Observations on the synthesis of photochromic naphthopyrans

Article abstract of DOI:10.1002/ejoc.200390176

1-Naphthol reacts with 1,1-diarylprop-2-yn-1-ols 5a,b, under alumina catalysis, by two pathways to give the photochromic naphtho[1,2-b]pyrans 6a,b, together with the propenylidenenaphthalenones 7a,b, representatives of a new class of merocyanine dyes. With 2-methyl-1-naphthol, formation of the photochrome is suppressed; the only products are merocyanines 7c,d. The cyclocondensation of 2-naphthol with 5a,b proceeds much more efficiently, to give the naphtho[2,1-b]pyrans 14a,b. Pyran formation is not suppressed from either 1-bromo- or 1-(4 -methoxyphenyl)-2-naphthol; reaction with 5a,b merely results in expulsion of the C-1 substituent. An alternative pathway supervenes in the reaction of 1-methyl-2-naphthol with 5a to give the benz[e]indanone 17, the constitution of which was determined by X-ray crystallography. Reaction of the 1,3,3-triarylpropynols 19a,b with 1-naphthol affords the naphthopyrans 20 together with merocyanines 21, whilst the isomeric pyrans 23 are efficiently produced from 2-naphthol. The configuration of merocyanines 7a and 21a was unequivocally established by X-ray crystallography. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200390176

FULL PAPER
Observations on the Synthesis of Photochromic Naphthopyrans
Christopher D. Gabbutt,[a] B. Mark Heron,*[a] Alicia C. Instone,[a] David A. Thomas,[a]
Steven M. Partington,[b] Michael B. Hursthouse,[c] and Thomas Gelbrich[c]
Keywords: Heterocycles / Naphthopyran / Rearrangements / Photochromism / Dyes
1-Naphthol reacts with 1,1-diarylprop-2-yn-1-ols 5a,b, under
alumina catalysis, by two pathways to give the photochromic
naphtho[1,2-b]pyrans 6a,b, together with the propenylid-
enenaphthalenones 7a,b, representatives of a new class of
merocyanine dyes. With 2-methyl-1-naphthol, formation of
the photochrome is suppressed; the only products are mero-
An alternative pathway supervenes in the reaction of 1-
methyl-2-naphthol with 5a to give the benz[e]indanone 17,
the constitution of which was determined by X-ray crystallo-
graphy. Reaction of the 1,3,3-triarylpropynols 19a,b with 1-
naphthol affords the naphthopyrans 20 together with mero-
cyanines 21, whilst the isomeric pyrans 23 are efficiently pro-
cyanines 7c,d. The cyclocondensation of 2-naphthol with duced from 2-naphthol. The configuration of merocyanines
5a,b proceeds much more efficiently, to give the naphtho[2,1-
b]pyrans 14a,b. Pyran formation is not suppressed from
either 1-bromo- or 1-(4-methoxyphenyl)-2-naphthol; reaction
with 5a,b merely results in expulsion of the C-1 substituent.
7a and 21a was unequivocally established by X-ray crystallo-
graphy.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
Photochromic diarylnaphthopyrans can be accessed in a
single step using a substantially modified version of this
protocol. Thus, heating 1,1-diarylprop-2-yn-1-ols with a
naphthol in toluene containing an acidic catalyst, required
to promote the initial naphthyl propargyl ether formation,
affords the naphthopyrans directly in good yield.[5] This
chemistry has recently been adapted for the solid-state syn-
thesis of naphthopyrans.[6] From our earlier studies on the
formation of photochromic diaryl-substituted 3H-naph-
tho[2,1-b]pyrans 2 and the isomeric 2H-naphtho[1,2-b]pyr-
ans 3 it was apparent that the former were frequently ob-
tained in superior yield and with less extensive by-product
formation than the isomeric 2H-naphtho[1,2-b]pyrans.[7]
We now report our results and observations on the syn-
thesis of some naphthopyrans from the acid-catalysed reac-
tion between a prop-2-yn-1-ol and a naphthol.
Claisen first described the rearrangement of allyl phenyl
ethers to o- or p-allylphenols in 1912.[1] Since this report
the Claisen rearrangement has been widely employed in
synthesis and has featured in many review articles.[2] Our
interest in this rearrangement relates to the preparation of
photochromic naphthopyrans, which are obtained by the
Claisen rearrangement of a naphthyl propargyl ether
formed in situ from the acid-catalysed etherification of a
naphthol with a prop-2-yn-1-ol.[3] The first application of a
Claisen rearrangement to the synthesis of the naphthopyran
system
1 was reported by Iwae and Ide in 1962
(Scheme 1).[4]
Scheme 1
[a]
Results and Discussion
Department of Colour Chemistry, The University of Leeds,
Leeds LS2 9JT, England
[b]
James Robinson Ltd.,
Heating 1-naphthol 4a with a 1,1-bis(4-methoxyphen-
yl)prop-2-yn-1-ol (5a), which was conveniently prepared by
the addition of lithium trimethylsilylacetylide (LTSA) to a
P. O. Box B3, Hillhouse Lane, Huddersfield HD1 6BU, England
[c]
Department of Chemistry, The University of Southampton,
Highfield, Southampton SO17 1BJ, England
1220
 2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ193X/03/0407Ϫ1220 $ 20.00ϩ.50/0 Eur. J. Org. Chem. 2003, 1220Ϫ1230

Observations on the Synthesis of Photochromic Naphthopyrans
FULL PAPER
substituted benzophenone with subsequent removal of the may be rationalised by a para-Claisen rearrangement of the
trimethylsilyl group,[8] in toluene containing acidic alu- intermediate ether, which requires the presence of both an
1
minium oxide for 1.5 h gave a deep-red multi-component exchangeable and an acetylenic signal in its H NMR spec-
gum after workup. Elution of this gum from silica gave the trum. The absolute structure of the red product was un-
expected naphthopyran 6a as pale pink microcrystals in equivocally established by X-ray crystallography (Figure 1)
37% yield (Scheme 2). A more polar, intense red fraction as the enone 7a.[13] The observed s-trans geometry is in
1
was also isolated. The H NMR spectrum of this red com- agreement with the minimisation of steric interactions be-
pound was complex due to a number of overlapping signals tween the diarylprop-2-enyl function and the naph-
in the aromatic region. However, it was apparent this com- thalenone moiety. It is noteworthy that one of the aryl
pound did not contain a pyran ring, since the signals of the groups is significantly twisted out of the main plane of the
AB system of the pyran unit which appear at δ ϭ 6.11 and molecule to alleviate the steric interactions between the or-
6.69 ppm with 9.8 Hz coupling for 3-H and 4-H, respec- tho-protons in a planar array. This contrasts with the tri-
tively, in 6a, were absent. The pyran ring protons of the phenylmethine dye unit where all three of the aryl rings are
2H-naphtho[1,2-b]pyrans typically resonate at δ ϭ 6.1 and twisted out of the plane to minimise steric interactions be-
6.8 ppm with J ഠ 10 Hz.[9] Additionally, the methoxy tween the ortho-protons.[14]
groups of the red product are nonequivalent giving rise to
signals at δ ϭ 3.86 and 3.92 ppm unlike the methoxy groups
of 6a which are equivalent and resonate at δ ϭ 3.76 ppm.
1
The H NMR spectrum of the red product also displays
the following well-resolved signals in the aromatic region: a
doublet of doublets (J ϭ 10.2, 1.5 Hz) at δ ϭ 6.62 ppm, a
doublet (J ϭ 10.2 Hz) at δ ϭ 8.11 ppm and a doublet of
doublets (J ϭ 7.9, 1.6 Hz) at δ ϭ 8.27 ppm. The 13C NMR
spectrum displays a low-field signal at δ ϭ 184.9 ppm which
is indicative of a carbonyl function.[10] The presence of the
CϭO group was corroborated by infrared spectroscopy
which gave a CϭO stretching band at 1630 cmϪ1 suggestive
of an α,β-unsaturated CϭO group.[11] High resolution elec-
tron impact mass spectrometry gave a molecular mass of
395.1642 for [M ϩ H]ϩ (C27H22O3) identical to that re-
quired for the naphthopyran 6a, but did not give a fragmen-
tation pattern in keeping with the presence of a pyran unit.
2H[1]-Benzopyrans and naphthopyrans readily fragment by
loss of one of the geminal substituents to give a pyrylium-
Figure 1. X-ray crystallographic structure of enone 7a
type radical cation which then suffers multiple fragmen-
tation.[12] However, the chemical ionisation mass spectrum
gave very similar fragmentation patterns, with both the
Similar results were obtained when 1-naphthol was
naphthopyran 6a and the unknown compound showing treated with prop-2-yn-1-ol 5b. The naphthopyran 6b was
loss of a 4-methoxyphenyl group. The UV/Vis spectrum of obtained after chromatography as pale blue microcrystals
this unknown exhibits λmax (EtOH) ϭ 462 nm with εm
ϭ
in 43% yield together with an intense red dye 7b [λmax
4.0 ϫ 104 molϪ1·dm3·cmϪ1
.
(EtOH) ϭ 556 nm with εm ϭ 3.8 ϫ 104 molϪ1dm3cmϪ1].
Clearly the spectroscopic data accrued for this unknown The bathochromic shift in λmax of 94 nm compared with
are incongruous with both a pyran ring containing struc- dye 7a is in agreement with the increasing strength of the
ture such as 6a and with compound 8. The formation of 8 donor groups (NMe2 vs. OMe). The signals in the aromatic
Scheme 2
Eur. J. Org. Chem. 2003, 1220Ϫ1230
1221

B. M. Heron et al.
FULL PAPER
1
region of the H NMR spectrum of 7b are well resolved viously suggested the involvement of alkynyl and allenyl
1
and could be unequivocally assigned from a H-1H COSY cations such as 9 and 10, derived from 1,1-diarylprop-2-
experiment. The cis relationship between 2-H (δ ϭ yn-1-ols, in electrophilic addition reactions to dimedone to
6.60 ppm) and 3-H (δ ϭ 8.16 ppm) of 7b was confirmed by afford merocyanine dyes,[8] and allenyl cations derived from
the 10 Hz coupling constant. As in example 7a, the signal 1,1-disubstituted O-(trimethylsilyl)prop-2-yn-1-ols upon re-
for 2-H shows long range coupling (ca. 1.4 Hz) to 1Ј-H. action with trimethylsilyl trifluoromethanesulfonate have
The s-trans arrangement of the propenylidene function was recently been implicated in novel CϪC bond-forming reac-
confirmed by the coupling between 1Ј-H and 2Ј-H of tions.[20] Olah and co-workers have investigated the struc-
11.9 Hz.[8]
ture and reactivity of alkynyl-allenyl cations and have con-
The formation of the naphthopyrans 6 and dyes 7 is out- cluded that such cations may be considered as alkyne-sub-
lined in the mechanism depicted in Scheme 3. Protonation stituted carbocations.[21] The reactivity of such cations has
and loss of water from the propynol 5 generates the carboc- recently been reviewed and their ambident reactivity is
ation 9. The currently accepted mechanism for the forma- markedly dependent upon their substitution pattern.[22]
tion of the naphthopyran follows route A, which com-
It was of interest to attempt to improve the formation of
mences with interception of 9 by 1-naphthol to afford the the novel dye system 7. Clearly, blocking the 2-position of
propargyl naphthyl ether 11. A Claisen rearrangement and 1-naphthol, the normal terminus of the Claisen rearrange-
subsequent enolisation afford the allenylnaphthol 12. The ment, with a methyl group should prevent the formation of
naphthopyran 6 results from a 1,5-H shift and an electro- the pyran ring (routes A and B) and favour route C. Thus,
cyclisation. Alternatively, 9 may be intercepted by either the heating 2-methyl-1-naphthol (4b), obtained by the reductive
2- or 4-position of 1-naphthol (route B and route C, respec- cleavage of 1-acetoxy-2-methylnaphthalene[23] with LiAlH4
tively) in a MeyerϪSchuster[15] or Rupe-like[16] rearrange- in THF, with prop-2-yn-1-ol (5a) gave the new enone 7c in
ment. The overall process is suggestive of participation of an improved but still low yield (19%) together with some
an allenyl cation 10. In the former route the common allen- unchanged starting material. The 1H NMR spectrum of
ylnaphthol intermediate 12 is obtained, which ultimately af- this compound displays a singlet at δ ϭ 2.2 ppm assigned
fords 6. However, in the latter an alternative allenylnaph- to the methyl group and a singlet at δ ϭ 8.0 ppm assigned
thol, 13, is obtained, which undergoes a 1,7-H shift to af- to 3-H. 1Ј-H appears as a doublet at δ ϭ 7.60 ppm with J ϭ
ford the dye 7. The loss of aromaticity in one ring of the 12.0 Hz, indicative of the s-trans arrangement; the signal for
allenylnaphthol 13 is presumably balanced by the stabilis- 2Ј-H is superimposed on the aromatic signals. The corre-
ation gained on generation of the extended conjugated sys- sponding dye from 4b and prop-2-yn-1-ol 5b was obtained
tem in 7.
in 27% yield. The greater yield of both the dyes and naph-
thopyrans derived from propynol 5b would infer that the
formation of the cations (9 and 10) is favoured when the
aryl rings contain efficient electron-releasing groups.
We next investigated the Claisen rearrangement of naph-
thyl propargyl ethers derived from 2-naphthols and diaryl-
prop-2-yn-1-ols. The reaction of 2-naphthol (4c) with propy-
nol 5a under the standard conditions gave the photoch-
romic naphthopyran 14a in 82% yield together with a small
amount of the unsaturated aldehyde 15a after elution from
silica and recrystallisation (Scheme 4). Repeating the reac-
tion with propynol 5b gave the expected naphthopyran 14b
1
(85%) and the aldehyde 15b (4%). The H NMR spectra
of these 3H-naphtho[2,1-b]pyrans 14 are similar, with 2-H
resonating at δ ഠ 6.2 ppm, upfield of the signal for 1-H,
which appears at about δ ϭ 7.3 ppm and with J1,2
ϭ
10 Hz.[9] The formation of the aldehydes 15 can be rational-
ised by interception of the alkyne-substituted carbocation 9
(Schemes 3 and 4) with water and subsequent tautomeris-
Scheme 3
Claisen rearrangements are markedly dependent upon ation (a MeyerϪSchuster rearrangement[15]). It is note-
the reaction solvent[17] and are particularly facile in tri- worthy that higher yields of the naphthopyrans were ob-
fluoroacetic acid.[18] However, it has been suggested that served when 2-naphthol was employed and also that a
when Claisen rearrangements are performed with an added greater amount of the α,β-unsaturated aldehyde was ob-
Lewis acid catalyst, the mechanism may no longer be cyclic tained when a more electron-rich propynol (5b) was used.
and instead follows the typical FriedelϪCrafts-type pro-
We explored the reaction between 1-bromo-2-naphthol
cess.[19] It may well be the case that under the present reac- (4d) and 5a in which the terminus of the Claisen rearrange-
tion conditions both electrophilic aromatic substitution (of ment is blocked with a bromine atom. The major product
1-naphthol by the alkyne-substituted carbocation 9) and a isolated from the reaction mixture was the naphthopyran
Claisen cyclic mechanism are in operation. We have pre- 14a in a yield of 19%; a considerable amount of intractable
1222
Eur. J. Org. Chem. 2003, 1220Ϫ1230

Observations on the Synthesis of Photochromic Naphthopyrans
FULL PAPER
Examination of the literature revealed that substituted
cyclopentanones have been observed from the Claisen re-
arrangement of propargylic ethers 18a[26] and 18b[27] in
which the favoured terminus of the rearrangement is
blocked. A multi-step sequence has been proposed to ac-
count for this remarkable annelation reaction.[26]
Scheme 4
polar material remained irreversibly bound to the chroma-
tography silica. The elimination of the bromine atom as Brϩ
from 1-bromo-2-naphthol during the acid-catalysed forma-
tion of fluorans has been reported.[24]
1-(4-Methoxyphenyl)-2-naphthol (4e) was conveniently
obtained in 56% yield by the Suzuki coupling[25] of 4d with
(4-methoxyphenyl)boronic acid. Heating 4e with propynol
5a followed by elution of the complex reaction mixture
With this information to hand we proposed the substi-
from silica also gave naphthopyran 14a in low yield (8%); tuted indanone 17 as the crystalline product from 4f and
no pure components, other than some recovered 4e and 5a, 5a. The structure of 17[28] was confirmed by X-ray crystal-
were isolated. It is likely that the 4-methoxyphenyl function lography (Figure 2).
has been lost as the resonance-stabilised carbocation, [4-
MeOC6H4]ϩ, which would have been intercepted by any nu-
cleophilic species in the reaction mixture. However, examin-
ation of the crude reaction mixture by GC-MS for compo-
nents that contained the MeOC6H4 fragment was incon-
clusive.
The reaction between 1-methyl-2-naphthol (4f) and pro-
pynol 5a was next investigated (Scheme 5) since the likeli-
hood of elimination of CH3ϩ is remote. A single pure major
1
component was obtained by flash chromatography. The H
NMR spectrum is particularly simple and displays a doub-
let at δ ϭ 1.54 ppm (J ϭ 7.5 Hz) coupled with a quadruplet
at δ ϭ 4.07 ppm. A singlet at δ ϭ 3.75 ppm and another at
δ ϭ 3.78 ppm, each accounting for a methoxy group, are
also present. The 14 aromatic protons appear in the range
δ ϭ 6.7Ϫ7.9 ppm. Infrared spectroscopy (ν˜max ϭ 1746
cmϪ1) and 13C NMR spectroscopy (δCϭO ϭ 218.8 ppm)
confirm the presence of a CϭO group in the molecule. Elec-
tron impact mass spectrometry gave [Mϩ] at m/z ϭ
408.1719 corresponding to
a molecular formula of
Figure 2. X-ray crystallographic structure of benzindanone 17
C28H24O3. Whilst this molecular formula would accommo-
date a linear naphthopyran of structure 16 the remaining
spectroscopic data would suggest an alternative structure.
The involvement of a diarylpropargyl naphthyl ether 11
(Scheme 3) in the mechanism proposed for the formation of
the naphthopyrans is widely accepted.[3] However, to the
best of our knowledge the isolation of such diaryl-substi-
tuted ethers has never been reported, whereas simple naph-
thyl propargyl ethers have been obtained.[4] The Mitsunobu
coupling reaction offers an efficient route for the coupling
of phenols with alcohols to form alkyl aryl ethers using the
ylide generated in situ from Ph3P and diethyl azodicarbox-
ylate (DEAD).[29] The mild conditions typically used to ef-
fect this transformation (room temp. and ether solvents)
were thought to be ideal for the preparation of ethers 11.
Thus, stirring a solution of propynol 5b with 2-naphthol in
anhydrous THF containing Ph3P and DEAD according to
Scheme 5
Eur. J. Org. Chem. 2003, 1220Ϫ1230
1223

B. M. Heron et al.
FULL PAPER
the general procedure[30] gave a complex mixture (TLC) singlet at δ ϭ 6.96 ppm, assigned to the alkenic proton of
which, remarkably, contained a photochromic component. 24. Complete rearrangement of propynol 19b to 24 was ef-
Aqueous workup and elution of the crude mixture from fected on heating 19b in toluene containing acidic alumina
silica gave some recovered 2-naphthol and alkynol 5b to- for 2 h (Scheme 6). Heating 19a with 1-naphthol under the
gether with the naphthopyran 14b in low yield (16%). None standard conditions gave two pure fractions after elution
of the desired propargyl ether was obtained. The use of a from silica. The least-polar fraction was identified as the
weak acid catalyst has recently been advocated to improve naphthopyran 20a (32%) with 3-H resonating at δ ϭ
the yield of the Mitsunobu reaction.[31] When the procedure 6.11 ppm as a singlet. The more polar, major fraction, was
was repeated with the addition of 0.2 equiv. of 4-nitrophe- intensely coloured [λmax (EtOH) ϭ 471 nm, εmax ϭ 2.6 ϫ
nol the naphthopyran 14b was obtained in an improved 104 molϪ1dm3cmϪ1] and was presumed to be the dye 21a
yield (27%). The formation of the naphthopyran under rather than the isomer 22a in which there is significant
these mild conditions is quite remarkable and would ac- crowding between the peri-proton (5-H) of the naphthalene
count for the fact that ethers such as 11 have not been ob- unit and the proximate aryl group at C-1Ј. The geometry
served, since under the standard conditions employed for of the dye 21a[33] was determined by X-ray crystallography
the synthesis of naphthopyrans (PhMe, heat, Hϩ cat.), they (Figure 3). It is significant that in this triaryl-substituted
readily rearrange. Investigation of the modified coupling merocyanine dye all three of the aryl rings are twisted out
procedure with 1-naphthol and 5b gave a complex intensely of the main plane of the molecule to alleviate steric interac-
coloured product from which the naphthopyran 6b (11%) tions. It is noteworthy that the dye 21a is the major compo-
and the dye 7b in 3% yield were isolated.
nent (36%) obtained from the reaction, a feature that im-
There are relatively few examples of the synthesis of tri- plies that a FriedelϪCrafts-type process predominates.
aryl-substituted naphthopyrans from naphthols and triaryl-
substituted propynols.[32] Triaryl-substituted propynols 19a,
b were conveniently obtained by the addition of a lithium
arylacetylide to 4,4Ј-dimethoxybenzophenone. Propynol
19a was obtained as a viscous gum (93%) that gradually
solidifies on standing. Repeating the procedure with lithium
(4-methoxyphenyl)acetylide afforded 19b (92%) also as a
viscous gum, but which does not solidify on standing. In
an attempt to encourage crystallisation a small quantity of
this propynol was eluted from flash chromatography silica.
1
The H NMR spectrum of the resulting compound indi-
cated that some 19b had rearranged to afford the α,β-un-
saturated ketone 24, the methoxy groups of which are non-
equivalent and give rise to signals at δ ϭ 3.73, 3.76 and
3.78 ppm, whereas in propynol 19b the methoxy groups of
the geminal aryl rings are equivalent and give rise to a signal
at δ ϭ 3.80 ppm, with the remaining methoxy group reson-
ating at δ ϭ 3.83 ppm. The most striking difference between
1
the H NMR spectra of 24 and 19b is the presence of a Figure 3. X-ray crystallographic structure of 21a
Scheme 6
1224
Eur. J. Org. Chem. 2003, 1220Ϫ1230

Observations on the Synthesis of Photochromic Naphthopyrans
FULL PAPER
A similar result was noted when tris(4-methoxyphe-
nyl)prop-2-yn-1-ol (19b) was treated with 1-naphthol, with
the naphthopyran 20b isolated in 25% yield. 3-H in this
compound resonates slightly upfield of that for 20a at δ ϭ
6.07 ppm. The dye 21b, isolated in 42% yield, has λmax
slightly bathochromically shifted [λmax (EtOH) ϭ 479 nm,
Conclusion
The reaction of 1,1-diaryl- and 1,1,3-triarylprop-2-yn-1-
ols with 1-naphthol proceeds to afford 2H-naphtho[1,2-
b]pyrans in moderate yield together with a novel merocyan-
ine dye system. Naphthopyran formation is suppressed
when 2-methyl-1-naphthol is treated with a prop-2-yn-1-ol.
The formation of these merocyanines is rationalised by nu-
cleophilic interception of an alkyne-substituted carbocation
9 by the 4-position of 1-naphthol and a subsequent 1,7-H
shift. A similar process initiated by interception of the al-
kyne-substituted cation by C-2 of 1-naphthol may operate
1
∆λ ϭ 8 nm] compared with 21a. In the H NMR spectra
of dyes 21 the coupling constant for the cis-disposed pro-
tons, 2-H and 3-H, is about 10.2 Hz, and 8-H resonates
furthest downfield at δ ϭ 8.44 ppm due to the proximity of
these protons to the peri-carbonyl function.
Heating 2-naphthol with propynols 19a and 19b gave the
3H-naphtho[2,1-b]pyrans 23a,b in 76 and 97% yield, respec- to afford an allenylnaphthol which leads to the naphthopy-
tively. In the 1H NMR spectra of these naphthopyrans 2-H
resonates at δ ഠ 6.1 ppm. The furthest downfield signal in
the 1H NMR spectra of 23a,b appears at δ ഠ 7.7 ppm,
shifted upfield from the corresponding signal in naphthopy-
ran 14a (δ ϭ 7.93 ppm) and is assigned to 10-H. It would
appear that 10-H lies in the shielding zone of the 1-aryl
ring, suggesting that the disposition of this ring must ap-
proach perpendicularity with respect to the major plane of
the molecule.[34] None of the rearranged product 24 was
isolated from either of these reaction mixtures.
The photochromic properties of the simple naphthopyr-
ans 6 and 14 derived from the 1,1-diarylprop-2-yn-1-ols 5
are well documented.[3] The novel isomeric triaryl-substi-
tuted naphthopyrans 20 and 23 derived from the 1,1,3-triar-
ylprop-2-yn-1-ols 19 failed to display any photochromic re-
sponse in toluene solution at room temperature. It was
thought that the thermal reversion of the ring-opened forms
to the pyran ring would be rapid at room temperature as a
consequence of the steric pressures exerted in the crowded
isomeric ring-opened forms, which are minimised in the py-
ran tautomers. This results in a photostationary state with
an undetectably low level of the ring-opened isomers. Re-
examination of the toluene solutions after cooling to Ϫ30
°C also failed to give a detectable photochromic response
(Scheme 7).
ran, a mechanism that may occur in tandem with the widely
accepted mechanism for the formation of naphthopyrans
involving a Claisen rearrangement.
The 3H-naphtho[2,1-b]pyrans are obtained more ef-
ficiently than their [1,2-b]-isomers, from the corresponding
reaction of a prop-2-yn-1-ol with a 2-naphthol. Only a
small amount of an α,β-unsaturated aldehyde accompanied
the formation of the 3H-naphtho[2,1-b]pyrans. When the 1-
position of 2-naphthol is substituted with a Br atom or a
4-methoxyphenyl group, the naphthopyran is obtained in
reduced yield through expulsion of the 1-substituent. How-
ever, when 1-methyl-2-naphthol is employed a complex re-
arrangement ensues to afford a benz[e]indanone.
Experimental Section
General: Flash chromatography was performed using chromatogra-
phy silica (40Ϫ63 micron particle size distribution) supplied by
Fluorochem Ltd. according to the published procedure.[35] Melting
points were recorded in capillaries and are uncorrected. Infrared
spectra were recorded with a PerkinϪElmer 882 infrared spectro-
photometer in KBr discs unless otherwise specified. NMR spectra
were recorded in CDCl3, unless stated otherwise, using a Bruker
Avance 400 MHz instrument; coupling constants are quoted in Hz.
Visible spectra were recorded in either spectroscopic grade ethanol
Scheme 7
Eur. J. Org. Chem. 2003, 1220Ϫ1230
1225

B. M. Heron et al.
FULL PAPER
or toluene in 1-cm quartz cells using a PerkinϪElmer λ5 spectro-
photometer.
Preparation of 1,1-Diarylprop-2-yn-1-ols: n-Butyllithium (2.5  in
hexanes) (33 mmol) was added slowly with a syringe to a cold (Ϫ10
°C), stirred solution of (trimethylsilyl)acetylene (33 mmol) in anhy-
drous tetrahydrofuran (100 mL) under nitrogen. On completion of
the addition (ca. 5 min) the cold solution was stirred for 1 h. The
benzophenone (30 mmol) slurried in anhydrous tetrahydrofuran
(50 mL) was then added in a single portion and the mixture stirred
until no benzophenone remained by TLC (ca. 3 h.). The reaction
mixture was then recooled to 0 °C and a solution of methanolic
potassium hydroxide was added [from potassium hydroxide
(40 mmol) in methanol (30 mL)] in a single portion. The cooling
bath was then removed and the mixture warmed to room tempera-
ture, after ca. 15 min TLC indicated that deprotection was com-
plete. The mixture was acidified to pH ഠ 7 using glacial acetic
acid and then poured into water (400 mL). The organic layer was
separated and the aqueous layer extracted with ethyl acetate (3 ϫ
75 mL), the organic phases were combined, washed with water (2
ϫ 100 mL) and dried (anhyd. Na2SO4). Removal of the solvent
gave the prop-2-yn-1-ol, which was sufficiently pure for subsequent
use. Analytically pure compounds were obtained by recrystallis-
ation from hexane and ethyl acetate. The following alkynols were
obtained in this way.
Crystal Structure Determination: Details of the crystal data and a
summary of data collection parameters for compounds 7a, 17a and
21a are given below.[13,28,33] Intensity data were recorded at 150 K,
using a Nonius-Kappa CCD area-detector diffractometer mounted
at the window of a rotating anode FR591 generator with a molyb-
˚
denum anode (λ ϭ 0.71073 A); ϕ- and ω-scans were carried out to
fill the Ewald sphere. An empirical absorption correction was ap-
plied using SORTAV. The structures were solved by direct methods
and refined on F2 by full-matrix least-squares refinements.[36]
Preparation of Intermediates
2-Methyl-1-naphthol (4b): A solution of 1-acetoxy-2-methylnaph-
thalene (25 mmol) in anhydrous THF (15 mL) was added over 30
min to a suspension of LiAlH4 (50 mmol) in anhydrous THF (50
mL). The resulting suspension was refluxed for 1 h. EtOAc (5 mL)
was cautiously added to the cooled (5 °C) reaction mixture fol-
lowed by water (100 mL). The resulting mixture was acidified (HCl,
2  aq.) and extracted with EtOAc (3 ϫ 50 mL). The combined
organic extracts were washed with water (100 mL), dried (anhyd.
Na2SO4) and the solvents evaporated to yield 4b as an off-white
solid which gradually darkened on standing in air (2.77 g, 70%),
1,1-Bis(4-methoxyphenyl)prop-2-yn-1-ol (5a): From 4,4Ј-dimeth-
oxybenzophenone. Isolated as an off-white powder (7.56 g, 94%)
after recrystallisation from ethyl acetate and hexane, m.p.
90.0Ϫ92.0 °C (ref.[40] 89.0Ϫ91.5 °C). IR (KBr): ν˜max ϭ 3472, 3245,
1
m.p. 58Ϫ60 °C (ref.[37] 61.5Ϫ62 °C). H NMR: δ ϭ 2.41 (s, 3 H,
2-Me), 5.10 (br. s, 1 H, OH), 7.23 (m, 1 H, 4-H), 7.44 (m, 3 H, Ar-
H), 7.76 (d, J ϭ 7.8 Hz, 1 H, 5-H), 8.10 (d, J ϭ 7.8 Hz, 1 H, 8-
H) ppm.
1
1608 cmϪ1. H NMR: δ ϭ 2.76 (s, 1 H, alkynic-H), 2.86 (s, 1 H,
OH), 3.79 (s, 6 H, OMe), 6.85 (m, 4 H, Ar-H), 7.49 (m, 4 H, Ar-
H) ppm.
1-(4-Methoxyphenyl)-2-naphthol
(4e):
1-Bromo-2-naphthol
(9.0 mmol) was added to a suspension of Pd(PPh3)4 (0.3 mmol) in
1,2-dimethoxyethane (40 mL) and the mixture stirred for 10 min.
To this mixture was added (4-methoxyphenyl)boronic acid
(13.5 mmol) in ethanol (5 mL) followed by aqueous sodium car-
bonate (2 , 9 mL). The resulting solution was refluxed until no
starting material remained (ca. 4 h). The solution was cooled to
room temperature, poured into water (500 mL), and then acidified
to pH ഠ 4 by addition of hydrochloric acid before extraction with
ethyl acetate (4 ϫ 50 mL). The organic extracts were dried (anhyd.
Na2SO4) and the solvent was removed to give a brown tar. Elution
from silica with 30% ethyl acetate/hexane afforded 4e as fine white
microcrystals (1.26 g, 56%), m.p. 101Ϫ103 °C (ref.[39] 98.5Ϫ100.5
°C). 1H NMR: δ ϭ 3.91 (s, 3 H, OMe), 5.17 (s, 1 H, OH), 7.12 (m,
2 H, Ar-H), 7.32 (m, 5 H, Ar-H), 7.78 (m, 3 H, Ar-H) ppm.
HRMS: [M ϩ H]ϩ 250.0991, C17H14O2 requires [M ϩ H]ϩ
250.0994.
1,1-Bis(4-dimethylaminophenyl)prop-2-yn-1-ol (5b): From 4,4Ј-
bis(dimethylamino)benzophenone. Isolated as an off-white powder
(7.59 g, 86%) after recrystallisation from ethyl acetate and hexane,
m.p. 158.0Ϫ160.0 °C (ref.[41] 154Ϫ155 °C). IR (Nujol): ν˜max
ϭ
3351, 3252, 2110, 1611 cmϪ1. H NMR: δ ϭ 2.75 (s, 1 H, alkynic-
H), 2.97 (br. s, 1 H, OH), 3.23 [s, 12 H, (NMe2)2], 6.88 (m, 4 H,
Ar-H), 7.64 (m, 4 H, Ar-H) ppm.
1
Preparation of 1,1,3-Triarylprop-2-yn-1-ols: n-Butyllithium (2.5 
in hexanes) (33 mmol) was added slowly with a syringe to a cold
(Ϫ10 °C), stirred solution of the arylacetylene (33 mmol) in anhy-
drous tetrahydrofuran (100 mL) under nitrogen. On completion of
the addition (ca. 5 min) the cold solution was stirred for 1 h. The
benzophenone (30 mmol) slurried in anhydrous tetrahydrofuran
(50 mL) was then added in a single portion and the mixture stirred
until no benzophenone remained by TLC (ca. 2 h.). The mixture
was poured into water (300 mL) containing ammonium chloride
solution (50 mL aq. sat.). The organic layer was separated and the
aqueous layer extracted with ethyl acetate (3 ϫ 75 mL), the organic
phases were combined, washed with water (2 ϫ 100 mL) and dried
(anhyd. Na2SO4). Removal of the solvent gave the triarylprop-2-
yn-1-ol, which was sufficiently pure for subsequent use. Analyti-
cally pure material was obtained by recrystallisation from hexane
and ethyl acetate. The following alkynols were obtained in this way.
1-Methyl-2-naphthol (4f): A stirred solution of 2-methoxy-1-naph-
thaldehyde (54 mmol), hydrazine hydrate (100 mmol) in diethylene
glycol (60 mL) was heated at 100 °C for 5 min and then cooled
to room temperature. KOH (54 mmol) was added and the mixture
refluxed for 1.5 h. The cooled mixture was poured into water (400
mL) and extracted with toluene (5 ϫ 50 mL). The combined ex-
tracts were washed with HCl (3 ϫ 50 mL, 2  aq.), water (2 ϫ 50
mL) and dried (anhyd. Na2SO4). Removal of the toluene gave crude
2-methoxy-1-methylnaphthalene, which was refluxed for 30 min
with pyridine hydrochloride (265 mmol). The cooled mixture was
diluted with water (200 mL) and extracted with CH2Cl2 (5 ϫ 50
mL). Removal of the dried (anhyd. Na2SO4) solvent gave 4f as a
pale brown solid (6.74 g, 79%), m.p. 105Ϫ107 °C (ref.[38] 110Ϫ111
1,1-Bis(4-methoxyphenyl)-3-phenylprop-2-yn-1-ol (19a): From 4,4Ј-
dimethoxybenzophenone and phenylacetylene. Isolated as a viscous
oil (9.60 g, 93%) which gradually solidified on standing m.p. 92Ϫ94
°C (ref.[42] 96 °C). IR (KBr): ν˜max ϭ 3442, 1606, 1243 cmϪ1 1H
.
NMR: δ ϭ 2.79 (s, 1 H, OH), 3.79 (s, 6 H, OMe), 6.86 (m, 4 H,
Ar-H), 7.31 (m, 3 H, Ar-H), 7.50 (m, 6 H, Ar-H) ppm. HRMS:
[Mϩ] 344.1409, C23H20O3 requires [Mϩ] 344.1412(5).
1
°C). H NMR: δ ϭ 2.52 (s, 3 H, 1-Me), 4.56 (br. s, 1 H, OH), 7.07
(d, J ϭ 8.8 Hz, 1 H, Ar-H), 7.32 (m, 1 H, Ar-H), 7.47 (m, 1 H,
Ar-H), 7.58 (d, J ϭ 8.8 Hz, 1 H, Ar-H), 7.73 (dd, J ϭ 8.5, 1.4 Hz,
1 H, Ar-H), 7.88 (dd, 1 H, J, 8.7, 1.4, Ar-H) ppm.
1,1,3-Tris(4-methoxyphenyl)prop-2-yn-1-ol (19b): From 4,4Ј-di-
methoxybenzophenone and 4-methoxyphenylacetylene. Isolated as
1226
Eur. J. Org. Chem. 2003, 1220Ϫ1230

Observations on the Synthesis of Photochromic Naphthopyrans
FULL PAPER
a viscous oil (10.32 g, 92%). IR (KBr): ν˜max ϭ 3677, 1242 cmϪ1
.
[Mϩ] 420.2201, C29H28N2O requires [Mϩ] 420.2202. C29H28N2O
1H NMR: δ ϭ 2.88 (br. s, 1 H, OH), 3.80 (s, 6 H, OMe), 3.83 (s, (420.30): calcd. C 82.8, H 6.7, N 6.7; found C 82.6, H 6.6, N 6.4.
3 H, OMe), 6.85 (m, 6 H, Ar-H), 7.41 (m, 2 H, Ar-H), 7.55 (m, 4
3: From 2-methyl-1-naphthol and 1,1-bis(4-methoxyphenyl)prop-2-
H, Ar-H) ppm. HRMS: [M ϩ H]ϩ 375.1594, C24H22O4 requires
yn-1-ol after elution from silica with 20% EtOAc/hexane. Fraction
[M ϩ H]ϩ 375.1596. C24H22O4 (374.22): calcd. C 77.0, H 5.93;
1:
4-[(E)-3Ј,3Ј-Bis(4-methoxyphenyl)-2-methylprop-2-enylidene]-
found C 76.6, H 5.8.
4H-naphthalen-1-one (7c) as orange needles from EtOAc/hexane
(248 mg, 19%), m.p. 198Ϫ200 °C. IR (KBr): ν˜max ϭ 1626 cmϪ1
.
General Method for the Acid-Catalysed Reaction of Naphthols with
Prop-2-yn-1-ols: A stirred solution of the naphthol (3.2 mmol) and
the prop-2-yn-1-ol (3.2 mmol) in toluene (40 mL) was warmed to
50 °C. Acidic alumina (2.5 g) was added and the mixture was re-
fluxed until none of the prop-2-yn-1-ol remained by TLC (ca. 1.5
h). The cooled mixture was filtered and the alumina was washed
with hot toluene (2 ϫ 50 mL). Removal of the toluene from the
combined washings and filtrate gave a deep red gum that was eluted
from silica to afford the pure products. The following compounds
were obtained using this protocol.
UV (EtOH): λmax (εm) ϭ 458 nm (4.4 ϫ 104 molϪ1dm3cmϪ1). H
NMR: δ ϭ 2.24 (s, 3 H, 2-Me), 3.85 (s, 3 H, OMe), 3.92 (s, 3 H,
OMe), 6.90 (m, 2 H, Ar-H), 7.00 (m, 2 H, Ar-H), 7.26 (m, 3 H,
Ar-H, 2Ј-H), 7.43 (m, 4 H, Ar-H), 7.60 (d, J ϭ 12.0 Hz, 1 H, 1Ј-
H), 7.66 (d, J ϭ 7.9 Hz, 1 H, Ar-H), 7.97 (s, 1 H, 3-H), 8.29 (dd,
J ϭ 7.8, 1.3 Hz, 1 H, 8-H). HRMS: [M ϩ H]ϩ 409.1809, C28H24O3
requires [M ϩ H]ϩ 409.1803. C28H24O3 (408.24): calcd. C 82.3, H
5.9; found C 82.1, H 5.7.
1
4: From 2-methyl-1-naphthol and 1,1-bis(4-dimethylaminophenyl)-
prop-2-yn-1-ol after elution from silica 20% EtOAc/hexane. Frac-
tion 1: 4-[(E)-3Ј,3Ј-Bis(4-dimethylaminophenyl)-2-methylprop-2-en-
ylidene]-4H-naphthalen-1-one (7d) as deep red crystals from
EtOAc/hexane (375 mg, 27%), m.p. 235Ϫ236 °C. IR (KBr): ν˜max ϭ
1650 cmϪ1. UV (EtOH): λmax (εmax) ϭ 542 nm (4.6 ϫ 104
1: From 1-naphthol and 1,1-bis(4-methoxyphenyl)prop-2-yn-1-ol
after elution from silica with 40% EtOAc/hexane. Fraction 1: 2,2-
Bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran (6a) as salmon-
pink microcrystals from EtOAc/hexane (467 mg, 37%), m.p.
137Ϫ138 °C (ref.[44] 143Ϫ145 °C). IR (KBr): ν˜max ϭ 1607, 1252,
1176 cmϪ1 1H NMR: δ ϭ 3.76 (s, 6 H, OMe), 6.11 (d, J ϭ 9.8
.
1
molϪ1dm3cmϪ1). H NMR: δ ϭ 2.24 (s, 3 H, 2-Me), 3.04 (s, 6 H,
Hz, 1 H, 3-H), 6.69 (d, J ϭ 9.8 Hz, 1 H, 4-H), 6.83 (m, 4 H, Ar-
H), 7.14 (d, J ϭ 8.2 Hz, 1 H, Ar-H), 7.31 (d, J ϭ 8.1 Hz, 1 H, Ar-
H), 7.42 (m, 6 H, Ar-H), 7.70 (dd, J ϭ 7.5, 1.8 Hz, 1 H, Ar-H),
8.30 (dd, J ϭ 7.6, 1.5 Hz, 1 H, Ar-H) ppm. Fraction 2: 4-[(E)-3Ј,3Ј-
Bis(4-methoxyphenyl)prop-2-enylidene]-4H-naphthalen-1-one (7a)
as maroon needles from EtOAc/hexane (12.6 mg, 1%), m.p.
148Ϫ150 °C. IR (KBr): ν˜max ϭ 1630 cmϪ1. UV (EtOH): λmax
(εm) ϭ 462 nm (4.0 ϫ 104 molϪ1dm3cmϪ1). 1H NMR: δ ϭ 3.86 (s,
3 H, OMe), 3.92 (s, 3 H, OMe), 6.62 (dd, J ϭ 10.2, 1.5 Hz, 1 H,
2-H), 6.91 (m, 2 H, Ar-H), 7.02 (m, 2 H, Ar-H), 7.27 (m, 2 H, Ar-
H), 7.36 (m, 2 H, Ar-H), 7.45 (m, 2 H, Ar-H, alkenic-H), 7.53 (m,
1 H, Ar-H), 7.68 (m, 2 H, Ar-H, alkenic-H), 8.11 (d, J ϭ 10.2 Hz,
1 H, 3-H), 8.27 (dd, J ϭ 7.9, 1.6 Hz, 1 H, 8-H), δC ϭ 55.4, 113.8,
113.9, 120.6, 121.9, 126.6, 126.7, 127.3, 127.6, 130.2, 131.2, 131.6,
132.6, 134.6, 135.6, 136.0, 136.1, 152.2, 160.2, 160.7, 185.0 ppm.
HRMS: [M ϩ H]ϩ 395.1642, C27H22O3 requires [M ϩ H]ϩ
395.1647. C27H22O3 (394.22): calcd. C 82.2, H 5.6; found C 81.9,
H 5.4.
NMe2), 3.08 (s, 6 H, NMe2), 6.70 (m, 2 H, Ar-H), 6.78 (m, 2 H,
Ar-H), 7.25 (m, 2 H, Ar-H), 7.37 (m, 3 H, Ar-H, 2Ј-H), 7.47 (m, 2
H, 6-H, 7-H), 7.73 (d, J ϭ 7.5 Hz, 1 H, Ar-H), 7.76 (d, J ϭ 12.0
Hz, 1 H, 1Ј-H), 8.02 (s, 1 H, 3-H), 8.31 (dd, J ϭ 7.8, 1.4 Hz, 1 H,
8-H) ppm. HRMS: [M ϩ H]ϩ 435.2433, C30H30N2O requires [M
ϩ H]ϩ 435.2463. C30H30N2O (434.32): calcd. C 82.9, H 7.0, N 6.4;
found C 82.7, H 6.6, N 6.2.
5: From 2-naphthol and 1,1-bis(4-methoxyphenyl)prop-2-yn-1-ol
after elution from silica with 30% EtOAc/hexane. Fraction 1: 3,3-
Bis(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran (14a) as colourless
crystals from EtOAc/hexane (1.03 g, 82%), m.p. 173Ϫ174.5 °C
(ref.[45] 175Ϫ176 °C). IR (KBr): ν˜max ϭ 16808, 1250, 1176 cmϪ1
.
1
UV (PhMe): λmax ϭ 475 nm. H NMR: δ ϭ 3.75 (s, 6 H, OMe),
6.19 (d, J ϭ 10.0 Hz, 1 H, 2-H), 6.82 (m, 4 H, Ar-H), 7.15 (d, J ϭ
8.8 Hz, 1 H, 5-H), 7.27 (d, J ϭ 10.0 Hz, 1 H, 1-H), 7.29 (m, 1 H,
Ar-H), 7.38 (m, 4 H, Ar-H), 7.43 (m, 1 H, Ar-H), 7.62 (d, J ϭ 8.8
Hz, 1 H, 6-H), 7.69 (d, J ϭ 7.8 Hz, 1 H, Ar-H), 7.93 (d, J ϭ 8.6
Hz, 1 H, 10-H) ppm. Fraction 2: 3,3-Bis(4-methoxyphenyl)prop-2-
enal (15a) as orange-red crystals from EtOAc/hexane (8.6 mg, 1%),
m.p. 58Ϫ60 °C (ref.[46] 56Ϫ57 °C). IR (KBr): ν˜max ϭ 1659, 1609
2: From 1-naphthol and 1,1-bis(4-dimethylaminophenyl)prop-2-yn-
1-ol after elution from silica 20% EtOAc/hexane. Fraction 1: 2,2-
Bis(4-dimethylaminophenyl)-2H-naphtho[1,2-b]pyran (6b), as pale
blue microcrystals from EtOAc/hexane (578 mg, 43%), m.p.
203Ϫ205 °C. IR (KBr): ν˜max ϭ 1608, 1359, 1226, 1166 cmϪ1. UV
1
cmϪ1. H NMR: δ ϭ 3.84 (s, 3 H, OMe), 3.88 (s, 3 H, OMe), 6.49
(d, J ϭ 8.1 Hz, 1 H, 2-H), 6.89 (m, 2 H, Ar-H), 6.96 (m, 2 H, Ar-
H), 7.24 (m, 2 H, Ar-H), 7.32 (m, 2 H, Ar-H), 9.48 (d, J ϭ 8.1 Hz,
1 H, 1-H) ppm.
1
(PhMe): λmax ϭ 567, 447 nm. H NMR (CD3CO2D): δ ϭ 3.12 (s,
12 H, NMe2), 6.36 (d, J ϭ 9.8 Hz, 1 H, 3-H), 6.87 (d, J ϭ 9.8 Hz,
1 H, 4-H), 7.23 (d, J ϭ 8.4 Hz, 1 H, Ar-H), 7.41 (m, 5 H, Ar-H),
7.51 (m, 2 H, Ar-H), 7.66 (m, 4 H, Ar-H), 7.77 (d, J ϭ 7.9 Hz, 1
H, Ar-H), 8.37 (d, J ϭ 8.2 Hz, 1 H, 10-H) ppm. HRMS: [Mϩ]
420.2209, C29H28N2O requires [Mϩ] 420.2202. C29H28N2O
(420.30): calcd. C 82.8, H 6.7, N 6.7; found C 82.6, H 6.4, N 6.5.
Fraction 2: 4-[(E)-3Ј,3Ј-Bis(4-dimethylaminophenyl)prop-2-enylid-
ene]-4H-naphthalen-1-one (7b) as lustrous maroon crystals from
EtOAc/hexane (134 mg, 10%), m.p. 137Ϫ138 °C. IR (KBr): ν˜max ϭ
1626 cmϪ1. UV (EtOH): λmax (εmax) ϭ 556 nm (3.8 ϫ 104
6: From 1-bromo-2-naphthol and 1,1-bis(4-methoxyphenyl)prop-2-
yn-1-ol after elution from silica with 30% EtOAc/hexane. Fraction
1: 3,3-Bis(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran (14a) as
colourless crystals from EtOAc/hexane (240 mg, 19%), m.p.
173Ϫ174 °C (ref.[45] 175Ϫ176 °C). Spectroscopically identical in all
aspects to the sample prepared previously.
7: From 1-(4-methoxyphenyl)-2-naphthol and 1,1-bis(4-methoxy-
phenyl)prop-2-yn-1-ol after elution from silica with 5% EtOAc/
PhMe. Fraction 1: 3,3-Bis(4-methoxyphenyl)-3H-naphtho[2,1-b]py-
ran (14a) as colourless crystals from EtOAc/hexane (101 mg, 8%),
m.p. 173Ϫ175 °C (ref.[45] 175Ϫ176 °C). Spectroscopically identical
in all aspects to the sample prepared previously.
1
molϪ1dm3cmϪ1). H NMR: δ ϭ 3.03 (s, 6 H, NMe2), 3.09 (s, 6 H,
NMe2), 6.60 (dd, J ϭ 10.0, 1.3 Hz, 1 H, 2-H), 6.68 (m, 2 H, Ar-
H), 6.78 (m, 2 H, Ar-H), 7.24 (m, 2 H, Ar-H), 7.37 (m, 3 H, Ar-
H, 2Ј-H), 7.41 (m, 1 H, 7-H), 7.49 (m, 1 H, 6-H), 7.75 (d, J ϭ 8.1
Hz, 1 H, 5-H), 7.85 (d, J ϭ 11.9 Hz, 1 H, 1Ј-H), 8.16 (d, J ϭ 10.0 8: From 2-naphthol and 1,1-bis(4-dimethylaminophenyl)prop-2-yn-
Hz, 1 H, 3-H), 8.29 (dd, J ϭ 7.9, 1.2 Hz, 1 H, 8-H) ppm. HRMS: 1-ol after elution from silica with 20% EtOAc/hexane. Fraction 1:
Eur. J. Org. Chem. 2003, 1220Ϫ1230
1227

B. M. Heron et al.
FULL PAPER
3,3-Bis(4-dimethylaminophenyl)-3H-naphtho[2,1-b]pyran (14b) as 501.2066. C34H28O4 (500.28): calcd. C 81.6, H 5.6; found C 81.4,
pale green microcrystals from EtOAc/hexane (1.14 g, 85%), m.p. H 5.8. Fraction 2: 4-[(E)-1Ј,3Ј,3Ј-Tris(4-methoxyphenyl)prop-2-en-
221.0Ϫ222.0 °C (ref.[47] 223 °C). UV (PhMe): λmax ϭ 558, 446 nm. ylidene]-4H-naphthalen-1-one (21b) as maroon needles from
IR (KBr): ν˜max ϭ 1613, 1358, 1234, 1167 cmϪ1 1H NMR: δ ϭ EtOAc/hexane (672 mg, 42%), m.p. 155.5Ϫ158.0 °C. IR (KBr):
.
2.96 [s, 12 H, (NMe2)2], 6.26 (d, J ϭ 10.0 Hz, 1 H, 2-H), 6.75 (m, ν˜max ϭ 1625, 1250 cmϪ1. UV (EtOH): λmax (εm) ϭ 479 nm (5.4 ϫ
1
4 H, Ar-H), 7.21 (d, J ϭ 8.8 Hz, 1 H, 5-H), 7.38 (m, 7 H, Ar-H,
104 molϪ1dm3cmϪ1). H NMR: δ ϭ 3.73 (s, 3 H, OMe), 3.74 (s, 3
1-H), 7.67 (d, J ϭ 8.8 Hz, 1 H, 6-H), 7.74 (m, 1 H, Ar-H), 7.99 H, OMe), 3.84 (s, 3 H, OMe), 6.35 (d, J ϭ 10.2 Hz, 1 H, 2-H),
(m, 1 H, Ar-H) ppm HRMS: [Mϩ] 420.2209, C29H28N2O requires
[Mϩ] 420.2201(6). C29H28N2O (420.30): calcd. C 82.7, H 6.7, N
6.7; found C 82.8, H 6.7, N 6.8. Fraction 2: 3,3-Bis(4-dimethylamin-
6.54 (m, 2 H, Ar-H), 6.60 (m, 2 H, Ar-H), 6.84 (m, 2 H, Ar-H),
6.88 (m, 2 H, Ar-H), 7.03 (m, 2 H, Ar-H), 7.31 (m, 3 H, Ar-H, 2Ј-
H), 7.44 (d, J ϭ 10.2 Hz, 1 H, 3-H), 7.52 (m, 2 H, 6-H, 7-H), 8.33
ophenyl)prop-2-enal (15b) as yellow-green plates from EtOAc/hex- (dd, J ϭ 7.7, 1.5 Hz, 1 H, 5-H), 8.44 (d, J ϭ 7.9 Hz, 1 H, 8-H)
ane (37.7 mg, 4%), m.p. 170Ϫ172 °C (ref.[48] 171Ϫ172 °C). IR (Nu-
ppm. HRMS: [M ϩ H]ϩ 501.2065, C34H28O4 requires [M ϩ H]ϩ
1
jol): ν˜max ϭ 2895, 2747, 1649, 1607 cmϪ1. H NMR: δ ϭ 3.02 (s, 501.2066. C34H28O4 (500.28): calcd. C 81.6, H 5.6; found C 81.4,
6 H, NMe2), 3.04 (s, 6 H, NMe2), 6.41 (d, J ϭ 8.0 Hz, 1 H, 2-H),
6.50 (m, 2 H, Ar-H), 6.72 (m, 2 H, Ar-H), 7.20 (m, 2 H, Ar-H),
7.30 (m, 2 H, Ar-H), 9.46 (d, J ϭ 8.0 Hz, 1 H, 1-H) ppm.
H 5.4.
12: From 2-naphthol and 1,1-bis(4-methoxyphenyl)-3-phenylprop-
2-yn-1-ol after recrystallisation from EtOAc/hexane. 3,3-Bis(4-
methoxyphenyl)-1-phenyl-3H-naphtho[2,1-b]pyran (23a) as colour-
less microcrystals (1.14 g, 76%), m.p. 195Ϫ197 °C. IR (KBr):
9: From 1-methyl-2-naphthol and 1,1-bis(4-methoxyphenyl)prop-2-
yn-1-ol after elution from silica with 30% EtOAc/hexane. Fraction
1: 3,3-Bis(4-methoxyphenyl)-1-methylbenz[e]indan-2-one (17) as
colourless plates after recrystallisation from EtOAc/hexane
(850 mg, 65%), m.p. 135Ϫ137 °C. IR (KBr): ν˜max ϭ 1746 cmϪ1. 1H
NMR: δ ϭ 1.54 (d, J ϭ 7.5 Hz, 3 H, 1-Me), 3.75 (s, 3 H, OMe),
3.78 (s, 3 H, OMe), 4.07 (q, J ϭ 7.5 Hz, 1 H, 1-H), 6.79 (m, 4 H,
Ar-H), 6.95 (m, 2 H, Ar-H), 7.15 (m, 2 H, Ar-H), 7.23 (d, J ϭ 8.7
Hz, 1 H, 4-H), 7.55 (m, 2 H, 7-H, 8-H), 7.81 (d, J ϭ 8.7 Hz, 1 H,
5-H), 7.91 (m, 2 H, 6-H, 9-H) ppm. 13C NMR: δ ϭ 18.4 (Me), 46.6
(C-1), 55.2 (OMe), 67.5 (C-3), 113.6, 113.7, 124.3, 124.4, 126.0,
126.7, 128.6, 129.2, 129.6, 130.0, 130.2, 133.3, 134.4, 135.6, 137.2,
141.5, 147.1, 158.4, 158.6, 218.8 ppm. HRMS: [Mϩ] 408.1719,
C28H24O3 requires [Mϩ] 408.1719. C28H24O3 (408.44): calcd. C
82.4, H 5.9; found C 82.3, H 6.0.
ν˜max ϭ 1609, 1251 cmϪ1. H NMR: δ ϭ 3.74 (s, 6 H, OMe), 6.13
1
(s, 1 H, 2-H), 6.84 (m, 4 H, Ar-H), 7.06 (m, 2 H, Ar-H), 7.30 (m,
7 H, Ar-H), 7.41 (m, 4 H, Ar-H), 7.67 (m, 2 H, Ar-H) ppm.
HRMS: [Mϩ] 470.1876, C33H26O3 requires [Mϩ] 470.1882.
C33H26O3 (470.26): calcd. C 84.2, H 5.6; found C 84.3, H 5.6.
13: From 2-naphthol and 1,1,3-tris(4-methoxyphenyl)prop-2-yn-1-
ol after elution from silica with 20% EtOAc/hexane. Fraction 1:
1,3,3-Tris(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran (23b) as
colourless microcrystals after recrystallisation from EtOAc/hexane
(1.55 g, 97%), m.p. 111Ϫ113 °C. IR (KBr): ν˜max ϭ 1608, 1250
cmϪ1. H NMR: δ ϭ 3.76 (s, 6 H, OMe), 3.86 (s, 3 H, OMe), 6.11
1
(s, 1 H, 2-H), 6.81 (m, 4 H, Ar-H), 6.89 (m, 2 H, Ar-H), 7.02 (m,
1 H, Ar-H), 7.18 (m, 1 H, Ar-H), 7.29 (m, 4 H, Ar-H), 7.44 (m, 4
H, Ar-H), 7.68 (m, 2 H, Ar-H) ppm. HRMS: [M ϩ H]ϩ 501.2061,
C34H28O4 requires [M ϩ H]ϩ 501.2066. C34H28O4 (500.28): calcd.
C 81.6, H 5.6; found C 81.5, H 5.6.
10: From 1-naphthol and 1,1-bis(4-methoxyphenyl)-3-phenylprop-
2-yn-1-ol after elution from silica with 20% EtOAc/hexane. Frac-
tion 1: 2,2-Bis(4-methoxyphenyl)-4-phenyl-2H-naphtho[1,2-b]pyran
(20a) as colourless microcrystals from EtOAc/hexane (428 mg,
1
32%), m.p. 103Ϫ105 °C. IR (KBr): ν˜max ϭ 1252 cmϪ1. H NMR:
Acid-Catalysed Rearrangement of 1,1,3-Tris(4-methoxyphenyl)prop-
2-yn-1-ol: A stirred solution of 1,1,3-tris(4-methoxyphenyl)prop-2-
yn-1-ol (1.34 mmol) in toluene (20 mL) was warmed to 50 °C.
Acidic alumina (0.8 g) was added and the mixture was refluxed
until none of the prop-2-yn-1-ol remained by TLC (ca. 2 h). The
cooled mixture was filtered and the alumina was washed with hot
toluene (2 ϫ 50 mL). Removal of the toluene from the combined
washings and filtrate gave 1,3,3-tris(4-methoxyphenyl)prop-2-en-
one (24) as a viscous oil[43] (446 mg, 89%). IR (KBr): ν˜max ϭ 1651,
1595, 1244, 1165 cmϪ1. 1H NMR: δ ϭ 3.73 (s, 3 H, OMe), 3.76 (s,
3 H, OMe), 3.78 (s, 3 H, OMe), 6.76 (m, 2 H, Ar-H), 6.82 (m, 4
H, Ar-H), 6.96 (s, 1 H, 2-H), 7.11 (m, 2 H, Ar-H), 7.31 (m, 2 H,
Ar-H), 7.90 (m, 2 H, Ar-H) ppm.
δ ϭ 3.75 (s, 6 H, OMe), 6.11 (s, 1 H, 3-H), 6.82 (m, 4 H, Ar-H),
7.17 (d, J ϭ 8.3 Hz, 1 H, Ar-H), 7.44 (m, 12 H, Ar-H), 7.68 (dd,
J ϭ 8.5, 2.1 Hz, 1 H, Ar-H), 8.41 (dd, J ϭ 8.3, 2.2 Hz, 1 H) ppm.
HRMS: [M ϩ H]ϩ 471.1951, C33H26O3 requires [M ϩ H]ϩ
471.1960. C33H26O3 (470.26): calcd. C 84.2, H 5.6; found C 84.0, H
5.7. Fraction 2: 4-[(E)-3Ј,3Ј-Bis(4-methoxyphenyl)-1Ј-phenylprop-2-
enylidene]-4H-naphthalen-1-one (21a) as maroon needles from
EtOAc/hexane (542 mg, 36%), m.p. 201.0Ϫ202.5 °C. IR (KBr):
ν˜max ϭ 1635, 1249 cmϪ1. UV (EtOH): λmax (εmax) ϭ 470 nm (2.6
ϫ 104 molϪ1dm3cmϪ1). 1H NMR: δ ϭ 3.73 (s, 3 H, OMe), 3.84 (s,
3 H, OMe), 6.34 (d, J ϭ 10.3 Hz, 1 H, 2-H), 6.53 (m, 2 H, Ar-H),
6.81 (m, 2 H, Ar-H), 6.87 (m, 2 H, Ar-H), 7.07 (m, 5 H, Ar-H),
7.31 (m, 4 H, Ar-H, 2Ј-H), 7.52 (m, 2 H, 6-H, 7-H), 8.33 (d, J ϭ
7.3 Hz, 1 H, Ar-H), 8.44 (d, J ϭ 7.8 Hz, 1 H, 8-H) ppm. HRMS:
[M ϩ H]ϩ 471.1957, C33H26O3 requires [M ϩ H]ϩ 471.1960.
C33H26O3 (470.26): calcd. C 84.2, H 5.6; found C 84.2, H 5.4.
Mitsonubu Coupling Procedure: Diethyl azodicarboxylate
(6.9 mmol) was added to
(3.5 mmol), 1,1-bis(4-dimethylaminophenyl)prop-2-yn-1-ol (5b)
(3.5 mmol) and triphenylphosphane (6.9 mmol) in tetrahydrofuran
(25 mL) at room temperature. The resultant brown solution was
a stirred solution of 2-naphthol
11: From 1-naphthol and 1,1,3-tris(4-methoxyphenyl)prop-2-yn-1-
ol after elution from silica with 5% EtOAc/toluene. Fraction 1: stirred until no further reaction was observed by TLC (ca. 5 h).
2,2,4-Tris(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran (20b) as
The mixture was then poured into water (200 mL) and extracted
with ethyl acetate (4 ϫ 50 mL). These extracts were then washed
with brine (2 ϫ 50 mL), water (100 mL) and dried (anhyd.
colourless microcrystals from EtOAc/hexane (400 mg, 25%), m.p.
1
118Ϫ120 °C. IR (KBr): ν˜max ϭ 1617, 1249 cmϪ1. H NMR: δ ϭ
3.75 (s, 6 H, OMe), 3.86 (s, 3 H, OMe), 6.07 (s, 1 H, 3-H), 6.81 (m, Na2SO4). Removal of the solvent gave a dark blue tar which, upon
4 H, Ar-H), 6.96 (m, 2 H, Ar-H), 7.18 (d, J ϭ 8.5 Hz, 1 H, Ar-H), elution from silica (5% ethyl acetate/toluene), afforded 3,3-bis(4-
7.28 (d, J ϭ 8.5 Hz, 1 H, Ar-H), 7.42 (m, 8 H, Ar-H), 7.70 (dd,
dimethylaminophenyl)-3H-naphtho[2,1-b]pyran as pale green mic-
J ϭ 7.5, 1.1 Hz, 1 H, Ar-H), 8.39 (dd, J ϭ 7.6, 0.9 Hz, 1 H, Ar-H) rocrystals 14b (235 mg, 16%), m.p. 223Ϫ225 °C, (ref.[47] 223 °C)
ppm. HRMS: [M ϩ H]ϩ 501.2061, C34H28O4 requires [M ϩ H]ϩ
identical in all aspects to the previously prepared material.
1228
Eur. J. Org. Chem. 2003, 1220Ϫ1230

Observations on the Synthesis of Photochromic Naphthopyrans
FULL PAPER
C. D. Gabbutt, J. D. Hepworth, B. M. Heron, S. M. Partington,
D. A. Thomas, Dyes Pigments 2001, 49, 65.
C. D. Gabbutt, T. Gelbrich, J. D. Hepworth, B. M. Heron, M.
B. Hursthouse, S. M. Partington, Dyes Pigments 2002, 54, 79.
F. W. Wehrli, A. P. Marchand, S. Wehrli, Interpretation of Car-
bon-13 NMR Spectra, 2nd ed., John Wiley & Sons, Chiches-
ter, 1988.
L. J. Bellamy, The Infrared Spectra of Complex Molecules, 3rd
ed., Chapman and Hall, London, 1975, vol. 1, p. 154.
J. D. Hepworth, C. D. Gabbutt, B. M. Heron in Comprehensive
Heterocyclic Chemistry, II (Ed.: A. McKillop), Pergamon Press,
Oxford, 1996, vol. 5, p. 325.
[8]
[9]
Modified Mitsonubu Coupling Procedure: Diethyl azodicarboxylate
(6.9 mmol) was added to
a stirred solution of 2-naphthol
(3.5 mmol), 1,1-bis(4-dimethylaminophenyl)prop-2-yn-1-ol (5b)
(3.5 mmol), triphenylphosphane (6.9 mmol) and 4-nitrophenol
(0.7 mmol) in tetrahydrofuran (35 mL) at room temperature. The
resultant brown solution was stirred until no further reaction was
observed by TLC (ca. 3.5 h). The mixture was then poured into
water (200 mL) and extracted with ethyl acetate (4 ϫ 50 mL). These
extracts were then washed with brine (2 ϫ 50 mL), water (100 mL)
and dried (anhyd. Na2SO4). Removal of the solvent gave a dark
blue tar which, upon elution from silica (5% ethyl acetate/toluene),
afforded 3,3-bis(4-dimethylaminophenyl)-3H-naphtho[2,1-b]pyran
(14b) as pale green microcrystals (397 mg, 27%), m.p. 222Ϫ224 °C,
(ref.[47] 223 °C) identical in all aspects to the previously prepared
material. The following compounds were obtained by this protocol.
[10]
[11]
[12]
[13]
Crystallographic data for compound 7a: Empirical formula
C27H22O3, M ϭ 394.45, monoclinic, space group P21/n, a ϭ
˚
˚
˚
16.8022(5) A, b ϭ 6.0430(2) A, c ϭ 20.1317(8) A, β ϭ
104.182(1)°, V ϭ 1981.8(1) A , Z ϭ 4; Dcalcd. ϭ 1.322 Mg/m3;
3
˚
µ (Mo-Kα) ϭ 0.085 mmϪ1. Red needle, crystal size 0.40 ϫ 0.02
ϫ 0.02 mm. θ range for data collection 3.1Ϫ25.1°, 11938 reflec-
tions of which 3461 are independent (Rint ϭ 0.096). Refinement
with 3461 data for 294 parameters, GOF ϭ 0.891, final R indi-
ces for data with [F2 Ͼ 2σ(F2)] R1 ϭ 0.052, wR2 ϭ 0.081; R
indices for all data R1 ϭ 0.1411, wR2 ϭ 0.0968. CCDC-195498
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223/336-033;
E-mail: deposit@ccdc.cam.ac.uk].
1: From 1-naphthol and 1,1-bis(4-dimethylaminophenyl)prop-2-yn-
1-ol after elution from silica with 10% ethyl acetate/toluene. Frac-
tion 1: 2,2-Bis(4-dimethylaminophenyl)-2H-naphtho[1,2-b]pyran
(6b) as pale blue microcrystals (162 mg, 11%), m.p. 203Ϫ205 °C.
This material was identical in all aspects to that obtained pre-
viously. Fraction 2: 4-[(E)-3Ј,3Ј-Bis(4-dimethylaminophenyl)prop-2-
enylidene]-4H-naphthalen-1-one (7b) as maroon crystals (44 mg,
3%), m.p. 137Ϫ138 °C. This material was identical in all aspects to
that obtained previously.
[14]
[15]
[16]
[17]
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Acknowledgments
We thank the EPSRC and James Robinson Ltd. (Huddersfield) for
an Industrial CASE award to D. A. T., the EPSRC for the provision
of an HRMS service at the University of Wales, Swansea and The
Worshipful Company of Clothworkers of the City of London for
a millennium grant for the purchase of Bruker Avance 400 MHz
NMR instrument.
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[4]
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[27]
[28]
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Crystallographic data for compound 17: Empirical formula
C28H24O3, M ϭ 408.47, monoclinic, space group P21/c, a ϭ
˚
˚
˚
10.0122(2) A, b ϭ 10.7974(2) A, c ϭ 19.8066(4) A, β ϭ
96.819(1)°, V ϭ 2126.06(7) A , Z ϭ 4, Dcalcd. ϭ 1.276 Mg/m3;
3
˚
µ (Mo-Kα) ϭ 0.082 mmϪ1. Colourless block, crystal size 0.25
ϫ 0.15 ϫ 0.15 mm3. θ range for data collection 3.1Ϫ25.0°,
15312 reflections collected of which 3701 are independent
(Rint ϭ 0.057). Refinement with 3701 data for 302 parameters,
GOF ϭ 1.065, final R indices for data with [F2 Ͼ 2σ(F2)] R1 ϭ
0.056, wR2 ϭ 0.138; R indices for all data R1 ϭ 0.087, wR2 ϭ
0.148. CCDC-195499 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223/336-033;
E-mail: deposit@ccdc.cam.ac.uk].
[6]
[7]
Eur. J. Org. Chem. 2003, 1220Ϫ1230
1229

B. M. Heron et al.
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solution and refinement: SHELXS-97: G. M. Sheldrick, Acta
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Sheldrick, University of Göttingen, Germany, 1997.
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[30]
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[31]
[32]
[38]
[39]
Helv. Chim. Acta 1997, 80, 725.
Crystalloghraphic data for compound 21a: Empirical formula
C33H26O3, M ϭ 470.54, monoclinic; space group P21/c, unit
[33]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
˚
˚
cell dimensions a ϭ 7.93070(10) A, b ϭ 11.8436(2) A, c ϭ
3
˚
˚
26.0902(5) A, β ϭ 98.4873(6)°, V ϭ 2423.76(7) A , Z ϭ 4,
Dcalcd. ϭ 1.289 Mg/m3, µ (Mo-Kα) ϭ 0.081 mmϪ1. Red prism,
red, crystal size 0.20 ϫ 0.15 ϫ 0.05 mm. θ range for data collec-
tion 2.9Ϫ26.0°; 18150 collected reflections of which 4750 are
independent (Rint ϭ 0.0849). Structure refinement with 4750
data for 352 parameters, GOF ϭ 0.991, final R indices for data
with [F2 Ͼ 2σ(F2)] R1 ϭ 0.042, wR2 ϭ 0.089; R indices for all
data R1 ϭ 0.058, wR2 ϭ 0.094. CCDC-195500 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (in-
ternat.) ϩ 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
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[34]
[47]
[48]
D. J. Zwanenburg, Th. A. M. M. Maas, Recl. Trav. Chim. Pays-
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[35]
[36]
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Received October 28, 2002
[O02599]
1230
Eur. J. Org. Chem. 2003, 1220Ϫ1230