl (1M, 75 mL), dried over Na2SO4, and
filtered, and the solvent was removed in vacuo to yield a white
solid (0.439 g, 99%).
Meth yla tion of Tyr osin e Oxim e (8). CH3I (0.7 mL, 11.3
mmol) was added to a stirred solution of oxime (7) (2.10 g,
9.41 mmol) and Cs2CO3 (3.06 g, 9.41 mmol) in anhydrous
acetone (70 mL). The mixture was heated at reflux until all
the starting material had been consumed as indicated by TLC
(SiO2, 100% Et2O) at which point the solvent was removed in
vacuo to give a brown gum. The gum was dissolved in EtOAc
(100 mL), washed successively with H2O (100 mL), HCl (1M,
2 × 100 mL), and H2O (100 mL), dried over Na2SO4, and
filtered; solvent was removed in vacuo to give a brown gum
(1.4 g). The mixture was separated by chromatography (SiO2,
hexanes/EtOAc, 3:1) to yield 12a (0.56 g, 20%) as a colorless
crystalline solid: mp 106.5-107.5 °C (CHCl3); IR (CHCl3
slurry) 3410, 2981, 1716, 1613 cm-1 1H NMR (300 MHz,
;
CDCl3) δ 7.11 (d, J ) 8.3 Hz, 2H), 6.76 (d, J ) 8.3 Hz, 2H),
4.28 (q, J ) 7.1 Hz, 2H), 4.11 (s, 3H), 3.88 (s, 2H), 1.29 (t, J )
7.1 Hz); 13C NMR (75 MHz, CDCl3) δ 163.6, 154.7, 151.1, 130.2,
127.4, 115.5, 63.4, 62.3, 30.5, 14.; HRMS (EI) calcd for C12H15
-
NO4 237.1001, found (M+) 237.0998. Also obtained was 12b
1
(0.41 g, 14%) as an orange oil: IR (neat) 2939, 1716 cm-1; H
NMR (300 MHz, CDCl3) δ 7.20 (d, J ) 8.8 Hz, 2H), 6.82 (d, J
) 8.8 Hz, 2H), 3.28 (q, J ) 7.1 Hz, 2H), 4.12 (s, 3H), 3.90 (s,
2H), 3.79 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 163.7, 158.2,
151.0, 130.0, 127.9, 114.1, 113.9, 63.3, 61.9, 55.3, 30.5, 14.3;
HRMS (EI) calcd for C13H17NO4 251.1158, found (M+) 251.1153.
Meth yl 3-(5-Br om o-4m eth oxyp h en yl)-2(E)-(h yd r oxy-
im in o)p r op a n oa te (14). TFA (0.048 mL, 0.62 mmol) was
added dropwise to the Boc-protected tyrosine derivative (13)23
(0.2 g, 0.52 mmol) in CH2Cl2 (3 mL). After the addition was
complete, the mixture was stirred for 3 h. The solvent was
removed in vacuo, and the residue was dissolved in EtOAc (50
mL). The EtOAc solution was washed with saturated NaHCO3
(2 × 50 mL) and saturated NaCl (50 mL), dried over Na2SO4,
and filtered, and the solvent was removed in vacuo to give an
orange gum (0.150 g). The crude amine was dissolved in
absolute EtOH (5 mL). Na2WO4 (0.172 g, 0.52 mmol) and then
30% aqueous H2O2 (5.2 mL, 52 mmol) were added, and the
mixture was stirred until reaction was complete as indicated
by TLC (SiO2, hexanes/Et2O/CH2Cl2, 5:5:1). The reaction
mixture was extracted with EtOAc (2 × 25 mL), and the
combined EtOAc extracts were washed with NaHSO3 (25 mL)
and H2O (25 mL), dried over Na2SO4 and filtered. The solvent
was removed in vacuo to yield a pale yellow solid (0.201 g).
Chromatography (SiO2, hexanes/Et2O/CH2Cl2, 10:5:1) gave the
title oxime (14) (0.102 g, 65%) as a colorless solid: mp 103.5-
104.5 °C (CHCl3); IR (CHCl3) 3287, 2952, 2848, 1721, 1602
cm-1; 1H NMR (300 MHz, CDCl3) δ 9.55 (br. s, 1H), 7.55 (d, J
) 2.2 Hz, 1H), 7.26 (dd, J ) 8.4, 2.2 Hz, 1H), 6.83 (d, J ) 8.4
Hz, 1H), 3.94 (s, 2H), 3.90 (s, 3H), 3.89 (s, 3H); 13C NMR (75
The crude acid (0.207 g, 0.54 mmol) was dissolved in CH2-
Cl2 (2 mL), and TFA (1 mL) was added dropwise. The resulting
solution was stirred until the reaction was complete as
indicated by TLC (SiO2, Et2O). The reaction was diluted with
H2O (20 mL), neutralized with saturated NaHCO3 (20 mL),
and extracted with EtOAc (2 × 50 mL). The combined organic
extracts were dried over Na2SO4 and filtered, and the solvent
was removed in vacuo to yield a yellow solid. Chromatography
(SiO2, hexanes/Et2O, 7:3) yielded a pale yellow solid (17) (0.161
1
g, 79%): mp 176.5-176.5 °C; IR (KBr) 3368, 2262 cm-1; H
NMR (300 MHz, (CD3)2CO) δ 7.60 (s, 2H), 3.94 (s, 2H); 13C
NMR (75 MHz, (CD3)2CO) δ 151.1, 132.8, 126.5, 118.6, 111.6,
21.0. Anal. Calcd for C8H5Br2NO: C, 33.03; H, 1.73. Found:
C, 32.85; H, 1.75.
3,5-Dibr om o-2-h yd r oxy-4-m eth oxyben za ld eh yd e (19b).
A solution of NBS (6.20 g, 34.8 mmol) in DMF (5 mL) was
added to a solution of 2-hydroxy-4-methoxybenzaldehyde (19a )
(2.41 g, 15.8 mmol) in DMF (5 mL) over 15 min at 0 °C. When
the addition was complete, the reaction mixture was diluted
with Et2O (200 mL), washed with H2O (3 × 50 mL) and
Na2S2O3 (3 × 50 mL), dried over Na2SO4, and filtered, and
the solvent was removed in vacuo to yield a pale yellow solid
(4.88 g). Recrystallization from hexanes/Et2O yielded a white
fibrous solid (4.76 g, 97%): mp 100-100.5 °C; IR (KBr) 3052,
2988, 2943, 1654, 1608 cm-1 1H NMR (300 MHz, CDCl3) δ
;
11.78 (s, 1H), 9.79 (s, 1H), 7.78 (s, 1H), 4.01 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 193.9, 161.0, 159.4, 136.2, 118.6, 107.9,

Synthesis of Verongamine and Purealidin N
J . Org. Chem., Vol. 66, No. 9, 2001 3117
107.8, 61.0. Anal. Calcd for C8H6Br2O3: C, 31.18; H, 1.96.
Found: C, 31.27; H, 2.01.
was removed in vacuo to yield a red oil (2.57 g). Purification
by chromatography (SiO2, hexanes/Et2O, 9:1) yielded a clear
1
2-(O-Met h oxym et h ylen oxy)-4-m et h oxyb en za ld eh yd e
(20a ). To a solution of 2-hydroxy-4-methoxybenzaldehyde (19a )
(1.05 g, 6.9 mmol) and EtNiPr2 (1.16 g, 8.9 mmol) in THF (5
mL) at 0 °C was added MOMCl (0.712 g, 8.9 mmol) over a 5
min period. The solution was stirred overnight, then diluted
with Et2O (75 mL), washed with H2O (2 × 50 mL) and HCl (1
M, 2 × 50 mL), dried over Na2SO4, and filtered, and the solvent
was removed in vacuo to yield a pale yellow liquid. Chroma-
tography (SiO2, hexanes/Et2O, 8:2) yielded the pure aldehyde
(20a ) as a colorless solid (84%): mp 62-62.5 °C; IR (KBr) 2940,
oil (21b) (1.95 g, 92%): IR (neat) 2930, 1732, 1634 cm-1; H
NMR (300 MHz, CDCl3) δ 7.22 (s, 1H), 6.19 (s, 1H), 4.91 (s,
2H), 3.75 (s, 3H), 3.53 (s, 3H), 3.47 (s, 3H), 0.84 (s, 9H), 0.09
(s, 6H); 13C NMR, (75 MHz, CDCl3) δ 164.5, 153.9, 152.4, 143.4,
132.6, 127.6, 114.2, 114.0, 112.1, 99.6, 60.5, 58.1, 51.8, 25.5,
18.2. Anal. Calcd for C19H28Br2O6Si : C, 42.38; H, 5.25.
Found: C, 42.29; H, 5.22.
Methyl 4-Hydroxy-3-{4-methoxy-2-(methoxymethylenoxy)-
ph en yl}-2-[{4-m eth oxy-2-(m eth oxym eth ylen oxy)p h en yl}-
m eth yl]-5-oxo-2,5-d ih yd r ofu r a n -2-ca r boxyla te (22).28 To
solution of silylenolether (21a ) (0.62 g,1.6 mmol) in MeOH (10
mL) was added Et3N‚HF (1 mL). When the reaction was
complete as indicated by TLC (hexanes/Et2O/CH2Cl2, 5:5:1),
the solvent was removed in vacuo. The resulting white solid
was dissolved in CH2Cl2 (75 mL), washed with H2O (2 × 50
mL) and NaHCO3 (saturated, 50 mL), dried over Na2SO4, and
filtered, and the solvent was removed in vacuo. The residue
was purified by chromatography (SiO2, hexanes/Et2O, 1:1) to
yield a colorless solid which crystallized directly from the
eluent (0.41 g, 95%): mp 152.5-154.0 °C; IR (KBr) 3314, 2943,
2846, 2759, 1684 1601 cm-1 1H NMR (300 MHz, CDCl3) δ
;
10.34 (s, 1H), 7.82 (d, 1H, J ) 9.0 Hz), 6.72 (d, 1H, J ) 2.3
Hz), 6.62 (dd, 1H, J ) 9.0, 2.3 Hz), 5.30 (s, 2H), 3.87 (s, 3H),
3.54 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 187.8, 165.6, 161.2,
129.9, 119.2, 107.5, 100.4, 94.4, 56.3, 55.5. Anal. Calcd for
C10H12O3: C, 61.20; H, 6.17. Found: C, 60.95; H, 6.16.
3,5-Dib r om o-2-(O-m et h oxym et h ylen oxy)-4-m et h oxy-
ben za ld eh yd e (20b). To a solution of dibromo aldehyde (19b)
(2.06 g, 6.6 mmol) and EtNiPr2 (1.28 g, 9.9 mmol) in THF (5
mL) at 0 °C was added MOMCl (0.80 g, 9.9 mmol) over a 5
min period. The solution was stirred overnight, then diluted
with Et2O (75 mL), washed with H2O (2 × 50 mL) and HCl
(1M, 2 × 50 mL), dried over Na2SO4, and the solvent was
removed in vacuo to yield a pale yellow solid (20b) (2.3 g,
99%): mp 79.9-80.1 °C; IR (KBr) 3000, 2949, 2889, 1734, 1684
cm-1; 1H NMR (300 MHz, CDCl3) δ 10.22 (s, 1H), 8.06 (s, 1H),
5.21 (s, 2H), 3.98 (s, 3H), 3.64 (s, 3H); 13C NMR (75 MHz,
CDCl3) δ 187.5, 159.8, 157.9, 131.3, 128.3, 115.0, 114.4, 101.0,
60.8, 58.3. Anal. Calcd for C10H10Br2O4: C, 34.10; H, 2.86.
Found: C, 33.99; H, 2.80.
1
1728, 1610 cm-1; H NMR (300 MHz, CDCl3) δ 7.58 (d, J )
8.9 Hz, 1H), 6.87 (d, J ) 2.5 Hz, 1H), 6.82 (d, J ) 8.4 Hz, 1H),
6.86 (dd, J ) 8.9, 2.6 Hz, 1H), 6.56 (d, J ) 2.4 Hz, 1H), 6.38
(s, 1H), 6.36 (dd, J ) 8.9, 2.5 Hz, 1H), 5.16 (d, J ) 6.9 Hz,
1H), 5.08 (d, J ) 6.9 Hz, 1H), 4.83 (d, J ) 6.9 Hz, 1H), 4.53 (d,
J ) 6.9 Hz, 1H), 3.88 (s, 3H), 3.79 (s, 3H), 3.75 (s, 3H), 3.71
(d, J ) 14.8 Hz, 1H), 3.53 (s, 3H), 3.50 (d, J ) 14.7 Hz, 1H),
3.35 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 169.0, 168.3, 161.5,
159.7, 156.8, 155.1, 138.2, 132.0, 130.9, 126.7, 114.5, 112.6,
107.7, 105.9, 101.8, 101.0, 94.9, 94.4, 87.5, 56.7, 55.8, 55.6, 55.3,
53.0, 31.0.
Met h yl 2-{(1,1d im et h ylet h yl)d im et h ylsilyloxy}-3-{4-
m e t h oxy-2-(O-m e t h oxym e t h yle n oxy)p h e n yl}p r op -2-
en oa te (21a ). To a solution of diisopropylamine (3.79 g, 37.5
mmol) in THF (25 mL) at -78 °C was added n-BuLi (15 mL,
2.5 M, 37.5 mmol), and the resulting mixture was stirred for
1 h, allowed to warm to room temperature, and then recooled
to -78 °C. Methyl 2-(tert-butyldimethylsilyloxy)-2-(dimeth-
ylphosphono) acetate (18)26 (5.00 g, 25 mmol) in THF (10 mL)
was added over 10 min. The reaction mixture was stirred for
an additional 30 min and then a solution of the aldehyde (20a )
(8.6 g, 27.5 mmol) in THF (25 mL) was added. The mixture
was stirred until the reaction was complete as indicated by
TLC (SiO2, hexanes/Et2O/CH2Cl2, 5:5:1). The solution was
allowed to warm to room temperature, quenched with satu-
rated NH4Cl (20 mL), and diluted with EtOAc (200 mL). The
EtOAc solution was washed with saturated NH4Cl (3 × 100
mL) and H2O (3 × 100 mL), dried over Na2SO4, and filtered,
and the solvent evaporated in vacuo to yield a red oil (10.19
g). Chromatography (SiO2, hexanes/Et2O, 9:1) yielded a pale
yellow oil (21a ) (7.71 g, 99%): IR (CDCl3) 2952, 2857, 1732,
Meth yl 2(E)-(Hyd r oxyim in o)-3-(4-m eth oxy-2-O-m eth -
oxym eth ylen oxyp h en yl)p r op a n oa te (23a ). To a solution
of the silylenolether (21a ) (5.64 g,14.7 mmol) in MeOH (20 mL)
was added Et3N‚HF (2 mL). When the starting material had
been consumed as indicated by TLC (SiO2, hexanes/Et2O/CH2-
Cl2, 5:5:1), NH2OH‚HCl (1.22 g, 17.6 mmol) was added and
stirring was continued. When complete reaction was observed
TLC (SiO2, Et2O), the solvent was removed in vacuo. The
resulting solid was dissolved in CH2Cl2 (75 mL), washed with
H2O (2 × 50 mL) and NaHCO3 (saturated, 50 mL), dried over
Na2SO4, and filtered, and the solvent was removed in vacuo
to give a yellow solid (4.16 g, 99%): mp 68.5-69.5 °C; IR (KBr)
3314, 2949, 2837, 1726 cm-1 1H NMR (300 MHz, CDCl3) δ
;
7.05 (d, J ) 8.3 Hz, 1H), 6.67 (d, J ) 2.4 Hz, 1H), 6.47 (dd, J
) 2.4, 8.3 Hz, 1H), 5.16 (s, 2H), 3.93 (s, 2H), 3.81 (s, 3H), 3.47
(s, 3H); 13C NMR (75 MHz, CDCl3) δ 163.8, 159.2, 155.4, 151.1,
130.0, 116.8, 105.1, 101.0, 94.2, 55.9, 55.2, 52.5, 25.1. Anal.
Calcd for C13H17NO6: C, 55.10; H, 6.05. Found: C, 55.05; H,
6.04.
1
1609 cm-1; H NMR (300 MHz, CDCl3) δ 7.18 (d, J ) 8.4 Hz,
Meth yl 3-{3,5-Dibr om o-4-m eth oxy-2-(O-m eth oxym eth -
ylen oxy)p h en yl}2(E)-(h yd r oxyim in o)p r op a n oa te (23b).
To a solution of silylenolether (21b) (1.94 g, 3.66 mmol) in
MeOH (5 mL) was added Et3N‚HF (1 mL). When the starting
material had been consumed as indicated by TLC (SiO2,
hexanes/Et2O/ CH2Cl2, 5:5:1), NH2OH‚HCl (0.305 g, 4.39 mmol)
was added and stirring was continued. When the reaction was
complete as indicated by TLC (SiO2, Et2O), the solvent was
removed in vacuo. The resulting white solid was dissolved in
EtOAc (75 mL), washed with H2O (2 × 50 mL) and saturated
NaHCO3 (50 mL), dried over Na2SO4, and filtered, and the
solvent was removed in vacuo to give a colorless solid (1.46 g,
90%): mp 138-139 °C (MeOH); IR (CDCl3) 3350, 2954, 1732,
1684 cm-1; 1H NMR (500 MHz, CDCl3) δ 8.52 (br. s, 1H), 7.21
(s, 1H), 5.15 (s, 2H), 4.06 (s, 2H), 3.86 (s, 3H), 3.85 (s, 3H),
3.66 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 163.6, 153.9, 153.4,
150.5, 131.6, 128.3, 114.6, 113.1, 99.9, 60.6, 58.1, 53.0, 25.3;
HRMS (FAB, m-nitrobenzyl alcohol) calcd for C13H16BrNO6 (M
+ H)+ 441.9344, found 441.9315. Anal. Calcd for C13H15Br2-
NO5: C, 35.54; H, 3.44. Found: C, 35.36; H, 3.36.
1H), 6.68 (d, J ) 2.4 Hz, 1H), 6.52 (dd, J ) 8.4, 2.4 Hz, 1H),
6.45 (s, 1H), 5.16 (s, 2H), 3.80 (s, 3H), 3.66 (s, 3H), 3.47 (s,
3H), 1.01 (s, 9H), 0.24 (s, 6H);13C NMR (75 MHz, CDCl3) δ
165.4, 160.1, 155.3, 140.8, 130.6, 116.9, 106.0, 101.1, 94.6, 56.0,
55.3, 51.4, 25.7, 18.3. Anal. Calcd for C19H30O6Si : C, 59.66;
H, 7.91. Found: C, 59.64; H, 7.93.
Meth yl 2-{(1,1d im eth yleth yl)d im eth ylsilyloxy}-3-{3,5-
d ibr om o-4-m eth oxy-2-(O-m eth oxym eth ylen oxy)p h en yl}-
p r op -2-en oa te (21b). To a solution of diisopropylamine (0.497
g, 4.9 mmol) in THF (5 mL) at -78 °C was added n-BuLi (2.5
M, mL, 4.9 mmol), and the resulting solution was stirred for
1 h. A solution of methyl 2-(tert-butyldimethylsilyloxy)-2-
(dimethylphosphono) acetate (18)26 (1.53 g, 4.9 mmol) in THF
(10 mL) was added via syringe over 10 min, and then the
reaction mixture was stirred for a further 30 min. A solution
of the dibromobenzaldehyde (20b) (1.45 g, 4.09 mmol) in THF
(10 mL) was added over 10 min, and the mixture was stirred
until the reaction was complete as indicated by TLC (SiO2,
hexanes/Et2O/CH2Cl2, 5:5:1). The reaction mixture was
quenched with saturated NH4Cl (20 mL), diluted with EtOAc
(75 mL), washed with H2O (3 × 50 mL) and saturated NH4Cl
(3 × 50 mL), dried over Na2SO4, and filtered, and the solvent
Met h yl 3-{5-Br om o-4-m et h oxy-2-(O-m et h oxym et h yl-
en oxy)p h en yl}-2(E)-(h yd r oxyim in o)p r op a n oa te (23c). To
a solution of (23a ) (0.254 g, 0.67 mmol) in DMF (2 mL) was

3118 J . Org. Chem., Vol. 66, No. 9, 2001
Boehlow et al.
added a solution of NBS (0.239 g, 1.3 mmol) in DMF (4 mL)
dropwise. The mixture was stirred until the reaction was
complete as indicated by TLC (SiO2, Et2O). It was then diluted
with Et2O (50 mL), washed with H2O (50 mL) and saturated
Na2S2O3 (50 mL), dried over Na2SO4, and filtered, and the
solvent was removed in vacuo to yield a deep purple oil (0.408
g). Purification by chromatography (SiO2, hexane/Ether, 8:3)
yielded a colorless solid (23c) (0.257 g, 87%): mp 108.8-109
(SiO2, solvent gradient 100% CH2Cl2 to 100% MeOH) to give
MOM-protected purealidin (27) as a colorless solid (0.113 g,
96%): mp 154.5-155.5 °C (MeOH), IR (KBr) 3395, 3219, 2938,
1652; 1H NMR (500 MHz, CD3OD) δ 7.60 (s, 1H), 7.18 (s, 1H),
6.85 (s, 1H), 5.13 (s, 2H), 3.97 (s, 2H), 3.81 (s, 3H), 3.63 (s,
2H), 3.51 (t, J ) 7.1 Hz, 2H), 2.82 (t, J ) 7.1 Hz, 2H); 13C
NMR (125 MHz, CD3OD) δ 165.7, 155.0, 154.9, 152.5, 136.2,
136.0, 132.9, 131.1, 118.0, 115.5, 113.8, 101.2, 61.2, 58.6, 40.5,
27.8, 25.2; HRMS (FAB, m-nitrobenzyl alcohol) calcd for
°C; IR (KBr) 3285, 2955, 1734 cm-1 1H NMR (300 MHz,
;
C
17H21Br2N4O5 (M + H)+ 520.9878, found 520.9844. A solution
CDCl3) δ 7.31 (s, 1H), 6.74 (s, 1H), 5.19 (s, 2H), 3.91 (s, 2H),
3.87 (s, 3H), 3.85 (s, 3H), 3.50 (s, 3H); 13C NMR (300 MHz,
CDCl3) δ 163.7, 155.1, 154.9, 150.7, 133.4, 118.2, 102.9, 99.5,
94.6, 56.3, 56.1, 52.7, 24.8. Anal. Calcd for C13H15Br2NO6: C,
43.11; H, 4.47. Found: C, 43.17; H, 4.43.
of the MOM-protected purealidin N (27) (0.1 g, 0.19 mmol)
and TsOH (0.005 g, 0.03 mmol) in MeOH (2 mL) was stirred
overnight at room temperature. The methanol was removed
in vacuo, and the residue was purified by chromatography
(SiO2, solvent gradient 100% CH2Cl2 to 100% MeOH) to give
purealidin N (28) (0.078 g, 87%) as a colorless hygroscopic
powder: mp 127.5-129 °C (MeOH), IR (KBr) 3396, 3226, 2930,
Meth yl 3-(3,5-Dibr om o-2-h yd r oxy-4-m eth oxyp h en yl)-
2(E)-(h yd r oxyim in o)p r op a n oa te (23d ).10,11 To a solution of
MOM ether (23b ) (1.03 g, 2.3 mmol) in MeOH (5 mL) was
added TsOH (4 crystals), and the resulting solution was stirred
overnight. The MeOH was removed in vacuo, and the resulting
solid was redissolved in EtOAc (50 mL), washed with H2O (25
mL), dried over Na2SO4, and filtered. The solvent was removed
in vacuo to yield a colorless solid (0.920 g, 99%): mp 146-147
°C (lit.11b 148-149 °C); IR (KBr) 3510, 3302, 3063, 2954, 1732
1
2869, 1650; H NMR (500 MHz, (CD3)2SO) δ 8.54 (t, J ) 5.7
Hz, 1H), 7.58 (s, 1H), 7.30 (s, 1H), 6.84 (s, 1H), 3.73 (s, 3H),
3.70 (s, 2H), 3.39 (dt, J ) 5.7, 7.1 Hz, 2H), 2.50 (t, J ) 7.1 Hz,
2H);1H NMR (500 MHz, CD3OD) δ 7.60 (s, 1H), 7.42 (s, 1H),
6.85 (s, 1H), 3.80 (s, 3H), 3.78 (s, 2H), 3.51 (t, J ) 7.1 Hz, 2H),
2.80 (t, J ) 7.1 Hz, 2H); 13C NMR (125 MHz (CD3)2SO) δ 164.6,
153.4, 152.8, 150.6, 132.6, 122.1, 107.7, 105.1, 60.1, 39.4
(masked by DMSO, visible in DEPT135), 26.5, 25.1;13C NMR
(125 MHz, CD3OD) δ 167.0, 155.1, 154.7, 151.7, 136.1, 134.9,
134.8, 123.0, 117.9, 109.0, 107.4, 60.9, 40.7, 27.7, 25.8; HRMS
(FAB, TFA) calcd for C15H16Br2N4O2 (M + H)+ 476.9596, found
476.9600.
1
cm-1; H NMR (300 MHz, CDCl3) δ 7.40 (s, 1H), 3.93 (s, 2H),
3.90 (s, 3H), 3.86 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 164.3,
153.6, 151.4, 149.8, 133.3, 119.3, 107.6, 60.6, 53.5, 25.6. Anal.
Calcd for C11H11Br2NO5: C, 33.43; H, 2.81. Found: C, 33.59;
H, 2.77.
Meth yl 7,9-Dibr om o-8-m eth oxy-6-oxo-1-oxa-2-azaspir o-
[4,5]d eca -2,7,9-tr ien e-3-ca r boxyla te (24).10,11 Meth od A. To
a room-temperature solution of phenolic oxime (23d ) (0.104
g, 0.26 mmol) in DMF (1 mL) was added NBS (0.056 g, 0.31
mmol). When the reaction was complete as indicated TLC
(SiO2, Et2O), the mixture was diluted with Et2O (2 × 50 mL),
washed with H2O (2 × 50 mL) and saturated NaHSO3 (2 × 50
mL), dried over Na2SO4, and filtered, and the solvent was
removed in vacuo to give the crude spiroisoxazoline (24) (0.096
g, 93%). The spectral properties were identical to those
reported by Nishiyama.11b
Ack n ow led gm en t. We thank the University of
Missouri Research Board for their financial support of
this project and the University of Missouri-St. Louis
Graduate School for a Summer Research Fellowship for
T.R.B.. We are grateful to the National Science Founda-
tion, the U.S. Department of Energy, and the University
of Missouri Research Board for grants to purchase the
NMR spectrometers (CHE-856671, CHE-9318696, DE-
FG02-92-CH10499) and the mass spectrometer (CHE-
9708640) used in this work. We thank Dr. R. E. K.
Winter and Mr. J . Kramer for assistance in obtaining
mass spectral data.
Meth od B. PSDIB (0.5 g, 3.5 mmol g-1, 1.75 mmol) was
stirred in MeCN (10 mL) and allowed to swell. A solution of
the phenolic oxime (23d ) (0.2 g, 0.51 mmol) in MeCN (1 mL)
was added via syringe to the swelled polymer, and the mixture
was stirred for 1 h. The polymer was removed by filtration,
washing with additional MeCN (3 × 25 mL). The solvent was
removed in vacuo to give a colorless solid (24) (0.198 g, 99%).
The spectral properties were identical to those reported by
Nishiyama.11b 1H NMR (300 MHz, CDCl3) δ 6.78 (s, 1H), 4.18
(s, 3H), 3.91 (s, 3H), 3.64 (d, J ) 17.8 Hz, 1H), 3.34 (d, J )
17.8 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 188.7, 163.3, 159.7,
150.0, 136.0, 121.0, 106.7, 87.0, 62.4, 53.4, 60.1, 44.6.
P u r ea lid in N (28).6 A solution of the oxime (25b) (0.1 g,
0.23 mmol) and histamine (0.1 g, 0.90 mmol) in MeOH (1 mL)
were heated for 72 h at 60 °C. The solvent was removed, and
the yellow gum obtained was purified by chromatography
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra for verongamine (15), purealidin N (28), and spiroisox-
azoline (24); a comparison of 1H and 13C NMR data for natural
and synthetic verongamine and purealidin N; a HPLC chro-
matogram for compound 24; and National Cancer Institute
biological screening data for compounds 23a , 23c, 23d , and
28. This material is available free of charge via the Internet
at http://pubs.acs.org.
J O010015V