Zinc derivative of ethyl diazoacetate in the synthesis of α-diazo-β- hydroxy esters

Article abstract of DOI:10.1055/s-1998-2105

Addition of diethylzinc to ethyl diazoacetate generates a zinc derivative which adds to aldehydes to give α-diazo-β hydroxy esters.

Full text of DOI:10.1055/s-1998-2105

SYNTHESIS
July 1998
1039
Zinc Derivative of Ethyl Diazoacetate in the Synthesis of α-Diazo-â-hydroxy Esters
Christopher J. Moody,* C. Neil Morfitt
Department of Chemistry, University of Exeter, Stocker Road, Exeter, EX4 4QD, U.K.
Fax +44(1392)263434; E-mail: c.j.moody@exeter.ac.uk
Received 8 December 1997; revised 16 January 1998
Abstract: Addition of diethylzinc to ethyl diazoacetate generates
a zinc derivative which adds to aldehydes to give α-diazo-β-
hydroxy esters.
ethyl diazoacetate with zinc bis[bis(trimethylsilyl)amide]
as an unstable oil, although no reactions of it were report-
ed.11 We now find that commercially available diethyl-
Key words: ethyl diazoacetate, diethylzinc, metalation of dia-
zinc can be used to generate a zinc derivative of ethyl
zoacetates, α-diazo-β-hydroxy esters
diazoacetate (1), which subsequently reacts with alde-
hydes to give α-diazo-β-hydroxy esters 2 in yields that are
Diazocarbonyl compounds are widely recognised as ver-
satile intermediates in synthesis since the derived car-
benes or metal carbenoids undergo a variety of useful
reactions.1 Although the synthesis of diazocarbonyl com-
pounds can be achieved by a number of routes,2 one po-
tentially attractive method is to carry out a substitution
reaction on a readily available diazo compound. Such re-
actions, in which the diazo carbon reacts as a nucleophile,
and the diazo group remains intact are known, and can in-
volve either the diazo compound itself, or prior metalla-
tion to give a more reactive organometallic diazo species
(Scheme 1). Organometallic derivatives of diazo carbonyl
compounds involving a variety of metals are known,2,3
but despite the fact that the first such compound –
bis(ethoxycarbonyldiazomethyl)mercury, a stable yellow
solid, was first prepared by Buchner over 100 years ago,
these metallated diazocarbonyl compounds have not
found widespread use in synthesis to date. Thus commer-
cially available ethyl diazoacetate can be readily lithiated
at low temperature and the resulting lithiated species re-
acted with electrophiles such as chlorotrimethylsilane,
alkyl halides, benzoyl chloride, and aldehydes and ke-
tones.4 The reaction with carbonyl compounds has been
widely investigated by Pellicciari and coworkers,5 and
can also be carried out in the presence of ethanolic KOH.6
The silver derivative of diazoacetate can also be alkylated
with SN1 active halides,7 and recently it has been shown
that the trialkylstannyl derivatives8 of ethyl diazoacetate
can be cross coupled with aryl iodides with palladium(0)
catalysis.9
comparable or better than those obtained from other meth-
ods (Scheme 2, Table).
Scheme 2
Table. Reaction of Ethyl Diazoacetate with Aldehydes
Prod-
uct
R
Method (%)
Et2Zn
LDA
KOH6
2a
2b
2c
2d
2e
2f
2g
2h
2i
Pha
2-MeC6H4
4-MeC6H4
C5H11
c-C6H11
Me2CH
59
86
74
45
80
85
12
74
64
68
72
20
45
–
15
23
–
60
–
–
–
90
80
–
–
–
2-pyridyl
4-MeO2CC6H4
(R)-2,2-dimethyl-1,3-
dioxolan-4-ylb
–
a
Use of n-BuLi or MeMgI as base gave 41–95% yield of the prod-
uct.4b
b
Reaction carried out in the absence of base afforded 59% of the
product.12
The reaction is carried out by adding a solution of dieth-
ylzinc in hexanes to ethyl diazoacetate keeping the tem-
perature at –50°C or below. The aldehyde was added at
low temperature; warming to room temperature and aque-
ous workup gave the α-diazo-β-hydroxy esters 2. The
yields are shown in the Table, together with literature
yields for the preparation of 2a, 2e, and 2f via the lithiated
diazo compound or using the potassium hydroxide meth-
od. In cases where no direct comparison was available,
we also carried out the addition reactions using lithium di-
isopropylamide (LDA) as base; the results are also shown
in the Table. In all cases, except one, ethyl group transfer
was not observed; in the case of pyridine-2-carbaldehyde
the major product was 1-(2-pyridyl)propan-1-ol (55%)
with the α-diazo-β-hydroxy ester 2g being formed in only
12% yield. Although ethyl diazolithioacetate adds to ke-
Scheme 1
In continuation of our interest in the diastereoselectivity
of reactions of rhodium carbenoids derived from diazo-
carbonyl compounds,10 we required a series of α-diazo-β-
hydroxy esters. Although these are available by reaction
of ethyl diazolithioacetate with aldehydes as referred to
above, we decided to investigate an alternative approach
based on zinc derivatives of ethyl diazoacetate in the ex-
pectation that such species might offer more selectivity
than their lithiated counterparts. Bis(ethoxycarbonyldiazo-
methyl)zinc has been prepared previously by reaction of

SYNTHESIS
Papers
1040
tones,4b,6 the organozinc derivative is selective for alde-
hydes, ketones (cyclohexanone, acetone, pentan-2-one,
pentan-3-one, acetophenone) failing to react. Hence when
an equimolar mixture of benzaldehyde and acetophenone
was treated with ethyl diazoacetate/diethylzinc, the hy-
droxy diazoester 2a was isolated in 72% yield, with the
acetophenone recovered unchanged in quantitative yield.
The α,β-unsaturated aldehyde trans-hex-2-enal also
failed to react with the zinc derivative. Other carbonyl
groups such as esters do not interfere as evidenced by the
fact that methyl 4-formylbenzoate reacts solely at the al-
dehyde group to give the α-diazo-β-hydroxy ester 2h in
74% yield.
THF (1 equiv). The mixture was left to come to r.t. overnight and then
cooled in an ice bath to 0°C before the addition of satd aq NH4OH so-
lution. The resulting solution was extracted with CH2Cl2, the com-
bined organic extracts dried (MgSO4), filtered and concentrated. The
residual oil was chromatographed (10% EtOAc, 90% petroleum
ether) to give the products 2a–d,g.
Ethyl 2-Diazo-3-hydroxy-3-phenylpropanoate (2a):
(a) According to the general diethylzinc procedure using benzalde-
hyde (180 µL, 1.8 mmol), the title compound was obtained as a yel-
low oil (338 mg, 59%) (Lit.6 oil).
IR (neat): ν = 3441, 2983, 2098, 1693 cm–1.
1H NMR (250 MHz, CDCl3): δ = 1.28 (3 H, t, J = 7.1, CH3), 3.27 (1
H, br s, OH), 4.26 (2 H, q, J = 7.1, CH2), 5.90 (1 H, d, J = 3.4,
CHOH), 7.31–7.45 (5 H, m, ArH).
13C NMR (62.9 MHz, CDCl3): δ = 14.8 (CH3), 61.6 (CH2), 65.1
(CN2), 68.8 (CH), 126.1 (CH), 128.6 (CH), 129.1 (CH), 139.7 (C),
166.9 (CO).
Finally the zinc derivative was reacted with an aldehyde
containing a chiral centre at the α-position. Glyceralde-
hyde acetonide gave the α-diazo-β-hydroxy ester 2i in
64% yield and 66% diastereomeric excess; the corre-
sponding reaction of methyl diazoacetate carried out with-
out base is reported to proceed in 59% yield (68% de).12
MS: m/z (%) = 220 (M+, 10), 192 (13), 146 (86), 118 (49), 105 (100),
77 (66).
HRMS: calc. for C11H12N2O3: 220.0848; found: 220.0848.
(b) According to the general LDA method with benzaldehyde
(180µL, 1.8 mmol); yellow oil (390 mg, 68%), spectral data as above.
Ethyl 2-Diazo-3-hydroxy-3-(2-tolyl)propanoate (2b):
(a) According to the diethylzinc procedure, 2-tolualdehyde (220 µL,
1.9 mmol) reacted to give 2b (380 mg, 86%) as a yellow oil.
IR (neat): ν =3442, 2098, 1694, 1670, 1373, 1292, 1102 cm–1.
1H NMR (300 MHz, CDCl3): δ = 1.31 (3 H, t, J = 6.0, CH3), 2.29 (3
H, s, ArCH3), 3.07 (1 H, br s, OH), 4.29 (2 H, m, AB of ABX3, CH2),
6.03 (1 H, d, J = 2.6, CHOH), 7.17–7.27 (3 H, m, ArH), 7.61 (1 H,
dd, J = 5.7, 1.7, ArH).
Commercially available reagents were used throughout without fur-
ther purification; solvents were dried by standard procedures. Light
petroleum refers to the fraction with bp 40–60°C. Reactions were
routinely carried out under a N2 atmosphere. Analytical TLC was car-
ried out using aluminium-backed plates coated with Merck Kieselgel
60 GF254. Plates were visualised under UV light (at 254 and/or
360 nm). Flash chromatography was carried out using Merck Kiesel-
gel 60 H or Matrex silica 60.
Fully characterised compounds were chromatographically homoge-
neous. IR spectra were recorded in the range 4000–600 cm–1 using
Nicolet Magna FT-550 spectrometers. 1H and 13C NMR spectra were
recorded using Bruker 250, 300 and 400 MHz instruments (1H fre-
quencies, corresponding 13C frequencies are 62.9, 75 and 100 MHz);
J values were recorded in Hz. High and low-resolution mass spectra
were recorded on a Kratos HV3 instrument, or at the EPSRC Mass
Spectrometry Service (Swansea).
13C NMR (75 MHz, CDCl3): δ= 14.4 (CH3), 18.8 (CH3), 41.2 (CH2),
62.4 (CN2), 65.9 (CH), 125.2 (CH), 126.2 (CH), 127.9 (CH), 130.5
(CH), 134.2 (C), 137.2 (C), 166.5 (CO).
MS: m/z (%) = 234 (M+, 1), 206 (67), 119 (100), 91 (82).
HRMS: calc. for C12H14N2O3: 234.1004; found: 234.1003.
(b) According to the standard LDA diazo procedure, 2-tolualdehyde
(220 µL, 1.9 mmol), gave 2b (323 mg, 72%) as a yellow oil, spectral
data as above.
Ethyl 2-Diazo-3-hydroxy-3-(4-tolyl)propanoate (2c):
(a) According to the standard diethylzinc procedure, 4-tolualdehyde
(230 µL, 1.9 mmol) reacted to give 2c (330 mg, 74%) as a yellow oil.
IR (neat): ν =3440, 2983, 2097, 1669, 1373, 1293, 1108, 1036, 771,
748 cm–1.
α-Diazo-â-hydroxy Esters 2a–i, Addition of Ethyl Diazoacetate to
Aldehydes Using Diethylzinc; General Procedure:
All glassware were dried in an oven before cooling in a desiccator. A
three-necked round bottomed flask fitted with a low temperature ther-
mometer, a magnetic stirrer bar and suba-seals was flushed with N2.
To this was added CH2Cl2 (5 mL) and ethyl diazoacetate (EDA)
(200 µL, 1.9 mmol), then cooled in a dry ice bath to approximately
–60°C before careful addition of the diethylzinc (1 equiv). As the
base was added the temperature was kept below –50°C, and the mix-
ture was left to stir for 30 min to allow time for complex formation.
After this time, the carbonyl compound (1 equiv) was added and the
mixture kept below –50°C for 2 h. The mixture was left to come to
r.t. overnight and then cooled in an ice bath to 0°C before the addition
of satd aq NH4OH solution. The resulting solution was extracted with
CH2Cl2, the combined organic extracts dried (MgSO4), filtered and
concentrated. The resulting oil was chromatographed (10% EtOAc,
90% petroleum ether) to give the products 2a–i.
1H NMR (250 MHz, CDCl3): δ= 1.29 (3 H, t, J = 7.1, CH2CH3), 2.35
(3 H, s, ArCH3), 3.43 (1 H, br s, OH), 4.26 (2 H, q, J = 7.1, CH2), 5.86
(1 H, s, CHOH), 7.24 (4 H, 2 x d, J = 7.9, ArH).
13C NMR (62.9 MHz, CDCl3): δ = 15.1 (CH3), 21.8 (CH3), 61.8
(CH2), 69.2 (CH), 126.3 (CH), 130.0 (CH), 136.6 (C), 138.6 (C),
167.2 (CO).
(b) A solution of EDA (200 µL, 1.9 mmol) in THF was cooled to
–78°C under N2, LDA (1 equiv) was added and the solution stirred for
30 min before the addition of 4-tolualdehyde (230 µL, 1.9 mmol).
Workup as in the general procedure gave 2c as a yellow oil (90 mg,
20%), spectral data as above.
Ethyl 2-Diazo-3-hydroxyoctanoate (2d):
(a) According to the standard diethylzinc procedure, hexanal
(200 µL, 1.7 mmol) reacted to give 2d (191 mg, 45%) as a yellow oil.
IR (neat): ν =3449, 2957, 2932, 2094, 1693 cm–1.
α-Diazo-â-hydroxy Esters 2a–d,g, Addition of Ethyl Diazoacetate
Using Lithium Diisopropylamide; General Procedure:
All glassware were dried in an oven before cooling in a desiccator. A
three-necked round bottomed flask fitted with a low temperature ther-
mometer, a magnetic stirrer bar and suba-seals was flushed with N2.
To this was added THF (5 mL), ethyl diazoacetate (EDA) (200 µL,
1.9 mmol) and carbonyl compound (1 equiv), then cooled in a dry ice
bath to approximately –60°C before the careful addition of LDA in
1H NMR (400 MHz, CDCl3): δ = 0.90 (3 H, t, J = 7.2, CH3), 1.26–
1.73 (13 H, m), 4.25 (2 H, q, J = 7.0, CH2), 4.67 (1 H, t, J = 6.5,
CHOH).
13C NMR (100 MHz, CDCl3): δ= 14.0 (CH3), 14.5 (CH3), 22.4 (CH),
25.3 (CH), 31.6 (CH), 34.1 (CH), 61.0 (CH2), 66.6 (CH), 166.7 (CO).

SYNTHESIS
July 1998
1041
(b) According to the general LDA procedure, EDA (1.1 mL,
10 mmol) with hexanal (1.2 mL, 10 mmol), gave 2d (960 mg, 45%)
as a yellow oil, spectral data as above.
MS: m/z (%) = 222 (M+, 53), 194 (100), 193 (25), 106 (21), 78 (92).
HRMS: calc. for C10H12N3O3: 222.0879; found: 222.0879.
Ethyl 2-Diazo-3-hydroxy-3-(4-methoxycarbonylphenyl)propanoate
(2h):
Ethyl 3-Cyclohexyl-2-diazo-3-hydroxylpropanoate (2e):
(a) According to the general diethylzinc procedure, cyclohexyl carb-
oxaldehyde (230 µL, 1.9 mmol) reacted to give 2e (342 mg, 80%) as
a yellow oil (Lit.6 oil).
According to the standard diethylzinc procedure, methyl 4-formyl-
benzoate (168 mg, 1 mmol) reacted to give 2h (207 mg, 74%).
IR (neat): ν =3461, 2984, 2950, 2099, 1723, 1696, 1612, 1437, 1401,
1372, 1340, 1281, 1189, 1110, 1038, 1018, 906, 748 cm–1.
1H NMR (400 MHz, CDCl3): δ= 1.23 (3 H, t, J = 7.1, CH2CH3), 3.86
(3 H, s, OCH3), 4.03 (1 H, br s, OH), 4.20 (2 H, q, J = 7.1, CH2Me),
5.90 (1 H, d, J = 2.0, CHOH), 7.46 (2 H, d, J = 8.2, ArH), 8.00 (2 H,
d, J = 8.2, ArH).
IR (neat): ν =3448, 2928, 2854, 2093, 1692, 1670, 1373, 1291,
1105 cm–1.
1H NMR (300 MHz, CDCl3): δ = 0.95–1.76 (8 H, m), 1.28 (3 H, t, J
= 6.0, CH3), 2.02 (1 H, d, J = 10.6), 2.57 (1 H, br s, OH), 4.23 (2 H,
q, J = 6.0, CH2), 4.29 (1 H, dd, J = 4.3, 2.77, CHOH).
13C NMR (75 MHz, CDCl3): δ= 14.3 (CH3), 25.6 (CH2), 25.8 (CH2),
26.2 (CH2), 29.0 (CH2), 29.1 (CH2), 42.1 (CH), 60.8 (CH2), 60.9
(CN2), 70.7 (CH), 166.8 (CO).
13C NMR (100 MHz, CDCl3): δ = 14.3 (Me), 52.1 (Me), 61.3 (CH2),
62.9 (CN2), 68.1 (CH), 125.7 (CH), 129.8 (C), 130.0 (CH), 144.5 (C),
166.1 (CO), 166.8 (CO).
MS: m/z (%) = 278 (M+, 2), 250 (4), 163 (100), 135 (16), 77 (30).
HRMS: calc. for C13H14N2O5: 278.0902; found: 278.0921.
HRMS: calc. for C11H18N2O3: 226.1317; found: 226.1327.
(b) According to the standard LDA procedure, cyclohexyl carboxal-
dehyde (230 µL, 1.7 mmol), gave 2e (127 mg, 30%) as a yellow oil,
spectral data as above.
Ethyl 2-Diazo-3,4,5-trihydroxy-4,5-(O-isopropylidene)pentanoate
(2i):
According to the diethylzinc procedure, glyceraldehyde diacetonide
(263 mg, 2 mmol) reacted to give 2i (284 mg, 64%; 66 % d.e.).
IR (neat): ν =3445, 2987, 2938, 2921, 2098, 1690, 1465, 1398, 1261,
1126, 1070 cm–1.
Ethyl 2-Diazo-3-hydroxy-4-methylpentanoate (2f):
(a) According to the general diethylzinc method, isobutyraldehyde
(150 µL, 1.7 mmol) reacted to give 2f (262 mg, 85%) as a yellow oil
(Lit.6 oil).
1H NMR (400 MHz; CDCl3): δ = 1.16 (3 H, t, J = 7.1, CH2CH3 of
major diastereomer), 1.20 (3 H, t, J = 7.1, CH2CH3 of minor diastere-
omer), 1.26 (3 H, s, acetonide CH3 of major diastereomer), 1.28 (3 H,
s, acetonide CH3 of minor diastereomer), 1.37 (3 H, s, acetonide CH3
of major diastereomer), 1.26 (3 H, s, acetonide CH3 of minor diaste-
reomer), 3.76–3.79 (1 H, m), 3.94–3.98 (2 H, m), 4.06–4.12 (3 H, m),
4.24 (1 H, d, J = 5.6); de determined by integration of acetonide CH3
signals.
IR (neat): ν =3455, 2964, 2049, 1671, 1467, 1373, 1291 cm–1.
1H NMR (250 MHz, CDCl3): δ = 0.87 [3 H, d, J = 6.7, CH(CH3)2],
0.99 [3 H, d, J = 6.7, CH(CH3)2], 1.26 (3 H, t, J = 7.1, CH3), 1.86 (1
H, m, CHMe2), 2.96 (1 H, s, OH), 4.21 (3 H, q, J = 7.1, CH2 + m,
CHO).
13C NMR (62.9 MHz, CDCl3): δ = 14.4 (CH3), 18.6 (CH3), 18.7
(CH3), 32.7 (CH), 60.9 (CH2), 60.1 (CN2), 72.1 (CH), 167.0 (CO).
MS: m/z (%) = 186 (M+, 2), 159 (2), 143 (30), 113 (33), 83 (100), 69
(42).
HRMS: calc. for C8H14N2O3: 186.1004; found: 186.1008.
(b) Using the same LDA addition method as for 4-tolualdehyde,
isobutyraldehyde (180 µL, 1.9 mmol) reacted to give 2f (51 mg, 15%)
as a yellow oil, spectral data as above.
13C NMR (100 MHz; CDCl3): δ = 14.2 (CH3), 24.9 (CH3), 26.2
(CH3), 58.7 (CN2), 60.9 (CH2), 66.5 (CH2), 67.4 (CH), 76.6 (CH),
109.8 (C), 166.5 (CO).
MS: m/z (%) = 244 (M+, 1), 202 (5), 171 (16), 101 (100).
HRMS: calc. for C10H16N2O5: 244.1059; found: 244.1058.
Ethyl 2-Diazo-3-hydroxy-3-(2-pyridyl)propanoate (2g):
According to the standard diethylzinc procedure, addition was per-
formed on pyridine-2-carboxaldehyde (190 µL, 2 mmol). The prod-
ucts from this reaction were purified by flash chromatography to give
two compounds.
We thank the E.P.S.R.C. for support of this work (studentship to
C.N.M.)
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IR (neat): ν =3387, 2967, 2934, 2897, 2243, 2101, 1597, 1436, 1049,
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732, 565 cm–1.
1H NMR (300 MHz, CDCl3): δ = 0.83 (3 H, t, J = 7.4, CH3), 1.58–
1.82 (2 H, m), 4.56 (1 H, dt, J = 5.0, 2.0, CHOH), 4.68 (1 H, br s, OH),
7.03–7.07 (1 H, m, ArH), 7.22 (1 H, d, J = 8.0, ArH), 7.53–7.58 (1 H,
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13C NMR (75 MHz, CDCl3): δ = 9.5 (CH3), 31.1 (CH2), 74.2 (CH),
120.4 (CH), 122.1 (CH), 136.6 (CH), 148.0 (CH), 162.7 (C).
MS: m/z (%) = 137 (M+, 21), 121 (100), 106 (38), 79 (73), 78 (63).
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IR (neat): ν =3414, 2984, 2098, 1740, 1695, 1297, 1112, 1041 cm–1.
1H NMR (300 MHz; CDCl3): δ =1.27 (3 H, t, J = 7.2, CH3), 4.24 (2
H, q, J = 7.2, CH2), 5.79 (1 H, br s, CHOH), 7.25–7.27 (1 H, m, ArH),
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13C NMR (75 MHz, CDCl3): δ = 14.5 (CH3), 61.0 (CH2), 67.3 (CH),
121.0 (CH), 123.3 (CH), 137.4 (CH), 148.2 (CH), 157.7 (C), 166.0
(CO).

SYNTHESIS
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