SCHEME 3
scale-up, since the desired aldehyde hydrate was slightly
adsorbed to the silica gel, requiring extensive rinsing with
dichloromethane or reduced product recovery. Rinsing
with reagent grade ether resulted in acetal formation,
while the use of more polar solvents such as ethyl acetate
or acetonitrile resulted in the leaching of iodine species
into solution. Finally, ethyl glyoxylate hydrate was
reliably prepared by the reaction of diethyl tartrate with
sodium periodate in 5:1 dichloromethane/water.9,10
Once pure 4 was in hand, the remainder of the
synthesis was easily carried out. Treatment of mono-Boc-
ethylenediamine (3) with 1.1 equiv of 4 in dichloro-
methane at 0 °C over 3 Å MS quantitatively afforded the
corresponding imine (5) within an hour. Filtration of this
solution, followed by addition of 0.05 equiv of Pd (10%
on activated carbon) and hydrogenation afford the PNA
backbone monomer (1). The imine solution has also been
stored overnight at -20 °C or evaporated and stored at
-20 °C with little or no effect on the purity of the final
product.
acetone, if necessary. As a solid, 1‚HCl is more conve-
niently stored and dispensed than neutral 1, which is
usually a viscous oil. We have prepared a few PNA
monomers from 1‚HCl (Scheme 3), and have observed no
disadvantage over the use of the neutral backbone.
In summary, we have developed a new synthesis of the
peptide nucleic acid monomer precursor, which is prefer-
able to reported methods due to its efficiency, reduced
labor, and especially the elimination of the need for
purification by chromatography. Furthermore, we report
the preparation, purification, and use of the hydrochlo-
ride salt of ethyl N-[(2-Boc-amino)ethyl]glycinate, a more
convenient form of this important compound.
When the hydrogenation was carried out in the pres-
ence of molecular sieves in an attempt to avoid the
filtration step, the quality of the crude product was
substantially decreased, containing a number of uniden-
tified side products.
To demonstrate the scalability of the procedure, 1 has
been prepared on 2-, 10-, and 38-g scales. On each scale,
the desired product was isolated pure and in essentially
quantitative yield. For scale-up, the amounts of reagents
and solvents were scaled equally, and reaction times
remained the same. The only difference between the
scales was the method of hydrogenation. On the 2-g scale,
hydrogenation was complete within 4 h of magnetic
stirring under a balloon of H2. However, on the 10-g scale,
these conditions afforded only 77% reduction based on
NMR analysis of the crude product. By increasing the
H2 pressure to 50 psi and providing vigorous shaking on
a Parr hydrogention apparatus, reduction on the 10-g
scale was complete within 4 h. On the 38-g scale, too large
for our hydrogenation apparatus, reduction was carried
out under a stream of H2, held back by a mercury
bubbler. Efficient agitation was achieved by the use of
strong magnetic stirring in a Morton-type flask. Under
these conditions, reduction was complete within 4 h. In
general, the major factor in the rate of the hydrogenation
appeared to be the efficiency with which H2 was trans-
ferred to the solution, and so strong agitation is of utmost
importance. For this reason, it is recommended (espe-
cially on larger scales) that completion of reduction be
verified by NMR analysis of a small, filtered, evaporated
aliquot of the reaction mixture prior to workup.
Exp er im en ta l Section
Ethyl glyoxylate hydrate,9 Boc-ethylenediamine,4 thymin-1-
ylacetic acid,3 N3-PMB-thymin-1-ylacetic acid,11 and 5-iodou-
racil-1-ylacetic acid12 were prepared according to literature
methods. Ethereal HCl was prepared by dropwise addition of a
large excess of concentrated HCl to an equal volume of concen-
trated H2SO4, and bubbling the gas thus formed through stirred,
ice-bath cooled diethyl ether. This reagent was stored at -20
°C and titrated prior to use. Molecular sieves were pulverized
and activated at 300 °C under vacuum for 3 days prior to use.
All other reagents and solvents were used as supplied, without
further purification.
Eth yl N-(2-Boc-a m in oeth yl) Glycin a te (1). To an ice-bath
cooled solution of ethyl glyoxylate hydrate (4, 5.37 g, 44.7 mmol)
in CH2Cl2 (∼90 mL) was added 3 Å MS (∼5 g), followed by
dropwise addition of Boc-ethylenediamine (3, 6.50 g, 40.6 mmol)
in CH2Cl2 (∼10 mL) over ∼10 min. The mixture was stirred at
0 °C for 1 h and then filtered through a short pad of Celite.
Evaporation of a few drops of this solution, followed by 1H NMR
analysis (600 MHz, CDCl3) revealed complete conversion of 3 to
imine 5: δ 7.60 (s, 1H), 4.93 (br s, 1H), 4.22 (q, J ) 7.2 Hz, 2H),
3.64 (t, J ) 5.3 Hz, 2H), 3.36 (app q, J ) 5.3 Hz, 2H), 1.31 (s,
9H), 1.24 (t, J ) 7.2 Hz, 3H). Palladium (10% on activated
carbon, 2.16 g, 2.03 mmol) was added to the filtered solution
and the mixture was hydrogenated at 50 psi. After 4 h, the
mixture was filtered through a hard-packed pad of Celite and
rinsed with MeOH under a stream of N2 (Ca u tion ! Pd/C is
pyrophoric in open air!). The solution was evaporated in vacuo
to afford 9.8 g (98%) of the desired product (1), a yellow oil that
partially crystallized on standing. The spectral data were in
agreement with those previously reported.5
When the procedure is carried out as described, the
PNA backbone monomer is obtained in highly pure form.
However, the crude product can be further purified by
dropwise addition of ethereal HCl to an ice-cooled ether
solution of crude 1. The backbone hydrochloride is a
stable white solid, which can be recrystallized from
1‚HCl. To an ice-bath cooled solution of 1 (8.71 g, 35.4 mmol)
in Et2O (150 mL) was added ethereal HCl (1.08 M, 35 mL, 37.8
mmol) dropwise over ∼5 min. The mixture was stirred at 0 °C
for 1 h, then filtered, rinsed with Et2O, and dried in vacuo to
afford 8.2 g (82%) of 1‚HCl, an air-stable, nonhygroscopic white
1
solid. Mp 121-124 °C dec. H NMR (400 MHz, D2O): δ 4.13 (q,
J ) 7.3 Hz, 2H), 3.85 (s, 2H), 3.27 (t, J ) 5.3 Hz, 2H), 3.07 (t, J
) 5.4 Hz, 2H), 1.27 (s, 9H), 1.12 (t, J ) 7.3 Hz, 3H). 13C NMR
(100 MHz, D2O): δ 167.09, 158.30, 81.83, 63.75, 47.70, 36.87,
(9) Bailey, P. D.; Smith, P. D.; Pederson, F.; Clegg, W.; Rosair, G.
M.; Teat, S. J . Tetrahedron Lett. 2002, 43, 1067-1070.
(10) Technical grade ethyl glyoxylate, ∼50% in toluene, is com-
mercially available from Fluka cat. #50705 and Lancaster Synthesis
cat. #19207, but this material “exists partly in the polymerized form”
(2001/2002 Fluka laboratory chemicals and analytical reagents cata-
log). We have not yet attempted to synthesize 1 using this product.
(11) Viirre, R. D.; Hudson, R. H. E. Org. Lett. 2001, 3, 3931-3934.
(12) Hudson, R. H. E.; Li, G.; Tse, J . Tetrahedron Lett. 2002, 43,
1381-1386.
J . Org. Chem, Vol. 68, No. 4, 2003 1631
Viirre, Russell D.
