Expanding the genetic alphabet: Pyrazine nucleosides that support a donor-donor-acceptor hydrogen-bonding pattern

Article abstract of DOI:10.1002/hlca.200490120

The 6-aminopyrazin-2(1H)-one, when incorporated as a pyrimidine-base analog into an oligonucleotide chain, presents a H-bond donor-donor-acceptor pattern to a complementary DNA or RNA strand. When paired with the corresponding acceptor-acceptor-donor puri

Full text of DOI:10.1002/hlca.200490120

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Expanding the Genetic Alphabet: Pyrazine Nucleosides That Support a
DonorÀDonorÀAcceptor Hydrogen-Bonding Pattern
by Ulrike von Krosigk1) and Steven A. Benner*
Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
The 6-aminopyrazin-2(1H)-one, when incorporated as a pyrimidine-base analog into an oligonucleotide
chain, presents a H-bond donorÀdonorÀacceptor pattern to a complementary DNA or RNA strand. When
paired with the corresponding acceptorÀacceptorÀdonor purine in oligonucleotides, the heterocycle selectively
contributes to the stability of the duplex, presumably by forming a base pair of WatsonÀCrick geometry joined
by a nonstandard H-bonding pattern, expanding the genetic alphabet. Reported here is a short, high yielding, b-
d-selective synthesis of a 6-aminopyrazin-2(1H)-one nucleoside via the glycine riboside derivative 28. The key
steps include a WittigÀHorner reaction of an appropriately protected ribose derivative (Scheme 10, 19 ! 21)
followed by a Michael-like ring closure (Scheme 12, 30 ! 1a and 32 ! 1b). Thus, a variety of pyrazine
nucleosides (Scheme 13) including the target 6-aminopyrazin-2(1H)-one riboside 1a, and its 5-methyl derivative
1b, 6-amino-5-methylpyrazin-2(1H)-one riboside, are obtained.
Introduction. In its most general form, the WatsonÀCrick base pair joins a six-
membered heterocyclic ring (in natural oligonucleotides, a pyrimidine) with a fused
five/six-membered ring system (in natural oligonucleotides, a purine) via three H-
bonds, one that joins the two central ring N-atoms of the paired heterocycles, and two
that join flanking exocyclic functional groups (Fig. 1). To hold the pair together, H-
bond donors in one heterocycle must be opposite H-bond acceptors in the other. With
three H-bonds, eight (ˆ23) H-bonding patterns and 16 independently replicable bases
are conceivable within the WatsonÀCrick geometry. Six H-bonding patterns, or 12
independently replicable bases, are readily accessible by using amino and carbonyl
functionality (Fig. 2) [1 3]. These form the components of an artificially expanded
genetic-information system (Aegis) [4].
In practice, pyrimidine analogs presenting acceptorÀdonorÀdonor and do-
norÀdonorÀacceptor H-bonding patterns are difficult to obtain [2][5]. First, to be
aromatic and, therefore, able to stack, the ring system must be joined to the sugar by a
CÀC bond (−C-nucleoside×). Several heterocycles might implement these H-bonding
patterns on a C-nucleoside (Fig. 3). The 6-aminopyridin-2(1H)-one structure, which
formally presents the correct H-bonding pattern, is readily oxidized, however, and did
not appear to be suitable as a heterocyclic system to support these patterns [6]. Adding
a ring N-atom to yield the 2-aminopyrimidin-4(3H)-one known as pseudocytidine de-
creases susceptibility to oxidation, but creates an unacceptable tautomeric ambiguity [7].
The donorÀdonorÀacceptor H-bonding patterns can also be implemented on a
pyrazine-ring system, for example as a 6-amino-pyrazin-2(1H)-one attached to ribose
at the 3-position. We speculated that the additional N-atom in the pyrazine would
1
)
Present address: Novartis, CH-4002 Basel, Switzerland.

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Fig. 1. Generalized WatsonÀCrick nucleobase pair with three protons between a large heterocycle and a small
heterocycle. Two inter-base H-bonds are formed between exocyclic functional groups; one is formed between
heteroatoms of the heterocycles. Dotted lines indicate the position of double bonds to complete the valences to
the heteroatoms, and to make the heterocycles aromatic.
Fig. 2. WatsonÀCrick geometry permits twelve nucleobases instead of the four found naturally. Pyrimidines are
designated by the prefix −py×, purines by the prefix −pu×. Following the prefix is the order, from the major groove
to the minor groove, of acceptor (A) and donor (D) groups. The standard A ¥ T base pair is missing an amino
group, of course. Amino A is shown to complete the H-bonding pattern. When Z and X are both CH, the
heterocycle is a pyridine that is sensitive to oxidation. When X is N, then the heterocycle is a pyrazine, and Z can
be either CH or substituted C.

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Fig. 3. Different heterocycle C-glycosides implement the pyrimidine donorÀdonorÀacceptor (pyDDA) H-
bonding scheme. Electron-withdrawing groups manage epimerization and oxidizability of these.
diminish both the basicity and the oxidizability of the system relative to the analogous
pyridines. When this work began, however, neither the parent 6-aminopyrazin-2(1H)-
one nor any pyrazine C-nucleosides were described in the literature. Townsend and co-
workers have recently reported a metallation route to prepare halo-substituted
pyrazine nucleoside analogs but without functionality suitable for supporting Wat-
sonÀCrick base pairing [8].
Components of Aegis have been used to increase the variety of amino acids that
can be incorporated into proteins via ribosome-based translation [9], expand the
potential of oligonucleotides binding in the major groove of double-stranded DNA
[10], and explore the specificity and fidelity of polymerases [11]. Aegis components
are now exploited in FDA-approved diagnostic assays to monitor the viral load of
patients infected with human immunodeficiency virus and hepatitis C [12]. For these
and other reasons, nucleic acid analogs that implement nonstandard H-bonding
patterns are attracting a new generation of researchers, eager to understand the rule-
based molecular recognition properties [13], whose prominence in nucleic acids is
emphasized by their absence in virtually every other class of organic molecule [14].
In view of these developments, we felt it timely to present here the results of a study
of the synthesis of pyrazine nucleoside analogs that could serve as Aegis components.

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Synthesis.
Retrosynthetic analysis suggested that amino acids carrying a
cyanomethyl substituent at the amino group might be readily cyclized to give pyrazine
ribosides (Scheme 1). To explore this approach, a model compound, 2a, was prepared
from phenylalaninamide (3a) or phenylalanine methyl ester (3b) (Scheme 2).
Compound 4a was obtained in 40 50% yield by treating the amino group of a
phenylalanine derivative with bromoacetonitrile in DMF. A Strecker-type condensa-
tion of 3a or 3b, formaldehyde, and KCN at pH 5 6 in dioxane/H2O mixtures proved,
however, to be more successful. The order of addition of the reagents was critical to the
success of the Strecker route. Maintaining a slight excess of KCN relative to
formaldehyde was needed to avoid the generation of substantial amounts of cyclic
and dimeric products. Under these conditions, 4a and 4b were obtained in 89 and 95%
yield, respectively.
Scheme 1
Scheme 2

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Direct cyclization of 4a should yield the dihydropyrazine 5 carrying keto and amino
functionalities at the appropriate positions, but requiring oxidation to give the desired
product 2a. We considered the possibility that dioxygen in the air might serve as an
oxidant. However, NaOMe in MeOH over Pd/C in the presence of air gave pyrazine 2a
in only 10% yield as an impure sample. Efforts were, therefore, undertaken to adjust
the oxidation level of the precursor before cyclization.
To this end, hydroxylamine was used to initiate the cyclization of 4b (Scheme 2).
The desired product 6 dominated when the reaction was run with a large excess of
reagent at 88. The molecule overall was in the desired oxidation state, and various
approaches were used to convert the dihydropyrazinedione monooxime to its
aminopyrazine isomer. Recognizing that the oxime tautomer is strongly preferred
over the N-imino-hydroxylamine tautomer, the compound was treated with phthaloyl
dichloride in the hope of creating the cascade of reactions shown in Scheme 3.
Unfortunately, only small amounts of a product that might possibly be assignable as the
phthaloyl-protected pyrazine derivative 7 were obtained.
Scheme 3
R ˆ PhCH2, TMSCl ˆ Me3SiCl.
Efforts were then made to increase the oxidation state of the pyrazine precursor by
attachment of a heteroatom to the secondary-amine N-atom. The N-chloro derivative 8
was obtained quantitatively by using tert-butoxy chloride (Scheme 4), and could be
isolated and analyzed as a crude product. Elimination by treatment with Et3N as a base
yielded imine 9. This compound could, however, not be cyclized, possibly because the
(E)-configuration of the CˆN bond of 9 is preferred.

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Scheme 4
Alternative leaving groups were then examined. Treatment of both 4a and 4b with
2-nitrobenzenesulfenyl chloride and 1,2,2,6,6-pentamethylpiperidine (PMP) in THF
under reflux yielded 10a and 10b in quantitative yield (Scheme 5). Even though the
products appeared as a single compound by TLC, two sets of signals were observed in
the NMR spectra at room temperature. In the ester 10b, these signals coalesced at 1008
in (D6)DMSO, suggesting that the multiple signals arose from conformational isomers.
Reaction of 10a with NaOMe in MeOH at room temperature yielded pyrazine 2a via
cyclization and elimination of 2-nitrothiophenol in 92% yield. No by-products were
detectable. An intermediate, seen only by TLC, was presumed to be the intermediate
10c. Solutions of 2a in the chromatography solvent mixture CH2Cl2/MeOH 95 :5 or in
CDCl3 turned dark red upon standing at room temperature for 24 h. This decom-
position was accompanied by broadening and, later, disappearance of NMR signals. In
its solid form, 2a decomposed more slowly. Nevertheless, the decomposition permitted
only partial characterization of this 3-benzyl-pyrazin-2(1H)-one.
Scheme 5
An analogous decomposition was not observed with the 5-benzyl-4-methylpyrazin-
2(1H)-one 12 prepared via rearrangement of the pyrazine N-oxide 11 (Scheme 6) [2].
Two possibilities to explain the differential stability were considered. First, decom-
position might be catalyzed by contaminants arising from the reaction sequence used to
prepare 2a. Alternatively, the propensity to decompose might reflect the detailed
nature of the substituents, with 5-benzyl-3-methylpyrazin-2(1H)-one 12 being more
stable than 3-benzylpyrazin-2(1H)-one.
To test the possibility that the decomposition was catalyzed by contaminants arising
from the reaction sequence used to prepare 2a, 12 was prepared again via yet a different
route (Scheme 7). Alaninamide 13 was converted via a Strecker reaction with
benzeneacetaldehyde and KCN into 14. Sulfenylation proceeded poorly with PMP in
THF in this case, but proceeded in nearly quantitative yield in pyridine at room
temperature. The resulting (phenylthio)amide 15 was cyclized with NaOMe in MeOH,

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Scheme 6
or with lithium diisopropylamide (LDA) in THF, to give 12 as slightly yellow crystals
after chromatography and recrystallization. This compound proved to be identical
spectroscopically to the product from the rearrangement of the pyrazine N-oxide. As
with the material 12 obtained via rearrangement of the N-oxide 11, no decomposition
was observed in CDCl3 solution for three days at room temperature.
Scheme 7
To prepare 6-amino-3-benzyl-5-methylpyrazin-2(1H)-one (18), phenylalaninamide
(3a) was treated with acetaldehyde to give 16 (Scheme 8). Sulfenylation in pyridine and
subsequent cyclization gave 18 as slightly beige crystals after recrystallization. Pyrazin-
2(1H)-one 18 was also stable under the conditions where the methyl-unsubstituted 2a
was not. This suggested that alkyl substituents increase the stability of aminopyrazino-
nes.
With an improved understanding of the reactivity of the aminohydroxypyrazine
ring system, we turned to appending this system to a ribose skeleton. Schmidt and co-
workers [15] developed a simple, general route for the synthesis of a-amino carboxylic
acid derivatives, whose key step is the WittigÀHorner reaction of an a-phosphoryl-
glycine ester 20 with various aldehydes to yield aminodihydro acid derivatives
(Scheme 9). The scope of this method is very broad and includes aromatic, aliphatic,
and heterocyclic aldehydes; ketone do not react [16]. In general, the (Z)-isomer is the

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Scheme 8
Scheme 9
major product (Scheme 9). Schmidt and co-workers used phosphoranes obtained from
the a-hydroxyglycine esters via a-phosphoryl esters 20 [17], and the N-[(benzyloxy)-
carbonyl]-protected methyl ester of glycine is commercially available.
As starting materials for the WittigÀHorner reaction with 20 (R'ˆZˆ
PhCH2OCO), both 2,3-O-isopropylideneribose 19a and the 5-O-(tert-butyl)dimeth-
ylsilyl-protected derivative 19b were considered (Scheme 10), based on the b/a-d-
ratios reported by Ohrui and Kane. Both were readily prepared by known methods
[18]. The WittigÀHorner reaction was found to proceed only poorly with the ribose
derivative 19a as the electrophile. The reaction was not complete and gave 22 and 24
(mixture of stereoisomers a and b with respect to the glycine moiety) in only 19% yield.
In contrast, 19b reacted with 20 (R'ˆZ) under the preferred conditions of Schmidt and
co-workers to give a mixture of three products in ca. 60% yield. Better yields were
achieved if 20 (R'ˆZ) was deprotonated first at À788 in CH2Cl2 and then treated
slowly with a solution of 19b at 08. The two major products 23a/b were obtained in a
combined yield of 90%, whose ratio varied between 1.5 :1 and 3 :1, and who could be
partially resolved. The NMR spectra corresponded to those expected for 23 and/or 25
as a mixture of stereoisomers a and b with respect to the glycine moiety. Analysis of the
configuration of the anomeric centers showed, however, that both major products 23a
and 23b had the b-d-configuration. This suggested that their structure differed only by
their configuration at C(2). Determination of this configuration was not undertaken, as
the stereogenic center was subsequently lost in the synthetic sequence. The first
substance to elute on chromatography (silica gel) was also the major stereoisomer and
arbitrarily designated as 23a; the other major product was designated as 23b. The minor
products were assigned by 1H-NMR to be the a-d-anomers 25a/b. A sufficient amount
of these was not obtained pure for analysis. Nevertheless, we can conclude that the
Wittig-Horner reaction had a b-d-selectivity of ꢀ 30:1 at a yield of ca. 90% in this case.
Ammoniolysis of esters 23a/b in MeOH led in almost quantitative yield to the Z-
protected amides 26a/b, which could be partly separated (Scheme 11). The Z protecting

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Scheme 10
group in 26a/b could be removed without epimerization via catalytic hydrogenation
with Pd and activated charcoal to yielded the free aminoamides 28a/b in ca. 90% yield.
The transformation of 23a/b to 28a/b proceeded better if the order of steps was
reversed (Scheme 11). Thus, catalytic hydrogenation of esters 23a and 23b with Pd on
activated charcoal led to the a-amino acid esters 27a and 27b, respectively, in 93% yield
without epimerization. Reaction of the a-amino esters 27a and 27b with ammonia in
Scheme 11
TBDMS ˆ tBuMe2Si

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MeOH gave exclusively amides 28a and 28b in almost quantitative yield, again without
epimerization. By this route, both aminoamides 28a/b were obtained from ribose in an
overall yield of 58% in five steps. This new route to glycine ribosides is short, should be
applicable to the synthesis of both of the 6-aminopyrazin-2(1H)-one nucleosides
needed, and should, with minor adaptations, be applicable to the synthesis of other C-
glycosides.
The glycine riboside derivative 28 (stereoisomer mixture) was then treated under
Strecker conditions with formaldehyde and cyanide to yield 29 as a mixture of
stereoisomers (Scheme 12). Sulfenylation yielded 30, which was transformed crude by
treatment with MeONa/MeOH to give the target pyrazine riboside derivative 1a.
Through flash chromatography, 1a was isolated in ca. 35% yield as an orange-brown
solid, the color presumably arising through partial oxidation. As a component of a
DNA molecule cannot have this property, no heroic efforts were made to exclude
oxygen. Further, solutions of 1a in MeCN colored quickly. Via reversed-phase
chromatography, 1a could be obtained pure as an almost colorless solid and
characterized, even though the solid, when exposed to air, slowly turned yellow. A
Scheme 12
TBDMS ˆ tBuMe2Si

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solution in CDCl3 turned dark yellow in less than 1 h, and deep black after 2 days. This
coloring led to broader signals and a shift in the aromatic HÀC(5) signals in the NMR
spectrum.
Work with the model system had suggested that an additional Me substituent at the
pyrazine ring would suppress the decomposition reaction. To prepare the methylated
pyrazine, 28a was treated with acetaldehyde and HCN under the Strecker conditions
developed in the model system to give a the secondary amines 31 in over 90% yield
(from 28a/b) as a mixture of stereoisomers arbitrarily designated as aa, ab, ba, and bb
(Scheme 12). These could be transformed with 2-nitrobenzenesulfenyl chloride in
pyridine in 99% yield to 32 (from ribose in an overall yield of 90%), again as a mixture
of stereoisomers whose configurations were not assigned. Cyclization with LDA in
THF led exclusively to the target pyrazine riboside 1b, which was isolated by
chromatography in 91% yield. Compound 1b was then precipitated as nearly colorless
flakes in 81% yield. Consistent with expectations based on the model studies, 1b could
be chromatographed without decomposition and without special care. Solutions of 1b
in CDCl3 showed no change in either color or NMR spectrum over at least three days at
room temperature. The same reaction sequence proceeded analogously with the C(2)
epimer 28b to yield 1b in approximately the same yield. Significant differences in the
reactivity of the two epimers could not be detected at any step.
Compound 1b was then used as a starting material for the synthesis of protected
pyrazine nucleoside derivatives suitable for the solid-phase synthesis of DNA
(Scheme 13). Because the 2-deoxypyrazine nucleoside was prone to epimerization
under conditions used to synthesize DNA, the 2'-OH group was retained. To prevent it
from participating in base-catalyzed cleavage reactions, and to favor thermodynami-
cally the b-d-epimer, the 2'-OH group was blocked as the methyl ether. The protecting
group manipulations depicted in Scheme 13 (via 33 41) and described in the Exper.
Part followed a strategy standard in the field, and yielded a 5'-dimethyoxytritylated 2-
cyanoethyl diisopropylphosphoramidite 42 with the heterocyclic exocyclic amino group
protected as its dimethylformamidine derivative and the exocyclic O-function
protected as an allyl ether.
Discussion.
The experiments described provide a convenient procedure for
preparing nucleoside analogs presenting a H-bond donorÀdonorÀacceptor array. The
pyrazine nucleoside 1b is prepared in an overall yield of ca. 42% in just eight steps from
ribose. The synthesis is selective for the b-d-epimer, which has a configuration
analogous to that found in natural nucleosides. The compounds reported here also
serve as starting points for oligonucleotide synthesis that contain the nonstandard base
pair.
With respect to the stability of the pyrazine, it is remarkable that the addition of a
single Me substituent (compare 1a with 1b) has such a significant effect on the stability
of the nucleoside. This example illustrates the need, when designing extra letters in a
genetic alphabet, to optimize the structure with respect to such substitution. Earlier
work in these laboratories had shown that a Me substitution at the 5-position of
isocytidine derivatives also stabilized the heterocycle, slowing deamination under
alkaline conditions.

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Scheme 13
TBDMS ˆ tBuMe2Si, DEAD ˆ diethyl diazenedicarboxylate, TIPS-Cl ˆ 1,3-dichloro-1,1,3,3-tetraisopropyldisi-
loxane, DMTˆ (MeO)2Tr ( Tr ˆ trityl).

Helvetica Chimica Acta Vol. 87 (2004)
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Outlook. The particular pyrazine, 1b, may be suited to develop a new class of
molecules based on polymerases that incorporate an epimerizing nucleotide. Many
polymerases are believed to identify, as a recognition element, an unshared pair of
electrons (or, perhaps better termed, −electron density×) protruding from the
nucleobase in the minor groove. The four standard nucleobases found in natural
DNA (adenine, guanine, cytosine, and thymine) present this to the minor groove of the
DNA double helix from N(3) of the purines and the exocyclic O-atom of the
pyrimidines [19]. This pair of electrons is a H-bond acceptor and can, therefore,
interact with a H-bond donating group presented by a polymerase to the minor groove.
Because it is present in all standard nucleobases, this electron pair can be the basis of a
−common site× interaction between the polymerase and whatever nucleobase is present
in the active site at any point in the polymerase catalytic cycle. Indeed, the electron pair
appears to be the only such contact that all standard nucleobases can make in the same
way. Therefore, the interaction between the unshared electron pair and the polymerase
is expected to be used by polymerases generally to enforce the geometry of the base
pair without discriminating between different substrates. This might be a key to
polymerase fidelity.
Crystallographers have found evidence for such H-bonding interactions in the
minor groove for various polymerases, including Taq [20][21] and Bst [22] from the A
evolutionary family of polymerases, and RB69 from the family-B polymerases [23].
The residues from Taq and Bst that form H-bonds with the minor groove are conserved
within most known family-A DNA polymerases [24]. These consist of a) an arginine (at
position 573 in Taq) that forms a H-bond with a nucleotide immediately after
incorporation and its template complement (N ‡ 1, T ‡ 1; Fig. 1), b) a glutamine (at
position 754 in Taq) that can also form a H-bond with the template at position T‡ 1, c)
an asparagine (at position 583 in Taq) that forms a H-bond with the elongating DNA
strand three sites from the site of triphosphate addition (N ‡ 3), and d) a lysine (at
position 540in Taq) that can form H-bonds with the nucleotides four and five positions
away from the site of triphosphate addition (N ‡ 4, N ‡ 5).
Minor-groove contacts are also suggested for family-B polymerases by crystallog-
raphy. For example, a lysine residue at position 706 in RB69 (a family-B polymerase)
forms a H-bond with a nucleotide on the elongating strand two positions from the site
of triphosphate addition (N ‡ 2). A second lysine (at position 734 in RB69) forms a
bond in the minor groove to a nucleotide four positions away from the addition site via
a H2O molecule (N ‡ 4). A tyrosine (at position 567 in RB69) forms a bond to the
minor groove of the template strand via a H2O molecule one position away from the
site of addition (T‡ 1). The lysine that interacts with the unshared electron pair carried
by nucleobase N ‡ 1 and the tyrosine that interacts with nucleobase T‡ 1 are conserved
among family-B polymerases. The lysine that interacts in the minor groove with
nucleobase N ‡ 4 is an arginine in most archaebacterial and mammalian family-B DNA
polymerases [25].
The pyrazine heterocycle reported here presents the unshared electron pair in the
minor groove. Thus, it should be accepted as a triphosphate by polymerases where this
electron pair is indeed a specificity determinant. Here, epimerization is not a key issue.
The epimerization will interconvert the a- and b-d-isomers of the triphosphate
substrates, and the polymerase will select the b-d-triphosphate for incorporation,

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ignoring the a-d-triphosphate. As the b-d-triphosphate is consumed, however, the a-d-
triphosphate will epimerize to create more.
Once incorporated, however, the epimerization reaction will lead to the formation
of the a-d-anomer within the strand. As a-d-anomeric nucleotides accumulate within a
strand, the affinity of the strand for its complement should be diminished. This, in turn,
will cause the copy to disassociate from the template, permitting the template to serve
as such for the synthesis of another product oligonucleotide.
Separating a product strand from a template strand is, of course, the purpose of the
heating cycle in a polymerase chain reaction. Here, the product (having multiple a-d-
anomeric nucleotides) would presumably not serve as a good template. Thus, the result
would be a linear amplification without thermal cycling of an oligonucleotide
containing nonstandard nucleotides. This may have applications in diagnostics and
taggants.
Experimental Part
1. General. Unless otherwise mentioned, reagents were purchased from Fluka or Aldrich at highest quality
(puriss. or purum). THF and toluene were freshly distilled from Na, MeCN and CH2Cl2 from CaH2. All other
solvents were purchased from Fluka or Aldrich in the highest quality. TLC: Merck TLC silica gel 60 F254 (d ˆ
0.25 mm) and Waters K6F silica gel 60 (d ˆ 0.25 mm); visualization either with UV light (l 254 nm) or staining
with either a soln. of phosphomolybdic acid/ceric(IV) sulfate tetrahydrate/conc. H2SO4 soln. or vanillin/EtOH/
conc. H2SO4 soln. and subsequent heating. Flash chromatography (FC): 50 100-fold silica gel 60 (Merck,
0.040 0.063 mm, or Fisher Davisil, 0.035 0.070 mm) with 0.2 0.3 bar pressure. HPLC: semiprep. Merck-
Septech-Novaprep-5000 instrument, with silica-gel-Merck-Lichrospher-Si (60-7 mm) column; semiprep. Waters-
PrepLC-4000 instrument with Waters-486 tunable absorbance detector on and Waters-Prep-Nova-Pak-HR-C18
column (60ä, 25 Â 100 mm); Waters-616 pump with Waters-600-S controller, Waters-996 photodiode-array
detector, and Shodex-RSpak-D18-613 column (6 Â 150mm) or Waters-Nova-Pak-C18 column (3.9 Â 150mm).
UV/VIS Spectra: Varian Cary-1-Bio UV/VIS spectrophotometer with a Cary temperature controller and
Shimadzu-UV/VIS-160 spectrophotometer; lmax (e) in nm. NMR Spectra: Bruker-AMX-500, Varian-Unity-500,
Varian-EM-390, Varian-XL-300, Varian-Gemini-300, and Varian-VXR-300 instruments; d in ppm rel. to SiMe4
as internal standard, J in Hz; starred (* or **) attributions may be interchanged multiplicity of the 13C-NMR
signals by DEPT. Anal. GC/MS; Hewlett-Packard gas Chromatograph 5710A combined with a mass
spectrometer 5710B as detector; tR in min. MS: VG-Tribrid (EI, 70eV), VG-ZAB2-SEQ and Finnigan-MAT-
95 (FAB, 3-nitrobenzyl alcohol (NOBA) matrix), Finnigan-MAT-LCQ (ESI), and Bruker Reflex (MALDI
TOF; matrices mentioned below) instruments; in m/z (rel. %).
2. Model Compounds. N-(Cyanomethyl)phenylalaninamide (4a). KCN (716 mg, 11 mmol) was added to a
soln. of phenylalaninamide (3a; 1.64 g, 10mmol) in dioxane/H 2O 3 :1 (40ml). The pH was carefully adjusted to
6 with AcOH. Formalin (0.9 ml, ca. 11.7 mmol) was then slowly added within 1 h, and the mixture was stirred
overnight at r.t. Sat. aq. NaHCO3 soln. (ca. 50ml) was then added, and the aq. phase was extracted with CH 2Cl2.
The combined org. phase was dried (Na2SO4) and evaporated, the residue suspended in CH2Cl2 (ca. 50ml), the
soln. filtered, and the filtrate evaporated. The residue was dried under high vacuum: 4a (1.808 g, 89%), pure by
TLC and 1H-NMR. A smaller portion was recrystallized from acetone to give an anal. sample. Colorless solid.
M.p. 1508. TLC (CH2Cl2/MeOH 9 :1): Rf 0.4. IR (KBr): 3370, 3350, 3320, 3190, 3030, 2930, 2860, 2230, 1640,
1495, 1475, 1455, 1425, 1345, 1330, 1255, 1200, 1135, 1090, 1075, 1030, 1015, 960, 935, 920, 885, 875, 855, 845, 820,
780, 740, 700, 620, 575, 520, 485. 1H-NMR ((D6)DMSO): 2.65 2.75 (m, with dd at 2.72, J ˆ 7.8, Jgem ˆ 13.7, NH,
1 H of PhCH2); 2.88 (dd, J ˆ 5.7, Jgem ˆ 13.7, 1 H of PhCH2); 3.31 3.37 (m, >1 H, HÀC(2), H2O); 3.43 (dd, J ˆ
6.9, Jgem ˆ 17.4, 1 H of CH2CN); 3.59 (dd, J ˆ 6.3, Jgem ˆ 17.4, 1 H of CH2CN); 7.10(br. s, 1 H of CONH2); 7.16
7.30( m, 5 arom. H); 7.41 (br. s, 1 H of CONH2). 13C-NMR ((D6)DMSO): 35.21 (t, CH2CN); 38.67 40.34
(m, DMSO, and t of CH2Ph); 62.06 (d, NCHCONH2); 118.84 (s, CN); 126.13 (d, arom. CH); 128.00
‡
(d, arom. CH); 129.17 (d, arom. CH); 138.08 (s, arom. C); 173.97 (s, CONH2). MS: 204 (0.13, [M ‡ 1] ), 203
‡
‡
‡
(0.18, M ), 176 (1, [M À HCN] ), 160 (12), 159 (100, [M À CONH2] ), 147 (11), 132 (35), 119 (12), 112 (34),
105 (11), 92 (13), 91 (38), 85 (35), 65 (12). Anal. calc. for C11H13N3O (203.24): C 65.01, H 6.45, N 20.67; found:
C 65.02, H 6.51, N 20.67.

Helvetica Chimica Acta Vol. 87 (2004)
1313
N-(Cyanomethyl)phenylalanine Methyl Ester (4b). As described for 4a, with KCN (0.72 g, 11 mmol),
phenylalanine methyl ester hydrochloride (3b; 2.16 g, 10mmol), dioxane/H 2O 1:1 (40ml), and formalin
(0.75 ml, 9.75 mmol; addition within 2.5 h). The residue obtained after the 1st evaporation was purified by FC
(silica gel, AcOEt/hexane 3 :7): 4b (2.02 g, 95% rel. to formalin). Colorless oil. TLC (hexane/AcOEt 1 :1):
Rf 0.42. IR (CHCl3): 3350, 3010, 2950, 1740, 1605, 1495, 1475, 1455, 1440, 1360, 1335, 1280, 1265, 1215, 1180,
1
1140, 1090, 1030, 1020, 995, 880. H-NMR (CDCl3): 1.86 (s, NH); 2.92 (dd, J ˆ 7.8, Jgem ˆ 13.7, 1 H of PhCH2);
3.10( dd, J ˆ 5.4, Jgem ˆ 13.8, 1 H of PhCH2); 3.54 (s, CH2CN); 3.72 3.76 (m, and s at 3.74, HÀC(2), MeO);
7.17 7.25 (m, 5 arom. H). 13C-NMR (CDCl3): 35.96 (t, CH2CN); 39.17 (t, PhCH2); 52.16 (q, MeO); 61.12
(d, NCHCOOMe); 117.25 (s, CN); 127.16 (d, arom. CH); 128.71 (d, arom. CH); 129.16 (d, arom. CH); 136.25
‡
‡
(s, arom. C); 173.31 (s, COOMe). MS: 218 (4, M ), 160 (12), 159 (100, [M À COOMe] ), 132 (26), 127 (77,
‡
‡
[M À PhCH2] ), 100 (23), 92 (10), 91 (34, PhCH2 ), 66 (18), 65 (10). Anal. calc. for C12H14N2O2 (218.25):
C 66.04, H 6.47, N 12.84; found: C 66.00, H 6.07, N 12.61.
3-Benzylpiperidine-26-dione 6-Oxime (6). A soln. of hydroxylamine hydrochloride (765 mg, 11 mmol) in
MeOH (25 ml) was placed in a glass tube at 08 and treated with 5.4m MeONa in MeOH (1.8 ml, 9.72 mmol).
After ca. 5 min stirring, 4b (1.09 g, 5 mmol) was added, the tube sealed, and the mixture heated overnight at 608
in an oil bath with stirring. The products were then adsorbed on silica gel (2.3 g) and separated by FC (silica gel,
CH2Cl2/MeOH 95 :5): 6 (892.4 mg, 81%). Colorless crystals. M.p. 1778. TLC (CH2Cl2/MeOH 9 :1): Rf 0.47. IR
(KBr): 3300, 3240, 3025, 1660, 1640, 1495, 1465, 1455, 1430, 1400, 1380, 1320, 1245, 1145, 985, 960, 935, 900, 825,
790, 750, 740, 700, 590, 570, 500, 485. 1H-NMR ((D6)DMSO): 2.69 (m, NH); 2.78 (dd, J ˆ 9.7, Jgem ˆ 14.1, 1 H of
PhCH2); 3.18 (dd, J ˆ 3.7, Jgem ˆ 14.1, 1 H of PhCH2); 3.34 (dd, J(5,NH) ˆ 8.4, Jgem ˆ 15.5, 1 H of
NCH2C(NOH)); 3.45 (dd, J(5,NH) ˆ 5.5, Jgem ˆ 15.7, 1 H of NCH2C(NOH)); 3.58 (dt-like m, HÀC(2));
7.17 7.30( m, 5 arom. H); 9.5, 10.1 (2 br. s, NH, NOH). 13C-NMR (CDCl3): 35.74 (t, PhCH2); 41.92
(t, NCH2C(NOH)); 59.79 (d, NCHCONH); 125.92 (d, arom. CH); 127.93 (d, arom. CH); 129.22 (d, arom. CH);
‡
138.77 (s, arom. C); 144.21 (s, C(NOH)NH); 170.40 (s, CONH). MS: 219 (28, M ), 129 (11), 128 (100, [M À
‡
‡
CH2Ph] ), 100 (21), 91 (23, PhCH2 ). Anal. calc. for C11H13N3O2 (219.24): C 60.26, H 5.98, N 19.17, O 14.60;
found: C 59.87, H 6.02, N 19.03.
N-Chloro-N-(cyanomethyl)phenylalaninamide (8). A suspension of 4a (20mg, 103.4 mmol) in THF (1 ml)
was cooled to À788 and treated with tert-butoxy chloride (12 mg, 110.53 mmol). The mixture was slowly (within
1 h) brought to r.t., yielding a clear colorless soln. The mixture was evaporated and dried under high vacuum to
yield 8 (ca. 24 mg), which was analyzed only as crude product. Colorless oil. TLC (CH2Cl2/MeOH 9 :1): Rf 0.53.
1H-NMR (200 MHz, CDCl3): 3.23 (d, J ˆ 6.8, PhCH2); 3.83 (t, J ˆ 6.8, HÀC(2)); 4.01 (s, CH2CN); 5.69, 5.89
(2 br. s, each 1 H, CONH2); 7.21 7.36 (m, >5 H, arom. H). 13C-NMR (50MHz, CDCl 3): 39.79 (PhCH2); 48.36
(CH2CN); 74.07 (NCHCONH2); 113.88 (CN); 127.11 (arom. CH); 128.64 (arom. CH); 128.98 (arom. CH);
139.71 (arom. C); 169.84 (CONH2).
a-[(Cyanomethyl)imino]benzenepropanamide (9). A suspension of 4a (51 mg, 251 mmol) in THF (2.5 ml)
was cooled to À788 and treated with tert-butoxy chloride (30mg, 276 mmol). The mixture was slowly brought to
r.t. within 1 h. The resulting clear, colorless soln. was cooled again to À788 and treated with Et3N (70 ml,
504 mmol). The mixture slowly became cloudy. After stirring at À788 for 1 h, the mixture was slowly (within ca.
2 h) brought to r.t., the yellow suspension filtered with a pipette and glass wool, and the residue washed several
times with dry THF. Evaporation of the filtrate gave 56 mg of crude product which was purified by FC (silica gel,
CH2Cl2/MeOH 95 :5): 9 (35 mg, 69%). Dark yellow oil. TLC (CH2Cl2/MeOH 9 :1): Rf 0.51. 1H-NMR (CDCl3):
4.00 (s, PhCH2); 4.30( s, CH2CN); 5.88 (br. s, 1 H, CONH2); 7.12 (m, 1 H of CONH2, Ph); NH could not be
determined with D2O exchange because 9 decomposed in the presence of H2O. 13C-NMR (CDCl3): 32.30, 39.69
(2t, CH2CN, PhCH2); 116.47 (s, CN); 127.25 (d, arom. CH); 128.51 (d, arom. CH); 129.23 (d, arom. CH); 133.63
‡
(s, arom. C); 165.28, 168.53 (2s, CONH2, CˆNCH2CN). MS (C11H11N3O, 201.23): 201 (16, M ), 158 (32), 157
(29), 117 (12), 116 (19), 92 (13), 91 (100), 65 (13).
N-(Cyanomethyl)-N-[(2-nitrophenylthio]phenylalaninamide (10a). A soln. of 2-nitrobenzenesulfenyl
chloride (73 mg, 385 mmol), 4a (50mg, 246 mmol), and 1,2,2,6,6-pentamethylpiperidine (PMP; 70 ml, 387 mmol)
in THF (3 ml) was heated under reflux (908 oil bath), until consumption of 4a was complete (3 h, TLC
monitoring). The mixture was diluted with a little MeOH, and the entire mixture was adsorbed on silica gel
(0.12 g) and subjected to FC (silica gel, CH2Cl2/MeOH 95 :5): 10a (86 mg, 98%). Yellow crystals. TLC (CH2Cl2/
MeOH 95 :5): Rf 0.38. 1H-NMR ((D6)DMSO): not evaluated due to complexity (inversion and rotation
isomers). 13C-NMR ((D6)DMSO; all signals doubled at r.t.): 36.07, 36.28 (2t, CH2CN); 40.69, 44.60 (2t, PhCH2);
68.10, 71.74 (2d, NCHCONH2); 117.08, 117.24 (s, CN); 124.01, 124.84, 125.25, 125.40, 125.62, 125.80, 126.20,
126.69, 128.02, 128.50, 128.83, 129.22, 133.89, 134.34 (14d, arom. CH); 137.26, 138.05, 141.19, 141.62, 141.82,
‡
143.37 (6s, arom. C); 172.42, 172.81 (s, CONH2). MS (C17H16N4SO3, 356.40): 356 (<1, M ), 312 (10,

1314
Helvetica Chimica Acta Vol. 87 (2004)
‡
[M À CONH2] ), 202 (24), 159 (20), 157 (10), 155 (15), 154 (100), 148 (11), 138 (14), 106 (23), 98 (29), 96 (16),
92 (13), 91 (67), 65 (13).
N-(Cyanomethyl)-N-[(2-nitrophenylthio]phenylalanine methyl ester (10b). As described for 10a, with 2-
nitrobenzenesulfenyl chloride (280mg, 1.48 mmol), 4b (218.3 mg, 1 mmol), PMP (0.27 ml, 1.49 mmol), and
THF (5 ml) for 2 h (by TLC). FC (residue adsorbed on silica gel (1.5 g); silica gel, CH2Cl2) gave 10b (352 mg,
95%). Yellow oil which slowly solidified upon standing. TLC (CH2Cl2): Rf 0.35. 1H-NMR ((D6)DMSO): 2.97
3.24 (m, PhCH2); 3.70, 3.76 (2s, MeO); 4.21 4.60( m, HÀC(2), CH2CN); 6.66, 7.14 7.24, 7.28 7.51 (d, m, m,
8 arom. H); 8.23 (dd, 1 arom. H). 1H-NMR (1008, (D6)DMSO): 3.10( dd, J ˆ 9.1, Jgem ˆ 14.5, 1 H of PhCH2);
3.22 (dd, J ˆ 6.4, Jgem ˆ 14.5, 1 H of PhCH2); 3.70( s, MeO); 4.33 (s-like m, CH2CN, HÀC(2)); 7.26 7.43
(m, 8 arom. H); 8.21 (dd, J1 ˆ 1.2, 8.3, 1 arom. H). 13C-NMR (CDCl3; all signals doubled): 36.31, 37.18, 41.60,
45.76 (4t, PhCH2, CH2CN); 52.61 (q, Me); 68.22, 72.01 (2d, NCHCOOMe); 115.73, 115.88 (2s, CN); 124.02,
124.66, 125.32, 125.76, 126.96, 127.49, 128.64, 128.83, 129.21, 134.05, 134.30 (11d, arom. CH); 136.35, 136.48,
141.49, 141.92, 142.35 (5s, arom. C); 171.65, 171.79 (2s, COOMe). GC/MS (P10020): tR 16.73; 312 (3, [M À
‡
COOCMe] ), 248 (15), 235 (12), 154 (75), 145 (15), 130 (14), 106 (27), 98 (62), 96 (24), 92 (15), 91 (100), 78
(20). MS (C18H17N3SO4, 371.41): 371 (0.7), 313 (3), 248 (13), 154 (92), 145 (16), 106 (26), 98 (35), 96 (18), 92
(14), 91 (100), 65 (11).
6-Amino-3-benzylpyrazin-2(1H)-one (2a). At 08, 5.4m MeONa (0.5 ml, 2.7 mmol) was added dropwise to a
soln. of 10a (387.6 mg, 1.088 mmol) in MeOH (15 ml). The mixture was slowly warmed under stirring to r.t. ( !
orange and precipitation). After total consumption of 10a (2 h, TLC monitored), the mixture was acidified with
2n HCl (pH ca. 6), concentrated in vacuo, and then evaporated under high vacuum. The residue was taken up in
CH2Cl2/MeOH 9 :1 and subjected to FC (silica gel, CH2Cl2/MeOH 9 :1): 2a (184.4 mg, 92%). Brownish foam.
TLC (CH2Cl2/MeOH 9 :1): Rf 0.29. 1H-NMR (200 MHz, CDCl3): 3.97 (s, PhCH2); 4.7 (br. s, NH2); 6.90
(s, HÀC(5)); 7.17 7.27 (m, Ph). 13C-NMR (50MHz, CDCl 3): 38.81 (t, CH2); 110.71 (d, C(5)); 126.56
(d, arom. CH); 128.79 (d, arom. CH); 128.84 (d, arom. CH); 139.63, 139.69 (2s, arom. C, arom. C(py));
‡
144.84 (s, arom. C(py)); 156.42 (s, arom. C(py)). GC/MS (P15020): tR 5.7; 201 (100, M ), 173, 156, 130, 91.
N-(1-Cyano-2-phenylethyl)alaninamide (14). KCN (360mg, 5.53 mmol) was added to a soln. of l-
alaninamide (13; 641 mg, 5 mmol) in dioxane/H2O 5 :3 (40ml). The pH was carefully adjusted to 6 with AcOH.
A soln. of benzeneacetaldehyde (50% in diethyl phthalate; 1.3 ml, 5.8 mmol) was added dropwise within 1 h,
with an additional amount (0.5 ml, 2.2 mmol) added after 1 h. The mixture was poured into an aq. Na2CO3 soln.,
the aq. phase extracted with CH2Cl2, and the combined org. phase washed with sat. aq. NaCl soln., dried, and
evaporated: 12 (811.7 mg, 75%). Colorless crystals. M.p. 107 1088. TLC (CH2Cl2/MeOH 95 :5): Rf 0.29. IR
(KBr): 3380, 3350, 3200, 3030, 2990, 2970, 2930, 2870, 2230, 1635, 1495, 1455, 1400, 1370, 1325, 1270, 1150, 1100,
1075, 1030, 1010, 985, 915, 840, 810, 790, 740, 700, 600, 575, 560, 510, 475. 1H-NMR (CDCl3): 1.32, 1.35 (2d,
3 Me); 2.96 3.15 (m, 2 H, PhCH2); 3.37 3.47 (m, 1 H, CHCONH2); 3.61 3.70, 3.82 3.90 (2m, 1 H, CHCN);
5.53, 5.77, 6.22, 6.52 (4 br. s, 2 H, NH); 7.26 7.39 (m, 5 H, PhCH2). 13C-NMR (CDCl3): 18.80, 19.84 (2q, Me);
39.33, 40.04 (2t, PhCH2); 50.08, 50.79 (2d, CHCN); 55.88, 56.74 (2d, CHCONH2); 119.19, 119.58 (2s, CN);
127.86, 128.92, 129.03, 129.37, 129.52 (5d, arom. CH); 134.60, 134.95 (2s, arom. C); 176.10( s, CONH2). MS: 217
(<1), 173 (16), 146 (53), 136 (11), 130(13), 99 (100), 91 (26), 73 (22), 71 (20). Anal. calc. for C
12H15N3O
(217.27): C 66.34, H 6.96, N 19.34; found: C 66.60, H 6.78, N 19.24.
N-(1-Cyano-2-phenylethyl)-N-[(2-nitrophenyl)thio]alaninamide (15). Amide 14 (1.00 g, 4.6 mmol) was
taken up in pyridine (4 ml) and then evaporated under high vacuum. The residue was redissolved in pyridine,
and the soln. was treated with 2-nitrobenzenesulfenyl chloride (1.3 g, 6.86 mmol) and a few crystals of N,N-
dimethylpyridin-4-amine (DMAP). After 1 h, the mixture was diluted with MeOH, dried under high vacuum,
and subjected to FC (silica gel, CH2Cl2/MeOH 95 :5): 15 (1.64 g, 96%). Yellow foam. TLC (CH2Cl2/MeOH
95 :5): Rf 0.34. 1H- and 13C-NMR (CDCl3): not assigned due to complexity (rotation and inversion isomers). MS
‡
(C18H18N4O3S, 370.43): 370 (1, M ), 326 (13), 216 (14), 171 (12), 155 (10), 153 (100), 106 (19), 98 (26), 96 (10),
91 (53), 84 (10), 49 (10).
6-Amino-5-benzyl-3-methylpyrazin-2(1H)-one (12). A soln. of 15 (270mg, 729 mmol) in THF (2 ml) was
treated with 1.5m LDA in cyclohexane (1 ml, 1.5 mmol) at À788. The mixture, which immediately turned deep
red, was stirred at r.t. overnight. The entire soln. was then added dropwise to 2m phosphate buffer (pH 7.1; 5 ml).
The mixture was extracted several times with AcOEt, the combined org. extract dried (Na2SO4) and
evaporated, and the residue recrystallized from CH2Cl2 to yield the first batch of 12 (77.9 mg, 50%). The
material in the mother liquor was subjected to FC (silica gel, CH2Cl2/MeOH 95 :5): 12 as an orange foam which
was recrystallized from MeCN (28.7 mg, 18%). The material was identical to an authentic sample [2]. Total yield
of 12: 106.6 mg (68%). M.p. 207 2088. TLC (CH2Cl2/MeOH 9 :1): Rf 0.30. 1H-NMR (200 MHz, (D6)DMSO):
2.10( s, Me); 3.82 (s, PhCH2); 5.57 (br. s, NH2); 7.14 7.28 (m, Ph); 10.7 (br. s, NH). 13C-NMR (50MHz,

Helvetica Chimica Acta Vol. 87 (2004)
1315
(D6)DMSO): 18.17 (q, Me); 36.79 (t, CH2); 122.74 (s, C(5)); 125.95 (d, C(4')); 128.35, 128.64 (2d, C(2'), C(3'));
‡
130.78 (s, C(3)); 140.52 (s, C(1')); 146.23 (s, C(6)); 155.35 (s, C(2)). GC/MS (P10020); tR 7.82; 2.15 (100, M ),
186, 169, 145, 129, 110, 91.
N-(1-Cyanoethyl)phenylalaninamide (16). A soln. of 3a (1.64 g, 10mmol) in dioxane/H 2O 3 :1 (60ml) was
treated with KCN (720mg, 11.06 mmol) and its pH carefully adjusted with AcOH ( ca. 4 ml) to pH 6. Within 1 h
was slowly added acetaldehyde (1 ml, 17.7 mmol) by means of a cooled syringe, and the mixture was stirred at r.t.
for ca. 1 h, until complete consumption of 3a (TLC monitoring). Sat. aq. Na2CO3 soln. (ca. 50ml) was added,
the mixture extracted with CH2Cl2, the combined org. phase dried (Na2SO4) and evaporated, the residue taken
up in CH2Cl2, the soln. treated with hexane until it became cloudy, and stored in the refrigerator for
crystallization. More product was isolated by repeated recrystallization of the mother liquor. From the rest of
the mother liquor, a second batch of 16 (295 mg, 14%) was isolated by FC (silica gel, CH2Cl2/MeOH 95 :5). The
amounts of the two diastereoisomers varied significantly from batch to batch. Total yield of 16: 1.787 g (82%).
Small, colorless needles. M.p. 1098. TLC (CH2Cl2/MeOH 9 :1): Rf 0.41. IR (KBr): 3380, 3315, 3200, 3085, 3030,
2980, 2930, 2870, 2230, 1635, 1495, 1470, 1460, 1440, 1390, 1380, 1325, 1295, 1255, 1200, 1160, 1150, 1125, 1090,
1070, 1030, 935, 860, 820, 790, 720, 695, 605, 580, 550, 515, 485. 1H-NMR (CDCl3): 1.31, 1.42 (2d, J ˆ 7.0, 7.1, 3 H,
Me); 1.59, 1.78 (d s, 1 H, NH); 2.82, 2.87 (2dd, 1 H of PhCH2); 3.22, 3.27 (2dd, 1 H of PhCH2); 3.46 3.69
(m, 2 H, MeCHCN, HÀC(2)); 5.9, 6.0(br. s, 1 H of CONH2); 6.6, 6.7 (br. s, 1 H of CONH2); 7.22 7.39
(m, PhCH2). 13C-NMR (CDCl3): 19.20, 19.94 (2q, Me); 39.10, 39.26 (2t, PhCH2); 44.41, 44.51 (2d, MeCHCN);
61.57, 61.99 (2d, NCHCONH2); 119.87, 120.10 (2s, CN); 127.38, 127.48 (2d, arom. CH); 128.10, 129.09, 129.22
‡
(4d(ˆ 3d), arom. CH); 135.89, 136.44 (2s, arom. C); 175.04, 175.32 (2s, CONH2). MS: 218 (0.22, [M ‡ 1] ), 190
‡
‡
(5.4, [M À HCN] ), 173 (34, [M À CONH2] ), 149 (20), 147 (20), 146 (94), 131 (12), 130 (14), 126 (10), 105
(18), 104 (13), 103 (13), 99 (100), 91 (42), 77 (14), 65 (14), 54 (13), 51 (10), 44 (39). Anal. calc. for C12H15N3O
(217.27): C 66.34, H 6.96, N 19.34; found: C 66.58, H 6.78, N 19.35.
N-(1-Cyanoethyl)-N-[(2-nitrophenyl)thio]phenylalaninamide (17). Compound 16 (434.5 mg, 2 mmol) was
dissolved in dry pyridine (2 ml) and evaporated under high vacuum. The residue was again taken up in dry
pyridine (2 ml), treated with a few crystals of DMAP and 2-nitrobenzenesulfenyl chloride (417 mg, 2.2 mmol),
and stirred at r.t. Following complete reaction (ca. 1 h, TLC monitoring), a few drops of MeOH were added, and
the mixture was evaporated under high vacuum. FC of the residue (silica gel, CH2Cl2/MeOH 95 :5) gave 17
(735.7 mg, 99%). Light yellow foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.22 0.45. 1H- and 13C-NMR (CDCl3): not
‡
‡
assigned due to complexity (rotation and inversion isomers). MS: 370( <1, M ), 326 (2.7, [M À CONH2] ), 216
‡
(19), 215 (100, [M À NO2C6H4SH] ), 214 (13), 186 (11), 154 (11), 153 (23), 137 (17), 91 (29). Anal. calc. for
C18H18N4O3S (370.43): C 58.36, H 4.90, N 15.12; found: C 59.06, H 5.15, N 15.23.
6-Amino-3-benzyl-5-methylpyrazin-2(1H)-one (18). a) To a soln. of 17 (180mg, 486 mmol) in THF (1 ml),
1.5m LDA in cyclohexane (0.33 ml, 0.5 mmol) was added dropwise at À788 ( ! immediately deep red soln. and
precipitation (starting material)). Following complete addition, the mixture was slowly warmed, the starting
material again went into soln., and the mixture was stirred for 2 h at r.t. Then 1-(trimethylsilyl)-1H-imidazole
(0.37 ml, 2.52 mmol) was added dropwise, and the mixture was further stirred overnight at r.t. The entire
mixture was added to a sat. aq. NH4Cl soln. (10ml), and the mixture was extracted with AcOEt. The combined
org. phase was dried (Na2SO4) and evaporated, the residue (0.19 g) taken up in MeCN, and the product
precipitated by scratching: 18 (61.3 mg, 59%) as a light yellow powder. The remaining mother liquor was
subjected to FC (silica gel, CH2Cl2/MeOH 9 :1): 18 (28 mg, 27%) as a light brown foam. Total yield of 18: 85%.
b) To a soln. of 17 (196 mg, 529 mmol) in THF (2 ml), 1.5m LDA in cyclohexane (0.36 ml, 0.54 mmol) was
carefully added at À788 ( ! immediately deep red soln. and precipitation (starting material)). Following
complete addition, the mixture was slowly warmed and stirred for 2 h at r.t. The mixture was again cooled to
À788, a further equivalent of LDA (0.36 ml,0.54 mmol) added dropwise, and the mixture again stirred at r.t.
Following a further 2 h, the entire mixture was added to 2m phosphate buffer (pH 7.1; 10ml), adjusted with 2 n
HCl to pH 7, and extracted with AcOEt. The combined org. phase was dried (Na2SO4) and evaporated, and the
residue subjected to FC: 18 (113 mg, 99%) as a yellowish foam. Precipitation from MeCN yielded a light brown
solid (92 mg, 80%).
Data of 18: Yellowish to brown foam. M.p. 187 1888. TLC (CH2Cl2/MeOH 9 :1): Rf 0.26. UV (EtOH): 373
(9366), 238.5 (10781), 335 (sh). IR (KBr): 3320, 3200, 3080, 3060, 3030, 2920, 1630, 1530, 1495, 1455, 1360, 1340,
1255, 1230, 1165, 1070, 1030, 1000, 950, 860, 750, 700, 590, 550, 505. 1H-NMR (CDCl3): 2.12 (s, Me); 3.79
(s, PhCH2), 5.64 (br. s, NH2); 7.10 7.25 ( m, 5 arom. H). 13C-NMR ((D6)DMSO): 18.39 (q, Me); 37.0 6 (t, CH2);
122.33 (s, arom. C(py)); 125.44, 127.93, 128.35 (3d, arom. CH); 130.46 (s, arom. C(py)); 140.68 (s, arom. C);
‡
147.65 (s, arom. C(py)); 154.66 (s, arom. C(py)). MS: 216 (13), 215 (100, M ), 214 (10), 187 (10), 186 (12), 99

1316
Helvetica Chimica Acta Vol. 87 (2004)
(17), 91 (37), 77 (9), 44 (13), 43 (17), 42 (19). Anal. calc. for C12H13N3O (215.25): C 66.96, H 6.09, N 19.52;
found: C 66.79, H 6.09, N 19.78.
3. Pyrazine Nucleoside Analogs. 2,3-O-Isopropylidene-d-ribofuranose (19a) [18]. A suspension of ribose
(3.0g, 20mmol) in 0.2% H 2SO4/acetone (60ml) was stirred at r.t. until a clear soln. had formed and no more
ribose was present (by TLC; ca. 1 h). For workup, solid anh. Na2CO3 (2 g, 18.9 mmol) was added, and the
mixture was stirred until the soln. was neutral. Salts were filtered off, and the clear soln. was evaporated to a
colorless oil (3.79 g), which could be used in the following reaction without further purification. For a higher-
quality product, the crude material was purified by FC (CH2Cl2/acetone 8 :2): 19a (3.22 g, 85%) as an a/b-d-
anomer mixture. TLC (CH2Cl2/MeOH 9 :1): Rf 0.42. 1H-NMR (200 MHz, CDCl3; only signals of the main
component; d of OH varied): 1.32 (s, 3 H, Me2CO2); 1.49 (s, 3 H, Me2CO2); 3.55 (dd, J ˆ 4.1, 6.1, OHÀC(5));
3.65 3.81 (m, CH2(5)); 4.42 (m, HÀC(4)); 4.59 (d, J ˆ 5.9, HÀC(2)*); 4.69 (d, J ˆ 6.4, OHÀC(1)); 4.84 (d, J ˆ
5.9, HÀC(3)*); 5.42 (d, J ˆ 6.4, HÀC(1)).
5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-d-ribofuranose (19b). Method A: To a soln. of 19a
t
(2.95 g, 15.51 mmol) and 1H-imidazole (2.112 g, 31 mmol) in CH2Cl2 (10ml) at À508, a soln. of BuMe2SiCl
(2.57 g, 17.05 mmol) in CH2Cl2 (3 ml) was slowly added within 1 h. The mixture was stirred at À30to À508 until
the reaction was complete (ca. 6 h). Adsorption on silica gel (ca. 7 g) and FC (Et2O/hexane 3 :7) yielded 19b as a
colorless oil (3.53 g, 75%) which crystallized on standing, besides the 1,5-O-disilylated compound (a- and b-d
together: 611.5 mg, 9%). Product 19b crystallized after FC from a conc. soln. as fine white needles, which then
were anomerically pure.
Method B: To a soln. of 19a (2.91 g, 15.3 mmol) in CH2Cl2 (10ml) at À708, Et3N (2.55 ml, 18.37 mmol) and
t
DMAP (75 mg, 0.61 mmol) were slowly added. The mixture was warmed to r.t., and a soln. of BuMe2SiCl
(2.538 g, 16.84 mmol) in CH2Cl2 (5 ml) was added within 1 h. After ca. 6 h of stirring at À30to À508, TLC
(Et2O/hexane 6 :4) showed completion. Adsorption on silica gel (ca. 8 g) and FC (Et2O/hexane 2 :8) yielded
19b (3.90g, 84%) as a colorless oil besides a small amount of the 1- O-silylated product. Even though Method B
was usually higher yielding, the product from Method A was easier to purify.
Data of 19b: Crystals. M.p. 49 508. TLC (Et2O/hexane 3 :7): Rf 0.22. 1H-NMR (CDCl3; the anomers are
arbitrarily named a and b; ratio a/b typically 5 :1): 0.06 (s, 6 H, b-Me2Si); 0.15 (2s, 6 H, a-Me2Si); 0.89 (s, 9 H, b-
tBuSi); 0.93 (s, 9 H, a-tBuSi); 1.33, 1.49 (2s, each 3 H, a-Me2CO2); 1.40, 1.56 (2s, each 3 H, b-Me2CO2); 3.65
(dd, 1 H, b-CH2(5)); 3.72 3.81 (m, 3 H, a-CH2(5), b-CH2(5)); 3.91 (d, J ˆ 11.5, 1 H, b-OH); 4.15 (m, 1 H, b-
HÀC(4)); 4.36 (m, 1 H a-HÀC(4)); 4.51 (d, J ˆ 5.9, 1 H, a-HÀC(2)*); 4.55 (dd, J ˆ 4.0, 6.1, 1 H, b-HÀC(2));
4.70( d, J ˆ 5.9, 1 H, a-HÀC(3)*); 4.73 (dd, J ˆ 0.8, 6.1, 1 H, b-HÀC(3)); 4.76 (d, J ˆ 11.9, 1 H, a-OH); 5.29
(d, J ˆ 11.9, 1 H, a-HÀC(1)); 5.46 (dd, J ˆ 4.0, 11.5, 1 H, b-HÀC(1)). 13C-NMR (CDCl3): À5.65 (q, Me2Si);
18.29 (s, Me3CSi); 24.96, 26.51 (2q, Me2CO2); 25.80( q, Me3CSi); 64.87 (t, C(5)); 81.80, 87.04, 87.69 (3d, C(2),
‡
C(3), C(4)); 103.52 (d, C(1)); 112.10( s, Me2CO2). MS: 303 (<1, M ), 287 (11), 247 (19), 189 (16), 171 (23), 159
(12), 143 (50), 131 (15), 129 (72), 117 (57), 105 (10), 102 (13), 101 (31), 97 (13), 89 (25), 85 (10), 75 (100), 73
(51), 69 (26), 59 (43), 57 (10), 55 (21), 43 (33), 41 (15).
Data of 1,5-O-Disilylated By-product: TLC (Et2O/hexane 3 :7): Rf 0.64, 0.74.
Data of 1-O-Silylated By-product: TLC (hexane/Et2O 4 :6): Rf 0.54; Rf of 19b 0.60.
N-[(Benzyloxy)carbonyl]-2-(2,3-O-isopropylidene-b-d-ribofuranosyl)glycine Methyl Ester (22a/b). KOtBu
(134.3 mg, 1.21 mmol) was suspended at À788 in CH2Cl2 (1.5 ml). Phosphorylglycinate 20 (401.3 mg, 1.2 mmol)
in CH2Cl2 (1.5 ml) was added dropwise. After ca. 15 min stirring, the soln. was taken up in a syringe and added
dropwise under cooling via syringe (dry ice) at 08 within 25 min to a soln. of 19a (190.4 mg, 1 mmol) in CH2Cl2
(1 ml). The mixture was stirred for 0.5 h at 08 and then for 6 days at r.t. Although the starting materials were not
completely consumed, the mixture was worked up by dilution with CH2Cl2 (7 ml). The org. phase was washed
with sat. aq. NaCl soln. (2 Â 10ml), dried (MgSO 4), and evaporated, and the residue subjected to FC (silica gel,
1
CH2Cl2/acetone 8 :2): stereoisomer mixture 22a/b (75 mg, 19%). Colorless oil H-NMR (CDCl3): 1.32 (s, 3 H,
Me2CO2); 1.53 (s, 3 H, Me2CO2); 2.90(br. s, OH); 3.65 3.83 (m, and s at 3.77, 5 H, CH2(5'), MeO); 4.02
(m, 1 H, HÀC(2)*); 4.43 (t, J ˆ 3.4, HÀC(1')*); 4.60 4.68 ( m, 3 H, HÀC(2'), HÀC(3'), HÀC(4')*); 5.12 (2s,
2 H, PhCH2); 6.23 (d, J ˆ 2.4, 1 H, NH); 7.28 7.35 (m, 5 H, Ph). 13C-NMR (CDCl3): 25.46 (q, Me2CO2); 27.43
(q, Me2CO2); 52.79 (q, MeO); 55.96 (d, NCHCO2Me); 62.41 (t, HOCH2*); 67.23 (t, PhCH2*); 81.01, 82.18,
84.32, 85.37 (4d, C(1'), C(2'), C(3'), C(4')); 114.15 (s, Me2CO2); 128.03, 128.19, 128.52 (3d, arom. CH); 136.22
(s, arom. C); 156.93 (s, NHCO2CH2Ph); 170.59 (s, CO2Me).
N-[(Benzyloxy)carbonyl]-2-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}gly-
cine Methyl Ester (23a/b). KOtBu (1.557 g, 13.87 mmol) was suspended at À788 in CH2Cl2 (20ml).
Phosphorylglycinate 20 (4.596 g, 13.87 mmol) in CH2Cl2 (12 ml) was then injected. After ca. 1 h stirring, the
cooling bath was replaced with an ice bath, and 19b (3.52 g, 11.56 mmol) in CH2Cl2 (23 ml) was slowly added

Helvetica Chimica Acta Vol. 87 (2004)
1317
dropwise within 1 h. The mixture was stirred for 6 h at 08 and then overnight at r.t. The mixture was diluted with
CH2Cl2 (ca. 150ml), the org. phase washed with sat. aq. NaCl soln. (2 Â 80ml), dried (MgSO 4), filtered, and the
crude product adsorbed on silica gel (13 g). FC (silica gel, Et2O/hexane 2 :8, then 4 :6) gave a small amount of
starting material (ca. 5%) and stereoisomer mixture 23a/b (5.313 g, 90%; ratio ca. 3 :2) as a colorless oil. A third
product, probably the a-d-anomer, was isolated in small amounts (< 3%). The mixture 23a/b could be partly
separated by FC.
Data of 23a: Colorless oil. TLC (hexane/Et2O 7:3): Rf 0.36. IR (CHCl3): 3420, 2990, 2950, 2930, 2900, 2860,
1755, 1725, 1510, 1470, 1460, 1455, 1435, 1385, 1375, 1335, 1290, 1260, 1230, 1170, 1155, 1135, 1115, 1080, 1030,
1005, 975, 910, 885, 855, 840, 815, 700. 1H-NMR (400 MHz, CDCl3): 0.03 (s, 1 MeSi); 0.05 (s, 1 MeSi); 0.84
(s, tBuSi); 1.33 (s, 3 H, Me2CO2); 1.53 (s, 3 H, Me2CO2); 3.72 (dd, J ˆ 2.4, Jgem ˆ 11.4, 1 H, CH2(5')); 3.76
(s, MeO); 3.83 (dd, J ˆ 2.6, Jgem ˆ 11.4, 1 H, CH2(5')); 3.97 (m, HÀC(4')); 4.52 4.66 (m, HÀC(2'), HÀC(3'),
HÀC(1'), HÀC(2)); 5.12 (s, 1 H of PhCH2); 5.13 (s, 1 H of PhCH2); 5.82 (d, J ˆ 9.5, NH); 7.30 7.37 ( m, Ph).
13C-NMR (100 MHz, CDCl3): À5.52 (q, MeSi); À5.42 (q, MeSi); 18.52 (s, Me3CSi); 25.55 (q, Me2CO2); 25.94
(q, Me3CSi); 27.60( Me2CO2); 52.55 (q, MeO); 55.87 (d, NCHCO2Me); 62.98 (t, C(5')); 67.20( t, PhCH2); 80.32,
82.01, 83.30, 84.97 (4d, C(1'), C(2'), C(3'), C(4')); 113.93 (s, Me2CO2); 128.12, 128.36, 128.39 (3d, arom. CH);
136.12 (s, arom. C); 156.75 (s, NHCO2CH2Ph); 170.32 (s, CO2Me). GC/MS (P1001020): tR 20.2; 452, 394, 344,
‡
t
‡
287, 213, 171, 117, 91 (100). MS: 509.2 (<1, M ), 452 (17, [M À Bu] ), 287 (14), 129 (10), 91 (100), 73 (10).
Data of 23b: Colorless oil. TLC (hexane/Et2O 7:3): Rf 0.33. 1H-NMR (CDCl3): 0.07 (2s, Me2Si); 0.89
(s, tBuSi); 1.33 (s, 3 H, Me2CO2); 1.51 (s, 3 H, Me2CO2); 3.68 3.76 (m, 5 H, MeO, CH2(5')); 4.06 4.11
(m, HÀC(2), HÀC(4')*); 4.51 4.61 (m, HÀC(1')*, HÀC(3')**); 4.70 4.74 ( m, HÀC(2')**); 5.11 (s, PhCH2);
5.51 (br. d, J ˆ 5.9, NH); 7.30 7.36 ( m, Ph). GC/MS (P1001020): tR 21.5; 452, 408, 394, 344, 287, 213, 171, 117, 91
(100).
N-[(Benzyloxy)carbonyl]-2-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl]gly-
cinamide (26a/b). A soln. of 23a/b (8.715 g, 17.1 mmol) in MeOH (35 ml) was placed in a glass tube and sat. at 08
with NH3 gas. The tube was sealed, and the mixture was stirred at r.t. for ca. 74 h. The soln. was evaporated and
the residue adsorbed on silica gel. FC (silica gel, Et2O/hexane 8 :2, then AcOEt/hexane 1:1) yielded
diastereoisomer mixture 26a/b (7.788 g, 92%) as a colorless oil. The diastereoisomers could be separated only
partially. Isomerically pure samples of 23 gave an epimer mixture 26a/b, indicating that epimerization occurred
upon ammoniolysis of the Z-protected derivative.
Data of 26a: TLC (Et2O/hexane 8 :2): Rf 0.23. IR (CHCl3): 3520, 3480, 3400, 3340, 3000, 2990, 2950, 2930,
2890, 2860, 1725, 1695, 1585, 1575, 1500, 1470, 1465, 1455, 1415, 1385, 1375, 1325, 1310, 1255, 1160, 1125, 1080,
1030, 1015, 980, 855, 835, 815, 700. 1H-NMR (CDCl3): 0.06 (s, Me2Si); 0.87 (s, tBuSi); 1.32 (s, 3 H, Me2CO2); 1.52
(s, 3 H, Me2CO2); 3.73 (dd, J ˆ 2.0, Jgem ˆ 11.3, 1 H, CH2(5')); 3.91 (dd, J ˆ 2.4, Jgem ˆ 11.3, 1 H, CH2(5')); 3.95
(m, HÀC(2)*); 4.46 (m, HÀC(1')*, HÀC(4')*); 4.53 (m, HÀC(3')**); 4.64 (dd, J ˆ 4.9, 6.4, HÀC(2')**); 5.12,
5.14 (2s, 1 H each, PhCH2); 5.82 (br. s, 1 H of CONH2); 6.00 (br. d, J ˆ 6.6, PhCH2OCONH); 6.57 (br. s, 1 H of
CONH2); 7.30 7.37 ( m, Ph). 13C-NMR (CDCl3): À5.38 (q, Me2Si); 18.58 (s, Me3CSi); 25.57 (q, Me2CO2); 26.01
(q, Me3CSi); 27.55 (Me2CO2); 55.64 (d, NCHCONH2); 62.49 (t, SiOCH2); 67.33 (t, PhCH2); 79.87, 81.74, 82.77,
84.48 (4d, C(1'), C(2'), C(3'), C(4')); 114.53 (s, Me2CO2); 128.28, 128.51 (2d, arom. CH); 136.02 (s, arom. C);
‡
‡
156.38 (s, NHCO2CH2Ph); 171.46 (s, CONH2). MS: 494.5 (2, M ), 479.5 (1, [M À Me] ), 450.5 (5, [M À
‡
t
‡
‡
CONH2] ), 437 (7, [M À Bu] ), 406 (5), 379 (3), 348 (5), 287 (7, [M À substituent at C(1')] ), 171 (6), 129
(9), 117 (8), 101 (5), 97 (6), 92 (9), 91 (100), 75 (13), 73 (17), 59 (12), 57 (12), 43 (19).
Data of 26b: TLC (Et2O/hexane 8 :2): Rf 0.29. 1H-NMR (CDCl3): 0.12 (s, MeSi); 0.13 (s, MeSi); 0.91
(s, tBuSi); 1.30( s, 3 H, Me2CO2); 1.51 (s, 3 H, Me2CO2); 3.73 (dd, J ˆ 4.9, Jgem ˆ 11.5, 1 H, CH2(5')); 3.83 (dd, J ˆ
3.3, Jgem ˆ 11.5, 1 H, CH2(5')); 4.01 (dd, J ˆ 2.7, 9.9, HÀC(2)*); 4.18 4.26 (m, HÀC(4'), HÀC(1')*); 4.64 4.66
(m, HÀC(3')*); 4.81 4.84 (m, HÀC(2')*); 5.12 (s, PhCH2); 5.50(br. s, 1 H of CONH2); 5.72 (br. s,
PhCH2OCONH); 6.73 (br. s, 1 H of CONH2); 7.30 7.37 ( m, Ph). 13C-NMR (CDCl3): À5.40( q, MeSi);
À5.31 (q, MeSi); 18.48 (s, Me3CSi); 25.48 (q, Me2CO2); 25.93 (q, Me3CSi); 27.29 (Me3CO2); 55.65
(d, NCHCONH2); 63.81 (t, SiOCH2); 67.20( t, PhCH2); 81.22, 83.35, 85.49, 86.37 (4d, C(1'), C(2'), C(3'),
C(4')); 113.56 (s, Me2CO2); 127.99, 128.16, 128.51 (3d, arom. CH); 136.12 (s, arom. C); 156.67 (s, NHCO2Ph);
171.44 (s, CONH2).
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl]glycine Methyl Ester (27a/b). A
soln. of 23a/b (1.80g, 3.53 mmol) in MeOH (14 ml) was treated with 10% Pd/C (120mg) with stirring under H
2
(slight overpressure with balloon) at r.t. After complete consumption of 23a/b (8 h, TLC monitoring), the soln.
was filtered through a bed of Celite and evaporated. The crude 27a/b (1.324 g, quant.) was sufficiently pure for
the next reaction. Chromatography (Et2O/hexane 8 :2) partly separated the two isomers 27a/b (yield after
chromatography 93%). By using isomerically pure samples of 23, epimerically pure products were obtained.

1318
Helvetica Chimica Acta Vol. 87 (2004)
Data of 27a: TLC (Et2O/hexane 8 :2): Rf 0.32. IR (CHCl3): 2990, 2960, 2930, 2900, 2860, 1740, 1600, 1470,
1465, 1440, 1385, 1375, 1260, 1175, 1160, 1140, 1075, 1000, 975, 940, 900, 840, 815. 1H-NMR (CDCl3): 0.07
(s, Me2Si); 0.90 (s, tBuSi); 1.35 (s, 3 H, Me2CO2); 1.54 (s, 3 H, Me2CO2); 1.67 (br. s, NH2); 3.57 (d, J ˆ 3.2,
HÀC(2)); 3.72 (dd, J ˆ 3.5, Jgem ˆ 11.2, 1 H, CH2(5')); 3.75 (s, MeO); 3.80( dd, J ˆ 3.3, Jgem ˆ 11.2, 1 H, CH2(5'));
3.98 (ddd(ˆdt), J ˆ 3.4, 4.6, HÀC(4')); 4.41 (dd(ˆt), J ˆ 3.4, HÀC(1')); 4.67 (dd, J ˆ 4.6, 6.4, HÀC(3')); 4.82
(dd, J ˆ 3.6, 6.4, HÀC(2')); NOE data were used to assign configuration. 13C-NMR (CDCl3): À5.50( q, MeSi);
À5.41 (q, MeSi); 18.39 (s, Me3CSi); 25.64 (q, Me2CO2); 25.90( q, Me3CSi); 27.61 (Me2CO2); 52.25 (q, MeO);
56.46 (d, NCHCO2Me); 63.06 (t, SiOCH2); 80.90, 82.42, 84.68, 85.17 (4d, C(1'), C(2'), C(3'), C(4')); 113.72
(s, Me2CO2); 174.12 (s, CO2Me). GC/MS (P15020): tR 5.6; 360, 318, 287, 242, 200, 171, 129, 73 (100). MS: 377
‡
‡
t
(12), 376 (49, [M ‡ 1] ); 360(13, [ M À Me] ), 319 (20), 318 (98, [M À Bu), 317 (17), 316 (19), 288 (10), 287 (48,
‡
t
‡
[M À NH2CHCO2Me] ), 286 (12), 260(16, [ M À BuMe2Si] ), 229 (10), 200 (10), 187 (13), 172 (10), 171 (17),
159 (10), 143 (10), 131 (12), 130 (16), 129 (81), 128 (14), 126 (10), 117 (55), 116 (18), 115 (23), 114 (11), 101
(27), 97 (24), 89 (39), 88 (51), 85 (16), 84 (10), 81 (10), 75 (72), 74 (12), 73 (100), 70 (11), 69 (16), 59 (42), 58
(14), 57 (25), 56 (13), 55 (23), 43 (35), 41 (21), 33 (14).
Data of 27b: TLC (Et2O/hexane 8 :2): Rf 0.24. IR (CHCl3): 2990, 2960, 2930, 2900, 2860, 1740, 1600, 1470,
1465, 1440, 1385, 1375, 1260, 1175, 1160, 1140, 1080, 1010, 975, 860, 840, 815. 1H-NMR (CDCl3): 0.07 (s, Me2Si);
0.90 (s, tBuSi); 1.34 (s, 3 H, Me2CO2); 1.53 (s, 3 H, Me2CO2); 1.64 (br. s, NH2); 3.72 (dd, J ˆ 3.5, Jgem ˆ 11.2, 1 H,
CH2(5')); 3.73 (d, J ˆ 5.9, HÀC(2)); 3.75 (s, MeO); 3.78 (dd, J ˆ 3.2, Jgem ˆ 11.2, 1 H, CH2(5')); 4.03 (q-like m,
J ˆ 3.3, HÀC(4')); 4.18 (dd-like m, J ˆ 3.1, 5.8, HÀC(1')); 4.64, 4.65 (2m, HÀC(2'), HÀC(3')); NOE data were
used to assign configuration. 13C-NMR (CDCl3): À5.50( q, MeSi); À5.41 (q, MeSi); 18.42 (s, Me3CSi); 25.61
(q, Me2CO2); 25.94 (q, Me3CSi); 27.54 (Me2CO2); 52.12 (q, MeO); 56.46 (d, NCHCO2Me); 63.42 (t, SiOCH2);
81.28, 82.06, 85.10, 85.75 (4d, C(1'), C(2'), C(3'), C(4')); 113.72 (s, Me2CO2); 173.35 (s, CO2Me). MS: 3.77 (4),
‡
‡
t
‡
376 (16, [M ‡ 1] ), 360(13, [ M À Me] ), 319 (12), 318 (62, [M À Bu] ), 316 (12), 288 (12), 287 (56, [M À
t
‡
NH2CHCO2Me), 260(30, [ M À BuMe2Si] ), 229 (11), 187 (15), 172 (18), 172 (12), 170 (20), 143 (12), 136 (10),
133 (11), 131 (10), 130 (15), 129 (92), 117 (52), 116 (12), 115 (21), 114 (12), 101 (28), 97 (22), 89 (37), 88 (62),
85 (17), 84 (10), 81 (11), 75 (72), 74 (11), 73 (100), 70 (11), 69 (15), 59 (39), 58 (13), 57 (24), 56 (12), 55 (22), 43
(33), 41 (19), 33 (14).
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}glycinamide (28a/b). Meth-
od A, by Removal of the Z-Group from 26a/b: To a soln. of 26a/b (6.84 g, 13.8 mmol) in MeOH (80ml) was
added Pd/C (6.9 g). The mixture was stirred for 48 h under H2 at r.t. The mixture was filtered through a bed of
Celite, which was washed with MeOH. The filtrate and washings were evaporated. FC (silica gel, CH2Cl2/MeOH
95 :5) of the residue yielded 28a/b (4.45 g, 89%) as a colorless oil. The reaction proceeded without
epimerization.
Method B, by Aminolysis of 27a/b: A soln. of 27a/b (1.132 g, 3.015 mmol) in MeOH (30 ml) in a glass tube
was saturated at 08 with NH3 gas. The tube was sealed and the mixture stirred for 2 d at r.t. The mixture was
evaporated and the residue purified by FC (silica gel, CH2Cl2/MeOH 95 :5): 28a/b (1.082 g, quant.) as a colorless
oil. The reaction proceeded without epimerization.
Data of 28a: TLC (CH2Cl2/MeOH 9 :1): Rf 0.44. IR (CHCl3): 3500, 3380, 3000, 2960, 2935, 2860, 1685, 1590,
1
1550, 1470, 1465, 1385, 1375, 1260, 1160, 1135, 1080, 1005, 980, 970, 905, 860, 840, 815. H-NMR (CDCl3): 0.07
(s, MeSi); 0.08 (s, MeSi); 0.90 (s, tBuSi); 1.35 (s, 3 H, Me2CO2); 1.54 (s, 3 H, Me2CO2); 1.77 (br. s, NH2); 3.52
(d, J ˆ 3.9, HÀC(2)); 3.75 (dd, J ˆ 2.6, Jgem ˆ 11.3, 1 H, CH2(5')); 3.86 (dd, J ˆ 2.7, Jgem ˆ 11.3, 1 H, CH2(5')); 4.04
(ddd(ˆdt), J ˆ 2.6, 3.9, HÀC(4')); 4.36 (dd(ˆt), J ˆ 4.1, HÀC(1')); 4.63 (dd, J ˆ 4.4, 6.6, HÀC(2')); 4.69
(dd, J ˆ 4.0, 6.7, HÀC(3')); 5.49 (br. s, 1 H of CONH2); 7.31 (br. s, 1 H of CONH2). 13C-NMR (CDCl3): À5.50
(q, MeSi); À5.41 (q, MeSi); 18.42 (s, Me3CSi); 25.61 (q, Me2CO2); 25.93 (q, Me3CSi); 27.54 (Me2CO2); 55.97
(d, NCHCONH2); 63.02 (t, SiOCH2); 80.83, 82.09, 84.59, 84.78 (4d, C(1'), C(2'), C(3'), C(4')); 114.21
(s, Me2CO2); 175.19 (s, CONH2). GC/MS (P1001020): tR 9.4. GC/MS (P10010): tR 15.1; 360, 345, 318, 303, 245,
171, 97, 73 (100). MS: 362 (4), 361 (5), 318 (11), 317 (36), 316 (57), 305 (24), 304 (51), 303 (53), 287 (22), 259
(11), 258 (21), 245 (10), 171 (39), 131 (10), 130 (12), 129 (55), 126 (10), 117 (39), 116 (11), 115 (18), 101 (18), 98
(11), 97 (25), 89 (35), 85 (16), 76 (10), 75 (70), 74 (23), 73 (100), 70 (10), 69 (12), 59 (33), 58 (13), 57 (20), 56
(11), 55 (16), 43 (23), 41 (14).
Data of 28b: TLC (CH2Cl2/MeOH 9 :1): Rf 0.43. IR (CHCl3): 3500, 3380, 3010, 2950, 2930, 2900, 2880, 2860,
1685, 1585, 1555, 1470, 1460, 1385, 1375, 1255, 1160, 1135, 1080, 1005, 970, 935, 860, 835, 810. 1H-NMR (CDCl3):
0.08 (s, MeSi); 0.09 (s, MeSi); 0.91 (s, tBuSi); 1.35 (s, 3 H, Me2CO2); 1.53 (s, 3 H, Me2CO2); 1.77 (br. s, NH2); 3.52
(d, J ˆ 6.7, HÀC(2)); 3.73 (dd, J ˆ 3.3, Jgem ˆ 11.3, 1 H, CH2(5')); 3.83 (dd, J ˆ 2.8, Jgem ˆ 11.3, 1 H, CH2(5')); 4.11
(m, HÀC(4'), HÀC(1')); 4.64 (dd, J ˆ 3.6, 6.5, HÀC(2')*); 4.75 (dd, J ˆ 4.0, 6.5, HÀC(3')*); 5.7 (br. s, 1 H of
CONH2); 6.9 (br. s, 1 H of CONH2). 13C-NMR (CDCl3): À5.48 (q, MeSi); À5.37 (q, MeSi); 18.39 (s, Me3CSi);

Helvetica Chimica Acta Vol. 87 (2004)
1319
25.57 (q, Me2CO2); 25.93 (q, Me3CSi); 27.53 (Me2CO2); 56.58 (d, NCHCONH2); 63.54 (t, SiOCH2); 81.12, 82.41,
85.16, 85.81 (4d, C(1'), C(2'), C(3'), C(4')); 113.88 (s, Me2CO2); 174.91 (s, CONH2). GC/MS (P1001020): tR 9.3.
GC/MS (P10010): tR 15.0; 360, 345, 318, 303, 258, 171, 97, 73 (100). MS: 361 (13), 345 (12), 317 (12), 316 (52),
304 (17), 303 (82), 287 (22), 258 (15), 245 (30), 173 (10), 171 (38), 129 (52), 117 (27), 116 (10), 115 (14), 101
(14), 97 (21), 89 (28), 86 (10), 85 (13), 84 (13), 75 (59), 74 (20), 73 (100), 70 (10), 69 (10), 59 (28), 58 (12), 57
(14), 55 (16), 43 (23), 41 (13).
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-N-(cyanomethyl)glycinamide
(29). A soln. of KCN (126 mg, 1.93 mmol) in H2O (4 ml) was acidified with AcOH to ca. pH 6, treated with a
soln. of 28a/b (632 mg, 1.75 mmol) in dioxane (4 ml), and acidified again with AcOH to pH 6. Formalin (135 ml,
ca. 1.755 mmol) was slowly added at r.t. within 2 h. After complete consumption of 28a/b (TLC monitoring),
the mixture was added to sat. aq. Na2CO3 soln., the aq. layer extracted with CH2Cl2, the extract dried (Na2CO3)
and evaporated, and the residue subjected to FC (silica gel, CH2Cl2/MeOH 95 :5): 29 (648 mg, 93%). Colorless
oil, which became solid upon standing. IR (CHCl3): 3510, 3480, 3390, 3340, 3000, 2960, 2940, 2900, 2860, 1690,
1585, 1560, 1470, 1465, 1385, 1375, 1260, 1160, 1130, 1080, 980, 865, 840, 815. TLC (CH2Cl2/MeOH 95 :5):
Rf 0.35. 1H-NMR (CDCl3): 0.10 (2s, Me2Si); 0.92 (s, tBuSi); 1.34 (s, 3 H, Me2CO2); 1.53 (s, 3 H, Me2CO2); 2.78
(ddd(ˆdt), J ˆ 3.1, 7.4, NH); 3.48 (dd, J ˆ 3.1, 5.3, HÀC(2)); 3.66 (d, J ˆ 7.4, CH2CN); 3.77 (dd, J ˆ 2.9, Jgem
ˆ
11.4, 1 H, CH2(5')); 3.87 (dd, J ˆ 2.9, Jgem ˆ 11.4, 1 H, CH2(5')); 4.00 (ddd(ˆdt), J ˆ 2.9, 4.2, HÀC(4')); 4.13
(dd, J ˆ 3.4, 5.3, HÀC(1')); 4.64 (dd, J ˆ 3.4, 6.8, HÀC(2')); 4.68 (dd, J ˆ 4.2, 6.8, HÀC(3')); 5.48 (br. s, 1 H of
CONH2); 6.99 (br. s, 1 H of CONH2). 13C-NMR (CDCl3): À5.38 (q, Me2Si); 18.48 (s, Me3CSi); 25.41
(q, Me2CO2); 25.96 (q, Me3CSi); 27.39 (Me2CO2); 36.09 (t, CH2CN); 61.79 (d, NCHCONH2); 62.54 (t, SiOCH2);
80.28, 82.09, 83.97, 84.87 (4d, C(1'), C(2'), C(3'), C(4')); 114.59 (s, Me2CO2); 117.12 (s, CN); 171.76 (s, CONH2).
‡
‡
‡
‡
MS: 401 (14); 400 (52, [M ‡ 1] ), 399 (6, M ), 384 (13, [M À Me] ), 373 (35, [M À HCN] ), 357 (12), 356 (22),
‡
t
‡
355 (63, [M À CONH2] ), 344 (12), 343 (38), 342 (79, [M À Bu] ), 328 (16), 316 (28), 315 (74), 297 (28), 288
‡
‡
(12), 287 (45, [M À CNCH2NHCHCONH2] ), 286 (11), 284 (41, [M À TBDMS] ), 270(35), 258 (14), 257 (34),
229 (13), 223 (13), 221 (11), 187 (13), 183 (15), 171 (64), 155 (17), 143 (29), 130(18), 129 (64), 117 (62), 115
(26), 112 (22), 109 (19), 101 (29), 99 (18), 97 (39), 89 (51), 85 (41), 81 (20), 75 (74), 73 (75), 69 (42), 67 (20), 59
(61), 58 (21), 57 (46), 56 (22), 55 (27), 45 (24), 44 (21), 43 (72), 42 (35), 41 (61), 39 (28).
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-N-(cyanomethyl)-N-[(2-nitro-
phenyl)thio]glycinamide (30). To a soln. of 2-nitrobenzenesulfenyl chloride (149 mg, 786 mmol) in THF (2.5 ml)
was added amide 29 (100 mg, 250 mmol) and PMP (145 ml, 802 mmol). The mixture was stirred under reflux at
908 for 5.5 h. Sat. aq. Na2CO3 soln. (10ml) was added, the mixture extracted with Et 2O (3 Â 10ml), the
combined org. phase dried (Na2CO3) and evaporated, and the residue subjected to FC (silica gel, CH2Cl2/MeOH
97:3): 30 (100 mg, 72%). Light yellow foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.54. 1H- and 13C-NMR (CDCl3):
not assigned due to complexity (inversion and rotation isomers).
6-Amino-3-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}pyrazin-2(1H)-one
(1a). To a soln. of 30 (196 mg, 355 mmol) in MeOH (4 ml) 5.4m MeONa in MeOH (0.1 ml, 540 mmol) was added
at 08 ( ! immediately deep red). After 3.5 h, the mixture was carefully neutralized at 08 with 2n HCl to pH 7 and
evaporated. FC (silica gel, CH2Cl2/MeOH 9 :1) yielded 1a (69.7 mg, 49%) as an orange yellow foam. A second
FC purification (reversed-phase silica gel (Merck, Art. 7719, silica gel 60 silanized, 70 230mesh ASTM)) gave
1a (23 mg, 16%). Colorless sample which on exposure to air became yellow. TLC (CH2Cl2/MeOH 9 :1): Rf 0.26.
1H-NMR (200 MHz, CDCl3): 0.05 (s, Me2Si); 0.88 (s, tBuSi); 1.38 (s, 3 H, Me2CO2(−exo×)); 1.57 (s, 3 H,
Me2CO2(−endo×)); 3.63 (dd, J ˆ 6.5, 11.0, 1 H, CH2(5')); 3.75 (dd, J ˆ 4.2, 10.8, 1 H, CH2(5')); 4.12 (m, HÀC(4'));
4.56 (dd, J ˆ 4.1, 6.7, HÀC(3')); 5.11 (d, J ˆ 4.5, HÀC(1')); 5.29 (dd, J ˆ 4.7, 6.6, HÀC(2')); 5.6 (br. s, NH2); 6.99
(s, HÀC(5)). 13C-NMR (50MHz, CDCl 3): À5.12 (q, Me2Si); 18.70( s, Me3CSi); 25.67 (q, Me2CO2); 26.16
(q, Me3CSi); 27.54 (q, Me2CO2); 64.70( t, C(5')); 81.64, 82.10, 82.26, 85.94 (4d, C(1'), C(2'), C(3'), C(4')); 111.56
(d, C(5)*); 114.71 (s, Me2CO2); 132.61 (s, C(6)*); 147.70( s, C(3)**); 156.64 (s, C(2)**). GC/MS (P15020):
tR 11.0; 396, 354 (100), 325, 287, 208, 124.
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-N-(1-cyanoethyl)glycinamide
(31). A soln. of 28a/b (721 mg, 2 mmol) and KCN (155 mg, 2.38 mmol) in dioxane/H2O 2 :1 (13.5 ml) was
carefully acidified to pH 5 with AcOH. Within 30min, acetaldehyde (0.16 ml, 2.83 mmol) was added slowly by
means of a cooled syringe. The mixture was then stirred at r.t. for ca. 1 h (complete consumption of 28a/b). Sat.
aq. Na2CO3 soln. (ca. 50ml) was added, the mixture extracted with CH 2Cl2 (4 Â 50ml), the combined org. phase
dried (Na2SO4) and evaporated, and the residue subjected to FC (silica gel, CH2Cl2/MeOH 95 :5): 31 (747.8 mg,
90%), mixture of the four epimers (at C(2) and C(2'')) as a colorless foam. With 28a as starting material, the
epimers 31aa/ab were obtained partly pure and separately analyzed. With 28b as starting material, both epimers
31ba/bb could be better separated via FC (AcOEt/hexane/EtOH 50:47:3).

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Helvetica Chimica Acta Vol. 87 (2004)
Data of 31aa: TLC (CH2Cl2/MeOH 9 :1): Rf 0.49. 1H-NMR (CDCl3): 0.09, 0.10 (2s, Me2Si); 0.91 (s, tBuSi);
1.34 (s, 3 H, Me2CO2); 1.52, 1.53, 1.55 (1s, 1d, 6 H, MeCHCN, Me2CO2); 2.78 (dd, J ˆ 2.0, 10.3, NH); 3.59
(dd, J ˆ 2.3, 4.6, HÀC(2)); 3.71 3.82 (m, 2 H, MeCHCN, CH2(5')); 3.90 3.94 ( m, 2 H, HÀC(4'), CH2(5'));
4.12 (dd, J ˆ 3.8, 4.4, HÀC(1')); 4.54 (dd, J ˆ 3.7, 6.8, HÀC(2')); 4.67 (dd, J ˆ 5.3, 6.8, HÀC(3')); 5.54 (br. s, 1 H
of CONH2); 7.15 (br. s, 1 H of CONH2). 13C-NMR (CDCl3): À5.41 (q, MeSi); 18.55 (s, Me3CSi); 19.65
(q, MeCHCN); 25.51 (q, Me2CO2); 25.97 (q, Me3CSi); 27.48 (q, Me2CO2); 44.19 (d, CHCN); 59.80( d, C(2));
62.01 (t, C(5')); 79.52, 81.96, 83.59, 84.49 (4d, C(1'), C(2'), C(3'), C(4')); 114.72 (s, Me2CO2); 120.23 (s, CN);
‡
171.93 (s, CONH2). FAB-MS (C19H35N3O5Si, 413.59; pos.): 436 (13, [M ‡ Na] ), 416 (11), 415 (30), 414 (100,
‡
[M ‡ H] ), 402 (11), 388 (14), 387 (52), 357 (11), 356 (43), 329 (21), 171 (10), 137 (13), 136 (12), 129 (16), 126
(11), 117 (14), 115 (14), 101 (11), 99 (18), 97 (12), 89 (28), 83 (13), 75 (32), 74 (10), 73 (83), 59 (17), 57 (11), 55
(11).
Data of 31ab: TLC (CH2Cl2/MeOH 9 :1): Rf 0.44. 1H-NMR (CDCl3; sample contaminated with ca. 15% of
31aa): 0.10 (2s, Me2Si); 0.92 (s, tBuSi); 1.34 (s, 3 H, Me2CO2); 1.52 (s, 3 H, Me2CO2); 1.53 (d, J ˆ 7.0, MeCHCN);
2.50( dd, J ˆ 3.4, 8.7, NH); 3.48 (dd, J ˆ 3.4, 6.4, HÀC(2)); 3.66 3.83 (m, MeCHCN, CH2(5')); 4.02
(m, HÀC(4')); 4.07 (dd, J ˆ 3.9, 6.4, HÀC(1')); 4.65 (dd, J ˆ 4.0, 6.6, HÀC(3')); 4.74 (dd, J ˆ 3.9, 6.7,
HÀC(2')); 5.70(br. s, 1 H of CONH2); 6.85 (br. s, 1 H of CONH2). 13C-NMR (CDCl3): À5.40, À5.31 (2q,
Me2Si); 18.46 (s, Me3CSi); 20.04 (q, MeCHCN); 25.41 (q, Me2CO2); 25.96 (q, Me3CSi); 27.35 (q, Me2CO2); 44.45
(d, CHCN); 62.38 (d, C(2)); 62.94 (t, C(5')); 81.01, 81.90, 84.98 (3d, C(1'), C(2'), C(3'), C(4')); 114.28
(s, Me2CO2); 120.10 (s, CN); 172.21 (s, CONH2).
Data of 31ba: TLC (AcOEt/hexane/EtOH 50:47:3): Rf 0.32. 1H-NMR (300 MHz, CDCl3): 0.13 (s, MeSi);
0.14 (s, MeSi); 0.94 (s, tBuSi); 1.35 (s, Me2CO2); 1.49 (d, J ˆ 7.0, MeCHCN); 1.52 (s, 3 H, Me2CO2); 2.55 (dd, J ˆ
6.4, 8.9, NH); 3.36 (dd, J ˆ 6.4, 7.4, HÀC(2)); 3.70 3.80 ( m, 2 H, MeCHCN, CH2(5')); 3.85 (dd, J ˆ 2.7, Jgem
ˆ
11.4, 1 H, CH2(5')); 4.03 (dd, J ˆ 3.6, 7.4, HÀC(1')); 4.16 (m, HÀC(4')); 4.63 (dd, J ˆ 3.2, 6.4, HÀC(3')); 4.69
(dd, J ˆ 3.6, 6.4, HÀC(2')); 5.64 (br. s, 1 H of CONH2); 6.70(br. s, 1 H of CONH2). 13C-NMR (75 MHz, CDCl3):
À5.36, À5.24 (2q, Me2Si); 18.52 (s, Me3CSi); 19.65 (q, MeCHCN); 25.61 (q, Me2CO2); 26.02 (q, Me3CSi); 27.42
(q, Me2CO2); 45.13 (d, CHCN); 61.28 (d, C(2)); 63.90( t, C(5')); 81.24, 82.84, 85.46, 85.59 (4d, C(1'), C(2'),
C(3'), C(4')); 113.89 (s, Me2CO2); 120.26 (s, CN); 172.76 (s, CONH2). MS: 387 (2), 371 (14), 330(20), 329
(100), 271 (14), 171 (27), 129 (24), 117 (10), 101 (11), 75 (13), 73 (11).
Data of 31bb: TLC (AcOEt/hexane/EtOH 50:47:3): Rf 0.25. 1H-NMR (300 MHz, CDCl3): 0.13 (s, MeSi);
0.14 (s, MeSi); 0.93 (s, tBuSi); 1.37 (s, 3 H, Me2CO2); 1.52 (s, 3 H, Me2CO2); 1.53 (d, J ˆ 6.9, MeCHCN); 2.19
(dd, J ˆ 5.8, 7.6, NH); 3.39 (dt, J ˆ 8.0, HÀC(2)); 3.68 (dd, J ˆ 5.8, 6.9, MeCHCN); 3.75 (dd, J ˆ 3.6, Jgem ˆ 11.5,
1 H, CH2(5')); 3.85 (dd, J ˆ 2.9, Jgem ˆ 11.5, 1 H, CH2(5')); 3.97 (dd, J ˆ 3.5, 8.2, HÀC(1')); 4.16 (m, HÀC(4'));
4.66 (dd, J ˆ 3.3, 6.4, HÀC(3')); 4.80( dd, J ˆ 3.5, 6.4, HÀC(2')); 5.67 (br. s, 1 H of CONH2); 6.76 (br. s, 1 H of
CONH2). 13C-NMR (75 MHz, CDCl3): À5.31, À5.24 (2q, Me2Si); 18.52 (s, Me3CSi); 19.88 (q, MeCHCN); 25.57
(q, Me2CO2); 26.03 (q, Me3CSi); 27.39 (q, Me2CO2); 44.64 (d, CHCN); 61.93 (d, C(2)); 63.71 (t, C(5')); 80.99,
83.34, 84.68, 85.85 (4d, C(1'), C(2'), C(3'), C(4')); 113.91 (s, Me2CO2); 120.39 (s, CN); 172.73 (s, CONH2). MS:
386 (5), 371 (15), 330 (23), 329 (100), 287 (10), 271 (16), 171 (40), 157 (14), 129 (43), 117 (24), 101 (32), 71
(18), 99 (29), 98 (13), 97 (11), 89 (15), 85 (13), 75 (44), 73 (59), 59 (23), 57 (12), 56 (10), 55 (13).
2-{5-O-[(tert-Butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-N-(1-cyanoethyl)-N-[(2-nitro-
phenyl)thio]glycinamide (32). Amine 31 (639.7 mg, 1.547 mmol) as a mixture of stereoisomers was dissolved in
dry pyridine (5 ml), evaporated under high vacuum, and again dissolved in dry pyridine (5 ml). This soln. was
treated with 2-nitrobenzenesulfenyl chloride (440mg, 2.32 mmol) and a few small crystals of DMAP, and stirred
at r.t. After 30min, a precipitate began to form, and the reaction was complete (TLC monitoring). Following
addition of MeOH (0.5 ml), the mixture was evaporated under high vacuum, and the residue subjected to FC
(silica gel, CH2Cl2/acetone 95 :5): 32 (865.3 mg, 99%) as a mixture of stereoisomers. Light yellow foam. TLC
(CH2Cl2/MeOH 95 :5): Rf 0.56 (32a); 0.68 and 0.47 (32b). IR (identical for 32a and 32b; CHCl3): 3480, 3340,
3000, 2960, 2940, 2860, 1695, 1595, 1570, 1515, 1450, 1385, 1340, 1310, 1255, 1160, 1080, 975, 930, 895, 835.
1H-NMR (CDCl3): not assigned due to complexity (rotation and inversion isomers). FAB-MS (pos.): 589 (5,
‡
‡
‡
[M ‡ Na] ), 567 (7, [M ‡ H] ); 540(16, [ M À Me] ), 509 (30), 412 (18), 388 (30), 387 (100), 330 (15), 329 (64),
171 (11), 155 (11), 154 (63), 138 (60), 136 (10), 129 (18), 117 (12), 115 (10), 106 (13), 99 (12), 98 (24), 97 (12), 89
(22), 75 (27), 73 (73), 59 (14). Anal. calc. for C25H38N4O7SSi (566.74): C 52.98, H 6.76, N 9.89, O 19.76; found
(mixture 32a): C 53.08, H 6.61, N 9.93; found (mixture 32b): C 53.08, H 6.93, N 9.76.
6-Amino-3-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene)-b-d-ribofuranosyl}-5-methylpyrazin-
2(1H)-one (1b). A soln. of 32 (168.4 mg, 297.14 mmol) in THF (1.5 ml) was treated at 08 dropwise with 1.5m
LDA (0.4 ml, 600 mmol) and stirred at r.t. After 3 h, traces of 32 were still evident (TLC monitoring). Thus, the
mixture was again cooled to 08, further treated with LDA (0.1 ml, 150 mmol), and further stirred at r.t. for 7 h

Helvetica Chimica Acta Vol. 87 (2004)
1321
(reaction complete). The deep red soln. was taken up in sat. aq. NH4Cl soln. (5 ml), the pH adjusted to 5 6 with
2n HCl, and the mixture extracted with CH2Cl2 (4 Â 5 ml). The combined org. phase was dried (Na2SO4) and
evaporated, and the residue subjected to FC (silica gel, CH2Cl2/MeOH 95 :5). The obtained brownish foam
(111.3 mg, 91%) was precipitated from Et2O/hexane: 1b (99.1 mg, 81%). Nearly colorless flakes. M.p. 134
1358. TLC (CH2Cl2/MeOH 9 :1): Rf 0.42. UV (EtOH): 372 (9257), 240(11675), 330(sh). IR (CHCl 3): 3500,
3410, 3330, 2990, 2960, 2930, 3360, 1645, 1615, 1535, 1480, 1435, 1385, 1370, 1260, 1185, 1160, 1135, 1075, 1005,
975, 835. 1H-NMR (CDCl3): 0.04, 0.05 (2s, Me2Si); 0.88 (s, tBuSi); 1.38 (s, 3 H, Me2CO2(−exo×)); 1.59 (s, 3 H,
Me2CO2(−endo×)); 2.16 (s, MeÀC(5)); 3.71 (dd, J ˆ 4.7, 11.0, 1 H, CH2(5')); 3.77 (dd, J ˆ 4.4, 11.0, 1 H, CH2(5'));
4.14 (q-like m, HÀC(4')); 4.70( dd, J ˆ 3.6, 6.6, HÀC(3')); 5.11 (d, J ˆ 4.9, HÀC(4')); 5.17 5.20( m, HÀC(2'),
NH2); NOE data were used to assign configuration. 13C-NMR (CDCl3): À5.36 (q, Me2Si); 18.00 (q, MeÀC(5));
18.45 (s, Me3CSi); 25.61 (q, Me2CO2); 25.99 (q, Me3CSi); 27.52 (q, Me2CO2); 63.50( t, C(5')); 81.98, 83.03, 83.45,
85.12 (4d, C(1'), C(2'), C(3'), C(4')); 114.15 (s, Me2CO2); 122.05 (s, C(5)*); 129.19 (s, C(6)*); 146.67 (s, C(3)**);
155.33 (s, C(2)**). GC/MS (P20005): tR 11.6; 411, 296, 262, 208, 180, 138 (100), 75. FAB-MS (pos.): 434 (37,
‡
‡
[M ‡ Na] ), 412 (100, [M ‡ H] ). Anal. calc. for C19H33N3O5Si (411.57): C 55.45, H 8.08, N 10.21, O 19.44;
found: C 55.63, H 7.89, N 10.04.
N-{5-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-1,6-dihydro-3-methyl-6-oxo-
pyrazin-2-yl}benzamide (33). Pyrazinone 1b (3.2 g, 7.78 mmol) was co-evaporated twice with pyridine and then
dissolved in pyridine (40ml). DMAP ( ca. 10mg) and benzoyl chloride (3 ml, 3.4 equiv.) were added at 0 8, and
the mixture was stirred at r.t. for ca. 2 h until completion (TLC monitoring). Addition of MeOH quenched
excess of reagent and dissolved precipitated pyridinium chloride. This mixture of di- and tribenzoylated
compounds was hydrolyzed in situ with ammonium hydroxide (ca. 8 ml) at r.t. After completion of the reaction,
the precipitated benzamide was filtered off and washed thoroughly with Et2O. The supernatant was evaporated
and the residue purified by FC (CH2Cl2/MeOH 97.5 :2.5): 33 (3.78 g, 94%). Colorless oil. TLC (CH2Cl2/MeOH
95 :5): Rf 0.39. 1H-NMR (300 MHz, CDCl3): 0.04, 0.06 (2s, Me2Si); 0.87 (s, tBuSi); 1.39 (s, 3 H, Me2CO2(−exo×));
1.61 (s, 3 H, Me2CO2(−endo×)); 2.41 (s, MeÀC(3')); 3.82 (m, CH2(5'')); 4.27 (q-like m, HÀC(4'')); 4.79 (dd, J ˆ
3.4, 6.5, HÀC(3'')); 5.07 (dd, J ˆ 4.4, 6.5, HÀC(2'')); 5.22 (d, J ˆ 4.4, HÀC(1'')); 7.50, 7.60, 7.91 (3m, Ph); 8.3
(1 H, NH). 13C-NMR (75 MHz, CDCl3): À5.45 (q, Me2Si); 18.44 (s, Me3CSi); 19.29 (q, MeÀC(3')); 25.63
(q, Me2CO2); 25.94 (q, Me3CSi); 27.52 (q, Me2CO2); 63.16 (t, C(5'')); 81.84, 84.13, 85.68, 85.84 (4d, C(1''), C(2''),
C(3''), C(4'')); 114.30( s, Me2CO2); 127.51, 128.97 (2d, Co and Cm(Ph)); 131.57, 138.97 (2s, C(2'), C(3')); 132.82
(d, Cp(Ph)); 133.11 (s, Cipso(Ph)); 140.53, 154.21 (2s, C(5'), C(6')); 166.04 (s, PhCO). FAB-MS (pos.): 1069 (2,
‡
‡
‡
‡
[2M ‡ Na] ), 1031 (1, [2M] ), 538 (10, [M ‡ Na] ), 516 (100, [M ‡ H] ).
N-{5-{5-O-[(tert-butyl)dimethylsilyl]-2,3-O-isopropylidene-b-d-ribofuranosyl}-3-methyl-6-(prop-2-enyloxy)-
pyrazin-2-yl}benzamide (34). Prior to use, 33 (104.6 mg, 202.8 mmol) and PPh3 (79.2 mg, 302 mmol) were dried
under high vacuum for at least 1 h at 508. The flask was then vented with Ar, and the compounds were dissolved
in dioxane (5 ml). Allyl alcohol (21 ml, 308.1 mmol) and DEAD (47 ml, 310.3 mmol) were added, and the mixture
was stirred for 30min. When the reaction was not complete, another 0.2 equiv. of DEAD was added. After
completion, excess of DEAD was quenched by addition of a few drops of H2O, and the mixture was evaporated.
Chromatography (hexane/Et2O 7:3) of the residue yielded 34 (74.3 mg, 66%). Colorless foam. TLC (Et2O/
pentane 1 :1): Rf 0.49. 1H-NMR (400 MHz, CDCl3): 0.03 (2s, Me2Si); 0.88 (s, tBuSi); 1.39 (s, 3 H, Me2CO2
(−exo×)); 1.60( s, 3 H, Me2CO2(−endo×)); 2.45 (s, MeÀC(3')); 3.68, 3.70(2 s, CH2(5'')); 4.20( m, HÀC(4'')); 4.78
(m, CH2ˆCHCH2O); 4.82 (dd, J ˆ 3.2, 6.4, HÀC(3'')); 5.22 5.25 (m, HÀC(2''), 1 H of CH2ˆCHCH2O); 5.38
(dq, 1 H of CH2ˆCHCH2); 5.37 (d, J ˆ 3.6, HÀC(1'')); 5.98 6.08 (ddt(ˆ m), CH2ˆCHCH2O); 7.51, 7.60, 7.93
(3m, Ph). 13C-NMR (100 MHz, CDCl3): À5.30( q, Me2Si); 18.40( s, Me3CSi); 20.35 (q, MeÀC(3')); 25.65
(q, Me2CO2); 25.96 (q, Me3Si); 27.47 (q, Me2CO2); 63.77 (t, C(5'')); 67.16 (t, CH2ˆCHCH2O); 81.24, 83.10,
83.42, 86.20(4 d, C(1''), C(2''), C(3''), C(4'')); 113.46 (s, Me2CO2); 117.94 (t, CH2ˆCHCH2O); 127.66, 128.86 (2d,
Co and Cm(Ph)); 132.48, 132.64 (2d, Cp(Ph), CH2ˆCHCH2O); 133.70, 138.61, 138.97, 141.40, 154.87 (5s,
‡
‡
Cipso(Ph), C(2'), C(3'), C(5'), C(6')); 165.56 (s, PhCO). FAB-MS (pos.): 1111.5 (<2, [2M] ), 556 (89, [M ‡ H] )
‡
105 (100, PhCO ).
N-[3-Methyl-6-(prop-2-enyloxy)-5-(b-d-ribofuranosyl)pyrazin-2-yl]benzamide (35).
A soln. of 34
(751 mg, 1.35 mmol) in 1n HCl/MeOH (40ml) was stirred overnight at r.t. The mixture was neutralized by
addition of solid NaHCO3, the salts were filtered off, and the supernatant was evaporated. FC (CH2Cl2/MeOH
39 :1) of the residue yielded 35 (760mg, 94%). Colorless oil. TLC (CH 2Cl2/MeOH 9 :1): Rf 0.36. 1H-NMR
(300 MHz, CD3OD): 2.40( s, MeÀC(3')); 3.74 (dd, J ˆ 3.0, 12.0, 1 H, CH2(5'')); 3.98 (dd, J ˆ 2.9, 12.0, 1 H,
CH2(5'')); 4.09 (m, HÀC(4'')); 4.26 4.31 (m, HÀC(2''), HÀC(3'')); 4.88 (m, > 2 H, CH2ˆCHCH2O, CD3OH);
5.23 5.30( m, 2 H, CH2ˆCHCH2O, HÀC(1'')); 5.44 (dq, 1 H, CH2ˆCHCH2O); 6.05 6.18 (ddt(ˆ m),
CH2ˆCHCH2O); 7.53, 7.62, 7.98 (3m, Ph). 13C-NMR (75 MHz, CD3OD): 19.65 (q, MeÀC(3')); 63.12

1322
Helvetica Chimica Acta Vol. 87 (2004)
(t, C(5'')); 68.43 (t, CH2ˆCHCH2O); 71.90; 76.88, 81.57, 85.29 (4d, C(1''), C(2''), C(3''), C(4'')); 118.41
(t, CH2ˆCHCH2O); 129.06, 129.80 (2d, Co and Cm(Ph)); 133.56, 134.11 (2d, Cp(Ph), CH2ˆCHCH2O); 134.95,
141.27, 141.85, 143.53, 156.03 (5s, Cipso(Ph), C(2'), C(3'), C(5'), C(6')); 169.17 (s, PhCO). FAB-MS (pos.): 825
‡
‡
‡
(<1, [2M ‡ Na] ), 803 (7, [2M] ), 402 (100, [M ‡ H] ). Anal. calc. for C20H23N3O6 (401.42): C 59.84, H 5.78,
N 10.47; found: C 59.33, H 5.82, N 10.21.
5-Amino-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl b-d Ribofuranoside (36). Benzamide 35 (745 mg,
1.86 mmol) in a glass autoclave was dissolved in conc. ammonium hydroxide (25 ml) and stirred overnight at
60 70 8. After cooling in an ice bath, the mixture was evaporated. FC (CH2Cl2/MeOH 95 :5) of the residue
yielded 36 (506.6 mg, 91.8%). Colorless oil. TLC (CH2Cl2/MeOH 9 :1): Rf 0.35. 1H-NMR (300 MHz, CD3OD):
2.24 (s, MeÀC(6')); 3.70( dd, J ˆ 2.7, 12.0, 1 H, CH2(5)); 3.93 (dd, J ˆ 2.9, 12.0, 1 H, CH2(5)); 4.00 (m, HÀC(4));
4.17 (dd, J ˆ 3.8, 4.9, HÀC(2)); 4.28 (dd, J ˆ 5.0, 6.1, HÀC(3)); 4.80( m, CH2ˆCHCH2O); 5.13 (d, J ˆ 3.8,
HÀC(1)); 5.21 (dq, J ˆ 10.5, 1 H, CH2ˆCHCH2O); 5.74 (dq, J ˆ 17.3, 1 H, CH2ˆCHCH2O); 6.07 (ddt, J ˆ 5.3,
10.5, 17.3, CH2ˆCHCH2O). 13C-NMR (75 MHz, CD3OD): 18.52 (q, MeÀC(6')); 63.35 (t, C(5)); 67.46
(t, CH2ˆCHCH2O); 72.09, 77.01, 81.09, 85.28 (4d, C(1), C(2), C(3), C(4)); 117.67 (t, CH2ˆCHCH2O);
128.12, 129.65, 153.47, 156.44 (4s, C(2'), C(3'), C(5'), C(6')); 134.89 (d, CH2ˆCHCH2O).
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl b-d-Ribofuranoside
(37). Dimethylformamide diethyl acetal (1.7 ml, 9.9 mmol) was added to a soln. of 36 (283 mg, 952 mmol) in
DMF (7 ml), and the mixture was stirred at r.t. for 24 h. The solvent was evaporated and the residue purified by
FC (CH2Cl2/MeOH 9 :1): 37 (315 mg, 94.2%). TLC (CH2Cl2/MeOH 9 :1): Rf 0.38. 1H-NMR (400 MHz,
CD3OD): 2.38 (s, MeÀC(6')); 3.10, 3.15 (2s, Me2N); 3.72 (dd, J ˆ 2.4, 12.0, 1 H, CH2(5)); 3.97 (dd, J ˆ 2.7, 12.0,
1 H, CH2(5)); 4.04 (m, HÀC(4)); 4.18 (dd, J ˆ 3.4, 4.8, HÀC(2)); 4.30( dd, J ˆ 4.9, 6.4, HÀC(3)); 4.85 (m,
>2 H, CH2ˆCHCH2O, CD3OH); 5.19 (d, J ˆ 3.4, HÀC(1)); 5.23 (dq, J ˆ 10.5, 1 H, CH2ˆCHCH2O); 5.41
(dq, J ˆ 17.3, CH2ˆCHCH2O); 6.10( ddt, J ˆ 5.3, 10.5, 17.3, CH2ˆCHCH2O); 8.46 (s, NCHN). 13C-NMR
(100 MHz, CD3OD): 18.83 (q, MeÀC(6)); 34.98, 41.10(2 q, Me2N); 63.10( t, C(5)); 67.55 (t, CH2ˆCHCH2O);
71.75, 77.21, 81.22, 85.13 (4d, C(1), C(2), C(3), C(4)); 117.67 (t, CH2ˆCHCH2O); 134.00, 138.81, 154.51, 155.64
(4s, C(2'), C(3'), C(5'), C(6')); 134.90( d, CH2ˆCHCH2O); 157.46 (d, NCHN). FAB-MS (pos.): 705 (7, [2M ‡
‡
‡
H] ), 353 (100, [M ‡ H] ). Anal. calc. for C16H24N4O5 (352.39): C 54.53, H 6.86, N 15.90; found: C 54.71,
H 6.76, N 15.84.
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl 3,5-O-(1,1,3,3-Tetraiso-
propyldisiloxane-1,3-diyl)-b-d-ribofuranoside (38). Ribofuranoside 37 (118 mg, 294 mmol) was twice co-
evaporated with pyridine (2 ml each) and then dissolved in pyridine (3 ml). TIPS-Cl (ˆ1,3-dichloro-1,1,3,3-
tetraisopropyldisiloxane; 107 ml, 341 mmol) was added at 08, and the mixture was slowly warmed to r.t. and
stirred overnight. Excess of reagent was quenched by adding a few drops of MeOH. The soln. was then
evaporated and the residue subjected to FC (Et2O/hexane 1:1): 38 (157 mg, 83%). Colorless oil. TLC (CH2Cl2/
1
MeOH 95 :5): Rf 0.72. H-NMR (400 MHz, CDCl3): 0.96 1.18 (m, iPrSi); 2.40( s, MeÀC(6')); 3.00 (d, J ˆ 2.3,
OHÀC(2)); 3.11 (s, Me2N); 3.97 4.03 (m, CH2(5), HÀC(4)); 4.44 4.47 (m, HÀC(2)); 4.75 4.86
(m, CH2ˆCHCH2O, HÀC(3)); 5.21 (dq, J ˆ 10.4, 1 H, CH2ˆCHCH2O); 5.28 (d, J ˆ 2.3, HÀC(1)); 5.36
(dq, J ˆ 17.2, 1 H, CH2ˆCHCH2O); 6.06 (ddt, J ˆ 5.3, 10.4, 17.2, CH2ˆCHCH2O); 8.36 (s, NCHN). FAB-MS
‡
‡
(pos.): 1188 (1, [2M] ) 595 (100, [M ‡ H] ).
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl 2-O-Methyl-3,5-O-
(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-b-d-ribofuranoside (39). Methyl iodide (2 ml) and Ag2O (175 mg,
5.1 equiv.) were added at 08 to a soln. of 38 (175 mg, 294 mmol) in MeCN (4 ml). The flask was closed tightly and
the mixture stirred at r.t. overnight in the dark. After addition of MeOH (2 ml), the mixture was filtered through
a small bed of silica gel and evaporated. The residue was subjected to FC (hexane/AcOEt 8 :2): 39 (107 mg,
59%). Slightly yellow oil. TLC (hexane/AcOEt 8 :2): Rf 0.48. 1H-NMR (400 MHz, CDCl3): 1.00 1.17
(m, iPrSi); 2.38 (s, MeÀC(6')); 3.11 (s, Me2N); 3.54 (s, MeOÀC(2)); 3.95 4.08 (m, CH2(5), HÀC(4),
HÀC(2)); 4.74 4.88 (m, CH2ˆCHCH2O, HÀC(3)); 5.20( dq, J ˆ 10.4, 1 H, CH2ˆCHCH2O); 5.36 (dq, J ˆ
17.2, 1 H, CH2ˆCHCH2O); 6.06 (ddt, J ˆ 5.3, 10.4, 17.2, CH2ˆCHCH2O); 8.35 (s, NCHN). 13C-NMR
(100 MHz, CDCl3): 12.64, 12.74, 13.13, 13.45 (4d, 4 Me2CHSi); 17.09 19.22 (9q, 4 Me2CHSi, MeÀC(6));
34.63, 40.76 (2q, Me2N); 58.94 (q, MeOÀC(2)); 61.23 (t, C(5)); 66.33 (t, CH2ˆCHCH2O); 73.01, 79.49, 80.49,
84.11 (4d, C(1), C(2), C(3), C(4)); 117.16 (t, CH2ˆCHCH2O); 133.06, 138.87, 152.36, 154.28 (4s, C(2'), C(3'),
‡
C(5'), C(6')); 133.91 (d, CH2ˆCHCH2O); 155.32 (d, NCHN). FAB-MS (pos.): 1216 (1, [2M] ), 609 (100, [M ‡
‡
H] ).
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl 2-O-Methyl-b-d-ribofu-
ranoside (40). A soln. of pyridinium fluoride in pyridine (ca. 5.4m) was slowly added to a soln. of 39
(251.5 mg, 413 mmol) in pyridine (1.5 ml) in a plastic vial. After the reaction was complete (ca. 12 h), it was

Helvetica Chimica Acta Vol. 87 (2004)
1323
quenched with methoxytrimethylsilane (0.5 ml, 1.2 equiv. with respect to FÀ) at 08. The mixture was stirred at r.t.
for 15 min, transferred to a glass flask, and evaporated. The residue was subjected to FC (CH2Cl2/MeOH 95 :5).
The product was easily crystallized from CH2Cl2/pentane: 40 (141.5 mg, 93.5%). M.p. 1908. TLC (CH2Cl2/
MeOH 95 :5): Rf 0.27. 1H-NMR (400 MHz, CDCl3): 2.44 (s, MeÀC(6)); 2.72 (d, J ˆ 8.1, OH); 3.11, 3.13 (2s,
Me2N); 3.52 (s, MeOÀC(2)); 3.79 (t, 1 H, CH2(5)); 3.87 (dd, J ˆ 2.4, 4.8, HÀC(2)); 4.08 4.20 (m, 2 H, CH2(5),
HÀC(4)); 4.50 4.55 ( m, HÀC(3)); 4.79 4.90(2 ddt, CH2ˆCHCH2O); 5.25 (dq, J ˆ 10.4, CH2ÀCHCH2O);
5.37 (m, 2 H, HÀC(1), CH2ˆCHCH2O); 6.05 (ddt, J ˆ 5.3, 10.4, 17.3, CH2ˆCHCH2O); 6.23 (d, J ˆ 10.5, OH);
8.38 (s, NCHN). 13C-NMR (100 MHz, CDCl3): 18.71 (q, MeÀC(6)); 34.70, 40.82 (2q, Me2N); 57.80
(q, MeOÀC(2)); 62.13 (t, C(5)); 66.54 (t, CH2ˆCHCH2O); 70.00, 76.43, 84.99, 85.86 (4d, C(1), C(2), C(3),
C(4)); 117.94 (t, CH2ˆCHCH2O); 132.99, 138.64, 152.89, 153.34 (4s, C(2'), C(3'), C(5'), C(6')); 133.29
‡
(d, CH2ˆCHCH2O); 155.26 (d, NCHN). FAB-MS (pos.): 367 (100, [M ‡ H] ). Anal. calc. for C17H26N4O5
(366.42): C 55.73, H 7.15, N 15.29; found: C 56.03, H 6.97, N 15.11.
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl 5-O-(4,4'-dimethoxytri-
tyl)-2-O-methyl-b-d-ribofuranoside (41). Ribofuranoside 40 (135.7 g, 370.3 mmol) was co-evaporated twice
with pyridine (2 3 ml each) and then dissolved in pyridine (2.5 ml). A soln. of 4,4'-dimethoxytrityl chloride
(150mg, 1.2 equiv.) in pyridine (1 ml) was added, and the mixture was stirred at r.t. overnight. After quenching
with MeOH, the solvent was evaporated and the residue purified by FC (hexane/AcOEt 1:1): 41 (182 mg,
73%). Colorless foam. TLC (hexane/AcOEt 1:1): Rf 0.45. 1H-NMR (200 MHz, CDCl3): 2.35 (s, MeÀC(6));
2.71 (d, J ˆ 5.3, OH); 3.12 (s, Me2N); 3.26 (dd, J ˆ 4.8, 9.9, 1 H, CH2(5)); 3.35 (dd, J ˆ 4.3, 9.8, 1 H, CH2(5)); 3.42
(s, MeOÀC(2)); 3.77 (s, (MeO)2Tr); 4.12 (m, HÀC(4)); 4.36 (m, HÀC(3), HÀC(2)); 4.83 (dq,
CH2ˆCHCH2O); 5.20( dq, J ˆ 10.5, 1 H, CH2ˆCHCH2O); 5.31 (d, J ˆ 4.3, HÀC(1)); 5.35 (dq, J ˆ 17.3, 1 H,
CH2ˆCHCH2O); 6.04 (ddt, J ˆ 5.3, 10.5, 17.3, CH2ˆCHCH2O); 6.75 7.51 (m, 13 H, (MeO)2Tr); 8.39
(s, NCHN).
5-{[(Dimethylamino)methylene]amino}-6-methyl-3-(prop-2-enyloxy)pyrazin-2-yl 5-O-(4,4'-dimethoxytri-
tyl)-2-O-methyl-b-d-ribofuranoside 3-[2-Cyanoethyl Diisopropylphosphoramidite] (42). Ribofuranoside 41
(130mg) was co-evaporated with pyridine (3 Â with 1 ml) and subsequently dried overnight in a desiccator over
P2O5 under high vacuum. The desiccator was vented with Ar and the residue dissolved in MeCN (1 ml). DMAP
(ca. 0.1 equiv.) and H¸nig×s base (0.15 ml, 4.5 equiv.) were added, followed by 2-cyanoethyl diisopropylphos-
phoramidochloridite (62 ml, 1.4 equiv.) at 08, and the reaction was stirred at r.t. After completion, the mixture
was diluted with AcOEt (10ml), the soln. washed with 2 m aq. phosphate buffer (pH 7.0; 10 ml) and brine
(5 ml), dried (Na2SO4), and evaporated, and the residue subjected to FC (hexane/AcOEt/Et3N 60:40:2):
42 (144 mg, 85%). Slightly yellow oil. TLC (hexane/AcOEt 1:1): Rf 0.28. 1H-NMR (500 MHz, CDCl3;
two diastereoisomers): 1.01 1.19 (m, 2 Me2CH); 2.31 2.39 (2s ‡ m, MeÀC(6), CNCH2); 2.67 2.70( m, 1 H,
CNCH2); 3.11 (s, Me2N); 3.25 3.43, 2.52 3.66, 3.89 3.99 (3m, POCH2, 2 Me2CH, CH2(5)); 3.38, 3.39 (2s,
MeOÀC(2')); 3.76, 3.77 (3s, (MeO)2Tr); 4.22 4.28 (m, HÀC(4)); 4.40 4.67 (3 m, HÀC(2), HÀC(3)); 4.81
4.85 (m, CH2ˆCHCH2O); 5.19 5.22 (dm, 1 H, CH2ˆCHCH2O); 5.34 5.40( m, 2 H, HÀC(1), CH2ˆCHCH2);
6.03 6.10 (m, CH2ˆCHCH2O); 6.74 6.78, 6.97 7.49 (m, 13 H, (MeO)2Tr), 8.37, 8.38 (2s, NCHN). 31P-NMR
‡
‡
(200 MHz, CDCl3): 149.6, 150.18. FAB-MS (pos.): 1737 (<1, 2M ), 869 (14, [M ‡ H] ).
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Received January 14, 2004