N-(diazoacetyl)oxazolidin-2-thiones as sulfur-donor reagents: Asymmetric synthesis of thiiranes from aldehydes

Article abstract of DOI:10.1002/anie.201204771

Sulfur tyranny: Thiiranes, instead of oxiranes, can be obtained in a highly stereoselective manner through the cycloaddition reaction of N-acyl oxazolidine tethered diazo thione compounds with aldehydes catalyzed by Rh^II. Thus, this reaction provides versatile adducts S functionalized at both the α and β position, with concomitant generation of two contiguous stereocenters. Copyright

Full text of DOI:10.1002/anie.201204771

Angewandte
Chemie
DOI: 10.1002/anie.201204771
Asymmetric Synthesis
N-(Diazoacetyl)oxazolidin-2-thiones as Sulfur-Donor Reagents:
Asymmetric Synthesis of Thiiranes from Aldehydes**
Israel Cano, Enrique Gꢀmez-Bengoa, Aitor Landa, Miguel Maestro, Antonia Mielgo,
Iurre Olaizola, Mikel Oiarbide, and Claudio Palomo*
Sulfur-containing compounds are widespread among natural
products and bioactive substances, and also useful ligands in
asymmetric catalysis.[1] Therefore considerable efforts have
ꢀ
been devoted to develop stereocontrolled C S bond-forming
procedures.[2] Two common approaches consist of the electro-
philic sulfenylation of enolates or equivalents[3] and the
conjugate addition of S nucleophiles to Michael acceptors;[4]
these routes afford S-functionalized carbonyls at either the
a or b position. Methods to access a,b-thioepoxy carbonyls[5]
would not only provide versatile adducts S-functionalized at
both the a and b position, but also imply generation of two
contiguous stereocenters (Scheme 1). However, as far as we
are aware there is virtually no method for achieving such
a goal in a direct and stereocontrolled fashion.[6] Herein, we
Scheme 1. Common strategies for the stereoselective sulfenylation of
carbonyls at the a or b position, and our proposal for the direct
simultaneous a and b sulfenylation.
describe N-(diazoacetyl)oxazolidin-2-thiones as new sulfur-
donor reagents that in combination with aldehydes and a RhII
catalyst are capable of producing a,b-thioepoxy carbonyls in
a highly stereoselective manner.
Inspired by the ability of the oxazolidin-2-thione group to
act as both an intramolecular sulfur-donor reagent and
a stereodirecting group (Scheme 2a),[7] we envisaged that
ꢀ
N-(diazoacetyl)oxazolidin-2-thiones might serve as both C C
ꢀ
and C S bond-forming reagents while controlling the reac-
tion stereochemistry. The assumption was that thiocarbonyl
ylide I (Scheme 2b), generated from N-(diazoacetyl)oxazoli-
din-2-thione upon treatment with a metal catalyst,[8] would
react with an aldehyde to afford the zwiterionic intermediate
II, which may follow either path A or B to provide either
epoxide or thioepoxide product. Although path A (epoxide
formation) seemed to be the preferred route for both sulfide
[*] Dr. I. Cano, Dr. E. Gꢀmez-Bengoa, Dr. A. Landa, Dr. A. Mielgo,
I. Olaizola, Prof. Dr. M. Oiarbide, Prof. Dr. C. Palomo
Departamento de Quꢁmica Orgꢂnica I, Universidad del Paꢁs Vasco
Manuel Lardizabal 3, 20018 San Sebastiꢂn (Spain)
E-mail: claudio.palomo@ehu.es
Scheme 2. Working hypothesis for stereoselective thiirane synthesis by
ꢀ
sulfur transfer with concomitant C C bond formation.
Dr. M. Maestro[+]
Departamento de Quꢁmica Fundamental, Facultad de Ciencias
Universidade da CoruÇa
ylides[9] and carbonyl ylides,[10] and implies no sulfur transfer,
we speculated that path B might also be possible, likely
through rearrangement of intermediate III.
Starting thione diazo compounds were readily prepared
by reaction of oxazolidin-2-thiones with 2-(2-tosylhydrazo-
no)acetyl chloride in yields of 47–67%. Initial screening of
catalysts and reaction conditions revealed that both RhII and
CuII salts catalyzed the reaction of 1 with benzaldehyde in
CH2Cl2 at 08C to afford thiirane 5a in yields from 50% to
62% upon isolation and cis/trans ratios from 85:15 to 88:12
Campus Zapateira s/n, 15071A CoruÇa (Spain)
[+] X-Ray analyses.
[**] Financial support was provided by the University of the Basque
Country UPV/EHU (UFI 11/22), Basque Government (GV grant No
IT-291-07), and Ministerio de Ciencia e Innovaciꢀn (MICINN, Grant
CTQ2007-68095-C02), Spain. A.L. thanks MICINN and the Euro-
pean Social Foundation for a Ramꢀn y Cajal contract. I.O. thanks
MCINN for a fellowship. We also thank SGIker (UPV/EHU) for
providing NMR, HRMS, X-ray, and computational resources.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204771.
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(Table 1, entries 1, 2, and 7). Remarkably, in both cases no
oxirane product was detected in the corresponding crude
reaction mixtures.[10] This process is thus particularly signifi-
We next investigated the scope of the reaction with
respect to the aldehyde component. As the results in Table 2
show, reactions of a range of aromatic aldehydes bearing
either electron-donating, neutral, or electron-withdrawing
ꢀ
cant in that the new C S s-bond formation occurs in
Table 1: Screening of catalysts for the reaction of N-(diazoacetyl)-2-
oxazolidinethiones 1–3 and benzaldehyde.[a]
Table 2: Scope of the reaction.[a]
Entry Substrate R1
Product cis/trans[b] Yield [%][c]
Entry
R
Substrate Catalyst
T
[8C]
Prod. cis/
trans[b]
Yield
[%][c]
1
2
3
4
1
1
1
1
1
1
1
1
1
1
1
2
2
2
1
2
1
Ph
5a
5b
5c
5d
5e
5 f
5g
5h
5i
93:7
65[d]
60
61
61
31[e]
63
57
56
61
56
65
75
69
60
1
2
3
4
5
6
7
8
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
tBu
Ph
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
Rh2(OAc)4
Rh2(OAc)4·2H2O
0
0
5a
5a
86:14
88:12
94:6
97:3
–
50
62
4-MeC6H4
3,5-MeC6H3
4-MeOC6H4
4-TBSOC6H4
4-ClC6H4
82:18
83:17
1:99
Rh2(OAc)4·2H2O ꢀ10 5a
64[d]
62[d]
0[e]
0
Rh2(OAc)4·2H2O ꢀ20 5a
5
1:99
6[f]
7
88:12
86:14
91:9
92:8
91:9
72:28
83:17
86:14
85:15
62:38
83:17
–
Rh2(OCOCF3)4
CoCl2
0
0
0
0
0
0
0
0
0
5a
5a
5a
5a
5a
5a
5a
5a
5a
–
3-ClC6H4
Cu(acac)2
Cu(OTf)2
CuCl
FeCl2·4H2O
AuCl
85:15
–
91:9
–
99:1
–
–
53
8
9
4-BrC6H4
4-NO2C6H4
4-CNC6H4
PhC C
PhC C
PhC C
3-ClC6H4C C 6l
3-furyl
3-furyl
0[e]
40
9
10
11
12[g]
13[h]
14[g]
15
16[g]
17
5j
10
11
12
13
14
15
0[e]
18
ꢁ
5k
6k
6k
ꢁ
AgOTf
Pd(OAc)2
0[e]
17
ꢁ
ꢁ
n.d.[i]
70
0[j]
Rh2(OAc)4·2H2O ꢀ20 6a
94:6
92:8
60[d]
45[d]
5m
6m
5n
Rh2(OAc)4·2H2O ꢀ20 7a
3-pyridyl
[a] The reactions were performed on a 0.30 mmol scale. [b] Determined
by 1H NMR spectroscopy. [c] Yield of isolated major isomer after
chromatography. [d] Using 2 mol% of catalyst. [e] Extensive decompo-
sition was observed.
[a] Reaction conditions: 1 (0.5 mmol), 4 (3 equiv, 1.5 mmol), Rh2-
(OAc)4·2H2O (2 mol%), ꢀ208C, 16–18 h in CH2Cl2 (1 mL). [b] Deter-
mined by 1H NMR spectroscopy. [c] Yields of isolated compounds 5 or 6
after column chromatography. [d] Reaction carried out at 2 mmol scale.
[e] Yield not optimized; partial desilylation occurred during chromato-
graphy (SiO2). [f] 91:9 diastereoselectivity in the presence of 2,2’-
bipyridyl as additive. [g] Reaction run at ꢀ608C. [h] Reaction run at
ꢀ788C. [i] n.d.=not determined. [j] Unchanged starting material recov-
ered.
ꢀ
detriment of the C O s-bond and with concomitant gener-
ation of two contiguous stereocenters. The nature of the
counterion of the transition metal salt used has an influence
on the catalytic activity: both Rh2(OAc)4·2H2O and
Cu(acac)2 were active and induced good reaction yields,
whereas no reaction at all was observed with either Rh2-
(OCOCF3)4 or Cu(OTf)2 salts (Table 1, entries 1, 2, and 7 vs. 5
and 8). The use of other divalent metal salts such as CoCl2,
FeCl2·H2O, and Pd(OAc)2, which are potentially capable of
inducing ylide formation, led to sluggish reactions or no
reaction at all (Table 1, entries 6, 10, 13).
On the other hand, some metals in the oxidation state + 1
were also effective catalysts. For instance, although no
reaction was observed with AgOTf, both CuCl and AuCl
promoted the reaction to give product 5a with cis/trans ratios
of 91:9 and 99:1, respectively, albeit in these cases the yields
were low (Table 1, entries 12, 9, and 11). Further optimization
of the reaction conditions with Rh2(OAc)4·2H2O as catalyst
indicated that a lower (2 mol%) catalyst loading could be
used and the cis/trans ratio could be improved by lowering the
temperature (up to 97:3 cis/trans ratio; Table 1, entries 3 and
4). Finally, diazo-oxazolidinethiones 2 and 3, bearing a tert-
butyl and a phenyl substituent, respectively, were also
tolerated, although in the case of the phenyl analogue 3
a relatively lower yield and selectivity was observed (Table 1,
entries 14 and 15).
substituents all produced the corresponding thiirane product
smoothly within 16–18 h at ꢀ208C. In each case a mixture of
cis/trans isomers was formed from which the major isomer
was obtained in 57–75% yield upon isolation. Interestingly, in
most cases cis-thiirane was obtained as the major isomer (cis/
trans ratio from 97:3 to 82:18; Table 2, entries 1–3, 6–10),
whereas in the case of p-anisidine, and p-tert-butyldimethyl-
silyloxybenzaldehyde, the trans-configured thiirane (5d and
5e) was the exclusive product (Table 2, entries 4 and 5). This
unusual reversal of the reaction stereochemistry observed for
benzaldehydes bearing electron-donating substituents could
be explained on the basis of the proposed reaction mechanism
(see below). The catalytic generation of thiiranes 5 and 6 also
worked for other nonenolizable aldehydes, such as alkynyl
and heteroaryl aldehydes (Table 2, entries 11–16). Pyridyl-
carbaldehyde was an exception (Table 2, entry 17). Assign-
ment of the cis/trans relative configuration of the formed
thiirane ring was primarily made by correlation of the
coupling constants between the two vicinal H nucleus in
1H NMR spectroscopy: 7.4 Hz to 7.7 Hz for the cis-thiirane
2
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systems; 4.80 Hz to 4.90 Hz for the trans isomer. In addition,
an X-ray single-crystal structure analysis of compound cis-5a
confirmed the proposed structure.[11]
Next, reaction conditions for the selective opening of the
thiirane ring and the release of the oxazolidinone auxiliary
were explored. For example, treatment of thiiranes cis-5a and
cis-5h with acetyl chloride in CH2Cl2 at room temperature in
the presence of CoCl2 (10 mol%), according to the procedure
of Iranpoor, Firouzabadi, and Jafari,[12] gave the b-chloro-a-
thio imide derivatives 8a and 8c in 88% and 65% yields,
respectively, after chromatography (Scheme 3). Similarly,
Scheme 4. Recovery of the auxiliary.
A DFT investigation was carried out at the B3LYP level of
theory, and provided support for a plausible pathway for this
intriguing thiirane-forming reaction. Calculations predict that
the corresponding rhodium carbenoid species[11] formed upon
treatment of diazo thione compound 1 with Rh2(OAc)4,
evolves into bicyclic ylide 11 (Scheme 5)[13] with no activation
Scheme 3. Thiirane ring opening of adducts 5.
treatment of cis-5a with acetyl bromide in the presence of
CoBr2, as catalyst, afforded the corresponding bromo deriv-
ative 8b in 70% yield upon isolation. In all these three cases
a minor amount (6–10%) of the corresponding regioisomeric
ring-opening product was also observed in the respective
crude reaction mixture. It is important to note that substitu-
tion at the b carbon atom during ring opening leading to
products 8 occurred with retention of configuration, perhaps
ꢀ
by a double inversion pathway involving a transient C O
adduct. Interestingly, acid-promoted cyclization of com-
pounds 8 to restore the thiirane ring took place very
efficiently, with inversion of the configuration of b carbon
atom. For instance, the treatment of 8a with methanolic HCl
afforded trans-5a in 70% yield. Accordingly, a two-step
thiirane-ring isomerization from cis to the more stable trans
isomer is feasible.
On the other hand, the removal of the oxazolidinone
moiety from thiirane adducts 5 through imide hydrolysis or
alcoholysis under standard reaction conditions led to exten-
sive loss of the sulfur atom. This problem could be circum-
vented by performing imide hydrolysis on the ring-opened
adducts 8 instead (Scheme 4). Thus, saponification with
LiOH/H2O2 of adducts 8a and 8c proceeded with restoration
of the thiirane ring, to afford the corresponding acids 9, which
were isolated as crystalline bench stable dicyclohexylamine
salts 10. In this transformation, oxazolidinone was also
formed and it could be recovered, transformed into the
thione auxiliary, and recycled.[7] Unambiguous determination
of the structure of salt 10b and compound 8a by X-ray single-
crystal structure analysis[11] further confirmed the identity of
the products as well as the stereochemical outcome of the
reactions involved.
Scheme 5. Principal reaction pathways found by DFT investigation at
the B3LYP level of theory for the Rh-catalyzed reaction between
diazocompound 1 and either benzaldehyde or p-anisaldehyde. Values
of Gibbs energy are given in kcalmolꢀ1
.
barrier, probably because of the high charge delocalization
exhibited by this particular ylide.[14] According to calculations,
subsequent reaction of 11 with either benzaldehyde or
p-anisaldehyde would generate a unique tricyclic adduct,[15]
and among the four possible relative orientations of the ylide
and aldehyde component during the cycloaddition, those
leading to anti, exo and anti, endo isomers are preferred. The
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[2] a) D. J. Procter, J. Chem. Soc. Perkin Trans. 1 2001, 335 – 354; for
asymmetric synthesis of tertiary thiols and thioethers, see: b) J.
Clayden, P. MacLellan, Beilstein J. Org. Chem. 2011, 7, 582 – 595;
complementary syn transition states lie considerably higher in
energy because unfavorable interactions between the ylide
isopropyl substituent and the incoming aldehyde. The energy
differences between anti, exo and anti, endo approaches for
benzaldehyde and p-anisaldehyde, (1.2 and 0.8 kcalmolꢀ1,
respectively) would justify preferential formation of the
anti, exo adduct with expected diastereoselectivities near
90:10. Transformation of these tricyclic high energy inter-
mediates into the final thiirane products would follow a more-
or-less downhill energy profile, involving thiirane ring open-
ing, Sax!eq conformational switch, and internal SN2 displace-
ment. Accordingly, from a tricyclic anti, exo intermediate the
cis-configured thiirane would be formed; conversely, from the
less-favorable anti, endo precursor, the trans-thiirane would
be formed, a prediction that agrees with the experimentally
observed trend for most of the aldehydes tested. Interestingly,
calculations also offer a plausible explanation of the reversal
of the reaction stereochemistry observed experimentally for
p-anisaldehyde and other related electron-rich aromatic
aldehydes. Indeed, thiirane generation could occur through
an alternative SN1-type pathway, which is about 2.5 kcalmolꢀ1
less favorable than the SN2 pathway for benzaldehyde, but
conversely about 3.3 kcalmolꢀ1 more favorable than the SN2
pathway for p-anisaldehyde. As expected, SN1-type cycliza-
tion would preferentially lead to the most stable trans-thiirane
product.
ꢀ
for metal-catalyzed C S bond formation, see: c) T. Kondo, T.-a.
Mitsudo, Chem. Rev. 2000, 100, 3205 – 3220; d) W. Adam, R. M.
Bargon, Chem. Rev. 2004, 104, 251 – 261; e) Y. Zhang, J. Wang,
Coord. Chem. Rev. 2010, 254, 941 – 953.
[3] Leading examples of asymmetric electrophilic a-sulfenylations:
Stoichiometric: a) D. A. Evans, K. R. Campos, J. S. Tedrow, F. E.
Michael, M. R. Gagnꢀ, J. Am. Chem. Soc. 2000, 122, 7905 – 7920;
b) D. Enders, A. Schaadt, Synlett 2002, 498 – 500: organocata-
lytic: c) M. Marigo, T. C. Wabnitz, D. Fielenbach, K. A. Jørgen-
sen, Angew. Chem. 2005, 117, 804 – 807; Angew. Chem. Int. Ed.
2005, 44, 794 – 797; d) G. L. Zhao, R. Rios, J. Vesely, L. Eriksson,
A. Cordova, Angew. Chem. 2008, 120, 8596 – 8600; Angew.
Chem. Int. Ed. 2008, 47, 8468 – 8472; e) L. Fang, A. Lin, H. Hu,
C. Zhu, Chem. Eur. J. 2009, 15, 7039 – 7043; metal catalyzed:
f) M. Jereb, A. Togni, Chem. Eur. J. 2007, 13, 9384 – 9392.
[4] For a review on asymmetric sulfa-Michael additions, see: D.
Enders, K. Lꢁttgen, A. A. Narine, Synthesis 2007, 959 – 980.
[5] For a general review on thiiranes, see: “Thiiranes and Deriva-
tives” M. Saito, J. Nakayama in Science of Syntheses, Houben-
Weyl Methods of Molecular Transformations, Vol. 39 (Eds.: D.
Bellus, E. N. Jacobsen, S. V. Ley, R. Noyori, M. Regitz, P. J.
Reider, E. Schaumann, I. Shinkai, E. J. Thomas, B. M. Trost, N.
Kambe), Thieme, Stuttgart, 2008, pp. 589 – 658 and Ref. [2d].
[6] Asymmetric synthesis of thiiranes from chiral nonracemic
precursors: allenes: a) C. Zhou, C. Fu, S. Ma, Angew. Chem.
2007, 119, 4457 – 4459; Angew. Chem. Int. Ed. 2007, 46, 4379 –
4381; oxiranes: b) M. Lee, M. M. Bernardo, S. O. Meroueh, S.
Brown, R. Fridman, S. Mobashery, Org. Lett. 2005, 7, 4463 –
4465; b-thiocyanoalcohols: c) E. Łukowska, J. Plenkiewicz,
Tetrahedron: Asymmetry 2007, 18, 1202 – 1209; synthesis of
chiral nonracemic thiiranium ions: d) S. E. Denmark, T. Vogler,
Chem. Eur. J. 2009, 15, 11737 – 11745.
In conclusion, we have reported the first Rh-catalyzed
reaction of a diazoacetyl compound with aldehydes that
affords thiiranes, instead of oxiranes, as known before. This
unusual reactivity relies on the use of N-(diazoacetyl)oxazo-
lidin-2-thiones as new chiral sulfur-donor reagents and
enables the direct production of optically active thiiranes
with very high stereoselectivity. Work towards expanding the
scope of this sulfur-transfer protocol is currently underway in
our laboratory.
[7] a) C. Palomo, M. Oiarbide, F. Dias, A. Ortiz, A. Linden, J. Am.
Chem. Soc. 2001, 123, 5602 – 5603; b) C. Palomo, M. Oiarbide, F.
Dias, R. Lꢂpez, A. Linden, Angew. Chem. 2004, 116, 3369 – 3372;
Angew. Chem. Int. Ed. 2004, 43, 3307 – 3310; c) C. Palomo, M.
Oiarbide, R. Lꢂpez, P. B. Gonzꢃlez, E. Gꢂmez-Bengoa, J. M.
Saꢃ, A. Linden, J. Am. Chem. Soc. 2006, 128, 15236 – 15247.
ꢀ
[8] For C S bond formation mediated by sulfur ylides derived from
Experimental Section
metal carbene, see Ref. [2e]. For metal-catalyzed reactions of a-
diazocarbonyl compounds, see: a) M. P. Doyle, M. A. McKervey,
T. Ye, Modern Catalytic Methods for Organic Synthesis with
Diazo Compounds, Wiley-Interscience, New York, 1998; b) Z.
Zhang, J. Wang, Tetrahedron 2008, 64, 6577 – 6605; c) M. P.
Doyle, R. Duffy, M. Ratnikov, L. Zhou, Chem. Rev. 2010, 110,
704 – 724; d) H. M. L. Davies, J. R. Deton, Chem. Soc. Rev. 2009,
38, 3061 – 3071; for the generation of thiocarbonyl ylides, see:
e) A. Padwa, Chem. Soc. Rev. 2009, 38, 3072 – 3081.
General catalytic procedure for the synthesis of thiiranes 5–7:
Rhodium(II) acetate dihydrate (4.8 mg, 0.01 mmol, 2 mol%) was
added to a solution of the corresponding diazocompound 1–3
(0.50 mmol) and aldehyde 4a–p (3 equiv, 1.5 mmol) in anhydrous
CH2Cl2 (1.5 mL) at a given temperature, under argon atmosphere.
The reaction mixture was stirred for 16–18h at the same temperature
and afterwards quenched with saturated NaHCO3; the organic layer
was separated, dried with MgSO4, and filtered. The solvent was
evaporated under reduced pressure and the residue was purified by
flash chromatography on silica gel (eluent: AcOEt/hexanes, 1:4) to
afford the desired product.
[9] For a review, see: a) V. K. Aggarwal, M. Crimmin, S. Riches in
Science of Synthesis, Vol. 37 (Ed.: C. J. Forsyth), Thieme,
Stuttgart, 2008, pp. 321 – 406; for leading recent examples, see:
b) V. K. Aggarwal, J. P. H. Charmant, D. Fuentes, J. N. Harvey,
G. Hynd, D. Ohara, W. Picoul, R. L. Robiette, C. Smith, J.-L.
Vasse, C. L. Winn, J. Am. Chem. Soc. 2006, 128, 2105 – 2114 and
references therein. For a mechanistic investigation, see: c) D. R.
Edwards, P. Montoya-Peleaz, C. M. Crudden, Org. Lett. 2007, 9,
5481 – 5484.
[10] a) M. P. Doyle, W. Hu, D. J. Timmons, Org. Lett. 2001, 3, 933 –
935; b) W.-J. Liu, B.-D. Lv, L.-Z. Gong, Angew. Chem. 2009, 121,
6625 – 6628; Angew. Chem. Int. Ed. 2009, 48, 6503 – 6506.
[11] See the Supporting Information for details.
Received: June 18, 2012
Published online: && &&, &&&&
Keywords: asymmetric synthesis · diazo compounds · sulfur ·
.
thiiranes · ylides
[1] a) Organosulfur Chemistry in Asymmetric Synthesis (Eds.: T.
Toru, C. Bolm), Wiley-VCH, Weinheim, 2008; b) M. Mellah, A.
Voituriez, E. Schulz, Chem. Rev. 2007, 107, 5133 – 5209.
[12] N. Iranpoor, H. Firouzabadi, A. A. Jafari, Synth. Commun. 2003,
33, 2321 – 2327.
4
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Chemie
[13] For generation, and subsequent cycloaddition, of thiocarbonyl
ylides from the RhII-catalyzed cyclization of diazothiocarbonyl
compounds, see: Ref. [8e] and a) K. T. Potts, E. Houghton, U. P.
Singh, J. Org. Chem. 1974, 39, 3627 – 3631; b) A. Padwa, F. R.
Kinder, L. Zhi, Synlett 1991, 287 – 288; c) A. Padwa, A. C. Flick,
H. I. Lee, Org. Lett. 2005, 7, 2925 – 2928.
stable aromatic mesoionic system: K. T. Potts, P. Murphy, J.
Chem. Soc. Chem. Commun. 1984, 1348 – 1349.
[15] An alternative pathway to this tricyclic adduct would involve
attack of the thione group onto the epoxide initially generated
from the reaction between the aldehyde and the diazocarbonyl
moiety. However, the fact that no traces of epoxide product were
detected in the reaction mixtures and a 14.4 kcalmolꢀ1 energy
barrier is calculated for such epoxide formation (see Ref. [11]),
disfavors the epoxide pathway.
[14] It has been reported that the treatment of a diazothioamide
compound related to 1 with rhodium(II) acetate yielded an
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Communications
Asymmetric Synthesis
I. Cano, E. Gꢁmez-Bengoa, A. Landa,
M. Maestro, A. Mielgo, I. Olaizola,
M. Oiarbide, C. Palomo*
&&&& — &&&&
Sulfur tyranny: Thiiranes, instead of oxir-
anes, can be obtained in a highly stereo-
selective manner through the cycloaddi-
tion reaction of N-acyl oxazolidine teth-
ered diazo thione compounds with alde-
hydes catalyzed by RhII. Thus, this reac-
tion provides versatile adducts S func-
tionalized at both the a and b position,
with concomitant generation of two con-
tiguous stereocenters.
N-(Diazoacetyl)oxazolidin-2-thiones as
Sulfur-Donor Reagents: Asymmetric
Synthesis of Thiiranes from Aldehydes
6
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
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