Intramolecular carbenoid ylide forming reactions of 2-diazo-3-keto-4- phthalimidocarboxylic esters derived from methionine and cysteine

Article abstract of DOI:10.3762/bjoc.8.49

Methionine, S-benzylcysteine and S-allylcysteine were converted into 2-diazo-3-oxo-4-phthalimidocarboxylic esters 8a-c in three steps. Upon rhodium-catalysed dediazoniation, two intramolecular carbenoid reactions competed, namely the formation of a cyclic sulfonium ylide and that of a six-ring carbonyl ylide. The S-methyl and S-benzyl ylides 12a and b could be isolated, while S-allyl ylide 12c underwent a [2,3]-sigmatropic rearrangement. The short-lived carbonyl ylides derived from methionine and S-benzylcysteine formed head-to-tail dimers by a [3 + 3]-cycloaddition and could be trapped with external dipolarophiles, while the S-allyl derivative 14c yielded the pentacyclic compound 17 by an intramolecular [3 + 2]-cycloaddition reaction.

Full text of DOI:10.3762/bjoc.8.49

Intramolecular carbenoid ylide forming reactions of
2-diazo-3-keto-4-phthalimidocarboxylic esters
derived from methionine and cysteine
Marc Enßle, Stefan Buck, Roland Werz and Gerhard Maas*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 433–440.
Institute of Organic Chemistry I, University of Ulm,
Albert-Einstein-Allee 11, D-89081 Ulm, Germany
doi:10.3762/bjoc.8.49
Received: 11 January 2012
Accepted: 06 March 2012
Published: 22 March 2012
Email:
Gerhard Maas* - gerhard.maas@uni-ulm.de
* Corresponding author
This article is dedicated to Professor Bärbel Schulze on the occasion of
her birthday.
Keywords:
α-aminoacids; carbonyl ylides; cycloaddition; α-diazo-β-ketoesters;
Associate Editor: D. Dixon
sulfonium ylides
© 2012 Enßle et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Methionine, S-benzylcysteine and S-allylcysteine were converted into 2-diazo-3-oxo-4-phthalimidocarboxylic esters 8a–c in three
steps. Upon rhodium-catalysed dediazoniation, two intramolecular carbenoid reactions competed, namely the formation of a cyclic
sulfonium ylide and that of a six-ring carbonyl ylide. The S-methyl and S-benzyl ylides 12a and b could be isolated, while S-allyl
ylide 12c underwent a [2,3]-sigmatropic rearrangement. The short-lived carbonyl ylides derived from methionine and S-benzylcys-
teine formed head-to-tail dimers by a [3 + 3]-cycloaddition and could be trapped with external dipolarophiles, while the S-allyl
derivative 14c yielded the pentacyclic compound 17 by an intramolecular [3 + 2]-cycloaddition reaction.
Introduction
The synthetic potential of diazo compounds, in particular of concerned, thermally induced isomerisation, that is the 1,2-shift
α-diazoketones and α-diazoesters, is greatly widened by the of a substituent (Stevens rearrangement) and [2,3]-sigmatropic
ability of the derived carbene or metal-carbene intermediates to rearrangement of allylsulfonium ylides [9-11], and the use as C1
undergo inter- and intramolecular formation of N-, O-, S- and building blocks in epoxidation, aziridination and olefination
other ylides [1,2]. The benefit of these transformations, which reactions [12,13] are common reaction channels. The intramole-
are usually performed with rhodium- or copper-based catalysts, cular formation of sulfonium ylides from α-diazocarbonyl com-
is given by subsequent rearrangement or addition reactions of pounds tethered with alkylthio or arylthio groups has been
the reactive ylides; several recent reviews [3-8] illustrate the studied by the research groups of Davies [14], Moody [15], and
wide range of applications. As far as sulfonium ylides are West [16]. From α-diazo-β-ketoesters 1, stable four- to seven-
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Beilstein J. Org. Chem. 2012, 8, 433–440.
membered cyclic ylides 2 were obtained (Scheme 1); in the case A diazoketone such as 3 has the structural prerequisites to
of R2 = allyl, however, the ylides underwent a spontaneous undergo two types of intramolecular carbenoid ylide-forming
[2,3]-sigmatropic rearrangement.
reactions, yielding either a cyclic sulfonium ylide or a six-ring
carbonyl ylide. In fact, it has been found that Rh(II)-catalyzed
dediazoniation of α-diazo-β-ketoesters with γ-phthalimido
[18,19] or related [20] substituents gives rise to cyclic carbonyl
ylides, which were trapped by intermolecular cycloaddition
reactions. In this paper, we report that the formation of sulfo-
nium as well as carbonyl ylides are indeed competing pathways
in the Rh(II)-catalyzed dediazoniation of phthaloyl-protected
γ-amino-α-diazo-β-ketoesters derived from methionine,
S-benzylcysteine, and S-allylcysteine.
Scheme 1: Synthesis of cyclic sulfonium ylides 2; n = 0–3.
On the other hand, the conversion of methionine-derived
diazoketone 3 into the cyclic sulfonium ylide 4 was not accom-
plished through the carbenoid route, but rather by cyclisation of
a diazonium ion followed by deprotonation of the resulting
sulfonium ion (Scheme 2) [17].
Results and Discussion
Synthesis of diazoesters
The 2-diazo-3-oxo-4-phthalimidocarboxylic esters 8a–c were
prepared from L-methionine (5a), S-benzyl-L-cysteine (5b) and
S-allyl-L-cysteine (5c), respectively, in a three-step sequence
(Scheme 3) applied by us previously for other α-aminoacids
[18]. Acids 5a–c were converted into their phthalimido-substi-
tuted derivatives 6 followed by conversion into β-ketoesters 7
(yields: 30% (7a), 60% (7b) and 19% (7c)) and diazo group
transfer to give diazoesters 8 (61–99%). Phthalimido-free
diazoesters 11a and b, which were desired for comparative re-
activity studies, were prepared analogously; it should be noted
that different approaches were taken to obtain the methyl ester
analogue of 11b and other similar sulfur-containing α-diazo-β-
ketoesters [14-16]. We found that diazoester 8a can also be
Scheme 2: Non-carbenoid formation of sulfonium ylide 4.
Scheme 3: Conditions: (a) phthalic anhydride, NEt3 (10 mol %), toluene, reflux, 2 h; (b) 1. carbonyldiimidazole, THF, 16 h; 2. MgCl2, NEt3,
HOOC–CH2CO2Et, 3.5 h; (c) p-tosyl azide, NEt3, acetonitrile, rt, 16 h (with 7a) or imidazole-1-sulfonyl azide·HCl, NEt3, CH2Cl2, reflux, 16 h (with 7b,c
and 10). Alternative synthesis of 8a: 1. 6a, SOCl2 (10 equiv), CH2Cl2, reflux, 2 h; 2. N2=CHCO2Et (2 equiv), rt, 4 days.
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Beilstein J. Org. Chem. 2012, 8, 433–440.
prepared by acylation of ethyl diazoacetate (two equivalents, of excess chlorotrimethylsilane) on the latter route, had an enan-
one equivalent serving to trap HCl) with the acid chloride of 6a. tiomeric ratio of 83:17.
Although the yield was modest (28%), it was still better and
gave fewer byproducts than the β-ketoester route.
Rhodium-catalysed ylide-forming reactions
A sample of racemic diazoester 8a, prepared from DL-methio-
The phthaloylation of 5a–c was achieved with phthalic anhy- nine, was exposed to a catalytic amount of dirhodium tetraac-
dride in the presence of a catalytic amount of triethylamine to etate in boiling benzene. Two products were isolated and identi-
lower the reaction temperature, so that the reaction could be fied, the six-membered cyclic sulfonium ylide 12a (13% yield)
performed in toluene under reflux. For phenylalanine, it was and the carbonyl ylide dimer 13a (43%), (Scheme 4). In an
reported that no racemisation occurred under these conditions analogous manner, the five-membered cyclic sulfonium ylide
[21]. However, with some other α-aminoacids we have noted a 12b and polyheterocycle 13b were formed from diazoester 8b
small degree of racemisation [18]. In this work, we can confirm in yields of 37 and 25%, respectively (yields determined by 1H
the reported specific rotation value for N,N-phthaloyl-L-methio- NMR integration on the crude product mixture). In contrast to
nine (6a), but racemisation occurred to a significant extent in these successful transformations, we found that α-diazoketone 3
the case of S-benzyl-N,N-phthaloyl-L-cysteine (6b) (Supporting (Scheme 2) was smoothly decomposed by a catalytic amount of
Information File 1). Practically complete racemisation took Rh2(OAc)4 or Cu(I) triflate (e.g., 1 mol % of Rh2(OAc)4,
place during conversion of N-protected aminoacids 6a–c to the CH2Cl2, 40 °C, 17 h), but a complex mixture of unidentified
β-ketoesters 7a–c, and therefore, diazoesters 8a–c were also products was formed.
obtained in (almost) racemic form. As a consequence,
diazoester 8a was prepared subsequently from DL-methionine. The cyclic sulfonium ylides 12a,b were both obtained as a mix-
On the other hand, the preparation of N-phthaloylated ture of two diastereomers. For the six-ring ylide 12a (isomer
diazoester 8a via the aminoacid chloride route (Scheme 3) ratio A:B = 4:1) a stereochemical assignment was made based
seems to occur with only a little racemisation. A sample of 8a, on 1H NMR studies (Figure 1 and Table 1). We propose that the
which was prepared from a slightly racemised sample of 6a major isomer 12aA assumes a cyclohexene-like half-chair con-
(HPLC: 90:10 enantiomeric ratio of the derived methyl ester, formation, and the minor isomer 12aB, which has the opposite
which was prepared from 6a and dry methanol in the presence configuration at the phthalimido-substituted ring position, exists
Scheme 4: Rh(II)-catalysed carbenoid reactions of diazoesters 8a,b.
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Beilstein J. Org. Chem. 2012, 8, 433–440.
in a twisted-boat form. In both cases, the S-methyl group occu- The oxazapolycycles 13a and b are head-to-tail dimers of car-
pies the (pseudo-)axial position and the sterically demanding bonyl ylides, resulting from a [3 + 3]-cycloaddition. Their
phthalimido group is in the (pseudo-)equatorial position. These constitution and relative stereochemistry was assigned by NMR
conformations fit the observed nuclear Overhauser effects as comparison with the corresponding alanine-derived ring system,
well as the 3J coupling constants. Crystal structure determina- for which a structural proof was furnished by X-ray crystal-
tions of an acyclic [22] and a five-membered cyclic [23] sulfo- structure determination [18]. Due to the symmetry of the
nium ylide containing the R2 S+ –C− (COOR or dimers, the NMR spectra display signals for only half of the
COCH3)–C(=O)R moiety have shown that the sulfur atom is total number of carbon and hydrogen nuclei. Characteristics are
pyramidalised and the negative charge is delocalised over the the signals for the NCH proton (13a: δ = 4.60 ppm; 13b: δ =
π-system of the planar oxoenolate unit. This geometry agrees 4.67 ppm) and the two bridgehead carbons of the epoxy bridge
with the proposed conformation of 12aA. In the twisted-boat (13a: δ = 86.60 and 94.13 ppm; 13b: δ = 86.75 and 94.04 ppm).
structure of 12B, the negative charge may be stabilised mainly While a CI mass spectrum of 13b showed a weak molecular-ion
by the ester group and less so by the adjacent keto group. The peak for the dimer (the base peak representing the monomer
axial position of the methyl group at the pyramidalised sulfur unit), in the case of 13a, even under these mild conditions of
atom is remarkable. In this geometry, however, chemical ionisation, the spectrum showed no peak at m/z greater
p(Cylide)–σ*(S–C) conjugation of the unshared pair of electrons than that for the monomer, which was again the base peak. The
at the ylidic carbon atom would be feasible. While this inter- relative stereochemistry shown in Scheme 4, with the two
action is a key stabilisation factor for the parent sulfonium epoxy bridges in syn relationship and the mercaptoalkyl
ylides (e.g., H2S–CH2 and Me2S–CH2) according to theoretical substituent in the (equatorial) exo position, is explained by an
calculations [24], its energetic contribution in the case of sulfo- endo transition state 15 for the [3 + 3]-cycloaddition, in which
nium ylides stabilised by acceptor substituents is likely to be two cyclic carbonyl ylide molecules 14 approach each other
small.
with their sterically less-occupied faces (Scheme 5). Our find-
ings on carbonyl ylide dimers reported here and in a preceding
paper [18] tie in with earlier reports on the [3 + 3]-cyclodimeri-
sation of carbonyl ylides of the 2-benzopyrylium-4-olate type
[25,26].
A different product pattern results from the rhodium(II)-catal-
ysed dediazoniation of S-allyl-substituted diazoester 8c
(Scheme 6). In this case, the simultaneous formation of five-
ring sulfonium ylide 12c and six-ring carbonyl ylide 14c is
immediately followed by intramolecular pericyclic reactions
with participation of the allylic π-system. Sulfonium ylide 12c
rearranges by the expected [2,3]-sigmatropic rearrangement
Figure 1: Proposed relative configurations of the diastereomeric cyclic
sulfonium ylides 12aA and 12aB. 1H NMR shifts δH (ppm) and NOE
relationships are shown (E = CO2Et).
Table 1: 1H NMR data for isomers A (major) and B (minor) of cyclic sulfonium ylide 12a (DMSO-d6, 400.13 MHz).
Assignment
Isomer A
Isomer B
δ (ppm), multiplicity, J (Hz)a
δ (ppm), multiplicity, J (Hz)a
OCH2CH3
SCH3
1.13, t, J = 7.1
1.14, t, J = 7.1
2.80, s
2.83, s
SCHeq b
SCHax b
SCH2CHeq
SCH2CHax
NCHCO
OCH2
3.41–3.46, m, 2J = 14.1, 3J = 2.8, 2.8
3.77–3.83, m, 2J ≈ 3J ≈ 14.1, 3J = 2.7
2.35–2.41 dddd, 2J = 14.1, 3J = 5.7, 2.8, 2.8
3.05, dddd, 2J ≈ 3J ≈ 14.1, 3J = 12.7, 3J = 2.9
4.65, dd, 3J = 12.7, 5.7
3.41–3.46, ddd, 2J = 13.3, 3J = 9.7, 4.1
3.77–3.83, ddd, 2J = 13.3, 3J = 7.3, 4.3
2.57–2.72, m
2.57–2.72, m
4.75, dd, 3J = 9.1, 5.6
3.94–4.07, m (both isomers)
7.80–7.96, broadened “s” (both isomers)
CHaryl
aCoupling constants are given without sign.
bThe multiplet signals of SCHeq and SCHax of the two isomers coincided completely. A magnetisation transfer experiment (TOCSY) allowed visualisa-
tion of the individual signal patterns.
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Beilstein J. Org. Chem. 2012, 8, 433–440.
ylide with a nonactivated but suitably positioned olefinic bond
have been known for some time [3,27-35] and can be used for
the construction of multicyclic molecular frameworks (see also
those carbonyl ylides that are part of an isomünchnone ring
system [36-40]). In almost all cases encountered so far, the
olefinic dipolarophile was tethered directly to a terminus of the
carbonyl ylide dipole, and therefore the cycloaddition step gave
rise to an annelated bicyclic substructure. In contrast, in our
case as well as in [29], the olefinic dipolarophile is found as a
substituent at a remote ring position of the cyclic carbonyl
ylide, such that the intramolecular 1,3-dipolar cycloaddition
generates a bridged oxabicyclo[3.2.1]octane substructure.
Trapping of carbonyl ylides with dipo-
larophiles
In situ generated carbonyl ylides can be intercepted not only in
intramolecular (as discussed above) but also in intermolecular
[3 + 2] cycloaddition reactions (see, for example, [3,18-
20,41,42]). By analogy with other 2-diazo-3-oxo-4-phthalimi-
docarboxylic esters [18,19], treatment of diazoester 8a with
Scheme 5: Endo transition state for [3 + 3]-dimerisation of carbonyl
ylide 14.
[3,10,16] to form tetrahydrothiophene 16 (31% yield, two catalytic Rh2(OAc)4 in the presence of an equimolar amount of
diastereomers), and carbonyl ylide 14c is trapped by an N-phenylmaleimide (NPI) or dimethyl acetylenedicarboxylate
intramolecular [3 + 2]-cycloaddition to give the pentacyclic (DMAD) produced the cycloaddition products 18 and 19, res-
compound 17 (35% yield). The regioselectivity of the latter pectively (Scheme 7). The signal sets of sulfonium ylides 12aA
reaction was established by NMR studies: after assignment of and 12aB were absent from the 1H NMR spectra of the crude
proton resonances from HMBC spectra, a NOESY NMR product mixture. For adduct 18, only one diastereomer was
experiment indicated the correlation shown in Scheme 6. obtained (38% yield after workup), which, according to the
Intramolecular [3 + 2]-cycloaddition reactions of a carbonyl observed NOE effects, has the pyrrolidinedione ring and the
Scheme 6: Rh(II)-catalysed carbenoid reactions of diazoester 8c.
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Beilstein J. Org. Chem. 2012, 8, 433–440.
Rhodium-catalysed carbenoid reactions of
other mercapto-functionalised diazoesters
With respect to sulfonium ylide formation, aminoacid-derived
diazoesters 8a–c behave like the analogous mercapto-function-
alised diazoesters that are devoid of the phthalimido group
(Scheme 8). Thus, rhodium(II)-catalysed decomposition of
2-diazo-6-methylthio-3-oxohexanoate 11a gave the stable six-
ring sulfonium ylide 20 in practically quantitative yield. It has
recently been reported that ruthenium(II) porphyrins are suit-
able catalysts for carbenoid sulfonium ylide formation as
well [43]. In continuation of our comparative
studies of dirhodium(II,II) tetracarboxylate and tetracar-
bonyldiruthenium(I,I) complexes [44], we found that the
saccharinato complex [Ru2(CO)5[μ-sac)2]2 [45], the acetato
complex [Ru2(CO)4(μ-OAc)2]n and the trinuclear ruthenium(0)
complex Ru3(CO)12 (3 mol % of catalyst in each case) gave
yields of 74, 69, and 49% for the conversion of 11a to 20.
The rhodium-catalysed formation of a stable five-ring sulfo-
nium ylide from the methyl ester analogue of diazoester 11b has
previously been reported [15]. We confirm these results with
11b; however, in addition to sulfonium ylide 21 we also
isolated the dienol 22 as a minor byproduct. The latter com-
pound likely results from a thermally induced ring-opening
β-elimination reaction of ylide 21, as was previously reported
Scheme 7: Tandem cyclisation/intermolecular cycloaddition of
diazoester 8a. Conditions: (a) Rh2(OAc)4 (3 mol %), N-phenyl-
maleimide or DMAD (1 equiv), benzene, 80 °C, 4 h.
(CH2)2SCH3 chain in exo positions. This means that the cyclo- for other cyclic [14] and acyclic [46] sulfonium ylides.
addition has gone through an exo transition state with the dipo-
larophile approaching the carbonyl ylide from the face opposite Conclusion
to the sulfur-containing alkyl chain. In contrast, the NPI adducts Metal-carbene intermediates derived from α-diazoesters and
of analogous carbonyl ylides derived from norleucine and α-diazo-β-ketoesters are known for their high reactivity towards
alanine (14, R = n-C4H9 and CH3, respectively), were obtained a range of functional groups. It is therefore interesting to gather
as two diastereomers (the NCHR epimers) in approximately information about the chemoselectivity of these reactive inter-
equal ratio [18]. The DMAD adduct 19 was also formed as a mediates. For intramolecular reactions, the ring size of the prod-
2:1 mixture of two diastereomers; unfortunately, the highly ucts can make an additional contribution to the observed
viscous oil obtained could not be purified completely.
chemoselectivity. In this study, we have addressed the competi-
Scheme 8: Carbenoid formation of sulfonium ylides from diazoesters 11a,b. Conditions: (a) Rh2(OAc)4 (3 mol %), benzene, 80 °C, 2 h (11a) or 30
min (11b).
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Beilstein J. Org. Chem. 2012, 8, 433–440.
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namely the formation of cyclic sulfonium and carbonyl ylides.
To this end, 2-diazo-3-oxo-4-phthalimidocarboxylic esters 8a–c
were prepared from the mercapto-functionalised aminoacids
methionine, S-benzylcysteine, and S-allylcysteine. Rhodium(II)-
catalysed decomposition of the diazoesters was found to
produce both six- or five-membered cyclic sulfonium ylides and
six-membered carbonyl ylides. In a qualitative manner, it can be
stated that in the case of 8a formation of the six-membered
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the six-membered sulfonium ylide 12a, although the struc-
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