Trapping Phosphodiester - Quinone Methide Adducts through in Situ Lactonization

Article abstract of DOI:10.1021/jo9915168

The goal of in situ modification of DNA via phosphodiester alkylation has led to our design of quinone methide derivatives capable of alkylating dialkyl phosphates. A series of catechol derivatives were investigated to trap the phosphodiester-quinone methide alkylation adduct through in situ lactonization. The catechol derivatives were uniquely capable of characterizable p-quinone methide formation for mechanistic clarity. These investigations revealed that with a highly reactive lactonization group (phenyl ester), lactonization competed with quinone methide formation. Lactone-forming groups of lower reactivity (methyl ester, n-propyl ester, and dimethyl amide) allowed quinone methide formation followed by phosphodiester alkylation; however, they were ineffective at in situ lactonization to drain the phosphodiester alkylation equilibrium to the desired phosphotriester product. The derivatives tethered with lactone-forming functionality of intermediate reactivity (chloro-, trichloro-, and trifluoroethyl esters), allowed quinone methide formation, phosphodiester alkylation, and in situ lactonization to efficiently afford the trapped phosphotriester adduct.

Full text of DOI:10.1021/jo9915168

2022
J . Org. Chem. 2000, 65, 2022-2029
Tr a p p in g P h osp h od iester -Qu in on e Meth id e Ad d u cts th r ou gh in
Situ La cton iza tion
Qibing Zhou and Kenneth D. Turnbull*
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas, 72701
Received September 27, 1999
The goal of in situ modification of DNA via phosphodiester alkylation has led to our design of
quinone methide derivatives capable of alkylating dialkyl phosphates. A series of catechol derivatives
were investigated to trap the phosphodiester-quinone methide alkylation adduct through in situ
lactonization. The catechol derivatives were uniquely capable of characterizable p-quinone methide
formation for mechanistic clarity. These investigations revealed that with a highly reactive
lactonization group (phenyl ester), lactonization competed with quinone methide formation. Lactone-
forming groups of lower reactivity (methyl ester, n-propyl ester, and dimethyl amide) allowed
quinone methide formation followed by phosphodiester alkylation; however, they were ineffective
at in situ lactonization to drain the phosphodiester alkylation equilibrium to the desired
phosphotriester product. The derivatives tethered with lactone-forming functionality of intermediate
reactivity (chloro-, trichloro-, and trifluoroethyl esters), allowed quinone methide formation,
phosphodiester alkylation, and in situ lactonization to efficiently afford the trapped phosphotriester
adduct.
Sch em e 1
In tr od u ction
Quinone methides have been studied in an array of
alkylation and biomolecular alkylation processes.1,2 In-
vestigations of nucleic acid alkylation have focused on
the reaction of nucleobases with various quinone me-
thides including: (i) simple quinone methides with
minimal functionalization,3,4 (ii) quinone methides as
active intermediates related to natural products,5,6 and
(iii) quinone methides generated in situ from de novo
designed precursors.7 Most of these alkylation studies
required that the initial unstable nucleobase-quinone
methide adducts undergo oxidation to a quinone de-
rivative5a-c;6a-c;7a-c or be trapped as an acetate5e-g to
facilitate full characterization.
Previous investigations in our laboratories revealed
that the facile alkylation of phosphodiesters with p-
quinone methide 1 was promoted by a Brønsted acid
(Scheme 1).8 In situ modification of DNA might be readily
achieved through trialkyl phosphate formation,9 although
this has proven quite challenging by reported ap-
proaches.10 Further development of a reagent for the
selective alkylation of phosphodiesters has led us to study
derivatives designed to stabilize the phosphotriester
(1) For reviews on quinone methides, see: (a) Wan, P.; Barker, B.;
Diao, L.; Fischer, M.; Shi, Y.; Yang, C. Can. J . Chem. 1996, 74, 465-
475. (b) Volod′kin, A. A.; Ershov, V. V. Russian Chem. Rev. 1988, 57,
336-349. (c) Gru¨nanger, P. In Methoden der Organisch Chemie;
Muller, E., Bayer, O., Eds.; Thieme G. Verlag: Stuttgart, 1979;
Houben-Weyl, Vol. VII/3b, pp 395-521. (d) Wagner, H. U.; Gompper,
R. In The Chemistry of Quinonoid Compound; Patai, S., Ed.; J ohn
Wiley & Sons; New York, 1974; Vol. 1, pp 1145-1178. (e) Turner, A.
B. Quart. Rev. 1964, 18, 347-360.
(2) For reviews covering bioalkylations with quinone methides,
see: (a) Peter, M. G. Angew. Chem., Int. Ed. Engl. 1989, 28, 555-570.
(b) Moore, H. W.; Czerniak, R. Med. Res. Rev. 1981, 1, 249-280. (c)
Moore, H. W. Science 1977, 197, 527-532.
(3) (a) Rokita, S. E.; Yang, J .; Pande, P.; Greenberg, W. A. J . Org.
Chem. 1997, 62, 3010-3012. (b) Zheng, Q.; Rokita, S. E. J . Org. Chem.
1996, 61, 9080-9081. (c) Lewis, M. K.; Yoerg, D. G.; Bolton, J . L.;
Thompson, J . A. Chem. Res. Toxical. 1996, 9, 1368-1374.
(4) For a recent, detailed analysis of nucleobase alkylation with an
o-quinone methide and thorough overview of nucleobase alkylation in
general, see: Pande, P.; Shearer, J .; Yang, J .; Greenberg, W. A.; Rokita,
S. E. J . Am. Chem. Soc. 1999, 121, 6773-6779.
(5) Some examples in which the nucleic acid base-quinone methide
adducts were fully characterized: (a) Ouyang, A.; Skibo, E. B. J . Org.
Chem. 1998, 63, 1893-1900. (b) Egholm, M.; Koch, T. H. J . Am. Chem.
Soc. 1989, 111, 8291-8293. (c) Tomasz, M.; Chowdary, D.; Lipman,
R.; Shimotakahara, S.; Veiro, D.; Walker, V.; Verdine, G. L. Proc. Natl.
Acad. Sci. U.S.A. 1986, 83, 6702-6706. (d) Marques, M. M.; Beland,
F. A. Carcinogenesis 1997, 18, 1949-1954. (e) Angle, S. R.; Rainer, J .
D.; Woytowicz, C. J . Org. Chem. 1997, 62, 5884-5892. (f) Angle, S.
R.; Yang, W. J . Org. Chem. 1992, 57, 1092-1097. (g) Angle, S. R.; Yang,
W. J . Am. Chem. Soc. 1990, 112, 4524-4528.
(6) Examples in which quinone methides were proposed as active
intermediates: (a) Taatjes, D.; Gaudiano, G.; Resing, K.; Koch, T. H.
J . Med. Chem. 1996, 39, 4135-4138. (b) Mayalarp, S. P.; Hargreaves,
R. H. J .; Butler, J .; O′Hare, C. C.; Hartley, J . A. J . Med. Chem. 1996,
39, 531-537. (c) Boruah, R. C.; Skibo, E. B. J . Med. Chem. 1994, 37,
1625-1631. (d) Boger, D. L.; Nishi, T.; Teegarden, B. R. J . Org. Chem.
1994, 59, 4943-4949. (e) Nicolau, K. C.; Dai, W.-M. J . Am. Chem. Soc.
1992, 114, 8908-8921.
(7) (a) Chatterjee, M.; Rokita, S. E. J . Am. Chem. Soc. 1994, 116,
1690-1697. (b) Chatterjee, M.; Rokita, S. E. J . Am. Chem. Soc. 1991,
113, 5116-5117. (c) Chatterjee, M.; Rokita, S. E. J . Am. Chem. Soc.
1990, 112, 6397-6399. (d) Li, T.; Rokita, S. E. J . Am. Chem. Soc. 1991,
113, 7771-7773.
(8) Zhou, Q.; Turnbull, K. D. J . Org. Chem. 1999, 64, 2847-2851.
(9) (a) Skibo, E. B.; Islam, I.; Schulz, W. G.; Zhou, R.; Bess, L.;
Boruah, R. Synlett 1996, 297-309. (b) Skibo, E. B.; Xing, C. Biochem-
istry 1998, 37, 15199-15213.
(10) Two examples of attempts at DNA phosphodiester alkylation
and later retractions: (a) Gohil, R. N.; Roth, A. C.; Day, R. A. Arch.
Biochem. Biophys. 1974, 165, 297-312. (b) Bhat, G.; Roth, A. C.; Day,
R. A. Biopolymers 1977, 16, 1713-1724. (c) Moody, H. M.; Van
Genderen, M. H. P.; Koole, L. H.; Meijer, E. M.; Buck, H. M. Nucleic
Acids Res. 1989, 17, 4769-4782. (d) Buck, H. M.; Moody, H. M.;
Quaedflieg, P. J . L. M.; Koole, L. H.; Van Genderen, M. H. P.; Smit,
L.; J urriaans, S.; Geelen, J . L. M. C.; Goudsmit, J . Science 1990, 250,
125-126.
10.1021/jo9915168 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/16/2000

Phosphodiester-Quinone Methide Adducts
J . Org. Chem., Vol. 65, No. 7, 2000 2023
Sch em e 2
product in the quinone methide-phosphodiester alkyla-
tion equilibrium. For mechanistic clarity, the present
investigations have been limited to precursors that allow
the formation of characterizable p-quinone methide
intermediates. The reaction conditions for these inves-
tigations were intentionally conceived to allow analysis
of a single aspect of a multifunctional phosphodiester
alkylating reagent under systematic development.
Previous investigators have demonstrated approaches
to stabilize quinone methide alkylation products. Angle
and co-worker showed o-quinone methide-nucleic acid
base alkylation products of anthracycline analogues could
be isolated without decomposition if the phenol was
acetylated.5e-g Other investigators have examined ap-
proaches to stabilize benzylic trialkyl phosphates. Meier
and co-workers examined the stability of a series of di-
AZT-benzyl phosphotriesters as potential prodrugs.11
They investigated a series of para substituents on the
phenyl ring of the benzyl group ranging from a methyl
to a nitro group and found the half-life of the correspond-
ing phosphotriesters in a phosphate buffer (pH 7.4)
ranged from 0.37 to 744 h, respectively. Thomson and
co-workers showed that no hydrolysis of bis(4-acyloxy-
benzyl)-AZT phosphotriester was found in a phosphate
buffer-acetonitrile mixture (95:5 v/v, pH 7.4) at 37 °C
over 3 h.12 Givens and co-workers studied the solvolysis
of two diethylbenzyl phosphates in methanol over 5.5 h,
observing 88% solvolysis with a p-methoxy substituent
on the phenyl ring of the benzyl group, while no solvolysis
was observed with a m-methoxy substituent.13
Sch em e 3
lactonization. The intent was to produce a lactone
precursor of appropriate reactivity to allow quinone
methide formation and phosphodiester addition to the
quinone methide to produce trialkyl phosphate 4, fol-
lowed by a slower lactonization to afford trapped trialkyl
phosphate 5. This would allow the equilibrium addition
of the phosphodiester to the quinone methide to be
converted to the desired lactonized trialkyl phosphate
product.
We initially examined the feasibility of this design with
a simple methyl ester 7 (Scheme 3). Commercially
available methyl coumarin 6 was converted into the
methyl cinnamate in 41% yield by refluxing with sodium
methoxide.15 Hydrogenation of the intermediate cin-
namate produced phenol 7 in 86% yield. Conversion to
the desired p-quinone methide 8a was attempted by
oxidation of phenol 7 with Ag2O or PbO2. Unfortunately,
These studies suggest that the stability of phosphodi-
ester-quinone methide adducts, such as 2 (Scheme 1),
can be increased by lowering the resonance electron-
donating capacity of the p-hydroxy substituent. We report
the development of a catechol derivative designed to
alkylate a phosphodiester via a characterizable p-quinone
methide followed by in situ lactonization. This simple
application of Le Chatelier’s principle has allowed the
alkylation equilibrium to be drained to the desired
phosphotriester as the favored product.
Resu lts a n d Discu ssion s
After our study of phosphodiester alkylation with
p-quinone methide 1 (Scheme 1),8 we sought a modifica-
tion to trap out an isolable phosphotriester for complete
characterization. We initially investigated the inter-
molecular acetylation of the trialkyl phosphate 2 with
various activated acetylating reagents. However, due to
the direct acetylation of phosphodiesters and competitive
alkylation reaction with the quinone methide, no acety-
lated trialkyl phosphate from phosphodiester addition to
the quinone methide was observed by 1H NMR analysis.
An alternative intramolecular approach was designed
to trap the resulting trialkyl phosphate. The designed
p-quinone methide 3 (Scheme 2) contained a side chain
carrying an activated acetylating group14 for in situ
1
only complex products were observed by H NMR analy-
sis through the presumed o-quinone methide intermedi-
ate 8b. o-Quinone methides have been reported to
dimerize rapidly even at very low concentrations,16 as
used advantageously in the synthesis of several natural
products.17
Geminal dimethyl groups were added to the ortho
position in an attempt to block o-quinone methide forma-
tion. In addition, such geminal dimethyl substituents
have been reported to increase lactonization rates of
related derivatives by 103-fold18a according to the Thorpe-
Ingold effect.18b,c Condensation of 2,4-dimethylphenol
(15) Zubia, E.; Lius, F. R.; Massanet, G. M.; Collado, I. G. Tetrahe-
dron 1992, 48, 4239-4241.
(16) (a) Gardner, P. D.; Sarrafizadeh, R. H.; Brandon, R. L. J . Am.
Chem. Soc. 1959, 81, 5515. (b) Filar, L. J .; Winstein, S. Tetrahedron
Lett. 1960, 25, 9-16.
(17) (a) Chapman, O. L.; Engel, M. R.; Springer, J . P.; Clardy, J . C.
J . Am. Chem. Soc. 1971, 93, 6696-6698. (b) Chiba, K.; Sonoyama, J .;
Tada, M. J . Chem. Soc., Perkin Trans. 1 1996, 1435-1443.
(18) (a) Amsberry, K. L.; Borchardt, R. T. J . Org. Chem. 1990, 55,
5867-5877. (b) Milstein, S.; Cohen, L. A. J . Am. Chem. Soc. 1970, 92,
4377-4382. (c) Milstein, S.; Cohen, L. A. Proc. Natl. Acad. Sci. U.S.A.
1970, 67, 1143-1147.
(11) Meier, C.; Habel, L. W.; Balzarini, J .; Clercq, E. D. Liebigs. Ann.
1995, 2203-2208.
(12) Thomson, W.; Nicholls, D.; Irwin, W. J .; Al-Mushadani, J . S.;
Freeman, S.; Karpas, A.; Petrik, J .; Mahmood, N.; Hay, A. J . J . Chem.
Soc., Perkin Trans. 1 1993, 1239-1245.
(13) Givens, R. S.; Matuszewski, B.; Athey, P. S.; Stoner, M. R. J .
Am. Chem. Soc. 1990, 112, 6016-6021.
(14) For a recent example of activated esters used for lactonization,
see: Burke, S. D.; McDermott, T. S.; O’Donnell, C. J . J . Org. Chem.
1998, 63, 2715-2718.

2024 J . Org. Chem., Vol. 65, No. 7, 2000
Zhou and Turnbull
Sch em e 4
Sch em e 6
Sch em e 5
Sch em e 7
Sch em e 8
Ta ble 1. Str u ctu r es a n d Yield s of Ca tech ol Der iva tive 13
were also examined (Scheme 6). It was found that when
the polar protic solvent EtOH was used, a large amount
of undesired lactone 14 was produced. Use of the non-
polar solvent CCl4 and excess catalyst (1.5 equiv of Pd/C
by weight) afforded pure catechol derivatives 15d -f. In
the debenzylation of 13g, about 10% of lactone 14 was
with methyl dimethyl acrylate afforded tetramethyl
coumarin 9 in 93% yield (Scheme 4).19 However, ester
10, obtained upon refluxing 9 in NaOMe/MeOH, lacton-
ized back at such a rate to preclude p-quinone methide
formation.
1
produced based on H NMR area integration analysis.
Our attention turned to the development of catechol
derivatives in order to prevent competitive o-quinone
methide formation and slow the rate of lactonization in
these model studies (Scheme 5). Key intermediate 11 was
synthesized through benzyl protection of 2-hydroxy-3,5-
dimethylacetophenone20 followed by Baeyer-Villiger oxi-
dation21 and hydrolysis. Alkylation of 11 with bromo-
acetic acid afforded 12 in 90% yield. From intermediates
11 and 12, an amide and a variety of esters were
synthesized (Table 1). Dimethyl amide 13a was synthe-
sized by alkylating 11 with bromoacetamide followed by
exhaustive methylation in 80% overall yield.22 Methyl
ester 13b was synthesized by alkylating 11 with methyl
bromoacetate in 82% yield. Esters 13c-g were synthe-
sized by carbodiimide-assisted condensation of 12 with
the corresponding alcohols in 48-81% yield (Table 1).23
Debenzylation via palladium-catalyzed hydrogenation
of esters 13a -c in EtOAc proceeded smoothly to afford
15a -c (Scheme 6). However, lactone 14 was observed in
the debenzylation of esters 13d -g. Therefore, in addition
to EtOAc as a debenzylation solvent, EtOH and CCl4
Clean oxidation of phenols 15b-f to the corresponding
p-quinone methides 16b-f (Scheme 7) was accomplished
with lead(IV) oxide using low phenol concentrations (2.50
mM) in the presence of acetic acid (1 equiv). Silver(I)
oxide was used to oxidize amide 15a to quinone methide
16a . Quinone methide 16g (X ) OPh) was formed from
the oxidation of phenyl ester 15g with PbO2 along with
40% of lactone 14 based on the relevant resonance area
integration from 1H NMR analysis (Scheme 7). The
competitive lactonization of 15g precluded its further
investigation.
The addition of dibenzyl phosphoric acid to p-quinone
1
methides 16a -f was monitored by H NMR analysis in
CDCl3 (Scheme 8). A solution of dibenzyl phosphoric acid
(1.5 equiv) in CDCl3 was added to the solutions of quinone
methide 16a -f (2.50 mM in CDCl3). The final concentra-
tions of quinone methide 16a -f and dibenzyl phosphoric
acid were 2.1 and 3.2 mM, respectively. All of the quinone
methide derivatives investigated successfully alkylated
the phosphodiester to afford trialkyl phosphates 17a -f
at 24 °C (Scheme 8, Table 2). Conversion to trialkyl
phosphate 17a from quinone methide 16a was complete
in less than 5 min. Quinone methides 16b-f required
up to 2 h to reach alkylation equilibrium forming the
corresponding trialkyl phosphate 17b-f. Greater than
95% conversion to trialkyl phosphates 17a -f from qui-
none methides 16a -f was observed by 1H NMR analysis.
Relative to our previous studies of phosphodiester alkyl-
ation with 2,6-dimethylquinone methide,8 these results
revealed that catechol quinone methides were more
(19) Nicolau, M. G.; Yuan, C.; Borchardt, R. T. J . Org. Chem. 1996,
61, 8636-8641.
(20) Venuti, M. C.; Loe, B. E.; J ones, G. H.; Young, J . M. J . Med.
Chem. 1988, 31, 2132-2136.
(21) Koch, S. S. C.; Chamberlin, A. R. Synth. Commun. 1989, 19,
829-833.
(22) Boger, D. L.; Zhou, J . J . Org. Chem. 1996, 61, 3938-3939.
(23) (a) Woodward, R. B.; Heusler, K.; Gosteli, J .; Naegeli, P.;
Oppolzer, W.; Ramage, R.; Ranganathan, S.; Vorbru¨ggen, H. J . Am.
Chem. Soc. 1966, 88, 852-853. (b) Hassner, A.; Alexanian, V. Tetra-
hedron Lett. 1978, 46, 4475-4478.

Phosphodiester-Quinone Methide Adducts
J . Org. Chem., Vol. 65, No. 7, 2000 2025
Ta ble 2. Alk yla tion of Diben zyl P h osp h or ic Acid w ith
Qu in on e Meth id es 16 F ollow ed by in Situ La cton iza tion a
quinone
entry methide
trialkyl
X
phosphateb % convn to 18
1
2
3
4
5
6
16a
16b
16c
16d
16e
16f
N(CH3)2
OCH3
O(CH2)2CH3
OCH2CH2Cl
OCH2CCl3
OCH2CF3
17a c
17bd
17cd
17d d
17ed
17fd
e
f
e
51g
75g
85g
a
All experiments were carried out in CDCl3 with dibenzylphos-
b
phoric acid (1.5 equiv). The percent conversion to trialkyl phos-
phates 17 at 24 °C was >95% by 1H NMR analysis relative to an
internal standard (mesitylene). c The alkylation was complete
within 5 min. The alkylation was complete within 2 h. e No
d
lactonized product was evident by 1H NMR analysis at 24 °C after
48 h. f Less than 5% lactonized product was observed at 24 °C after
g
48 h. The in situ lactonization was monitored at 35 °C over 92 h
and the percent conversion was calculated relative to an internal
standard (mesitylene).
reactive than 2,6-dimethylquinone methide as the alkyl-
ation equilibrium was shifted to fully favor phosphodi-
ester alkylation.24 Minor impurities detected in the
1
alkylation reaction (<5% by H NMR analysis relative
to an internal standard) were consistent with hydrolysis
byproducts of 17 and 18.
The conversion of quinone methide 16a -f to trialkyl
phosphate 17a -f was clearly observed by 1H NMR
analysis. The disappearance of the characteristic p-
quinone methide alkylidene resonance of 16a -f (∼5.75
ppm)8,25 coincided with the appearance of two new
doublets at ∼4.86 ppm (3J H-P ) 8.6-8.9 Hz, 17 R, Scheme
8) and ∼4.97 ppm (3J H-P ) 7.2-8.2 Hz, 17 R′, Scheme 8)
in a 1:2 ratio. This is characteristic of the phosphorus
coupled benzylic hydrogen resonances of the trialkyl
phosphate and clearly revealed that phosphodiester
addition occurred.8
The in situ lactonization of intermediates 17a -f was
initially monitored by 1H NMR analysis at 24 °C over 48
1
h. Lactonized trialkyl phosphate 18 was evident by H
NMR analysis with intermediate phosphotriesters 17d -f
(Table 2). However, intermediate 17a and 17c showed
no sign of lactonization to 18 and methyl ester 17b
afforded less than 5% lactonized trialkyl phosphate 18
during this time frame. This suggested that in situ
lactonization of intermediates 17a -c was inefficient for
conversion of the trialkyl phosphate to the trapped
product. The efficiencies of the in situ lactonization of
intermediates 17d -f to product 18 were then examined
by 1H NMR analysis at 35 °C over 92 h (Table 2). Among
esters 17d -f (entries 4-5, Table 2), trifluoroethyl ester
17f afforded the highest percent conversion (85%) to the
lactonized trialkyl phosphate 18 while intermediate 17d
and 17e resulted in 51% and 75% conversion, respec-
tively. The percent conversions given in Table 2 were
determined relative to mesitylene as an internal stand-
ard.
F igu r e 1. Progression of the in situ lactonization of 17f to
1
form 18 as monitored by 1H NMR. (a) H NMR spectrum of
intermediate trialkyl phosphate 17f at room temperature. (b)
1H NMR spectrum after 14 h, 35 °C. (c) 1H NMR spectrum
1
after 40 h, 35 °C. (d) H NMR spectrum after 90 h, 35 °C.
to confirm the formation of 18. Progression of the
lactonization of 17f as monitored by 1H NMR analysis is
shown in Figure 1. Clean 1H NMR analysis of this
lactonization required that the benzylic resonances of the
residual dibenzyl phosphoric acid be shifted. This was
accomplished by adding 1 equiv of tetrabutylammonium
dibenzyl phosphate to the reaction mixture of 17f con-
taining residual dibenzyl phosphoric acid. Addition of the
salt appeared to have little effect on the reaction as the
observed rates of lactonization were nearly identical to
reactions without the added tetrabutylammonium diben-
zyl phosphate. The resulting buffered dibenzyl phosphate
19 [HOPO(OBn)2/(Bu)4N+ -OPO(OBn)2] appeared as a
broad doublet at 4.92 ppm in the 1H NMR spectra
throughout the reaction (Figure 1a-d). The benzylic
hydrogen resonances of the intermediate trialkyl phos-
phate 17f appeared as doublets at 4.86 ppm (3J H-P ) 8.9
Hz, 17f R) and at 4.97 ppm (3J H-P ) 8.2 Hz, 17f R′)
(Figure 1a). Over 90 h at 35 °C, the benzylic resonances
A more detailed study of the in situ lactonization of
1
intermediate 17f was carried out by H NMR analysis
(24) The increased reactivity of a methoxy-substituted p-quinone
methide relative to 2,6-dimethyl quinone methide has been reported
in hydrolysis studies. The half-life of p-quinone methides in a phos-
phate buffer (pH 7.4, 25 °C) revealed that 2,6-dimethylquinone methide
was about 20 times less reactive than the 2-methoxy derivative (26 vs
1.3 s) (a) Bolton, J . L.; Comeau, E.; Vukomanovic, V. Chem.-Biol.
Interact. 1995, 95, 279-290. (b) Bolton, J . L.; Valerio, L. G., J r.;
Thompson, J . A. Chem. Res. Toxicol. 1992, 5, 816-822.
(25) Dyall, L. K.; Winstein, S. J . Am. Chem. Soc. 1972, 94, 2196-
2200.

2026 J . Org. Chem., Vol. 65, No. 7, 2000
Zhou and Turnbull
organic layers were washed with brine (2 × 150 mL), dried
over MgSO4, and concentrated. Flash chromatography (EtOAc
in hexanes, 20%) afforded cinnamate (974 mg) in 41% yield
as a white solid: mp 135-137 °C; IR (film, cm-1) 3195 (br),
1677, 1583, 1298, 1213; 1H NMR (CDCl3, 270 MHz) δ 7.99 (d,
J ) 16.2 Hz, 1H), 7.25 (s, 1H), 7.02 (d, J ) 8.2 Hz, 1H), 6.74
(d, J ) 8.2 Hz, 1H), 6.60 (d, J ) 16.2 Hz, 1H), 6.24 (s, 1H),
3.81 (s, 3H), 2.26 (s, 3H); 13C NMR (68 MHz) δ 168.8, 153.2,
140.9, 132.2, 130.0, 129.5, 121.3, 117.9, 116.4, 51.8, 20.5; MS
(EI) m/z (relative intensity) 192 (M+, 29), 160 (88), 132 (100).
Meth yl (2′-Hyd r oxy-5′-m eth ylp h en yl)p r op ion a te (7).
To a solution of cinnamate (200 mg, 1.04 mmol) in MeOH (15.0
mL) was added palladium on activated carbon (10%, 200 mg).
The suspension was shaken under H2 (55 psi) on a Parr
hydrogenator for 18 h. The catalyst was removed by filtration
through a pad of Celite, and the filtrate was concentrated to
afford 7 in 86% yield as a faint yellow oil (174 mg): IR (film,
cm-1) 3405 (br), 2951, 1713, 1508, 1441, 1264; 1H NMR (CDCl3,
270 MHz) δ 7.00 (s, 1H), 6.90 (d, J ) 8.3 Hz, 1H), 6.77 (d, J )
8.3 Hz, 1H), 6.89 (s, 1H), 3.68 (s, 3H), 2.87 (t, J ) 6.7 Hz, 2H),
2.70 (t, J ) 6.7 Hz, 2H), 2.25 (s, 3H); 13C NMR (68 MHz) δ
176.0, 152.0, 131.1, 130.0, 128.5, 127.1, 116.9, 52.3, 35.1, 24.8,
20.5; MS (EI) m/z (relative intensity) 194 (M+, 25), 162 (69),
134 (100), 121 (38).
Tetr a m eth yld ih yd r ocou m a r in (9). To a mixture of 2,4-
dimethylphenol (2.00 g, 16.4 mmol) and methyl 3,3-dimethyl-
acrylate (2.00 g, 17.5 mmol) was added methanesulfonic acid
(5.0 mL). The resulting brown solution was stirred under N2
at 70 °C. After 24 h, the reaction solution was diluted with
Et2O (150 mL) and washed with H2O (150 mL), saturated
NaHCO3 (100 mL), and brine (100 mL). The organic layer was
dried over MgSO4 and concentrated. Crystallization (EtOH/
H2O) afforded 9 (3.11 g) in 93% yield as a white solid: mp
96-97 °C; IR (KBr, cm-1) 2960, 1762, 1474, 1266, 1206, 1116;
1H NMR (CDCl3, 270 MHz) δ 6.92 (s, 1H), 6.91 (s, 1H), 2.57
(s, 2H), 2.29 (s, 3H), 2.26 (s, 3H), 1.31 (s, 6H); 13C NMR (68
MHz) δ 168.7, 146.9, 133.7, 131.3, 130.4, 126.1, 122.3, 43.8,
33.3, 27.8, 21.0, 15.9; MS (EI) m/z (relative intensity) 204 (M+,
85), 189 (87), 162 (100), 147 (68).
of trialkyl phosphate 17f gradually disappeared while the
corresponding benzylic resonances of lactonized trialkyl
phosphate 18 grew in at 4.87 ppm (3J H-P ) 8.4 Hz, 18 R)
and at 5.01 ppm (3J H-P ) 8.1 Hz, 18 R′) (Figure 1b-d).
After 90 h at 35 °C, near complete conversion to lacton-
ized trialkyl phosphate 18 was evident by 1H NMR
analysis (Figure 1d). The lactonization also coincided
with the release of trifluoroethanol (methylene resonance
at 3.92 ppm) as the byproduct from the lactonization of
trifluoroester 17f (methylene resonance at 4.53 ppm).
This clearly indicated that in situ lactonization effectively
converted the intermediate trialkyl phosphate 17f to the
desired phosphotriester product 18.
Based on the above results, the synthesis of the
lactonized trialkyl phosphate 18 was carried out using
15f as the starting ester (20.0 mg). The quinone methide
solution 16f (2.5 mM in CHCl3) was formed with PbO2
oxidation in the presence of acetic acid (3.0 equiv).
Dibenzyl phosphoric acid (2.0 equiv) was added to the
reaction solution and the resulting reaction was allowed
to react at 35 °C for 6 days. Tetrabutylammonium acetate
(1.5 equiv) was then added to buffer the reaction solution
and the desired trialkyl phosphate 18 was purified
through a flash column as a faint yellow oil in 58%
isolated yield.
Con clu sion
The development of a DNA phosphodiester alkylating
reagent has driven our investigation of dialkyl phosphate
alkylation by p-quinone methides. These investigations
have shown that in situ lactonization can effectively
convert the quinone methide-dialkyl phosphate alkyl-
ation intermediate to a trapped phosphotriester. A cat-
echol system has been developed to examine this in situ
lactonization process. Fast lactonization, which competed
with the formation of p-quinone methide, precluded the
study of derivative 15g tethered with a highly reactive
phenyl ester as the lactone precursor. Derivatives 15a -
c, tethered with less reactive lactone forming functional
groups, allowed the formation of p-quinone methides and
phosphodiester alkylation, yet were ineffective at lacton-
ization. Derivatives 15d -f, tethered with intermediate
reactive lactone-forming esters, allowed the formation of
p-quinone methides, alkylation of the phosphodiester,
and in situ lactonization to convert the alkylation equi-
librium to the desired lactonized phosphotriester 18.
These investigations have allowed a further understand-
ing of mechanistic details regarding reactivity and stabil-
ity of the alkylation process. This is allowing direct
applications under anhydrous conditions and guiding the
design of more advanced derivatives for projected in situ
modification of nucleic acid polymers under biologically
relevant conditions.
2-Ben zyloxy-3,5-d im eth ylp h en ol (11). To a solution of
3,5-dimethyl-2-hydroxyacetophenone26 (2.16 g, 13.2 mmol) and
benzyl bromide (2.0 mL, 16.8 mmol) in DMF (15.0 mL) in an
ice-salt bath was slowly added potassium hydride solid (710
mg, 17.7 mmol). After 12 h, the suspension was diluted with
a saturated NaHCO3 aqueous solution (100 mL) and extracted
with Et2O (3 × 100 mL). The organic layers were dried over
MgSO4 and concentrated. Flash chromatography (EtOAc in
hexanes, 2.5-4.5%) afforded the benzyl-protected phenol (2.85
g) in 85% yield as an oil: IR (film, cm-1) 2923, 1683, 1465,
1
1255, 1211; H NMR (CDCl3, 270 MHz) δ 7.43-7.28 (m, 5H),
7.21 (s, 1H), 7.13 (s, 1H), 4.79 (s, 2H), 2.56 (s, 3H), 2.30 (s,
3H), 2.28 (s, 3H); 13C NMR (68 MHz) δ 205.2, 154.1, 137.0,
135.8, 134.1, 133.9, 132.3, 128.8, 128.5, 128.3, 127.8, 76.7, 30.4,
20.3, 15.8; MS (EI) m/z (relative intensity) 254 (M+, 3), 236
(1), 211 (5), 149 (3), 91 (100).
To a solution of the benzyl-protected phenol (2.84 g, 11.2
mmol) in CH2Cl2 (25.0 mL) was added m-CPBA (65%, 6.10 g,
23.0 mmol). The resulting solution was cooled in an ice bath,
and trifluoroacetic acid (0.86 mL, 11.2 mmol) was slowly added.
The resulting suspension was stirred in the dark for 5 h. The
reaction solution was diluted in an aqueous Na2SO3 solution
(10%, 100 mL) and extracted with CH2Cl2 (3 × 100 mL). The
organic layers were washed with sat. NaHCO3 (150 mL), H2O
(150 mL) and brine (150 mL), dried over MgSO4, and concen-
trated. The resulting residue was dissolved in methanol (20.0
mL) and KOH solution (5 N, 20.0 mL) was added. The mixture
was stirred for 12 h, neutralized with 2 N HCl, and extracted
with CH2Cl2 (3 × 100 mL). The organic layers were dried over
MgSO4 and concentrated. Flash chromatography (EtOAc in
hexanes, 3-5%) afforded 11 (2.07 g) in 81% yield as a faint
yellow oil: IR (film, cm-1) 3512 (br), 2921, 1498, 1219, 1174;
Exp er im en ta l Section
All commercially available compounds were purchased from
Aldrich Chemical Co. (Milwaukee, WI), Acros Organics (Fisher
Scientific), or Lancaster Synthesis, Inc. (Windham, NH) and
used without purification, unless noted otherwise. 31P chemical
shifts are reported relative to 85% phosphoric acid.
Meth yl 2-Hyd r oxy-5-m eth ylcin n a m a te. To a mixture of
6-methylcoumarin (2.00 g, 125 mmol) and sodium methoxide
(3.20 g, 59.3 mmol) was slowly added dry MeOH (25.0 mL).
The resulting yellow solution was refluxed under N2 for 18 h.
The reaction was neutralized with an aqueous HCl solution
(2 N, 20 mL) and extracted with EtOAc (3 × 100 mL). The
(26) Cullinane, N. M.; Edward, B. F. R. J Appl. Chem. 1959, 133-
136.

Phosphodiester-Quinone Methide Adducts
J . Org. Chem., Vol. 65, No. 7, 2000 2027
1H NMR (CDCl3, 270 MHz) δ 7.47-7.33 (m, 5H), 6.59 (s, 1H),
6.53 (s, 1H), 5.36 (s, 1H), 4.85 (s, 2H), 2.29 (s, 3H), 2.23 (s,
3H); 13C NMR (68 MHz) δ 148.9, 142.2, 137.4, 134.8, 130.9,
129.1, 128.8, 128.4, 123.3, 113.9, 75.4, 20.7, 15.8; MS (EI) m/z
(relative intensity) 228 (M+, 7), 137 (5), 91 (100).
2-Ch lor oeth yl (2′-Ben zyloxy-3′,5′-d im eth yl)p h en oxy-
a ceta te (13d ). To a suspension of 12 (150 mg, 0.52 mmol) and
1,3-dicyclohexylcarbodiimide (152 mg, 0.74 mmol) in CH2Cl2
(10.0 mL) were added pyridine (42 µL, 0.52 mmol) and
2-chloroethanol (45 µL, 0.67 mmol). The suspension was stirred
at room temperature for 12 h. The precipitate was filtered,
and the filtrate was concentrated. Flash chromatography
(EtOAc in hexanes 10%) afforded 13d (89 mg) in 48% yield as
a white solid: mp 40-42 °C; IR (film, cm-1) 2915, 1769, 1496,
1210, 1154, 1118; 1H NMR (CDCl3, 270 MHz) δ 7.51-7.27 (m,
5H), 6.64 (s, 1H), 6.55 (s, 1H), 5.01 (s, 2H), 4.72 (s, 2H), 4.44
(t, J ) 5.5 Hz, 2H), 3.67 (t, J ) 5.5 Hz, 2H), 2.25 (s, 3H), 2.17
(s, 3H); 13C NMR (68 MHz) δ 168.7, 150.5, 144.4, 137.9, 133.4,
132.4, 128.3 (2C), 127.8, 125.0, 113.0, 74.5, 66.0, 64.5, 41.3,
21.1, 16.1; MS (EI) m/z (relative intensity) 350 (M + 2, 3), 348
(M+, 7), 257 (15), 149 (18), 137 (12).
(2-Ben zyloxy-3,5-d im eth yl)p h en oxya cetic Acid (12). To
a suspension of 11 (1.50 g, 6.58 mmol) and sodium hydride
(200 mg, 8.3 mmol) in DMF (10.0 mL) was cannulated a
suspension of bromoacetic acid (1.10 g, 7.89 mmol) and sodium
hydride (316 mg, 13.2 mmol) in DMF (10.0 mL). The resulting
suspension was stirred for 12 h. The reaction solution was
diluted in an aqueous acetic acid solution (2%, 100 mL) and
extracted with Et2O (3 × 100 mL). The organic layers were
washed with H2O (2 × 200 mL), brine (200 mL), dried over
MgSO4, and concentrated. Flash chromatography (MeOH in
CH2Cl2, 1-7%) afforded 12 (1.70 g) in 90% yield as a white
solid: mp 104-106 °C; IR (KBr, cm-1) 2918 (br), 1726, 1494,
2,2,2-Tr ich lor oeth yl (2′-Ben zyloxy-3′,5′-d im eth yl)p h e-
n oxya ceta te (13e). To a suspension of 12 (270 mg, 0.94 mmol)
and 1,3-dicyclohexylcarbodiimide (292 mg, 1.41 mmol) in
CH2Cl2 (10.0 mL) were added pyridine (76 µL, 0.94 mmol) and
2,2,2-trichloroethanol (117 µL, 1.22 mmol). The suspension was
stirred at room temperature for 12 h. The precipitate was
filtered, and the filtrate was concentrated. Flash chromatog-
raphy (EtOAc in hexanes, 5-10%) afforded 13e (311 mg) in
79% yield as a white solid: mp 68-69 °C; IR (film, cm-1) 2965,
1
1248, 1111; H NMR (CDCl3, 270 MHz) δ 7.46-7.34 (m, 5H),
6.68 (s, 1H), 6.59 (s, 1H), 4.97 (s, 2H), 4.68 (s, 2H), 2.26 (s,
3H), 2.19 (s, 3H); 13C NMR (68 MHz) δ 173.2, 150.9, 144.9,
137.5, 134.4, 132.9, 128.7, 128.6, 128.4, 126.0, 114.3, 75.2, 67.0,
20.8, 15.7. Anal. Calcd for C17H18O4: C, 71.31; H, 6.34.
Found: C, 71.47; H, 6.14.
N ,N -Dim e t h yl(2-b e n zyloxy-3,5-d im e t h yl)p h e n oxy-
a ceta m id e (13a ). To a solution of bromoacetamide (191 mg,
1.39 mmol) and 11 (300 mg, 1.32 mmol) in DMF (5.0 mL) in
an ice bath was added potassium hydride (79.0 mg, 2.00 mmol)
under N2. The resulting suspension was stirred for 3 h.
Potassium hydride (270 mg, 6.75 mmol) and methyl iodide (1.0
mL, 12 equiv) were then slowly added to the reaction solution
in an ice bath. After 5 h, the reaction solution was diluted in
a saturated NaHCO3 aqueous solution (100 mL) and extracted
with CH2Cl2 (3 × 100 mL). The organic layers were dried over
MgSO4 and concentrated. Flash chromatography (MeOH in
CH2Cl2, 4%) afforded 13a as an oil (334 mg) in 80% yield: IR
(film, cm-1) 2924, 1662, 1495, 1103; 1H NMR (CDCl3, 270 MHz)
δ 7.45-7.32 (m, 5H), 6.64 (s, 2H), 4.98 (s, 2H), 4.72 (s, 2H),
3.05 (s, 3H), 2.99 (s, 3H), 2.27 (s, 3H), 2.18 (s, 3H); 13C NMR
(68 MHz) δ 167.9, 150.9, 144.3, 138.0, 133.6, 132.2, 128.3,
128.2, 127.8, 124.5, 112.9, 74.6, 68.3, 36.6, 35.6, 21.2, 16.1; MS
(EI) m/z (relative intensity) 313 (M+, 2), 268 (3), 222 (23), 149
(8), 135 (5), 91 (61).
1
1783, 1496, 1219, 1155, 1124; H NMR (CDCl3, 270 MHz) δ
7.52-7.28 (m, 5H), 6.67 (s, 1H), 6.60 (s, 1H), 5.04 (s, 2H), 4.85
(s, 2H), 4.84 (s, 2H), 2.26 (s, 3H), 2.19 (s, 3H); 13C NMR (68
MHz) δ 167.6, 150.3, 144.4, 137.8, 133.4, 132.5, 128.3 (2C),
127.8, 125.1, 113.1, 94.4, 74.6, 74.0, 65.8, 21.1, 16.1; MS (EI)
m/z (relative intensity), 420 (M + 4, 0.8), 418 (M + 2, 3), 416
(M+, 3), 327 (4), 325 (3), 149 (12), 137 (9).
2,2,2-Tr iflu or oeth yl (2′-Ben zyloxy-3′,5′-d im eth yl)p h e-
n oxya ceta te (13f). To a suspension of 12 (150 mg, 0.52 mmol)
and 1,3-dicyclohexylcarbodiimide (151 mg, 0.738 mmol) in
CH2Cl2 (10.0 mL) were added pyridine (43 µL, 52 mmol) and
2,2,2-trifluoroethanol (46 µL, 0.64 mmol). The suspension was
stirred at room temperature for 12 h. The precipitate was
filtered, and the filtrate was concentrated. Flash chromatog-
raphy (EtOAc in hexanes, 10-15%) afforded 13f (138 mg) in
71% yield as a white solid: mp 48-50 °C; IR (film, cm-1) 2924,
1
1782, 1495, 1218, 1169; H NMR (CDCl3, 270 MHz) δ 7.52-
7.25 (m, 5H), 6.66 (s, 1H), 6.54 (s, 1H), 4.99 (s, 2H), 4.60 (s,
Met h yl (2-Ben zyloxy-3,5-d im et h yl)p h en oxya cet a t e
(13b). To a solution of methyl bromoacetate (55 µL, 0.60 mmol)
in DMF (5.0 mL) was cannulated a suspension of 11 (110 mg,
0.48 mmol) and sodium hydride (40 mg, 1.00 mmol) in DMF
(10.0 mL). The resulting suspension was stirred for 12 h. The
reaction solution was diluted in a saturated NaHCO3 aqueous
solution (100 mL) and extracted with Et2O (3 × 100 mL). The
organic layers were dried over MgSO4 and concentrated. Flash
chromatography (EtOAc in hexanes, 7-10%) afforded 13b (118
mg) in 82% yield as an oil: IR (film, cm-1) 2951, 1762, 1494,
2
2H), 4.56 (q, J H-F ) 8.4 Hz, 2H), 2.25 (s, 3H), 2.17 (s, 3H);
13C NMR (75 MHz) δ 167.7, 150.4, 144.6, 137.9, 133.5, 132.6,
1
128.3 (2C), 127.9, 125.4, 122.7 (q, J C-F ) 277.3 Hz), 113.5,
2
74.7, 65.9, 60.6 (q, J C-F ) 36.7 Hz), 21.0, 16.1; MS (EI) m/z
(relative intensity) 368 (M+, 12), 277 (9), 219 (5), 149 (12), 91
(100).
P h en yl (2-Ben zyloxy-3,5-d im et h yl)p h en oxya cet a t e
(13g). To a suspension of 12 (100 mg, 0.35 mmol) and 1,3-
dicyclohexylcarbodiimide (76 mg, 0.37 mmol) in CH2Cl2 (10.0
mL) were added pyridine (30 µL, 0.37 mmol) and phenol (33
mg, 0.35 mmol). The suspension was stirred at room temper-
ature for 12 h. The precipitate was filtered, and the filtrate
was concentrated. Flash chromatography (EtOAc in hexanes
5%) afforded 13g (102 mg) in 81% yield as a white solid: mp
58-60 °C; IR (film, cm-1) 2919, 1775, 1493, 1204, 1150, 1112;
1H NMR (CDCl3, 270 MHz) δ 7.51-7.09 (m, 10H), 6.67 (s, 1H),
6.65 (s, 1H), 5.03 (s, 2H), 4.91 (s, 2H), 2.28 (s, 3H), 2.19 (s,
3H); 13C NMR (68 MHz) δ 167.7, 150.7, 150.2, 144.6, 138.0,
133.6, 132.6, 129.6, 128.5, 128.4, 128.0, 126.3, 125.2, 121.4,
113.4, 74.7, 66.5, 21.2, 16.2; MS (EI) m/z (relative intensity)
362 (M+, 0.5), 269 (7), 213 (5), 184 (8), 178 (25), 150 (17), 91
(100).
1
1216; H NMR (CDCl3, 270 MHz) δ 7.47-7.27 (m, 5H), 6.62
(s, 1H), 6.52 (s, 1H), 4.99 (s, 2H), 4.66 (s, 2H), 3.78 (s, 3H),
2.24 (s, 3H), 2.16 (s, 3H); 13C NMR (75 MHz) δ 169.5, 150.7,
144.5, 138.0, 133.4, 132.4, 128.4, 128.3, 127.8, 124.9, 113.0,
74.5, 66.3, 52.1, 21.1, 16.1; MS (EI) m/z (relative intensity) 300
(M+, 9), 209 (23), 149 (28), 91 (100).
1-P r op yl (2-Ben zyloxy-3,5-d im et h yl)p h en oxya cet a t e
(13c). To a suspension of 12 (200 mg, 0.70 mmol) and 1,3-
dicyclohexylcarbodiimide (152 mg, 0.73 mmol) in CH2Cl2 (10.0
mL) were added pyridine (56 µL, 0.70 mmol) and 1-propanol
(57 µL, 0.76 mmol). The suspension was stirred at room
temperature for 12 h. The precipitate was filtered and the
filtrate was concentrated. Flash chromatography (EtOAc in
hexanes, 3-5%) afforded 13c (112 mg) in 49% yield as an oil:
La cton e (14). A suspension of 13g (52 mg, 0.29 mmol) and
potassium carbonate (425 mg) in CH2Cl2 (10 mL) was stirred
for 3 h. The reaction solution was diluted in CH2Cl2 (50 mL)
and washed with water (60 mL). The organic layer was dried
over MgSO4 and concentrated. Flash chromatography (EtOAc
in hexanes, 2%) afforded 14 (26 mg) in 76% yield as a faint
1
IR (film, cm-1) 2966, 1758, 1495, 1208, 1153, 1118; H NMR
(CDCl3, 270 MHz) δ 7.52-7.25 (m, 5H), 6.62 (s, 1H), 6.53 (s,
1H), 5.01 (s, 2H), 4.66 (s, 2H), 4.16 (t, J ) 6.7 Hz, 2H), 2.24 (s,
3H), 2.16 (s, 3H), 1.67 (m, 2H), 0.91 (t, J ) 7.7 Hz, 3H); 13C
NMR (68 MHz) δ 169.2, 150.7, 144.4, 138.0, 133.3, 132.3, 128.4,
128.3, 127.8, 124.7, 112.9, 74.5, 66.7, 66.2, 21.9, 21.1, 16.1, 10.3;
MS (EI) m/z (relative intensity) 328 (M+, 10), 237 (18), 195
(15), 149 (19), 137 (55), 91 (100).
1
yellow oil: IR (film, cm-1) 2920, 1773, 1495, 1330, 1201; H
NMR (CDCl3, 270 MHz) δ 6.68 (s, 1H), 6.67 (s, 1H), 4.60 (s,
2H), 2.25 (s, 6H); 13C NMR (75 MHz) δ 163.6, 142.0, 137.4,

2028 J . Org. Chem., Vol. 65, No. 7, 2000
Zhou and Turnbull
134.6, 126.7, 125.5, 115.0, 64.7, 20.8, 15.0; MS (EI) m/z
(relative intensity) 178 (M+, 49), 150 (68), 149 (100).
solid: mp 52-54 °C; IR (film, cm-1) 3463 (br), 2923, 1777,
1
1501, 1301, 1161; H NMR (CDCl3, 270 MHz) δ 6.65 (s, 1H),
2
N,N-Dim eth yl (2-h yd r oxy-3,5-d im eth yl)p h en oxya cet-
a m id e (15a ). To a solution of 13a (110 mg, 0.49 mmol) in
EtOAc (10.0 mL) was added palladium on activated carbon
(10%, 100 mg). The resulting suspension was shaken under
H2 (55 psi) on a Parr hydrogenator for 3 h. The catalyst was
removed by filtration through a pad of Celite, and the filtrate
was concentrated to afford 15a (74 mg) in 95% yield as a white
solid: mp 89-91 °C; IR (film, cm-1) 3144 (br), 2925, 1648,
1500, 1310; 1H NMR (CDCl3, 270 MHz) δ 9.16 (s, 1H), 6.67 (s,
1H), 6.64 (s, 1H), 4.70 (s, 2H), 2.98 (s, 3H), 2.90 (s, 3H), 2.22
(s, 3H), 2.20 (s, 3H); 13C NMR δ (68 MHz) 170.2, 146.9, 145.3,
128.1, 126.8, 126.0, 117.7, 71.5, 35.7, 35.3, 20.5, 15.8; MS (EI)
m/z (relative intensity) 223 (M+, 12), 178 (14), 150 (14), 87 (54).
Meth yl (2-Hydr oxy-3,5-dim eth yl)ph en oxyacetate (15b).
To a solution of 13b (98 mg. 0.47 mmol) in EtOAc (10.0 mL)
was added palladium on activated carbon (10%, 100 mg). The
resulting suspension was shaken under H2 (55 psi) on a Parr
hydrogenator for 3 h. The catalyst was removed by filtration
through a pad of Celite and the filtrate was concentrated to
afford 15b as a colorless oil (63 mg) in 91% yield: IR (film,
cm-1) 3481 (br), 2840, 1748, 1234; 1H NMR (CDCl3, 270 MHz)
δ 6.65 (s, 1H), 6.53 (s, 1H), 4.63 (s, 2H), 3.79 (s, 3H), 2.22 (s,
3H), 2.21 (s, 3H); 13C NMR (68 MHz) δ 171.0, 145.4, 143.2,
128.6, 126.0, 125.2, 114.2, 68.6, 52.5, 20.7, 15.6; MS (EI) m/z
(relative intensity) 210 (M+, 69), 178 (23), 150 (100), 137 (32).
1-P r op yl (2-H yd r oxy-3,5-d im et h yl)p h en oxya cet a t e
(15c). To a solution of 13c (109 mg, 0.46 mmol) in EtOAc (10.0
mL) was added palladium on activated carbon (10%, 50 mg).
The resulting suspension was shaken under H2 (55 psi) on a
Parr hydrogenator for 3 h. The catalyst was removed by
filtration through a pad of Celite and the filtrate was concen-
trated to afford 15c (78 mg) in 99% yield as a colorless oil: IR
(film, cm-1) 3383 (br), 2967, 1740, 1501, 1307, 1214; 1H NMR
(CDCl3, 270 MHz) δ 6.64 (s, 1H), 6.55 (s, 1H), 4.62 (s, 2H),
4.15 (t, J ) 6.7 Hz, 2H), 2.21 (s, 3H), 2.20 (s, 3H), 1.66 (m,
2H), 0.92 (t, J ) 7.4 Hz, 3H); 13C NMR (68 MHz) δ 171.1, 145.7,
143.4, 128.7, 126.1, 125.2, 114.5, 68.9, 67.3, 22.0, 20.7, 15.6,
10.2; MS (EI) m/z (relative intensity) 238 (M+, 23), 178 (32),
150 (100), 137 (31).
6.52 (s, 1H), 6.27 (br, 1H); 4.74 (s, 2H), 4.56 (q, J H-F ) 8.2
Hz, 2H), 2.21 (s, 6H); 13C NMR (68 MHz) δ 168.9, 144.9, 142.9,
1
128.9, 126.2, 125.3, 122.6 (q, J C-F ) 277.2 Hz), 113.3, 67.7,
2
61.0 (q, J C-F ) 36.9 Hz), 20.8, 15.6; MS (EI) m/z (relative
intensity) 278 (M+, 3), 178 (13), 149 (13).
P h en yl
(2′-H yd r oxy-3′,5′-d im et h yl)p h en oxya cet a t e
(15g). To a solution of 13g (48 mg) in CCl4 (10.0 mL) was
added palladium on activated carbon (10%, 50 mg). The
resulting suspension was shaken under H2 (55 psi) on a Parr
hydrogenator for 1 h. The catalyst was removed by filtration
through Celite, and the filtrate was concentrated to afford 15g
(32 mg) in 80% yield as a mixture containing lactone 14 (10%
based on the area integration in 1H NMR analysis): 1H NMR
(CDCl3, 270 MHz) δ 7.42-7.08 (m, 5H), 6.68 (s, 1H), 6.62 (s,
1H), 6.48 (br, 1H); 4.88 (s, 2H), 2.24 (s, 3H), 2.22 (s, 3H); 13C
NMR (68 MHz) δ 169.2, 150.0, 145.3, 143.2, 129.7, 128.8, 126.5,
126.2, 125.4, 121.3, 114.0, 68.7, 20.9, 15.7.
Gen er a l P r oced u r e for th e F or m a tion of Qu in on e
Meth id es (16a -g). Stock solutions (25.0 mM) of 15a -g in
CDCl3 containing acetic acid (d4, 1 equiv) were prepared and
diluted to 2.5 mM with CDCl3. The resulting solutions were
oxidized with PbO2 at room temperature for 5 min. The
suspensions were filtered with Acrodisc filter (13 CR, 0.45 µm)
to give the desired 2.5 mM solutions of quinone methides 16a -
g.
Qu in on e Meth id e (16a ). Silver(I) oxide was used as the
oxidizing reagent for 20 min: 1H NMR (CDCl3, 270 MHz) δ
6.97 (d, J ) 2.2 Hz, 1H), 6.45 (d, J ) 2.2 Hz, 1H), 5.81 (s, 1H),
5.73 (s, 1H), 4.67 (s, 2H), 3.10 (s, 3H), 2.94 (s, 3H), 2.03 (s,
3H).
Qu in on e m eth id e (16b): 1H NMR (CDCl3, 270 MHz) δ 6.98
(d, J ) 2.2 Hz, 1H), 6.24 (d, J ) 2.2 Hz, 1H), 5.77 (s, 1H), 5.74
(s, 1H), 4.62 (s, 2H), 3.79 (s, 3H), 2.03 (s, 3H).
Qu in on e m eth id e (16c): 1H NMR (CDCl3, 270 MHz) δ 6.98
(d, J ) 1.8 Hz, 1H), 6.24 (d, J ) 1.8 Hz, 1H), 5.75 (s, 1H), 5.73
(s, 1H), 4.62 (s, 2H), 4.15 (t, J ) 6.9 Hz, 2H), 2.03 (s, 3H), 1.66
(m, 2H), 0.91 (t, J ) 7.4 Hz, 3H).
Qu in on e m eth id e (16d ): 1H NMR (CDCl3, 270 MHz) δ 6.98
(d, J ) 2.0 Hz, 1H), 6.29 (d, J ) 2.0 Hz, 1H), 5.78 (s, 1H), 5.75
(s, 1H), 4.68 (s, 2H), 4.45 (t, J ) 5.7 Hz, 2H), 3.69 (t, J ) 5.7
Hz, 2H), 2.03 (s, 3H).
2-Ch lor oeth yl (2′-Hyd r oxy-3′,5′-d im eth yl)p h en oxya ce-
ta te (15d ). To a solution of 13d (74 mg) in CCl4 (10.0 mL)
was added palladium on activated carbon (10%, 72 mg). The
resulting suspension was shaken under H2 (55 psi) on a Parr
hydrogenator for 3 h. The catalyst was removed by filtration
through Celite and the filtrate was concentrated to afford 15d
(53 mg) in 97% yield as a colorless oil: IR (film, cm-1) 3420
(br), 2923, 1754, 1500, 1208; 1H NMR (CDCl3, 270 MHz) δ 6.65
(s, 1H), 6.55 (s, 1H), 4.86 (s, 2H), 4.44 (t, J ) 5.4 Hz, 2H), 3.69
(t, J ) 5.4 Hz, 2H), 2.22 (s, 3H), 2.21 (s, 3H); 13C NMR (68
MHz) δ 170.2, 145.2, 143.1, 128.7, 126.0, 125.1, 113.9, 68.3,
64.9, 41.2, 20.7, 15.6; MS (EI) m/z (relative intensity) 260 (M
+ 2, 0.4), 258 (M+, 1), 178 (1), 150 (4), 137 (2), 91 (0.4).
2,2,2-Tr ich lor oet h yl (2′-H yd r oxy-3′,5′-d im et h yl)p h e-
n oxya ceta te (15e). To a solution of 13e (47 mg) in CCl4 (10.0
mL) was added palladium on activated carbon (10%, 150 mg).
The resulting suspension was shaken under H2 (55 psi) on a
Parr hydrogenator for 1 h. The catalyst was removed by
filtration through Celite, and the filtrate was concentrated to
afford 15e (36 mg) in 99% yield as a faint yellow solid: mp
52-54 °C; IR (film, cm-1) 3441 (br), 2924, 1772, 1501, 130,
Qu in on e m eth id e (16e): 1H NMR (CDCl3, 270 MHz) δ 6.98
(d, J ) 2.2 Hz, 1H), 6.33 (d, J ) 2.2 Hz, 1H), 5.75 (s, 2H), 4.84
(s, 2H), 4.80 (s, 2H), 2.03 (s, 3H).
Qu in on e m eth id e (16f): 1H NMR (CDCl3, 270 MHz) δ 6.99
(d, J ) 2.5 Hz, 1H), 6.29 (d, J ) 2.5 Hz, 1H), 5.78 (s, 1H), 5.77
(s, 1H), 4.75 (s, 2H), 4.58 (q, 2J H-F ) 8.4 Hz, 2H), 2.03 (s, 3H).
Qu in on e Meth id e (16 g). The resulting quinone methide
16g solution contained lactone 14 (40%) based on the area
integration in 1H NMR analysis: 1H NMR (CDCl3, 270 MHz)
δ 7.42-7.08 (m, 5H), 7.00 (d, J ) 2.2 Hz, 1H), 6.41 (d, J ) 2.2
Hz, 1H), 5.81 (s, 1H), 5.78 (s, 1H), 4.87 (s, 2H), 2.05 (s, 3H).
Stu d y of th e Diben zyl P h osp h or ic Acid Alk yla tion by
Qu in on e Meth id es 16. To the CDCl3 solutions of quinone
methides 16a -f (2.5 mM, 600 µL each) containing mesitylene
as an internal standard was added a solution of dibenzyl
phosphoric acid in CDCl3 (100 µL each). The final concentra-
tions of dibenzyl phosphoric acid and quinone methides 16
were 3.2 and 2.1 mM, respectively. The reaction was monitored
1
by H NMR analysis. Greater than 95% conversion to trialkyl
1151; H NMR (CDCl3, 270 MHz) δ 6.65 (s, 1H), 6.54 (s, 1H),
phosphate 17 was observed by 1H NMR analysis. Minor
impurities detected in the alkylation reaction (<5% by 1H NMR
analysis relative to an internal standard) were consistent with
hydrolysis byproducts 17 and 18. Although the trialkyl phos-
phate 17 was not sufficiently stable to allow isolation, the
following solution characterizations were consistent with the
structural assignments.
1
6.28 (br, 1H); 4.83 (s, 2H), 4.80 (s, 2H), 2.21 (s, 6H); 13C NMR
(68 MHz) δ 168.8, 144.9, 142.8, 128.8, 126.1, 125.2, 113.2, 94.3,
74.4, 67.8, 20.9, 15.7; MS (EI) m/z (relative intensity) 332 (M
+ 6, 0.5), 330 (M + 4, 3), 328 (M + 2, 9), 326 (M+, 9), 178 (13),
149 (100), 137 (25).
2,2,2-Tr iflu or oet h yl (2′-H yd r oxy-3′,5′-d im et h yl)p h e-
n oxya ceta te (15f). To a solution of 13f (45 mg, 0.16 mmol)
in CCl4 (10.0 mL) was added palladium on activated carbon
(10%, 150 mg). The resulting suspension was shaken under
H2 (55 psi) on a Parr hydrogenator for 1 h. The catalyst was
removed by filtration through Celite, and the filtrate was
concentrated to afford 15f (30 mg) in 88% yield as a white
Trialkyl phosphate 17a : 1H NMR (CDCl3, 270 MHz) δ 7.33-
3
7.28 (m, 10H), 6.85 (s, 1H), 6.81 (s, 1H), 4.97 (d, J P-H ) 7.7
3
Hz, 4H), 4.87 (d, J P-H ) 8.7 Hz, 2H), 4.61 (s, 2H), 2.97 (s,
3H), 2.84 (s, 3H), 2.21 (s, 3H).
Trialkyl phosphate 17b: 1H NMR (CDCl3, 270 MHz) δ 7.35-
3
7.27 (m, 10H), 6.77 (s, 1H), 6.70 (s, 1H), 4.97 (d, J P-H ) 7.9

Phosphodiester-Quinone Methide Adducts
J . Org. Chem., Vol. 65, No. 7, 2000 2029
3
Hz, 4H), 4.85 (d, J P-H ) 8.9 Hz, 2H), 4.54 (s, 2H), 3.77 (s,
18 could be clearly resolved for observation. Progress of the
in situ lactonization was monitored by 1H NMR analysis in
the presence of 4 Å molecular sieves (10 mg).
3H), 2.22 (s, 3H).
Trialkyl phosphate 17c: 1H NMR (CDCl3, 270 MHz) δ 7.33-
3
7.26 (m, 10H), 6.77 (s, 1H), 6.21 (s, 1H), 4.97 (d, J P-H ) 7.9
La cton ized Tr ia lk yl P h osp h a te (18). To a solution of 15f
(20.0 mg, 0.07 mmol) in CHCl3 (35.0 mL) containing acetic acid
(14 µL, 0.21 mmol) was added lead(IV) oxide (2.50 g). The
suspension was stirred for 3 min, and the solid was filtered
through a fritted glass filter. Dibenzyl phosphoric acid (40.0
mg, 0.14 mmol) and 4 Å molecular sieves (50 mg) were added
to the resulting yellow solution. The reaction solution was
stirred at 25 °C for 4 h, and then dry acetonitrile (2.0 mL)
was added. In situ lactonization was carried out at 35 °C for
6 days. Tetrabutylammonium acetate (32.6 mg, 1.5 equiv) was
added, and the resulting solution was passed through a pad
of Florisil (200 mesh). Flash chromatography (Florisil, 200
mesh; EtOAc in CHCl3, 0-10%) gave 18 (18.3 mg) in 58% yield
as a yellow oil: IR (film, cm-1) 1783, 1328, 1265, 1204, 1017;
1H NMR (CDCl3) δ 7.37-7.25 (m, 10H), 6.80 (s, 2H), 5.01 (d,
3J P-H ) 8.1 Hz, 4H), 4.87 (d, 3J P-H ) 8.4 Hz, 2H), 4.59 (s, 2H),
2.25 (s, 3H); 13C NMR (75 MHz) δ 163.2, 142.2, 139.5, 135.8
(d, 3J P-C ) 7.5 Hz), 132.6 (d, 3J P-C ) 7.5 Hz), 128.7(2C), 128.0,
Hz, 4H), 4.85 (d, 3J P-H ) 8.6 Hz, 2H), 4.53 (s, 2H), 4.13 (t, J )
6.7 Hz, 2H), 2.20 (s, 3H), 1.65 (m, 2H), 0.91 (t, J ) 7.4 Hz,
3H).
Trialkyl phosphate 17d : 1H NMR (CDCl3, 270 MHz) δ 7.33-
3
7.24 (m, 10H), 6.76 (s, 1H), 6.70 (s, 1H), 4.98 (d, J P-H ) 8.2
Hz, 4H), 4.86 (d, 3J P-H ) 8.9 Hz, 2H), 4.59 (s, 2H), 4.42 (t, J )
5.7 Hz, 2H), 3.67 (t, J ) 5.7 Hz, 2H), 2.20 (s, 3H).
Trialkyl phosphate 17e: 1H NMR (CDCl3, 270 MHz) δ 7.33-
3
7.28 (m, 10H), 6.76 (s, 1H), 6.69 (s, 1H), 4.98 (d, J P-H ) 7.2
3
Hz, 4H), 4.85 (d, J P-H ) 8.7 Hz, 2H), 4.80 (s, 2H), 4.69 (s,
2H), 2.16 (s, 3H).
Trialkyl phosphate 17f: 1H NMR (CDCl3, 270 MHz) δ 7.33-
3
7.24 (m, 10H), 6.77 (s, 1H), 6.68 (s, 1H), 4.97 (d, J P-H ) 8.2
3
Hz, 4H), 4.86 (d, J P-H ) 8.9 Hz, 2H), 4.63 (s, 2H), 4.53 (q,
J H-F ) 8.4 Hz, 2H), 2.20 (s, 3H).
In vestiga tion of in Situ La cton iza tion of Tr ia lk yl
P h osp h a te 17a -f. Upon completion of the phosphate addi-
tion to quinone methides 16a -f, the study of in situ lacton-
ization was initially carried out at room temperature over 48
h. Intermediates 17d -f produced the desired lactonized tri-
alkyl phosphate product 18 as indicated by 1H NMR analysis.
However, no lactonized product 18 was produced with 17a and
17c. Less than 5% conversion was observed with 17b by 1H
NMR analysis. The efficiency of in situ lactonization of
intermediates 17d -f was further examined at 35 °C in the
presence of 4 Å molecular sieves (10 mg) over 92 h. The
reaction was monitored by 1H NMR analysis. The percent
conversion was calculated relative to mesitylene, the internal
standard.
2
2
127.5, 124.5, 114.4, 69.4 (d, J P-C ) 5.7 Hz), 68.5 (d, J P-C
)
6.4 Hz), 64.6, 15.2; 31P NMR δ 8.75; MS (DCI, NH3) m/z
(relative intensity) 472 (MNH4+, 34), 455 (MH+, 40), 296 (100),
279 (33), 240 (62), 223 (52), 194 (51), 177 (16), 149 (21), 108
(39), 106 (29), 91 (17); HRMS (DCI, NH3), m/z [MH+] calcd for
C
24H24O7P 455.1260, found 455.1240.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the National Science Foundation
(EHR-9108762 and CHE-9734187) and the Arkansas
Science and Technology Authority (98-B-03).
A detailed analysis of the in situ lactonization of intermedi-
ate 17f (prepared as described above) was studied by 1H NMR
analysis at 35 °C over 90 h to confirm the formation of 18.
Tetrabutylammonium dibenzyl phosphate (1.0 equiv to the
residual dibenzyl phosphoric acid) was added to the reaction
so that the benzylic resonances of trialkyl phosphates 17f and
Su p p or tin g In for m a tion Ava ila ble: NMR spectra for
compounds 7, 9, 11, 13a -g, 14, 15a -f, 15g/14, and 18. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O9915168