Addition reactions of protonic reagents to optically active 2-phenyl-1,3,2-oxazaborolines

Article abstract of DOI:10.1016/s0022-328x(99)00051-0

The addition reactions of water and methanol to (4R, 5S)-4-methyl-2,5-diphenyl-1,3,2-oxazaboroline 1 (derived from ( + )-norephedrine), (4R, 5S)-( + )-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaboroline 2 (derived from ( + )-ephedrine) and to (4R, 5R)-( + )-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaboroline 3 (derived from ( + )-pseudoephedrine) were studied. The reactions gave the corresponding (N-B)-phenyl(2-aminoethoxy)boronic acids (4-6) or methyl esters (7-9). In 4 and 7 the boron atom is a stereogenic center, therefore two diasteromeric derivatives are expected. In compounds 5, 6, 8 and 9 both nitrogen and boron atoms are stereogenic centers and four diasteromers are possible. The structures of the addition products have been established by ¹H-, ¹¹B- and ¹³C-NMR, and that of compound 6 by X-ray diffraction analysis. The reactions with methanol gave only one isomer (7-9). The reaction of 1 with water afforded both boron epimers (50/50 ratio). The reaction of 2 with water observed at - 50° showed both B-epimers in 80/20 ratio, whereas the same reaction with 3 gave only one isomer. In all cases the main isomer has the N-methyl trans to the C-methyl group and the B-phenyl cis to the N-methyl group. The X-ray diffraction molecular structure of compound 6 [(4R, 5R)-( + )-3,4-dimethyl-2,5-diphenyl-2-hydroxy-1,3,2-oxazaborolidine], confirmed the structure assignment made from the NMR data. Addition reactions to 1,3,2-oxazaborolines are an efficient method to introduce stereoselectively functional groups to boron atom in order to obtain new boron heterocycles.

Full text of DOI:10.1016/s0022-328x(99)00051-0

Journal of Organometallic Chemistry 581 (1999) 122–128
Addition reactions of protonic reagents to optically active
2-phenyl-1,3,2-oxazaborolines
Alberto Rosendo Rico, Margarita Tlahuextl, Angelina Flores-Parra,
Rosalinda Contreras *
Departamento de Qu´ımica del Centro de In6estigacio´n y de Estudios A6anzados del IPN, A.P. 14-740, CP 07000 Mexico, D.F., Mexico
Received 17 June 1998; received in revised form 8 December 1998
Abstract
The addition reactions of water and methanol to (4R,5S)-4-methyl-2,5-diphenyl-1,3,2-oxazaboroline 1 (derived from (+)-
norephedrine), (4R,5S)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaboroline 2 (derived from (+)-ephedrine) and to (4R,5R)-(+)-
3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaboroline 3 (derived from (+)-pseudoephedrine) were studied. The reactions gave the
corresponding (N–B)-phenyl(2-aminoethoxy)boronic acids (4–6) or methyl esters (7–9). In 4 and 7 the boron atom is a
stereogenic center, therefore two diasteromeric derivatives are expected. In compounds 5, 6, 8 and 9 both nitrogen and boron
atoms are stereogenic centers and four diasteromers are possible. The structures of the addition products have been established
1
by H-, 11B- and 13C-NMR, and that of compound 6 by X-ray diffraction analysis. The reactions with methanol gave only one
isomer (7–9). The reaction of 1 with water afforded both boron epimers (50/50 ratio). The reaction of 2 with water observed at
−50° showed both B-epimers in 80/20 ratio, whereas the same reaction with 3 gave only one isomer. In all cases the main isomer
has the N-methyl trans to the C-methyl group and the B-phenyl cis to the N-methyl group. The X-ray diffraction molecular
structure of compound 6 [(4R,5R)-(+)-3,4-dimethyl-2,5-diphenyl-2-hydroxy-1,3,2-oxazaborolidine], confirmed the structure as-
signment made from the NMR data. Addition reactions to 1,3,2-oxazaborolines are an efficient method to introduce stereoselec-
tively functional groups to boron atom in order to obtain new boron heterocycles. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Heterocycles; Ephedrine; Boronic acids; Chirality
an excellent tool to determine the molecular structure
of nitrogenated molecules by NMR, by examining the
steric or electronic effects of the boron over its neigh-
boring atoms, in this way N-borane groups function as
steric probes [10–13,27–29,36,39,41,46]. The borane
coordination works also as an anchor to stop the
conformational equilibrium in heterocyclohexanes al-
lowing to study anomeric effects [36,40,45,47].
On the preparation of N-borane adducts of oxazo-
lidines derived from ephedrines, we have obtained rela-
tively stable compounds [27–29], Scheme 1. We have
also prepared the ephedrine esters of catecholborane,
the nitrogen–boron bonding produces spiranic struc-
tures. The nitrogen atom configuration was also de-
duced from NMR data [10], Scheme 2. A similar study
was performed in (N–B)-diphenyl(2-aminoethoxy)-
boronic acid derivatives of ephedrine and pseu-
doephedrine. The analysis of the dependence of the
1. Introduction
Boraheteroazolines are very important heterocycles,
used widely as reagents for asymmetric catalysis or as
optically active reducing agents [1–9]. In this context
we have prepared optically active boron heterocycles
[10–20] and studied the nature of bonding between
boron and heteroatoms. We are interested especially in
the nature of the N–B bond [10–15,17–50] and in its
ability to form heterocycles via N–B coordination [10–
13,19,22,23,26,30,32,34–36,38,44–47]. We have been
working in the synthesis of molecules where boron or
nitrogen are stereogenic centers [10–13,19,22,25,27–
29,45–47]. On the other hand, borane coordination is
* Corresponding author. Tel.: +527-7-477000; fax: +525-7-
477113.
E-mail address: rcontrer@mail.clnesrav.mx (R. Contreras)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00051-0

A. Rosendo Rico et al. / Journal of Organometallic Chemistry 581 (1999) 122–128
123
Scheme 1. The structure of the N-epimers derived from optically
active N-borane oxazolidine can be deduced from the 13C-NMR data
shown here [27–29].
Scheme 4. Preparation of phenylboronic esters derived from
ethanolamines.
ordinate boron compounds. There are some previous
examples of addition reactions to the B–N bond in
boroxazolines. We have reported phenol addition reac-
tion to benzoxazaboroline, to give a phenol ester bear-
ing a N–B coordination bond [50], as is depicted in
Scheme 5. In this reaction the nucleophilic oxygen atom
of phenol adds to boron and the proton transfers to
nitrogen. We have also investigated on borane addition
to the covalent nitrogen–boron of 1,3,2-oxazaborolines
derived from ephedrine [15,17,18,20], Scheme 6. In this
addition the boron in borane coordinates to the nitro-
gen atom, and a BH3 hydride bridges to the endocyclic
boron. The assignment of stereochemistry for these
compounds is based on their earlier observations of
empirical chemical shifts correlations with structure
[10,14,15,17–20,27–29].
Compounds 1–3 react rapidly with water 4–6 or
methanol 7–9 to give the corresponding boroxazolidi-
nes. Compound 1 could form two boron epimers I–II,
whereas compounds 2 and 3 could give four dias-
teromers III–VI, because the products have nitrogen
and boron stereogenic centers. The stereochemistry of
all possible compounds is shown in Scheme 7.
Scheme 2. The two N-epimers derived from catecholborane and
ephedrine, and the main isomer derived from pseudoephedrine and
catecholborane [10].
stereochemistry and the energy of the N–B bond on the
nitrogen substitution and the nature of the ligands was
reported [19]. In all cases their stereochemistry was
deduced from the NMR data based on steric and
electronic effects produced by neighboring groups,
Scheme 3.
Herein we report the study of addition reactions of
water and methanol to the N–B bond to 2-phenyl-
1,3,2-oxazaborolines derived from (1S,2R)-(+)-
norephedrine 1, (1S,2R)-(+)-ephedrine
2
and
(1R,2R)-(+)-pseudoephedrine 3. The syntheses of
compounds 1 [8] and 2 [2] have been reported. They
have been used as catalyzing agents for reduction or
alkylations [2,3,6–9]. Compounds 1–3 were prepared
from the reactions of phenylboronic acid and the corre-
sponding ethanolamines in dry toluene in a Dean–
Stark trap, Scheme 4. The compounds were purified by
distillation. Their NMR data are listed in Tables 1 and
2.
In heteroborolines, the heteroatoms provide elec-
tronic density by p-donation to the electronic deficient
boron atom [21,42,48–50]. This gives some double
bond character to the boron sp2 bonds, and makes
them susceptible to addition reactions that result in
tetracoordinated boron structures. Presumably, the ad-
dition reactions are favored because the electronic defi-
ciency at boron atom is better relieved in the
tetrahedral structures resulting from nucleophilic addi-
tion, relative to stabilization by p-donation in three-co-
The reaction of compound 1 with D2O in an NMR
tube generates two compounds (4a and 4b) in a 1/1
ratio with one 11B-NMR signal at +6.7 ppm, which is
characteristic for tetrahedral boron. The boron-epimers
are in equilibrium as is denoted by the two sets of
broad signals in 1H- and 13C-NMR spectra. If the
mixture is sublimed then 4a is enriched (70%), but
equilibration of the chloroform solution gives 4b as the
predominant isomer (70%). In Scheme 8 the NMR data
Table 1
13C-NMR data of 1–5, 7–9 in CDCl3 and 6 in toluene–d8
Compound
C-5
C-4
CH3–C
CH3–N
CH3–O
1
2
3
4a
4b
5a
5b
6
82.8
81.5
85.6
77.1
77.2
76.3
77.0
80.8
77.6
77.3
80.7
54,6
61.8
20.5
15.6
19.2
17.9
16.0
12.0
9.0
11.9
16.0
13.5
13.5
31.0
30.5
66.4
51.9
53.0
60.0
59.0
67.6
53.8
61.1
68.0
33.0
31.3
35.6
7
8
9
50.0
50.2
50.1
33.0
36.5
Scheme 3. 13C-NMR data of diphenylborinic esters derived from
ephedrine and pseudoephedrine [19].

124
A. Rosendo Rico et al. / Journal of Organometallic Chemistry 581 (1999) 122–128
Table 2
1H-NMR data of compounds 1–5, 7–9 in CDCl3 and 6 in toluene-d8
Com-
H-5 H-4 CH3–C CH3–N N–H OH CH3–O
pound
1
2
3
4a
4b
5a
5b
6
5.63 4.06 0.66
5.61 3.78 0.70
4.89 3.36 1.28
4.40 3.05 0.87
4.73 3.21 0.72
5.22 2.11 0.58
5.39 3.07 0.41
4.88 1.77 0.54
5.35 3.70 0.80
5.36 3.40 0.77
3.84
5.6
Scheme 6. Borane adds to the B–N bond giving two tetrahedral
boron and nitrogen atoms [15,17,18,20].
2.90
2.80
tion and N–methyl and B–phenyl also trans, Scheme
10. The structure assignment was done by comparison
with the analogous diphenylborinic esters, Scheme 3
[19].
1.70
1.75
1.31
6.6
7
8
9
3.25
3.33
3.32
2.34
2.09
4.6
3.25 1.26
2. Variable temperature experiment in compound 6
The mobility of the two protons B–OH and N–H
in heterocycle 6 was calculated by variable tempera-
ture NMR experiments. The temperature dependence
of the proton chemical shifts (Dl/DT) was calculated.
These data give information about the nature of N–
H and O–H bonds. It is accepted that (Dl/DT) val-
ues higher than 4×10−3 ppm K−1 correspond to
intermolecular or solvent interactions, whereas lower
values than 3×10−3 ppm K−1 indicate that the pro-
ton is less labile or is coordinated intramolecularly
[51,52]. Calculations performed in compound 6 in
toluene gave values of 13.7×10−3 ppm K−1 for O–
H and 3.8×10−3 ppm K−1 for N–H. Theses results
indicate that the proton in N–H is attached strongly
to the nitrogen atom, and therefore the proton is not
as acidic as in a normal amine, whereas the O–H is
labile. The OH value is consistent with intermolecular
O–D bonding observed in the packing diagrams of
compound 6, Fig. 3.
assigned to each isomer is shown. On the other hand,
the reaction of 1 with methanol is stereoselective and
only one isomer (7, 11B= +6.7 ppm) whose structure
is similar to that of compound 4a was detected
(Scheme 8).
The reaction product of compound 2 with water in
a NMR tube shows one set of broad signals in the
1H- and 13C-NMR spectra in toluene at room tem-
perature (11B= +7.8 ppm). If the solution is cooled
to −50°C two sets of sharp signals appear in a ratio
20/80. These resonances were assigned to compounds
5a and 5b, respectively, and the NMR data is shown
in Scheme 9. The reaction of 2 with methanol gives
only one isomer 8 whose structure is the same as that
of compound 5b (11B= +7.8 ppm, Scheme 9). The
sharp NMR resonances undicate that compound 8 is
not in equilibrium with isomeric structures.
The reaction of compound 3 with water gives one
set of broad signals when the compound is observed
at room temperature. At −50° (400 MHz, in
toluene) one set of sharp signals appear that was as-
signed to heterocycle 6. Its molecular structure was
obtained by an X-ray diffraction study, Figs. 1–3.
The structure corresponds to that deduced from the
NMR spectra, Scheme 10. The reaction of compound
3 with methanol was also stereoselective, as it gave
only one compound, 9. Compounds 6 and 9 present
similar structures, with methyl groups in trans posi-
Scheme 7. The two possible B-epimers from reaction addition of
water or methanol to the phenylborinic ester derived from
norephedrine I–II and the four diasteromers derived from ephedrine
or pseudopehedrine III–VI are shown.
Scheme 5. Phenol addition to benz-1,3,2-oxazaborole affords tetraco-
ordinated nitrogen and boron atoms [50].

A. Rosendo Rico et al. / Journal of Organometallic Chemistry 581 (1999) 122–128
125
Scheme 8. 13C- and 1H-NMR data of B-epimers obtained from
reaction of water and compound 1.
3. X-ray diffraction molecular structure of compound 6
Compound 6 was prepared directly in an NMR tube
by addition of one equivalent of D2O to a solution of 3
in toluene-d8. The NMR tube was let aside for some
weeks, crystals were obtained, and the X-ray structure
obtained (Figs. 1–3, Tables 3–7). It shows a cyclic
compound with a coordinating B–N bond [bond
length 1.692(6)] similar to that found in the analogous
compound derived from diphenylborinic ester (B–
N)=1.66(1) [19]. The two phenyl groups and C-methyl
are in pseudoequatorial positions as shown by the
dihedral angles, Table 5. The bond length B–O1 is
Fig. 1. View of the molecular structure of 6.
ucts are stable but they can be completely hydrolyzed
in an excess of water. The water addition to the
norephedrine derivatives is not selective but in the
ephedrine derivative one isomer predominates. The
pseudoephedrine heterocycle gives completely stereose-
lective reactions. In all cases the methanol additions
are 100% stereoselective. In the preferred configuration
of all compounds the N-methyl is in trans position to
C-methyl group. At the same time the N-methyl group
drives the more stable configuration at the boron atom,
thus the B-phenyl group is placed cis to the N-methyl
group. In these compounds it seems that the phenyl
group prefers a pseudoequatorial position with the
plane of the phenyl orthogonal to the main ring plane,
inducing the stereoselectivity of water or methanol ad-
dition. The addition reactions to the B–N bond of
boroxazolines present interesting perspectives for the
stereoselective synthesis of boron heterocycles. Some
other addition reactions are under investigation in our
group.
,
1.450(6) A, whereas B–O2 of the hydroxyl group is
,
slightly shorter 1.441(6) A in accord with its pseudoax-
,
ial conformation. The B–C bond length is 1.595(7) A.
The large value for C5–N1–B1 117.8(4) can be at-
tributed to the steric compression between phenyl and
methyl groups and also from ring distortion in order to
put the C-methyl group in pseudoequatorial position.
The endocyclic angle B–N1–C3 is very close 104.6 (3).
The boron atom has also distorted angles. The angle
O1–B–N1 99.5(3) is more acute because is an endo-
cyclic angle in a five membered ring, and the O2–B–
N1 small angle 106.1(4) is due to the pseudoaxial
position of the B–OH group. The C12–B–N1 wide
angle 113.7(4) is due to the steric compression between
the N–Me and the B–phenyl groups. The packing
diagram indicates a deuterium bonding between the
deuterium of the OD and the endocyclic oxygen atom
of another molecule (OD—O atomic distance=3.24
,
A), Fig. 3.
4. Conclusions
In phenyl 1,3,2-oxazaboroline the addition reactions
of H2O and methanol are favored. The reaction prod-
Scheme 9. Structure, 13C- and 1H-NMR data of compounds 5a, 5b
and 8.
Fig. 2. View of the molecular structure of 6. The phenyl group
conformation is shown.

126
A. Rosendo Rico et al. / Journal of Organometallic Chemistry 581 (1999) 122–128
Table 3
,
Selected bond lengths (A) of compound 6
O1–C2 1.402(5)
O2–D2 0.88(5)
N1–D1 0.87(5)
N1–C3 1.508(6)
C2–H2 0.855(4)
B1–C12
C2–C3
C3–C4
O1–B1
C3–H3
1.595(7)
1.523(6)
1.508(7)
1.450(6)
1.095(5)
O2–B2
N1–C5 1.481(6)
N1–B1 1.692(6)
1.441(6)
C2–C6
1.493(6)
on diffractometer angles for 24 automatically centred
reflections. Absorption correction was not necessary,
corrections were made for Lorentz and polarization
effects. Solution and refinement: direct method
(SHELXS-86) for structure solutions SHELXS (Sheldrick)
[53] package was used for refinement and data output.
All hydrogen and deuterium atoms were found and
refined (Tables 5–7).
Fig. 3. View of the deuterium bonds between two molecules of
,
compound 6. The OD–O atomic distance is 3.24 A.
5.1. General procedure to 4-methyl-2,5-diphenyl-
1,3,2-oxazaborolines
A solution of the corresponding aminoalcohol in dry
toluene was treated with one equivalent of phenyl-
boronic acid and heated under reflux for three hours.
The solution was evaporated to dryness under reduced
pressure and the crude product distilled under vacuum.
The compounds were dissolved in CDCl3 or toluene
and the structure identified by NMR.
Scheme 10. Stereochemistry and NMR data of compounds 6 and 9
obtained by the addition of water and methanol to the phenylboronic
ester derived from pseudoephedrine.
5.2. (4R,5S)-4-methyl-2,5-diphenyl-1,3,2-oxaza-
boroline, 1
5. Experimental
All solvents were distilled freshly and dried before
use according to established procedures. The aminoal-
cohols and phenylboronic acids were commercially
available reagents. All reactions were handled under a
nitrogen atmosphere using dried glassware and sol-
vents. NMR spectra were obtained on a Jeol GXS 270
Et2O–BF3 or 400 MHz Eclipse spectrometers. The
X-ray diffraction study was performed on an Enraf–
(+)-Norephedrine 0.6 g (3.9 mmol), phenylboronic
acid 0.483 g (3.9 mmol) and 25 ml of toluene. The
distillation afforded a transparent viscous liquid 1 (0.81
g, 87%).
5.3. (4R,5S)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-
oxazaboroline, 2
(+)-Ephedrine 1.2 g (7.2 mmol), phenylboronic acid
0.885 g (7.2 mmol) and 40 ml of toluene. The distilla-
tion afforded a transparent viscous liquid 2 (1.53 g,
84%).
Nonius CAD4 diffractometer (uMo–K =0.71069,
a
monochromator: graphite T=293 K, −2q scan). Cell
parameters were determined by least-squares refinement
Table 4
Selected bond angles (°) of compound 6
C2–O1–B1
D1–N1–C5
C5–N1–C3
C5–N1–B1
O2–B1–O1
O1–B1–C12
O1–B1–N1
O1–C2–C6
C6–C2–C3
109.2 (3)
114.1 (29)
110.7 (4)
117.8 (4)
109.1 (4)
111.6 (4)
99.5 (3)
C6–C2–H2
C4–C3–N1
N1–C3–C2
N1–C3–H3
D2–O2–B1
D1–N1–C3
D1–N1–B1
C3–N1–B1
O2–B1–C12
109.6 (4)
112.5 (4)
103.7 (3)
96.5 (4)
130.0 (33)
104.2 (30)
104.1 (30)
104.6 (3)
115.5 (4)
O2–B1–N1
C12–B1–N1
O1–C2–C3–
O1–C2–H2
C3–C2–H2
C4–C3–C2
C4–C3–H3
C2–C3–H3
106.1 (4)
113.7 (4)
105.7 (4)
111.7 (4)
104.9 (4)
114.8 (5)
122.8 (5)
103.6 (4)
112.7 (3)
112.0 (4)

A. Rosendo Rico et al. / Journal of Organometallic Chemistry 581 (1999) 122–128
127
Table 5
Selected dihedral angles (°) of compound 6
C6–C2–O1–B1
O1–C2–C3–C4
C6–C2–C3–N1
C12–B1–N1–C3
167.47
−156.72
−156.69
130.47
C6–C2–C3–C4
C2–C3–N1–C5
C4–C3–N1–C5
80.16
139.94
−95.42
C4–C3–N1–B1
C12–B1–O1–C2
O1–B1–N1–C5
136.68
−154.39
−111.79
Table 6
Table 7
Crystal data and data collection parameters
Atomic coordinates (×104) of compound 6
x
y
z
U(eq)
Formula
Weight
C16H18BD2NO2
271.14
O1
O2
N1
B1
C2
C3
C4
C5
C6
−4389(3)
−2812(4)
−1098(5)
−3052(6)
−3529(6)
−1649(6)
−192(8)
−4177(3)
−2216(3)
−4446(4)
−3722(5)
−4202(5)
−4907(5)
−4549(10)
−5626(5)
−4950(5)
−4405(6)
−5167(11)
−6459(10)
−7026(7)
−6277(6)
−4302(5)
−3813(7)
−4212(8)
−5167(10)
−5711(8)
−5295(7)
−3729(49)
−1666(54)
−5319.8(12)
−4927(2)
−5168(2)
−4884(2)
−5870(2)
−5774(2)
−6225(3)
−4844(3)
−6320(2)
−6867(2)
−7272(2)
−7134(4)
−6599(4)
−6200(3)
−4250(2)
−3751(3)
−3194(3)
−3126(3)
−3609(3)
−4162(2)
−5225(19)
−4778(23)
50.0(8)
65.1(10)
56.2(10)
52.0(12)
52.6(11)
65.0(13)
120(3)
82(2)
55.7(11)
77.2(15)
110(2)
99(2)
100(2)
77(2)
55.7(11)
89(2)
102(2)
109(3)
100(2)
76.1(15)
59(13)
80(17)
Size
System
Space group
Unit cell dimensions
a (A)
b (A)
c (A)
h (°)
i (°)
k (°)
0.22×0.31×0.53
Orthorhombic
P21 21 21
,
,
7.1868 (10)
9.473 (2)
22.877 (5)
90.00
90.00
90.00
1557.5 (5)
4
1.156
0.007
576
05h58, −115k50, −285l50
52.58
293(2)
1231
1231
1111
0.0000
,
−223(7)
−4662(6)
−5013(8)
−6026(11)
−6697(9)
−6332(9)
−5347(8)
−3567(6)
−2728(9)
−3232(11)
−4634(11)
−5533(8)
−4977(7)
−367(67)
−1940(78)
C7
C8
C9
3
,
V (A )
Z
C10
C11
C12
C13
C14
C15
C16
C17
D1
D2
zcalc. (Mg m−3
)
[mm−1
]
F(000)
Index range
2q (°)
Temp (K)
Refl. collected
Refl. unique
Refl. observed (4|)
R(int.)
No. variables
Weight scheme x/y
w−1
P
GOF
189
0.1084/0.0998
|2F2o+(xP)2+yP
(F2o+2F2c)/3
1.088
5.4. (4R,5R)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-
oxazaboroline, 3
Final R (4|)
Final wR2
Larg. Res. Peak (e/A )
0.0568
0.1478
0.215
3
,
(−)-Pseudoephedrine 0.6 g (3.6 mmol), phenyl-
boronic acid 0.442 g (3.6 mmol) and 25 ml of toluene.
The distillation afforded a transparent viscous liquid 3,
(0.87 g, 91%).
Preparation of compounds 4–9 was performed di-
rectly in NMR tubes in CDCl3 or toluene-d8 by addi-
tion of one equivalent of water or deuterated water or
methanol to the corresponding compounds. The reac-
tion products were observed directly.
Compound 6 after adding D2O and standing for
several weeks in the tube crystallized from the CDCl3
solution and the X-ray diffraction molecular structure
was obtained.
Marco Leiva Ram´ırez for the X-ray diffraction
determination.
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