Asymmetric 1,3-dipolar cycloadditions of 2-diazocyclohexane-1,3-diones and alkyl diazopyruvates

Article abstract of DOI:10.1002/hlca.200590071

The 1,3-dipolar cycloaddition reactions of 2-diazocyclohexane-1,3-dione (7a; Table 1) and of alkyl diazopyruvates (11a-e; Table 3) to 2,3-dihydrofuran and other enol ethers have been investigated in the presence of chiral transition metal catalysts. With Rh^II catalysts, the cycloadditions were not enantioselective, but those catalyzed by [Ru^IICl ₂(1a)] and [Ru^IICl₂(1b)] proceeded with enantioselectivities of up to 58% and 74% ee, respectively, when diazopyruvates 11 were used as substrates. The phenyliodonium ylide 7c yielded the adduct 8a in lower yield and poorer selectivity than the corresponding diazo precursor 7a (Table2) upon decomposition with [Ru(pybox)] catalysts. This suggests that ylide decomposition by Ru^II catalysts, contrary to that of the corresponding diazo precursors, does not lead to Ru-carbene complexes as reactive intermediates. Our method represents the first reproducible, enantioselective 1,3-cycloaddition of these types of substrates.

Full text of DOI:10.1002/hlca.200590071

1010
Helvetica Chimica Acta ± Vol. 88 (2005)
Asymmetric 1,3-Dipolar Cycloadditions of 2-Diazocyclohexane-1,3-diones
and Alkyl Diazopyruvates
by Paul Müller* and Sabrina Chappellet
Department of Organic Chemistry, University of Geneva, 30, Quai Ernest-Ansermet, CH-1211 Geneva 4
(phone: ‡ 41-22-379 6527; fax: ‡ 41-22-379 3215; e-mail: paul.muller@chiorg.unige.ch)
The 1,3-dipolar cycloaddition reactions of 2-diazocyclohexane-1,3-dione (7a; Table 1) and of alkyl
diazopyruvates (11a ± e; Table 3) to 2,3-dihydrofuran and other enol ethers have been investigated in the
presence of chiral transition metal catalysts. With RhII catalysts, the cycloadditions were not enantioselective,
but those catalyzed by [RuIICl2(1a)] and [RuIICl2(1b)] proceeded with enantioselectivities of up to 58% and
74% ee, respectively, when diazopyruvates 11 were used as substrates. The phenyliodonium ylide 7c yielded the
adduct 8a in lower yield and poorer selectivity than the corresponding diazo precursor 7a (Table 2) upon
decomposition with [Ru(pybox)] catalysts. This suggests that ylide decomposition by RuII catalysts, contrary to
that of the corresponding diazo precursors, does not lead to Ru-carbene complexes as reactive intermediates.
Our method represents the first reproducible, enantioselective 1,3-cycloaddition of these types of substrates.
Introduction. ± The transition-metal-catalyzed decomposition of diazo compounds
carrying electron-accepting substituents differs significantly from that of simple diazo
esters or diazo ketones. Thus, ethyl diazoacetoacetate (ˆethyl 4-diazo-3-oxobuta-
noate) [1], ethyl diazopyruvate (ˆethyl 3-diazo-2-oxopropanoate) [2][3] or 2-
diazocyclohexane-1,3-diones [4] may react with olefins by formal 1,3-cycloadditions
to afford dihydrofuran derivatives. The corresponding phenyliodonium ylides undergo
the same 1,3-cycloadditions when exposed to transition metal catalysts [3][5]. Such
cycloadditions occur typically with enol ethers, enol acetates, furans, and other
polarizable olefins, but may also be observed with acetylenes [6][7], and even with
styrene [8]. There is evidence that, in some cases, the cycloadditions are concerted
[6][9]; however, a two-step mechanism involving an intermediate cyclopropane or
cyclopropene, or a zwitterionic intermediate, is also conceivable.
The enantioselectivity of these reactions is controversial. The first examples of
enantioselective cycloadditions of 2-diazodimedone (ˆ2-diazo-5,5-dimethylcyclohex-
ane-1,3-dione) to furan and dihydrofuran were reported in 1992 [10] and 1997 [11].
However, these initial results have not been confirmed.
Recently, we have re-investigated the cycloaddition of 2-diazocyclohexane-1,3-
diones under the conditions described in the literature [10][11], but were unable to
reproduce these results. In addition, only very modest enantioselectivities were
observed with a large selection of structurally different, chiral RhII catalysts [6][8]. This
failure was tentatively attributed to the highly electrophilic nature of the intermediate
metallocarbene, which is a consequence of the presence of the additional electron-
withdrawing carbonyl substituent of the carbene. It is known that carbenoid reactions
of diazoacetate esters carrying electron-donating substituents exhibit remarkably
enhanced enantioselectivities in comparison to those of unsubstituted diazoacetates or
ꢀ 2005 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta ± Vol. 88 (2005)
1011
to diazoacetates with electron-accepting substituents. The Hammett reaction constant 1
for the [RhI2I{(S)-dosp}4] catalyzed cyclopropanation of substituted styrenes with 4-
methoxyphenyl diazoacetate is À 1.3, while those for the corresponding phenyl and
vinyl esters are À 1.01). In contrast, 2-diazomalonate and diazoacetate esters have 1
constants of À 0.3 and ca. 0. The carbenes having high negative 1 values exhibit in
general high enantioselectivities, while those with low negative 1 values react with low
enantioselectivity [12]. In application of the reactivity/selectivity principle [13], higher
negative 1 values of more-stabilized carbenes have been attributed to a later transition
state for carbene transfer, and to a later transition state corresponds a higher selectivity.
Unfortunately, these more-enantioselective carbenes do not, in general, undergo 1,3-
dipolar cycloadditions.
In view of this, we turned our attention to Ru catalysts, which have been found to
catalyze the cyclopropanation of certain olefins effectively, with high levels of
asymmetric induction [14][15]. Actually, Ru has, besides Cu and Rh, emerged as the
third important metal for carbenoid decomposition of diazo compounds [16]. In
addition, Ru catalysts have recently been applied successfully to asymmetric nitrene
transfer reactions [17]. We were particularly struck by the high 1 value of À 2.5 for the
‡
cyclopropanation of substituted styrenes with the catalyst [RuIICl{pnnp}] reported by
Bachmann and Mezzetti2) [18]. This high negative reaction constant suggests that the
Ru-carbene complexes might be more selective than their Rh analogues. In previous
work, we had made the observation that the Ru-catalyzed diazo decomposition
produced indeed metallocarbenes of rather low reactivity [19]. It, therefore, appeared
plausible that the association of a highly reactive carbene with a Ru catalyst might
produce a more-selective, yet sufficiently reactive, metallocarbene to effect the desired
enantioselective cycloadditions of 2-diazocyclohexane-1,3-diones. On the other hand,
we were aware that the reactivity/selectivity principle is controversial [20], and that
extrapolations from Rh to Ru might be dangerous.
In this paper, we present our results concerning the asymmetric 1,3-dipolar
cycloaddition of 2-diazocyclohexane-1,3-diones and diazopyruvate esters with olefins
in the presence of various Ru catalysts containing the ligands 1 ± 6 (Figure).
Results and Discussion. ± 1. Ruthenium-Catalyzed Cycloaddition of 2-Diazodime-
done to 2,3-Dihydrofuran. The diazo decomposition of 2-diazodimedone (7a) by
5 mol-% of the [Ru(pybox)] complex3)of Nishiyama et al. [15], which was generated in
situ from [Ru(p-cymene)Cl2]2 and the chiral ligand 1b, proceeded very sluggishly in
toluene at room temperature. In the presence of 2,3-dihydrofuran, the cycloadduct 8a
was isolated after 3 d in 70 % yield (Table 1). No cyclopropane was detected in the
mixture. The reaction time could be reduced to 16 h when 10 mol-% of catalyst was
used. The enantioselectivity (in terms of enantiomeric excess; ee) varied from 55 ± 57%
ee at 258. Increasing the temperature to 458 shortened the reaction time to 17 h (with
5 mol-% of catalyst), but slightly lowered the enantioselectivity (52% ee; Table 1). At
1
)
)
)
dosp ˆ (S)-[N-(4-Dodecylphenyl)sulfonyl]prolinate.
2
3
pnnp ˆ N,N'-Bis[2-(diphenylphosphanyl)benzylidene]-1,2-diaminocyclohexane.
The term pybox stands for 2,6-bis(4,5-dihydro-1,3-oxazol-2-yl)pyridine.

1012
Helvetica Chimica Acta ± Vol. 88 (2005)
Figure. Ligands 1 ± 6 used in Ru catalyzed 1,3-dipolar cycloadditions. The values in parentheses refer to yield/
enantiomeric excess (%) of 8a in the reaction between 7a and 2,3-dihydrofuran (see Table 1 below).
a temperature of 08, basically no reaction took place, and the enantioselectivity was not
increased.
Neither addition of silver salts (AgPF6, AgBF4) nor other solvents resulted in any
improvement of the above transformation with ligand 1b. With the pybox ligands 1a
and 1c, similar results were obtained with respect to yield and selectivity as with 1b (see
Figure). The pnnp ligand [14] was also examined, but was found to be ineffective.
Several other ligands were tested, but no significant enantioselectivities were achieved.
In contrast, the attempted cycloadditions of several structurally related 2-diazo-1,3-
diones produced unsatisfactory results: methyl diazoacetoacetate, 2-diazocyclopen-
tane-1,3-dione, and 3-diazo-2H-chromene-2,4-dione were unreactive towards the
[Ru(pybox)] catalyst [RuCl2(1b)] (data not shown). 2-Diazocyclohexane-1,3-dione
(7b), in turn, reacted with 2,3-dihydrofuran with significantly lower yield and
selectivity than 7a (Scheme 1). The cycloaddition of the diazolactone 9 to 10 proceeded

Helvetica Chimica Acta ± Vol. 88 (2005)
1013
Table 1. Cycloaddition of Diazotized Dimedone 7a to 2,3-Dihydrofuran in the Presence of the [Ru(pybox)]
Catalyst [RuCl2(1b)]
Catalyst
[mol-%]
Solvent
Time
[h]
T
[8]
Yield
[%]
ee
[%]
5
5
10
5
5
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhCH3
HMIb)
THF
16
18
16
72
17
72
18
18
16
16
16
48
25
25
25
25
45
0
25
25
25
25
25
25
22
40
72
70
76
< 5
30
16
40
15
25
0
57
57
55
56
52
50
57
48
56
56
55
±
10a)
10a)
10
10
10
10c)
PhCH3
a) With AgPF6 as additive. b) 1-Hexyl-3-methylimidazolium hexafluorophosphate. c) With [RuCl(3a)](PF6).
in both low yield and selectivity in the presence of ligand 4a. No significant
enantioselectivities resulted upon cycloaddition of 7a to 3,4-dihydro-2H-pyran and
vinyl acetate (Scheme 1), and no diazodecomposition of 7a occurred in the presence of
furan, ethyl vinyl ether, and tert-butyl vinyl ether.
2. Ruthenium-Catalyzed Cycloaddition with a Phenyliodonium Ylide Derived from
Dimedone. In the case of RhII and CuI catalysts, the enantioselectivity for carbene
transfer with diazo compounds is identical to that observed with the corresponding
phenyliodonium ylides, although there are exceptions owing to occurrence of
secondary reaction pathways [21]. This suggests that the reactions proceed via the
same reactive intermediate, i.e., the Rh- or Cu-complexed carbene. Since phenyl-
iodonium ylides usually decompose under milder conditions than the corresponding
diazo compounds, we have examined the suitability of the ylide 7c as reagent for the
Ru-catalyzed cycloadditions, in the hope of being able to carry out the reaction at lower
temperature, thereby increasing the selectivity.
A series of cycloadditions to 2,3-dihydrofuran were carried out with 7c and catalyst
[RuCl2(1b)] (Table 2). However, the yield of 8a was disappointing and, in addition, the
enantioselectivity was significantly below that obtained with the diazo precursor 7a (see
Table 1). Again, some other ligands were tested, but the resulting enantioselectivities
were insignificant in all cases.
The poor yields indicate that the Ru catalysts were not sufficiently reactive to
decompose the ylide efficiently to afford a Ru-carbene complex. Since the enantio-

1014
Helvetica Chimica Acta ± Vol. 88 (2005)
Scheme 1
selectivity with ylide 7c is different from that of the diazo precursor 7a for a given
ligand, its reaction may not proceed through the same reactive intermediate. While for
the Ru-catalyzed diazo decomposition a metallocarbene intermediate is plausible [14 ±
16], the intermediate resulting from decomposition of 7c with the [Ru(pybox)]
catalysts must be a different species, presumably 7c associated to the Ru complex.
Apparently, this intermediate is less sensitive to the presence of the chiral ligands than
the (hypothetical) metallocarbene that results from decomposition of the diazo
precursor 7a. This contrasts with the situation observed in RhII and CuI catalyzed
reactions, where decomposition of the ylide and the corresponding diazo precursor
proceeds via the same intermediary metal-carbene complex.
3. Enantioselective Cycloadditions of Alkyl Diazopyruvates. In view of the low
reactivity of 2-diazodimedone (7a) in Ru-catalyzed carbene transfer, and in view of the
inability of Ru catalysts to decompose phenyliodonium ylides such as 7c efficiently, we
turned our attention to alkyl diazopyruvates of type 11 as substrates. It is well-known
that the diazo decomposition of precursors having a single substituent on the diazo C-
atom occurs under milder conditions than that of their disubstituted analogues [22].
Although diazopyruvates react with nonpolar olefins via cyclopropanation, they are
known to undergo formal 1,3-dipolar cycloadditions to polarizable olefins such as 2,3-
dihydrofuran [1][2].

Helvetica Chimica Acta ± Vol. 88 (2005)
1015
Table 2. Ru Catalyzed Cycloaddition of Phenyliodonium Ylide 7c to 2,3-Dihydrofuran. Conditions: catalyst,
10 mol-equiv.; 2,3-dihydropyran, 30-fold excess; ligand, three-fold excess rel. to Ru; in toluene at 208.
Catalyst
Time [h]
Yield [%]
ee [%]
[Ru(p-cymene)Cl2]2
[RuCl2(1b)]
[RuCl2(1b)]
[RuCl2(1b)]a)
[RuCl2(1b)]
[RuCl2(1b)]b)
[RuCl2(1b)]c)
[RuCl2(1b)]c)
[RuCl2(1f)]
[RuCl2(1g)]
[RuCl2(1e)]
[RuCl2(1k)]
[RuCl2(2)]
24
24
72
16
48
48
48
48
72
72
72
72
72
72
72
72
54
35
75
70
55
40
55
52
35
26
10
35
34
30
25
28
±
15
17
5
0
0
11
12
0
0
0
0
0
0
0
[RuCl2(3a)]
[RuCl2(3b)]
[RuCl2(6)]
12
c
a) Reaction conducted at 508. b) In fluorobenzene. ) With Ag PFÀ6 as an additive (1.5-fold excess rel. to
‡
catalyst).
Ethyl diazopyruvate (11a) was prepared according to a literature procedure (see
Exper. Part) [23]. The cycloaddition of 11a to 2,3-dihydropyran was first examined with
RhII catalysts known to be efficient for diazo decomposition (Table 3). This reaction
gave rise to the cycloadduct 12a in acceptable yields (70 ± 85%), but without significant
enantioselectivity. No cyclopropane was detected in the reaction mixture. The same
result was obtained with several CuI-based catalysts [24]. However, the diazo
decomposition of 11a proceeded smoothly at room temperature with different RuII
catalysts, giving rise to 12a ± e in medium-to-good yields within 15 h (Table 3), as
described earlier in a preliminary communication [25].
The cycloaddition of 11a to 2,3-dihydrofuran was optimized with the i-Pr
substituted pybox ligand 1b, giving rise to the catalyst [RuCl2(1b)]. The best results
were obtained in toluene, with 68% ee at room temperature. Upon lowering the
temperature to 08, 12a was obtained in 74% ee. Variation of the ligand substituents R at
the oxazolidine rings had a significant effect on the yield and ee value of the adduct. All
ligands, except 1d and 1e, were efficient in diazo decomposition, and afforded 12a in
yields of 40 ± 67%. When ligand 1e, with its bulky Ph groups, was added to [RuCl2(p-
cymene)]2, a color change was observed indicating formation of the corresponding Ru
catalyst. However, no reaction occurred, even after prolonged reaction times or raising

1016
Helvetica Chimica Acta ± Vol. 88 (2005)
Table 3. Optimization of the Cycloaddition of Diazopyruvates 11 to 2,3-Dihydrofuran. Conditions: catalyst,
10 mol-equiv.; 2,3-dihydropyran, 30-fold excess; ligand, threefold excess rel. to Ru; temperature, 258.
Product
Catalyst
Solvent
Yield [%]
ee [%]
12a
[Rh2(OAc)4]
[Rh2{(S)-tbsp}4]
[Rh2{(S)-ntv}4]
[RuCl2(1a)]
[RuCl2(1b)]
[RuCl2(1b)]
[RuCl2(1b)]a)
[RuCl2(1b)]
[RuCl2(1b)]
[RuCl2 (1b)]
[RuCl2(1d)]
[RuCl2(1e)]
[RuCl2(1f)]
[RuCl2(1g)]
[RuCl2(1h)]
[RuCl2(1i)]
PhCH3
PhCH3
PhCH3
PhCF3
Neat
PhCH3
PhCH3
CH2Cl2
C6H14
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
85
70
80
50
65
65
68
55
20
36
40
0
50
63
67
60
50
0
0
0
12
58
62
68
74
56
66
26
< 5
±
22
36
47
46
11
±
[RuCl2 (1j)]
[RuCl(3a)](PF6)
[RuCl2 (1b)]
[RuCl2 (1b)]
[RuCl2 (1b)]
[RuCl2 (1b)]
12b
12c
12d
12e
55
50
40
80
70
65
65
59
a) Reaction conducted at 08.
the temperature. This is most likely due to steric hindrance. In the case of the t-Bu-
substituted 1d, no color change was observed at all when the ligand was added to
[RuCl2(p-cymene)]2; apparently, the complex did not form. The negligible ee value of
< 5% indicated that diazo decomposition occurred with unreacted achiral [RuCl2(p-
cymene)]2.
The enantioselectivity of the above cycloaddition varied from 11 to 74% ee within
the series of Ru catalysts tested, the highest ee value (74%) occurring with ligand 1b
(R ˆ i-Pr). Sterically less-demanding ligands such as 1a (R ˆ Me) or 1f (R ˆ
C6H11CH2) afforded lower enantioselectivities (26 and 22% ee, resp.), while the
bulkier ligands resulted in a less-reactive (for 1c) or completely unreactive catalyst (for
1d). The larger, disubstituted pybox ligands 1h ± 1j, in turn, were found to be less
effective than 1b with respect to enantioselectivity. No cycloadduct was obtained with
‡
the [RuCl2{pnnp}] catalyst of Bachmann and Mezzetti [18]. The use of other alkyl
pyruvates 11b ± 11e had no significant effect on both the yield and enantioselectivity of
the reaction.

Helvetica Chimica Acta ± Vol. 88 (2005)
1017
The cycloaddition of 11a to ethyl vinyl ether in the presence of [RuCl2(1b)]
proceeded, by analogy, to compound 13 in 78% yield (Scheme 2), but with lower
enantioselectivity. Both yield and enantioselectivity dropped further with the more-
hindered tert-butyl vinyl ether, which gave rise to adduct 14. Finally, cycloaddition to
3,4-dihydro-2H-pyran afforded 15 [26] in lower yield and selectivity than in the case of
2,3-dihydrofuran (Scheme 2). However, no diazo decomposition of 11a occurred in the
presence of furan or vinyl acetate as substrates.
Scheme 2
4. Enantioselective Cycloaddition of Ethyl 3-Diazo-3-(2-nitrophenyl)pyruvate and
2,3-Dihydrofuran. Preliminary results have shown that ethyl 3-diazo-3-(2-nitrophen-
yl)pyruvate (16), which can be readily prepared from 17, undergoes cycloaddition to
2,3-dihydrofuran upon diazo decomposition with RhII and RuII catalysts to 18
(Scheme 3). The regioselectivity of the reaction was derived from the 1H-NMR
chemical shift of the single acetal H-atom of 18, which resonates at d(H) 5.59,
comparable to the corresponding H-atom in 12a (d(H) 5.84). Whereas 18 obtained
with the chiral RhII catalyst [Rh2{(S)-nttl}4]4) was racemic, a low degree of chiral
induction (5%) was obtained with [RuCl2(1b)].
Conclusions. ± To our knowledge, our findings represent the first reproducible,
enantioselective 1,3-dipolar cycloadditions of 2-diazocyclohexane-1,3-diones and alkyl
diazopyruvates to enol ethers. The enantioselectivity of the reactions are not yet
satisfactory, but with the introduction of novel RuII-based catalysts, this problem might
eventually be overcome. It was not possible, in the context of this project, to determine
the absolute configurations of the adducts. However, we think that this project is
sufficiently advanced and promising to be further elaborated in the future.
4
)
(S)-nttl ˆ (S)-N-1,8-naphthoyl-t-leucinate.

1018
Helvetica Chimica Acta ± Vol. 88 (2005)
Scheme 3
This work was supported by the Swiss National Science Foundation (projects No. 20-52581.97 and 2027-
048156). The support and sponsorship conceded by COST Action D24 (ꢁSustainable Chemical Processes:
Stereoselective Transition Metal Catalyzed Reactionsꢂ) are kindly acknowledged. The authors are indebted to
Dr. A. Mezzetti (ETHZurich) for clarifying discussions and for samples of the pnnp ligands, to J.-P. Saulnier and
A. Pinto for recording NMR spectra, and to A. Klink for recording mass spectra.
Experimental Part
1. General: see [27]. The following chiral ligands and catalysts were synthesized according to published
procedures: 1a [28]; 1b, 1d, 1e [29]; 1c [30]; 1i ± k and 1f [31]; 1g [32]; 1h [33]; 2 [34]; 3a and 6 [35]; 3b ± d [36];
3e [37]; 4a,b [38]; [Rh2{(S)-nttl}4] [39]; [Rh2{(S)-ntv}4] [40]; [Rh2{(S)-tbsp}4] [41]. Ligand 5 is commercially
available. All reactions were carried out under inert Ar atmosphere. Toluene, CH2Cl2, and hexane were dried by
passage through solvent-purification columns. 2,3-Dihydrofuran and (trifluoromethyl)benzene were distilled
before use. Flash chromatography (FC): silica gel 0.32 ± 0.636 mm (Merck). The enantiomeric excess (ee) was
determined by chiral GC on a Supelco b-dex column under isothermal conditions, or by chiral HPLC; retention
or elution times t in min. NMR Spectra were recorded on a Bruker AMX-300 spectrometer; d in ppm, J in Hz.
EI- and ESI-MS: Varian CH4 or SM1 spectrometers; HR-MS: VG-7070 analytical spectrometer; in m/z (rel.
%).
2. Cycloadditions of Compounds 7a, 7b,and 9. 2.1. Cycloaddition of 7a to 2,3-Dihydrofuran (see Table 1).
The ligand 2,6-bis[(S)-4,5-dihydro-4-isopropyloxazol-2-yl]pyridine (1b; 72 mg, 0.24 mmol) in CH2Cl2 (1.0 ml)
was added to a soln. of [RuCl2(p-cymene)]2 (36 mg, 0.06 mmol) in CH2Cl2 (1.0 ml). The dark red mixture was
stirred at r.t. for 1 h under Ar. The solvent was removed under reduced pressure, and the solid residue was taken
up in 2,3-dihydrofuran (1.8 ml). Compound 7a (100 mg, 0.6 mmol) [6] and toluene (3.0 ml) were added. After
16 h, the mixture was concentrated, and the residue was purified by FC (SiO2; AcOEt/pentane 60 :40) to afford
2,3,3a,4,5,6,7,8a-Octahydro-6,6-dimethylfuro[2,3-b][1]benzofuran-4-one (8a; 90 mg, 72%). Pale yellow solid.
M.p. 988. GC (Tˆ 1408): t1 ˆ 40.2, t2 ˆ 41.8. [a]D20 ˆ À124 for 57% ee (c ˆ 1.0, CHCl3). For spectroscopic data,
see [6].
2.2. 2,3,3a,4,5,6,7,8a-Octahydrofuro[2,3-b][1]benzofuran-4-one (8b) [6]. As described in Sect. 2.1, but from
7b and 2,3-dihydrofuran. Purification by FC (SiO2, AcOEt/pentane 60 :40) afforded 8b in 45% yield. Yellow
solid. [a]D20 ˆ À24 for 26% ee (c ˆ 1.1, CHCl3). GC (Tˆ 1508): t1 ˆ 26.0, t2 ˆ 27.6. For spectroscopic data, see
[6].
2.3. 2,3,3a,6,7,8a-Hexahydro-6,6-dimethylfuro[3',2':4,5]furo[3,2-c]pyran-4(4H)-one (10) [8]. As described
in Sect. 2.1, but from 9 and 2,3-dihydrofuran. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 10 in
45% yield. Yellow solid. M.p. 1258. GC (Tˆ 1458): t1 ˆ 67.1, t2 ˆ 69.4. [a]D20 ˆ À50 for 26% ee (c ˆ 1.2, CHCl3).
For spectroscopic data, see [8].
2.4. 3,4,4a,5,6,7,8,9a-Octahydro-7,7-dimethylpyrano[2,3-b][1]benzofuran-5(2H)-one (19) [5]. As described
in Sect. 2.1, but from 7a and 3,4-dihydro-2H-pyran. Purification by FC (SiO2, AcOEt/pentane 60 :40) afforded
19 in 25% yield. Yellow solid. M.p. 958. GC (Tˆ 1408): t1 ˆ 63.9, t2 ˆ 65.7. IR: 2960, 1640, 1220. 1H-NMR

Helvetica Chimica Acta ± Vol. 88 (2005)
1019
(300 MHz, CDCl3): 6.05 (d, J ˆ 7.5, 1 H); 3.80 (m, 1 H); 3.15 (m, 1 H); 2.40 (m, 2 H); 2.20 (m, 2 H); 1.95 (m,
2 H); 1.55 (m, 2 H); 1.17 (s, 6 H ).13C-NMR (75 MHz, CDCl3): 189.4 (s); 169.0 (s); 110.8 (s); 94.4 (d); 79.5 (s);
‡
65.3 (t); 51.5 (t); 44.1 (t); 40.3 (d); 29.9 (s); 27.5 (q); 27.2 (q); 22.5 (t); 19.5 (t). MS: 222 (100, M ).
2.5. 2-Acetoxy-2,3,4,5,6,7-hexahydro-6,6-dimethylbenzofuran-4-one (20) [42]. As described in Sect. 2.1, but
from 7a and vinyl acetate. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 20 in 20% yield. Yellow
solid. M.p. 908. GC (Tˆ 1408): t1 ˆ 50.3, t2 ˆ 50.9. IR: 2960, 1750, 1640, 1220. 1H-NMR (300 MHz, CDCl3): 6.58
(dd, J ˆ 5.5, 1 H); 3.08 (m, 1 H); 2.81 (m, 1 H); 2.41 (m, 1 H); 2.35 (m, 1 H); 2.29 (m, 2 H); 2.15 (s, 3 H); 1.27 (s,
6 H ).13C-NMR (75 MHz, CDCl3): 190.4 (s); 175.3 (s); 170.8 (s); 111.4 (s), 99.4 (d); 51.5 (t); 37.3 (t); 34.5 (t);
‡
32.1 (s); 29.5 (q); 28.4 (q); 21.2 (q). MS: 255.22 (100, M ).
3. Syntheses of Alkyl 3-Diazopyruvates 11. 3.1. Ethyl 3-Diazo-2-oxopropanoate (11a) [23]. To a mixture of
ethyl chloro(oxo)acetate (1.35 g, 9.9 mmol) in THF (20 ml) at 08 was added dropwise (trimethylsilyl)diazo-
methane (15 ml; 2m soln. in hexane). The mixture was stirred at r.t. for 3 h, the solvent was removed, and the
product was purified by FC (SiO2; AcOEt/pentane 30 :70 ! 50 :50) to afford 11a (900 mg, 70%). Pale yellow
solid. M.p. 758. IR: 3050, 2960, 2140, 1727, 1625. 1H-NMR (300 MHz, CDCl3): 6.20 (s, 1 H); 4.35 (q, J ˆ 7.1, 2 H) ;
1.40 (t, J ˆ 7.1, 3 H) . 13C-NMR (75 MHz, CDCl3): 176.9 (s); 160.4 (s); 62.9 (t); 56.9 (d); 14.0 (q). HR-MS:
‡
‡
143.0857 ([M ‡ 1] , C5H7N2O3 ; calc. 143.0457).
3.2. General Procedure for the Preparation of 11b ± 11d. To oxalyl chloride (ˆethanedioyl dichloride) was
added the appropriate alcohol (1 equiv.) in CH2Cl2 at 08 over 30 min. The mixture was stirred at r.t. for 1.5 h. The
solvent and residual oxalylchloride were removed. The resulting oil was dissolved in THF (20 ml), and
(trimethylsilyl)diazomethane (3 equiv; 2m soln. in hexane) was added dropwise. After stirring for 3 h at r.t., the
mixture was concentrated, and the residue was purified by FC (SiO2; AcOEt/pentane 70 :30).
Methyl 3-Diazo-2-oxopropanoate (11b) [43]. Yield 75%. M.p. 958. IR: 3050, 2965, 2142, 1720, 1621.
1H-NMR (300 MHz, CDCl3): 6.18 (s, 1 H); 3.57 (s, 1 H ).13C-NMR (75 MHz, CDCl3): 175.9 (s); 160.4 (s); 56.9
‡
(d); 50.9 (q). HR-MS: 128.0230 (C4H4N2O3 ; calc. 128.0222).
2,2-Dimethylpropyl 3-Diazo-2-oxopropanoate (11c). Yield: 65%. IR: 3047, 2955, 2150, 1720, 1625. 1H-NMR
(300 MHz, CDCl3): 5.98 (s, 1 H); 3.81 (s, 1 H); 0.84 (s, 9 H ).13C-NMR (75 MHz, CDCl3): 176.8 (s); 160.3 (s);
75.85 (t); 57.0 (d); 26.5 (q); 26.4 (q); 26.3 (q).
Phenyl 3-Diazo-2-oxopropanoate (11d) [44]. Yield 40%. M.p. 938. IR: 3320, 3047, 2955, 2130, 1705, 1625.
1H-NMR (300 MHz, CDCl3): 7.35 (m, 2 H); 7.25 (m, 1 H); 7.10 (m, 2 H); 6.15 (s, 1 H ).13C-NMR (75 MHz,
CDCl3): 176.2 (s); 159.3 (s); 150.1 (s); 129.2 (d); 126.5 (d); 121.2 (d); 56.8 (d).
Dicyclohexylmethyl 3-Diazo-2-oxopropanoate (11e). Yield 80%. M.p. 958. IR: 3120, 2945, 2130, 1715, 1615.
1H-NMR (300 MHz, CDCl3): 6.17 (s, 1 H); 3.85 (t, J ˆ 6, 1 H); 2.07 (m, 2 H); 1.77 ± 1.1 (m, 20 H ).13C-NMR
(75 MHz, CDCl3): 176.2 (s); 160.3 (s); 82.1 (d); 56.8 (d); 41.2 (d); 40.9 (d); 28.7 (t); 28.5 (t); 27.6 (t); 26.5 (t); 26.4
‡
‡
(t); 26.2 (t); 26.1 (t); 25.9 (t). HR-MS: 293.1813 ([M ‡ 1] , C16H25N2O3 ; calc. 293.1865).
4. Cycloadditions of Diazopyruvates 11 (see Table 3). 4.1. Reaction of 11a with 2,3-Dihydrofuran. Ligand 1b
(86 mg, 0.294 mmol) in CH2Cl2 (1.0 ml) was added to a soln. of [RuCl2(p-cymene)]2 (58 mg, 0.095 mmol) in
CH2Cl2 (1.0 ml). The dark-red mixture was stirred at r.t. for 1 h under Ar. The solvent was removed under
reduced pressure. Then, 2,3-dihydrofuran (1.4 ml) and 11a (100 mg, 0.95 mmol) dissolved in toluene (3.0 ml)
were added. The resulting suspension was stirred until the starting material disappeared (TLC). The mixture
was concentrated, and the residue was purified by FC (SiO2; AcOEt/pentane 60 :40) to afford ethyl 3a,4,5,6a-
tetrahydrofuro[2,3-b]furan-2-carboxylate (12a). Yield: 104 mg (68%). Pale yellow oil. GC (Tˆ 1408): t1 ˆ 43.9,
t2 ˆ 45.7. [a]D20 ˆ ‡17 for 74% ee (c ˆ 1.12, CHCl3). IR: 2983, 2281, 1737, 1633. 1H-NMR (300 MHz, CDCl3):
6.25 (d, J ˆ 8.0, 1 H); 5.84 (d, J ˆ 3.5, 1 H); 4.32 (q, J ˆ 9.3, 2 H); 4.11 (m, 1 H); 3.78 (m, 2 H); 2.13 (m, 1 H); 1.96
(m, 3 H ).13C-NMR (75 MHz, CDCl3): 159.7 (s); 148.8 (s); 111.7 (d); 110.5 (d); 67.1 (t); 61.4 (t); 47.6 (d); 31.2 (t);
‡
‡
14.2 (q). HR-MS: 184.0718 (M , C9H12O4 ; calc. 184.0736).
4.2. Methyl 3a,4,5,6a-Tetrahydrofuro[2,3-b]furan-2-carboxylate (12b). As described in Sect. 4.1, but with
11b. Purification by FC (SiO2, AcOEt/pentane 60 :40) afforded 12b in 55% yield. Yellow oil. GC (Tˆ 1408):
t1 ˆ 31.1, t2 ˆ 32.2. [a]D20 ˆ ‡17.0 for 70% ee (c ˆ 1.12, CHCl3). IR: 2958, 2880, 1753, 1633. 1H-NMR (300 MHz,
CDCl3): 6.17 (d, J ˆ 8.3, 1 H); 5.83 (d, J ˆ 2.9, 1 H); 4.50 (q, J ˆ 8.5, 2 H); 3.97 (m, 1 H); 3.80 (s, 3 H); 2.04 (m,
1 H); 1.79 (m, 1 H ).13C-NMR (75 MHz, CDCl3): 158.9 (s); 149.5 (s); 111.7 (d); 110.5 (d); 67.0 (t); 52.6 (d); 42.5
‡
‡
(q); 34.3 (t). HR-MS: 170.0622 (M , C8H10O4 ; calc. 170.0579).
4.3. 2,2-Dimethylpropyl 3a,4,5,6a-Tetrahydrofuro[2,3-b]furan-2-carboxylate (12c). As described in Sect. 4.1,
but with 11c. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 12c in 50% yield. Yellow oil. GC
(T ˆ 1408): t1 ˆ 53.2, t2 ˆ 54.8. [a]D20 ˆ ‡16.8 for 65% ee (c ˆ 1.2, CHCl3). IR: 2940, 2870, 2180, 1734, 1615.
1H-NMR (300 MHz, CDCl3): 6.24 (d, J ˆ 8.3, 1 H); 5.78 (d, J ˆ 3.5, 1 H); 4.09 (t, J ˆ 10.8, 1 H); 3.93 (s, 1 H );

1020
Helvetica Chimica Acta ± Vol. 88 (2005)
3.92 (s, 1 H); 3.75 (m, 2 H); 2.12 (m, 1 H); 1.96 (m, 1 H); 0.98 (s, 9 H ).13C-NMR (75 MHz, CDCl3): 159.8 (s),
148.8 (s), 111.3 (d), 110.3 (d), 74.3 (t), 67.2 (t), 47.5 (d), 31.8 (s), 31.2 (t), 26.4 (q).
4.4. Phenyl 3a,4,5,6a-Tetrahydrofuro[2,3-b]furan-2-carboxylate (12d). As described in Sect. 4.1, but with
11d. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 12d in 40% yield. Yellow oil. HPLC (OD-H
column; i-PrOH/hexane 95 :5): t1 ˆ 21.8, t2 ˆ 29.9. [a]D20 ˆ ‡16.4 for 65% ee (c ˆ 1.2, CHCl3). IR: 2920, 2840,
1705, 1598, 870. 1H-NMR (300 MHz, CDCl3): 7.43 (m, 3 H); 7.18 (d, J ˆ 11.8, 2 H); 6.32 (d, J ˆ 8.3, 1 H); 6.06 (d,
J ˆ 3.6, 1 H); 4.14 (m, 1 H); 3.86 (m, 2 H); 2.18 (m, 1 H); 2.04 (m, 1 H ).13C-NMR (75 MHz, CDCl3): 157.9 (s);
150.1 (s); 148.1 (s); 113.6 (d); 110.6 (d); 129.5 (s); 126.1 (s); 121.4 (s); 67.2 (t); 47.8 (d); 31.2 (t).
4.5. Dicyclohexylmethyl 3a,4,5,6a-Tetrahydrofuro[2,3-b]furan-2-carboxylate (12e). As described in Sect.
4.1, but with 11e. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 12e in 87% yield. Yellow solid. M.p.
63 ± 658. HPLC (OD-H column; i-PrOH/hexane 5 :95): t1 ˆ 17.9, t2 ˆ 26.6). [a]D20 ˆ ‡13.6 for 59% ee (c ˆ 0.70,
CHCl3). IR: 2890, 2830, 2150, 1759, 16120. 1H-NMR (300 MHz, CDCl3): 6.26 (d, J ˆ 6.1, 1 H); 5.79 (d, J ˆ 2.8,
1 H); 4.78 (m, 1 H); 4.12 (m, 1 H); 4.10 ± 3.72 (m, 2 H); 2.12 (m, 1 H); 1.96 (m, 1 H); 2.12 ± 1.55 (m, 22 H ).
13C-NMR (75 MHz, CDCl3): 158.8 (s); 147.9 (s); 110.0 (d); 109.3 (d); 82.6 (d); 67.1 (t); 47.6 (d); 37.3 (d); 37.2 (d);
31.2 (t); 29.9 (t); 29.8 (t); 29.6 (t); 27.6 (t); 27.4 (t); 26.4 (t); 26.23 (t); 26.21 (t); 26.1 (t); 26.0 (t). HR-MS: 334.2148
‡
‡
(M , C20H30O4 ; calc. 334.2144).
4.6. Ethyl 5-Ethoxy-4,5-dihydrofuran-2-carboxylate (13). As described in Sect. 4.1, but with 11a and ethyl
vinyl ether. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 13 in 78% yield. Yellow oil. GC (T ˆ
1308): t1 ˆ 56.4, t2 ˆ 57.4. [a]D20 ˆ ‡11.5 for 36% ee (c ˆ 1.1, CDCl3). IR: 2944, 2275, 1732, 1630. 1H-NMR
(300 MHz, CDCl3): 5.88 (dd, J ˆ 2.7, 2.9, 1 H); 5.59 (dd, J ˆ 7.0, 2.5, 1 H); 4.21 (qq, J ˆ 7.0, 1.8, 2 H); 3.86 (qtqt,
J ˆ 7.0, 1 H); 3.53 (qt, J ˆ 7.0, 1 H); 2.90 (dddd, J ˆ 7.0, 18, 2.5 Hz, 1 H); 2.6 (dddd, J ˆ 18, 2.9, 2.9, 1 H); 1.3 (t, J ˆ
7, 3 H) ; 1.2 ( t, J ˆ 7, 3 H) . 13C-NMR (75 MHz, CDCl3): 160.2 (s); 146.6 (s); 110.0 (d); 105.5 (d); 64.2 (t); 61.1 (t);
37.4 (t); 15.0 (q); 14.2 (q).
4.7. Ethyl 5-(tert-Butoxy)-4,5-dihydrofuran-2-carboxylate (14). As described in Sect. 4.1, but with 11a and
tert-butyl vinyl ether. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 14 in 55% yield. Yellow oil. GC
(Tˆ 1408): t1 ˆ 27.5, t2 ˆ 28.2. [a]D20 ˆ ‡12.9 for 22% ee (c ˆ 1.9, CDCl3). IR: 2954, 2235, 1742, 1610. 1H-NMR
(300 MHz, CDCl3): 5.94 (m, 2 H); 4.30 (q, J ˆ 9.3, 2 H); 3.01 (dddd, J ˆ 3.5, 9.8, 24, 1 H); 2.61 (dddd, J ˆ 4, 8, 24,
1 H); 1.34 (m, 12 H ).13C-NMR (75 MHz, CDCl3): 159.5 (s); 147.8 (s); 109.7 (d); 100.6 (d); 60.5 (t); 38.1 (t); 28.8
‡
‡
(q); 14.2 (q). HR-MS: 214.1223 (M , C11H18O4 ; calc. 214.1205).
4.8. Ethyl 3a,5,6,7a-Tetrahydro-4H-furo[2,3-b]pyran-2-carboxylate (15) [25]. As described in Sect. 4.1, but
with 11a and 3,4-dihydro-2H-pyran. Purification by FC (SiO2; AcOEt/pentane 60 :40) afforded 15 in 50% yield.
Yellow oil. GC (Tˆ 1408): t1 ˆ 41.8, t2 ˆ 43.5. [a]2D0 ˆ ‡12.3 for 34% ee (c ˆ 1.3, CDCl3). IR: 2920, 2854, 2180,
1
1732, 1632. H-NMR (300 MHz, CDCl3): 6.00 (d, J ˆ 3.8, 1 H); 5.88 (d, J ˆ 10.0, 1 H); 4.35 (m, 3 H); 3.90 (m,
2 H); 1.95 (m, 2 H); 1.61 (m, 2 H ).13C-NMR (75 MHz, CDCl3): 158.3 (s); 148.6 (s); 112.7 (d); 101.5 (d); 67.1 (t);
62.4 (t); 47.5 (d); 31.2 (t); 28.6 (d); 14.1 (q).
5. Cycloaddition of 16 to 2,3-Dihydrofuran. 5.1. Ethyl 3-Diazo-3-(2-nitrophenyl)-2-oxopropanoate (16). To
4-methylbenzenesulfonic acid (TsOH; 545 mg, 2.8 mmol) in EtOH (100 ml) was added dropwise ꢁ o-nitro-
phenylpyruvic acidꢂ (ˆ 3-(2-nitrophenyl)-2-oxopropanoic acid; 3.00 g, 14 mmol) in EtOH(50 ml). The mixture
was heated at reflux for 12 h. The solvent was removed, and the resulting residue was distilled at 1208/10À2 mm
Hg to afford ethyl 3-(2-nitrophenyl)-2-oxopropanoate. The latter (1.0 g, 4.2 mmol) was dissolved in MeCN
(60 ml) at 08, and 4-(acetylamino)benzenesulfonyl azide (1.00 g, 4.2 mmol) and Et3N (430 mg, 4.25 mmol) were
added. The mixture was stirred for 15 h. The solvent was evaporated, and the residue was purified by FC (SiO2;
AcOEt/pentane 70 :30) to afford 16 (330 mg, 45%). Colorless oil. IR: 3340, 2980, 2854, 1732, 1702, 1598, 1542.
1H-NMR (300 MHz, CDCl3): 8.19 (d, J ˆ 8.2, 1 H); 7.75 (m, 1 H); 7.60 (m, 21 H); 4.40 (m, 2 H); 1.35 (m, 3 H ).
13C-NMR (75 MHz, CDCl3): 189.4 (s); 171.2 (s); 160.6 (s); 133.9 (d); 133.7 (d); 132.6 (d); 125.6 (d); 103.6 (s);
63.4 (t);14.2 (q).
5.2. Ethyl 3a,4,5,6a-Tetrahydro-3-(2-nitrophenyl)furo[2,3-b]furan-2-carboxylate (18). Prepared as de-
scribed in Sect. 4.1, but with 16 and 2,3-dihydrofuran. Purification by FC (SiO2; AcOEt/pentane 60 :40)
afforded 18 in 50% yield. Yellowish solid. M.p. 1108. HPLC (OJ column, EtOH/hexane 10 :90): t1 ˆ 40.6, t2 ˆ
49.6. IR: 2980, 2854, 1732, 1620, 1542. 1H-NMR (300 MHz, CDCl3): 7.99 (dd, J ˆ 1.5, 7.8, 1 H); 7.62 (tt, J ˆ 1.5,
7.3, 1 H); 7.30 (tt, J ˆ 1.0, 7.6, 1 H); 7.10 (d, J ˆ 8.1, 1 H); 5.59 (d, J ˆ 5.1, 1 H); 4.36 (m, 3 H); 4.04 (m, 1 H); 3.96
(m, 1 H); 2.69 (m, 1 H); 2.41 (m, 1 H); 1.35 (t, J ˆ 7.1, 3 H) . 13C-NMR (75 MHz, CDCl3): 183.8 (s); 163.7 (s);
152.0 (s); 135.8 (d); 127.8 (d); 125.8 (d); 120.1 (s); 119.8 (d); 94.4 (d); 93.1 (s); 67.9 (d); 67.3 (t); 62.9 (t); 28.8 (t);
14.1 (q).

Helvetica Chimica Acta ± Vol. 88 (2005)
1021
REFERENCES
[1] M.-E. Alonso, A. Morales, A. W. Chitty, J. Org. Chem. 1982, 47, 3747.
[2] M.-E. Alonso, P. Jano, M. I. Hernandez, R. S. Greenberg, E. Wenkert, J. Org. Chem. 1983, 48, 3047; E.
Wenkert, M.-E. Alonso, B. L. Buckwalter, E. L. Sanchez, J. Am. Chem. Soc. 1983, 105, 2021; M.-E. Alonso,
M. I. Hernandez, M. Gomez, P. Jano, S. Pekerar, Tetrahedron 1985, 41, 2347.
[3] C. Batsila, G. Kostakis, L. P. Hadjarapoglou, Tetrahedron Lett. 2002, 43, 5997.
[4] M. C. Pirrung, J. Zhang, K. Lackey, D. D. Sternbach, F. Brown, J. Org. Chem. 1985, 60, 2112; M. C. Pirrung,
J. Zhang, A. T. McPhail, J. Org. Chem. 1991, 56, 6269; M. C. Pirrung, Y. R. Lee, Tetrahedron Lett. 1994, 35,
6231; M. C. Pirrung, Y. R. Lee, J. Am. Chem. Soc. 1995, 117, 4814; M. C. Pirrung, F. Blume, J. Org. Chem.
1999, 64, 3642; M. C. Pirrung, K. P. Kaliappan, Org. Lett. 2000, 3, 353.
[5] Y. R. Lee, B. S. Cho, Bull. Korean Chem. Soc. 2002, 23, 779; A. Asouti, L. P. Hadjiarapoglou, Tetrahedron
Lett. 1998, 39, 9073.
[6] P. Müller, Y. F. Allenbach, G. Bernardinelli, Helv. Chim. Acta 2003, 86, 3164.
[7] E. P. Gogonas, L. P. Hadjiarapoglou, Tetrahedron Lett. 2000, 41, 9299.
[8] P. Müller, Y. F. Allenbach, M. Ferri, G. Bernardinelli, ARKIVOC 2003, VII, 80.
[9] E. Wenkert, T. P. Anantanarayan, V. F. Ferreira, M. G. Hoffmann, H.-S. Kim, J. Org. Chem. 1990, 56, 4975.
[10] M. C. Pirrung, J. Zhang, Tetrahedron Lett. 1992, 33, 5987.
[11] H. Ishitani, K. Achiwa, Heterocycles 1997, 46, 153.
[12] H. M. L. Davies, S. A. Panaro, Tetrahedron 2000, 56, 4871.
[13] R. A. Y. Jones, ꢁPhysical and Mechanistic Organic Chemistryꢂ, 2nd edn., Cambridge University Press,
Cambridge, 1972.
[14] C. Bonaccorsi, S. Bachmann, A. Mezzetti, Tetrahedron: Asymmetry 2003, 14, 845; D. Huber, A. Mezzetti,
Tetrahedron: Asymmetry 2004, 15, 2193.
[15] H. Nishiyama, Y. Itoh, Y. Sugawara, K. Matsumoto, K. Aoki, K. Itoh, Bull. Chem. Soc. Jpn. 1995, 68, 1253.
[16] G. Maas, Chem. Soc. Rev. 2004, 183.
[17] J.-L. Liang, S.-X. Yuan, S.-S. Huang, C.-M. Che, J. Org. Chem. 2004, 69, 3610.
[18] S. Bachmann, A. Mezzetti, Helv. Chim. Acta 2001, 84, 3963.
[19] P. Müller, unpublished results.
[20] O. Exner, ꢁCorrelation Analysis of Chemical Dataꢂ, Plenum Press, New York, 1988; E. Buncel, H. Wilson, J.
Chem. Educ. 1987, 64, 475; A. Argile, A. R. E. Carey, G. Fukata, M. Harcourt, R. A. More OꢂFerrall, M. G.
Murphy, Isr. J. Chem. 1985, 26, 303.
[21] P. Müller, Acc. Chem. Res. 2004, 37, 243.
[22] M. P. Doyle, S. B. Davies, W. Hu, Org. Lett. 2000, 2, 1145; M. P. Doyle, W. Hu, ARKIVOC 2003, VII, 15.
[23] T. Ohsumi, H. Neurhoeffer, Tetrahedron 1992, 48, 5227.
[24] P. Müller, S. Chappellet, unpublished results.
[25] P. Müller, S. Chappellet, Synlett 2004, 2573.
[26] E. Wenkert, M. Alonso, B. Buchwalter, E. Sanchez, J. Am. Chem. Soc. 1983, 105, 2021.
[27] P. Müller, P. Nury, Helv. Chim. Acta 2001, 84, 662.
[28] Y. Motoyama, O. Kurihara, K. Murata, K. Aoki, H. Nishiyama, Organometallics 2000, 19, 1025.
[29] H. Nishiyama, M. Kondo, T. Nakamura, K. Itoh, Organometallics 1991, 10, 500.
[30] J. Li-Ren, Z. Jian-Feng, H. Shi-Jun, H. Pei-Qiang, Xiamen Daxue Xuebao 2003, 42, 741.
Â
[31] P. Müller, C. Bolea, Helv. Chim. Acta 2001, 84, 1093.
[32] A. J. Phillips, Y. Uto, P. Wipf, M. J. Reno, D. R. Williams, Org. Lett. 2000, 8, 1165.
[33] I. W. Davies, G. L. Gerena, N. Lu, R. D. Larsen, P. J. Reider, J. Org. Chem. 1996, 61, 9629.
[34] C. Bianchini, H. M. Lee, Organometallics 2000, 19, 1833.
[35] R. M. Stoop, S. Bachman, M. Valentini, A. Mezzetti, Organometallics 2000, 19, 4117.
[36] X. Yao, M. Qiu, W. Lu, H. Chen, Z. Zheng, Tetrahedron: Asymmetry 2001, 12, 197.
[37] F. Voegtle, E. Goldschmitt, Chem. Ber. 1976, 109, 1.
[38] L. Hortala, C. Moberg, V. Levacher, J. Bourguignon, G. Dupas, Tetrahedron Lett. 2002, 43, 1027.
[39] P. Müller, Y. Allenbach, E. Robert, Tetrahedron: Asymmetry 2003, 14, 779.
[40] P. Müller, G. Bernardinelli, Y. F. Allenbach, M. Ferri, H. D. Flack, Org. Lett. 2004, 6, 1725.
[41] G. H. P. Roos, A. E. McKervey, Synth. Commun. 1992, 22, 1751.
[42] Y. Mehmet, P. A. Tarik, Synth. Commun. 2001, 31, 3871.
[43] R. Gallucci, J. Maitland, J. Org. Chem. 1985, 50, 4404.
[44] V. Goodfellow, M. Settineri, R. Lawton, Biochemistry 1989, 28, 6346.
Received February 22,2005