Full text of DOI:10.1002/jlcr.484

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Labelled Cpd Radiopharm 2001; 44: 529–539.
DOI: 10.1002/jlcr.484
Synthesis and deuterium labeling of some
4-phenyl, 3-substituted isochromanes
Anne-Laure Guyard, Alain Valleix, Bernard Rousseau and
Jean-Christophe Cintrat*
! ! # !
CEA/Saclay, Service des Molecules Marquees, Bat. 547, Departement de
Biologie Cellulaire et Mole!culaire, 91191 Gif sur Yvette Cedex, France
Summary
The preparation of a number of 4-phenyl isochromanes as well as the
deuterated analogs is described using a simple strategy starting from easily
obtained acetals. The diasteresoselectivity and the enantioselectivity of the
adopted approach are both good. The deuterated analogs are prepared using
tributyltin deuteride or triethyl deuterosilane. Copyright # 2001 John Wiley
& Sons, Ltd.
Key Words: acetal; deuterium; isochromanes; tributyltin deuteride; triethyl
deuterosilane
Introduction
Unlike their natural analogues the coumarins, isochromanes (Scheme 1)
have received little attention. Nevertheless recently, isochromanes
Scheme 1.
!
*Correspondence to: J.-C. Cintrat, CEA/Saclay, Service des Molecules Marquees, Bat. 547,
Departement de Biologie Cellulaire et Moleculaire, 91191 Gif sur Yvette Cedex, France.
!
!
E-mail: jean-christophe.cintrat@cea.fr
Copyright # 2001 John Wiley & Sons, Ltd.
Received 7February 2001
Revised 5 April 2001
Accepted 11 April 2001
Published online \ \ \ \

530
A.-L. GUYARD ET AL.
bearing functionalities have been used either as fragrances (musk)1 or as
herbicides.2 This class of compound has also shown therapeutic activity
as potential ligands for the D1 or D4 receptor of dopamine.3
The ease of accessing isochromanes is related to their complexity.
In fact, the synthesis of the unsubstituted isochromane (3,4-dihydro-1H-
2-benzopyrane) is easily achieved starting from 2-(2-hydroxymethyl-
phenyl)ethanol.4,5 In a related manner, Thibault et al. have gained
monosubstituted isochromanes starting from a-chloro ethers.6 Now
the most commonly used approach arises from the work of Mohler
and Thompson.7 Their approach deals with the Lewis acid
catalyzed cyclization of acyclic acetals derived from phenethylalcohols
(Scheme 2). Using this route, enantio-enriched 1,3-substituted isochro-
manes can be obtained but the starting enantiopure alcohols are usually
difficult to obtain.
Scheme 2.
To obtain the corresponding isochromanes labeled on the pyranyl
ring following the previous sequences necessitates the introduction of a
deuterium or tritium atom at a very early stage of the synthesis. We
were intrigued as to whether we could apply the labeling approach
designed by Giles8 who started from cyclic acetals bearing nucleophilic
arylrings (Scheme 3):
Scheme 3.
Initially, we report our results on the preparation of some
isochromanes before reporting the extension of the method to
deuterium labeling.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 529–539

SYNTHESIS AND DEUTERIUM LABELING
531
Results and discussion
As in the Giles approach, our key step is the conversion of an acetal to
the isochromane skeleton (Scheme 4):
Scheme 4.
All the starting acetals 2 have been prepared either from the corres-
%
ponding cyclic orthoesters9 1 by substitution of the methoxy substituent
%
using tributyltin hydride in the presence of boron trifluoride etherate
(Scheme 5, route a) or via acetalization of an aldehyde (Scheme 5,
route b).
Scheme 5.
The results are summarized in Table 1.
The acetals 2 were obtained in fairly good yields, ranging from 65 to
%
95%. Each was synthesized as a racemic mixture or in an enantiopure
form (entries 1–2, 4, 8–9). We were able to obtain acetals bearing an
alkyl substituent on C5 (entries 1, 7), a benzyl group (entry 6) or no
substituents (entry 3). Most of these acetals bear a phenyl group at C5
(entries 2, 4–5, 8–9). Using either route a or b (Scheme 5), 2-substituted
acetals have been synthesized (entries 8 and 9), but using route b gives a
diastereomericmixture of trans and cis acetals (entry 9).
With these acetals in hand the next step was to convert the dioxolane
rings into isochromanes. This was achieved using a combination of
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532
A.-L. GUYARD ET AL.
Table 1. Synthesis of acetalsa
Entry R
R0 R00
Acetal
Yield Isochromane
(%)
Yield
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
Ph CH3
Ph Ph
(5S)-2a
% %
95
95
90
(3R, 4R)- 3a
% %
30%b
c
( ꢀ )-and (5S)-2b
( ꢀ )-and (3R, 4S)-3b 30%
% %
% %
H
H
H
Ph
( ꢀ )-2c
}
}
}
% %
Meso and (4S, 5S)-2d 89
}
25%c
}
% %
Ph p-CF3Ph ( ꢀ )-2e
Ph Benzyl
Ph Isopropyl ( ꢀ )-2g
80
65
95
91
93a
( ꢀ )-3e
}
% %
% %
( ꢀ )-2f
( ꢀ )-3g
}
}
5%c
}
% %
% %
% %
CH3 Ph Ph
Ph Ph Ph
(2R, 5S)-2h
% %
(5S)-2i
% %
}
a This is the sole acetal synthesized via route b giving a mixture of trans/cis compounds (trans/
cis=85/15.
b Trans/cis: 80/20.
c Trans/cis>95/5> (based on 1H NMR).
Figure 1. Chiral HPLC analysis of ( ꢀ )-3b and (3R, 4S)-3b
% %
% %
triethylsilane and titanium tetrachloride, according to Scheme 4. The
first obvious comment to make is that only acetals with two aryl rings at
C4 are converted into isochromanes (entries 1–2, 5, 7). When R00 is an
aromatic group as well, the diastereoselectivity of the cyclization is
excellent since only one single diastereomer was observed in the crude
1H NMR spectrum (entries 2, 5, 7). The trans configuration was
assigned on the basis of the H3/H4 coupling constant, estimated at
about 9 Hz.10 The enantioselectivity of the reaction, starting from
the enantiopure acetal 2b has also been checked. A chiral HPLC
% %
analysis showed no loss of the optical integrity of the starting material
(Figure 1).
If R00 is a smaller group, however, the diastereoselectivity decreases
and 20% of the cis compound is obtained along with the trans isomer
(entry 1). It is noteworthy that for the bulky isopropyl group a low yield
was obtained (entry 7).
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 529–539

SYNTHESIS AND DEUTERIUM LABELING
533
In order to rationalize the use of Et3SiH, we have run the experiments
with acetal 2b in the absence of the silane. To our great surprise and in
% %
contrast with previously described work,8 no isochromane formation
was detectable in the crude NMR spectrum. Thus, we postulate the
following mechanism (Scheme 6) for the conversion of the acetals into
the isochromanes:
Scheme 6.
Indeed if the reaction would proceed via route I, we should have
obtained the 4-hydroxy isochromane after hydrolysis of the reaction
1
mixture but this compound was not detected in the crude H NMR
spectrum when we omitted triethylsilane. Thus, since the presence of
Et3SiH is necessary, route I may be ruled out.
The stereochemical outcome of this reaction can also be rationalized
by the following transition state (Scheme 7):
Scheme 7.
First O3 of the acetal can be complexed by the Lewis acid, due to the
higher basicity of this oxygen and to a decomplexation of the axial H2
and Ph4 substituents. Then the postulated oxocarbenium intermediate
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534
A.-L. GUYARD ET AL.
A adopts a 6-membered transition state (boat like) with all the bulky
substituents in a pseudo-equatorial position. Finally, the phenyl ring
attacks the oxocarbenium yielding the trans isochromane.
The use of metallichydrides in our syntheticroute to isochromanes
allowed us to consider the preparation of deuterated (or tritiated)
isochromanes. Therefore, the synthesis was done again with commer-
cially available tributyltin deuteride in the first step to give [2-2H]- 2b as
% %
1
a single diastereomer (i.e.>95% based on MS and H NMR). Then
reaction of this 2-deutero acetal in the presence of Et3SiH and TiCl4
gives the corresponding 1-deutero isochromane but with complete
epimerization at C1. (Scheme 8)
Scheme 8.
In a similar vein, reaction of acetal 2b in the presence of Et3Si2H
% %
(freshly prepared from Et3SiCl and LiAl2H4)11 and TiCl4 gives
the corresponding 4-deutero isochromane [4-2H]-3b (i.e.>95%
% %
1
based on MS and H NMR) with excellent diastereoselectivity since
only one of the two expected diastereomers (the trans isomer) was
1
detectable on the H NMR spectrum of the crude reaction mixture
(Scheme 9).
Scheme 9.
Copyright # 2001 John Wiley & Sons, Ltd.
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SYNTHESIS AND DEUTERIUM LABELING
535
Conclusion
Starting from easily obtainable acetals, we have been able to synthesize
a number of 4-phenyl isochromanes. The diastereoselectivity favours the
trans isomer. The use of tributyltin deuteride or deuterated triethylsilane
yields the corresponding 1- or 4-deuterated analogs with excellent
isotopic enrichments. Since the introduction of the isotope mainly in the
4 position is achieved in the very last step of the synthesis of the
isochromane skeleton, this route would be of interest for the
preparation of tritiated isochromane.
Experimental
General methods
Non commercially available diols were prepared according to literature
procedures.12 CDCl3 was from Eurisotop. All other chemicals were
commercially available.
13
1H and C NMR spectra were recorded on a Bruker AC300
.
spectrometer operating, respectively, at 300.125 and 75.47 MHz.
Deuterium incorporation was measured by integration of the respective
1H NMR signals and by referring to the signal of another proton or
group of protons within the molecule as internal standard or via MS.
A typical procedure for the preparation of acetals 1 is as follows:
%
In a Schlenk tube, 127 mL (1.2 mmol) of BF3 ꢁ OEt2 is added to a
solution of 1 mmol of C2 epimeric 1 in 20 mL of anhydrous CH2Cl2, at
%
ꢂ788C. The resulting solution was stirred for 15 min at ꢂ788C and
295 mL (1.1 mmol) of Bu3SnH are then added dropwise. The reaction
was monitored by TLC, and when no more starting material could be
detected (typically after 7/8 h), the reaction mixture was diluted by
20 mL of CH2Cl2, quenched with 10 mL of a saturated aqueous solution
of NH4Cl, the aqueous layer was extracted with 2 ꢃ 50 mL of CH2Cl2
and the organicphases were combined and dried over Na 2SO4. After
filtration and evaporation of the organicphase, the residue was purified
on silica gel (eluent hexane/Et3N:100/1 then hexane/Et2O/Et3N:75/25/1,
Rf=0.3) giving compound 2.
%
(5S)-5-methyl-4,4-diphenyl-[1,3]-dioxolane 2a
% %
1H NMR: 1.07 (d, J=6.7 Hz, 3H, CH3); 5.21 (q, J=6.7 Hz, 1H, H5);
0
4.93 (s, 1H, H2 or H2 ); 5.43 (s, 1H, H2 or H2 );7.22–7.46 (m, 10H, Har).
0
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A.-L. GUYARD ET AL.
13C NMR: 17.37 (CH3); 79.51 (C5); 86.86 (C4); 93.30 (C2); 126.55;
126.94; 127.36; 127.55; 128.14 (CHar); 141.95; 143.18 (Cquat. ar).
M.S.: 258[M+NH4]+.
I.R.: 3087; 2981; 2934; 2872; 1599–1584–1493–1448; 1386; 1200–960;
755; 700.
( ꢀ )- and (5S)-4,4,5-triphenyl-[1,3]-dioxolane 2b
H NMR: 5.12 (s, 1H, H2); 5.68 (s, 1H, H2 ); 5.76 (s, 1H, H5);
% %
1
0
7.02–7.68(m, 15H, Har).
13C NMR: 85.18 (C5); 88.67 (C4); 93.88 (C2); 126.52; 126.87; 127.00;
127.20; 127.42; 127.59; 128.17 (CHar); 127.78; 140.30; 140.66; 142.72
(Cquat. ar).
M.S.: 320 [M+NH4]+.
I.R.: 3087; 2981; 1599–1492–1448; 1200 to 960; 752; 695
( ꢀ )-4-phenyl-[1,3]-dioxolane 2c
% %
1H NMR: 3.69 (dd, J=7.3 Hz, J=7.9 Hz, 1H, H5 or H5); 4.25 (dd,
J=6.7 Hz, J=7.9 Hz, 1H, H5 or H5); 5.00 (dd, J=6.7 , 7.3 Hz, 1H, H4);
0
5.09 (s, 1H, H2 or H2 ); 5.27 (s, 1H, H2 or H2 ); 7.25–7.36 (m, 5H,
0
Har).
13C NMR: 71.65 (C5); 77.41 (C4); 95.82 (C2); 125.87; 127.91; 128.39
(CHar); 139.17 (Cquat. ar).
Meso and (4S, 5S)-4,5-diphenyl-[1,3]-dioxolane 2d
% %
meso- 2d
% %
1
0
H NMR: 5.21 (s, 1H, H2 or H2 ); 5.29 (s, 2H, H4 or H5); 5.68 (s, 1H, H2
0
or H2 ); 6.93–7.25 (m, 10H, Har).
13C NMR: 81.59 (C4 and C5); 95.40 (C2); 126.52, 127.20, 127.36
(CHar); 136.64 (Cquat. ar).
(4S, 5S)-4,5-diphenyl-[1,3]- dioxolane 2d
% %
1H NMR: 4.72 (s, 2H, H2); 5.48 (s, 2H, H4 and H5); 7.25-7.40 (m, 10H,
Har).
( ꢀ )-4,4-diphenyl-5-(4-trifluoromethyl-phenyl)-[1,3]-dioxolane 2e
% %
1
0
H NMR: 5.07 (s, 1H, H5); 5.67 (s, 1H, H2 or H2 ); 5.81 (s, 1H, H2 or
0
H2 ); 7.00–7.58 (m, 14H, Har).
13C NMR: 83.88 (C5); 89.09 (C4); 93.97 (C2); 124.18 (q, J3F=258 Hz);
124.22; 126.62, 126.94, 127.65, 127.84, 128.33, 128.52, 128.69, 128.85,
129.04, 129.95, 130.76 (CHar); 139.04 (q, 3J3F=34.3 Hz); 141.95, 142.27
(Cquat. ar).
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SYNTHESIS AND DEUTERIUM LABELING
537
( ꢀ )-5-benzyl-4,4-diphenyl-[1,3]-dioxolane 2f
% %
1H NMR: 2.35 (dd, J=14, 10 Hz, 1H, H6 or H6’); 2.62 (dd, J=14, 2 Hz,
0
1H, H6 or H6 ); 4.76 (dd, J=2, 10 Hz, 1H, H5); 4.88 (s, 1H, H2 or H2 );
0
0
5.47 (s, 1H, H2 or H2 ); 7.17-7.33 (m, 15H, Har).
13C NMR: 38.46 (C6); 84.40 (C5), 86.93 (C4); 93.49 (C2); 126.23,
126.62, 127.07, 127.17, 127.46, 127.65, 128.17, 128.78 (CHar); 138.36,
141.82, 142.92 (Cquat.ar).
( ꢀ )-5-isopropyl-4,4-diphenyl-[1,3]-dioxolane 2g
% %
1H NMR: 0.77 (d, J=6.7 Hz, 3H, CH3); 0.85 (d, J=6.7 Hz; 3H, CH3);
1.64 (dsept, J=5.5, 6.7 Hz, 1H, H6); 4.46 (d, J=5.5 Hz, 1H, H5); 4.83 (s,
0
0
1H, H2 or H2 ); 5.37 (s, 1H, H2 or H2 ); 7.19–7.29 (m, 10H, Har)
13C NMR: 17.43; 18.18 (CH3); 20.70 (C6); 87.02 (C5); 87.12 (C4);
93.13 (C2); 126.91, 127.13, 127.33, 127.39, 128.07 (CHar); 141.53, 143.66
(Cquat.ar).
(2R,5S)-2-methyl-4,4,5-triphenyl-[1,3]-dioxolane (100% trans) 2h
% %
1H NMR: 1.82 (d, J=5 Hz, 3H, CH3); 5.38 (q, J=5 Hz, 1H, H2); 5.86
(s, 1H, H5); 7.04–7.75 (m, 15H, Har).
(5S)-2,4,4,5-tetraphenyl-[1,3]-dioxolane 2i
% %
1H NMR: Major diastereomer (85%): 6.01 (s, 1H, H5); 6.07 (s, 1H, H2);
6.74–7.80 (m, 15H, Har).
Minor diastereomer (15%): 5.65 (s, 1H, H5); 6.74 (s, 1H, H2); 6.74–
7.80 (m, 15H, Har).
A typical procedure for the conversion of acetals 1 into isochromanes
%
3 is as follows:
%
In a flame dried Schlenk tube, 150 mL (1.2 mmol) of TiCl4 is first
added to a solution of 1 mmol of acetal 2 in 20 mL of anhydrous
%
CH2Cl2, at ꢂ788C under nitrogen immediately followed by the
addition of 290 mL of Et3SiH (1.1 mmol). The reaction was monitored
by TLC, and when no more starting material could be detected
(typically after 7/8 h), the reaction mixture was diluted with 20 mL
of CH2Cl2, quenched with 10 mL of a saturated aqueous solution
of NH4Cl, the aqueous layer was extracted with 2 ꢃ 50 mL of
CH2Cl2 and the organicphases were ocmbined and dried over
Na2SO4. After filtration and evaporation of the organicphase, the
residue was purified on silica gel (eluent hexane: 100% then hexane/
Et2O: 95/5, Rf=0.3) then repurified using semi-preparative HPLC
(Zorbax C8 column, 4 mL/min, CH3CN/H2O 60/40) giving pure
compound 3:
%
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538
A.-L. GUYARD ET AL.
3-methyl-4-phenyl-isochromane (trans/cis 80/20) 3a
% %
1H NMR: Trans isomer: 1.19 (d, J=3.7 Hz, 3H, CH3); 3.84 (dq, J=3.7
and 6.1 Hz, 1H, H3); 3.85 (d, J=6.1 Hz, 1H, H4); 4.90 and 5.00 (AB
system, J=15 Hz, 2H, H1 ꢃ 2); 6.73–7.31 (m, 9H, Har).
Cis isomer: 1.18 (d, J=3.7 Hz, 3H, CH3); 3.85 (d, J=6.1 Hz, 1H, H4);
4.08 (dq, J=3.7, 6.1 Hz, 1H, H3); 4.95 et 5.03 (AB system, J=15 Hz,
2H, H1 ꢃ 2); 6.73–7.31 (m, 9H, Har).
13C NMR: 19.50 (CH3); 52.02 (C4); 68.32 (C1); 77.22 (C2); 123.51,
125.81, 126.91, 128.01, 128.49, 129.20, 129.40 (CHar); 134.35, 137.39,
142.34 (Cquat.ar)
3,4-diphenyl-isochromane (trans/cis=92/8), trans isomer data reported13
3b
% %
1H NMR: 4.24 (d, J=9.8 Hz, 1H, H3); 4.68 (d, J=9.8 Hz, 1H, H4);
5.03 and 5.14 (AB system, J=15 Hz, 2H, H1 ꢃ 2); 6.81–7.21 (m, 14H,
Har).
HPLC: 250 ꢃ 4.6 mm, 5m RP C8 column, eluted with 60/40 CH3CN/
H2O, UV 254 nm, 1 mL/min, tr=10.7 min.
The chiral separation was carried out on a Whelk (Pirkle type) (S,S)
column (250 ꢃ 4.6 mm) supplied by Regis Technologies. The eluent used
was a mixture of iso-octane/iso-propanol 98/2 (0.8 mL/min, 258C, UV
detection 220 nm).
3-(4-trifluoromethyl-phenyl)-4-phenyl-isochromane (trans/cis>95/5) 3e
% %
1H NMR (trans isomer): 4.18 (d, J=9 Hz, 1H, H4); 4.75 (d, J=9 Hz,
1H, H3); 5.07 and 5.17 (AB system, J=15 Hz, 2H, H1 ꢃ 2); 6.82–7.54
(m, 13H, Har).
3-(4-trifluoromethyl-phenyl)-4-phenyl-isochromane (trans/cis>95/5) 3g
% %
1H NMR (trans isomer): 0.96 (d, J=7 Hz, 3H, CH3); 0.99 (d, J=7 Hz,
3H, CH3); 1.66 (sept, J=7 Hz, 1H, H5); 3.62 (dd, J=7, 9 Hz, 1H, H3);
4.12 (d, J=9 Hz, 1H, H4); 5.07 and 5.17 (AB system, J=15 Hz 2H,
H1 ꢃ 2); 7.02–7.34 (m, 9H, Har).
(2R,5S)-2-deuterio-4,4,5-triphenyl-[1,3]-dioxolane (2R,5S)-2-[2H]-2b (i.e.
% %
=92%)
1H NMR: 5.63 (s, 1H, H5); 5.72 (s, 1H, H2); 7.01–7.63 (m, 15H, Har).
1-deutero-3,4-diphenyl-isochromane( ꢀ )-, epi-2, 2-[2H]-3b (i.e.=92%)
% %
1H NMR: 4.44 (d, J=9. Hz, 1H, H3); 4.94 (d, J=9 Hz, 1H, H4); 5.31
(s, 0.4H, H1); 5.42 (d, 0.6H, H1); 7.35–7.71 (m, 14H, Har).
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SYNTHESIS AND DEUTERIUM LABELING
539
13C NMR: 52.21 (C4); 68.65 (t, JD=26 Hz, C1); 84.34 (C3); 123.81,
126.20, 126.62, 126.75, 126.95, 127.66, 127.98, 128.18, 129.53, 129.86
(CHar); 134.58, 137.69, 140.34, 141.51 (Cquat.ar
)
4-deutero-3,4-diphenyl-isochromane (100% trans, i.e.>95%) ( ꢀ )-, 4-
[2H]- 3b
% %
1H NMR: 4.74 (s, 1H, H3); 5.07 and 5.18 (AB system, J=15 Hz, 2H,
H1 ꢃ 2); 6.86–7.26 (m, 14H, Har).
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Copyright # 2001 John Wiley & Sons, Ltd.
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