Diastereoselective cobalt-mediated acylation-cyclization of allenes

Article abstract of DOI:10.1055/s-2001-12335

Allenes with α-oxygen and γ-sulfonamide substituents were synthesized and cyclized to pyrrolidines using an acyl cobalt reagent. The reactions proceeded with very high trans selectivity.

Full text of DOI:10.1055/s-2001-12335

532
LETTER
Diastereoselective Cobalt-mediated Acylation-Cyclization of Allenes
Roderick W. Bates,a* Vachiraporn Satcharoenb1
aChulabhorn Research Institute, Vibhavadi-Rangsit Road, Laksi, Bangkok 10210, Thailand
Fax +66 2 574 2027; E-mail: roderick@tubtim.cri.or.th
bDepartment of Chemistry, Chulalongkorn University, Phaya Thai Road, Patumwan, Bangkok 10330, Thailand
Received 23 January 2001
Abstract: Allenes with -oxygen and -sulfonamide substituents
were synthesized and cyclized to pyrrolidines using an acyl cobalt
reagent. The reactions proceeded with very high trans selectivity.
Key words: allene, cobalt, cyclization, pyrrolidine, diastereoselec-
tivity
The acylation-cyclization reaction of allenes (1, X = H)
using acyl cobalt reagents was recently reported to be an
efficient route to pyrrolidines (2, X = H) and other cyclic
compounds (Equation 1).2 We were interested in the pos-
sibility of diastereoselectivity in the cyclization if there
was a substituent on the tether between the sulfonamide
nucleophile and the allene. Due to our interest in the syn-
thesis of hydroxylated pyrrolizidines, we chose to exam-
ine the effect of a protected alcohol group to the allene
(X = OPG).
Scheme 1 (a) NBS, CH3CN (97%); (b) TBSCl, imidazole, THF
(70%); (c) LDA (4 equiv). THF, -78 °C (51%); (d) i-Pr2NH, CuI,
(CH2O)n, dioxane, (89%); (e) MsCl, Et3N; (f) NaN3, DMF; (g) Zn,
AcOH, THF; filter then TsCl, Na2CO3, H2O, CH2Cl2 (from (7):
55%); (h) KF, Bu4NHSO4, CH3CN (73%); (i) BzCl, Et3N;
(j) MeOH, Et3N (from (8): overall 49%).
and O,N-dibenzoylated product (9) were obtained. The
O,N-dibenzoylated product (9) could be converted to the
desired O-benzoyl product (1b) by treatment with me-
thanolic triethylamine.
Equation 1
Attempts to O-alkylate 8 selectively were disappointing,
therefore, for ether derivatives, we returned to the inter-
mediate azide (10) (Scheme 2). This was deprotected and
the resulting azidoalcohol (11) was O-alkylated to give
the benzyl, MTM and TIPS ethers. These were converted
to the sulfonamides (1c, 1d, 1e) as before. It may be noted
that the PTC method10 is effective for the MTM group,
proceeding in 77% yield, although it has been reported
that formation of secondary MTM ethers in a Williamson
fashion is inefficient.11
Related cyclizations have been reported using other met-
als, especially palladium catalysts, although the exact
mechanism is not always clear3 (both 2 and 3 intermedi-
ates have been proposed). Modest to excellent diastereo-
selectivity has been observed.4 Recently, it has been
shown that lanthanide complexes can catalyze highly ste-
reoselective amino allene cyclizations.4b,5
The required substituted allenes 1 (X H) were prepared
from the known diol6 (3) (Scheme 1). Treatment with
NBS resulted in cyclization. The remaining hydroxyl
group was protected by conversion to TBS ether (5).
Elimination with ring opening was achieved by treatment
with an excess of LDA7 to give alkyne (6) which was fur-
ther converted to allene (7) using the method of Searles
and Crabbé.8 The allene (7) was converted to the desired
sulfonamide (1a) via its unstable azide.9
To obtain ester derivative 1b, the TBS group was removed
to give the alcohol 8. On treatment with benzoyl chloride,
some selectivity for O-benzoylation was achieved, but,
generally, mixtures of the desired O-benzoyl product (1b)
Scheme 2 (a) KF, Bu4NHSO4 (73%); (b) BnBr or MTMCl, KOH,
Bu4NHSO4, toluene; or TIPSCl, imidazole, DMAP; (c) Zn, AcOH,
THF; filter then TsCl, Na2CO3, H2O, CH2Cl2 (from (11): 54, 59 and
26% yields respectively).
Synlett 2001, No. 4, 532–534 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Diastereoselective Cobalt-mediated Acylation-Cyclization of Allenes
533
The sulfonamides (1a-e) were treated with acetyl tetracar-
bonyl cobalt and triethylamine according to our previous
procedure (Equation 2).2 In all cases the trans-isomer of
the pyrrolidine (2) was the major product, in some cases
the exclusive pyrrolidine product (Table).
Equation 2
Scheme 3
Table Acylation-Cyclization Diastereoselectivitya
cobalt atom by the sulfur disrupts the conformations. The
effect of S-Co coordination in the Pauson-Khand reaction
has been noted.15
The consistantly high diastereoselectivity of this cycliza-
tion and the synthetic value of the products16 makes this a
useful reaction for organic synthesis. Applications to alka-
loid synthesis are in hand.
a reaction conditions: AcCo(CO)4 (1.1 equiv), THF, r.t., 10 min,
then Et3N (1.2 equiv), overnight; b only one isomer obtained;
c 71% after recovered starting material
Acknowledgement
This research was generously supported by a grant from the Thai-
land Research Fund.
The stereochemistry could be unambiguously determined
by observation of the H2-H3 coupling constants.12 For the
trans-isomers these coupling constants are always about 0
Hz. Molecular models confirmed a dihedral angle of
about 90°. Curiously, H3 was observed as a doublet, im-
plying that one of the H3-H4 coupling constants is also 0
Hz. Once again, observation of molecular models con-
firmed the feasibility of one of the H3-C-C-H4 dihedral
angles being close to 90°. For the cis-isomers, where iso-
lated, the H2-H3 coupling constant was larger: about 4.5
Hz. In related pyrrolidines, Carretero et al. have observed
that the cis-isomers have a distinctly larger H2-H3 cou-
pling constant than the trans-isomers.13
References and Notes
(1) Present address: Department of Chemistry, Naresuan
University, Phitsanulok 65000, Thailand.
(2) Bates, R.W., Rama-Devi, T., Tetrahedron Letters 1995, 36,
509; Bates, R.W., Rama-Devi, T., Ko, H.-H., Tetrahedron
1995, 51, 12939.
(3) Karstens, W.F.J., Rutjes, F.P.J.T., Hiemstra, H., Tetrahedron
Lett. 1997, 38, 6275.
(4) a) 5- and 6-membered rings: Kang, S.K., Baik, T.G., Kulak,
A.N., Synlett 1999, 324; b) Ariedondo, V.M., McDonald,
F.E., Marks, T.J., J.Am.Chem.Soc. 1998, 120, 4871; c)
Meguro, M., Yamamoto, Y., Tetrahedron Lett. 1998, 39,
5421; d) Snider, B.B., He, F., Tetrahedron Lett. 1997, 38,
5453; e) Walkup, R.D., Guan, L., Mosher, M.D., Kim, S.W.,
Kim, Y.S., Synlett 1993, 88; f) Davies, I.W., Scopes, D.J.C.,
Gallagher, T., Synlett 1993, 85; g) Gallagher, T., Davies, I.W.,
Jones, S.W., Lathbury, D., Mahon, M.F., Molloy, K.C., Shaw,
R.W., Vernon, P., J. Chem. Soc, Perkin I 1992, 433; h) 4-
membered rings: Rutjes, F.P.J.T., Tjen, K.C.M.F., Wolf, L.B.,
Karstens, W.F.J., Schoemaker, H.E., Hiemstra, H., Org. Lett.
1999, 1, 717; i) 3-membered rings: Ma, S., Zhao, S., J. Am.
Chem. Soc. 1999, 121, 7943.
The predominance of the trans-isomer can be explained
by appeal to a simple conformational argument (Scheme
3). The conformation of the 3-cobalt intermediate (12t)
that leads to the trans-isomer places the oxygen substitu-
ent in a pseudo equatorial position. On the other hand, the
conformation (12c) leading to the cis-isomer results in a
pseudo-axial position for this substituent with a distinct
interaction with the acetyl substituent on the 3-allyl moi-
ety.14
(5) Arredondo, V.M., Tian, S., McDonald, F.E., Marks, T.J., J.
Am. Chem. Soc. 1999, 121, 3633.
(6) Tamaru, Y., Hojo, M., Kawamura, S.-i., Sawada, S., Yoshida,
Z.-i., J. Org. Chem. 1987, 52, 4062.
(7) Jones, E.R.H., Eglinton, G., Whiting, M.C., Org. Syn. Coll.
Vol. IV 1963, 755; Yadav, J.S., Deshpande, P.K., Sharma,
G.V.M., Tetrahedron 1990, 46,7033.
It may be noted that, although the TBS ether (1a) gives a
very high ratio, the somewhat bulkier TIPS ether (1e)
gives exclusively the trans-isomer, reflecting the greater
size of the TIPS group. In contrast, the MTM ether (1d)
gives a very low ratio. We believe that coordination of the
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534
R. W. Bates, V. Satcharoen
LETTER
(8) Searles, S., Li, Y., Nassim, B., Lopes, M.-T.R., Tran, P.T.,
Crabbé, P., J. Chem. Soc. Perkin I 1984, 747.
(9) azide reduction: Kirk, D.N., Wilson, M.A., J. Chem. Soc. (C)
1971, 414.
cis-(2a) 1H NMR -0.14 (3H, s, SiCH3), 0.10 (3H, s, SiCH3),
0.75 (9H, s, t-Bu), 1.25 (2H, m, H4), 2.34 (3H, s, COCH3),
2.42 (3H, s, ArCH3), 3.42-3.60 (2H, m, H5), 4.18 (1H, app. q,
J = 4.5 Hz, H3), 4.64 (1H, d, J = 4.5 Hz, H2), 6.28 (1H,
s, =CH), 6.31 (1H, s, =CH), 7.30 (2H, d, J = 8 Hz, Ar), 7.69
(2H, d, J = 8 Hz, Ar).
(10) Freedman, H.H., Dubois, R.A., Tetrahedron Lett. 1975, 3251.
(11) Corey, E.J., Bock, M.G., Tetrahedron Lett. 1975, 3269.
(12) data for trans-(2a): mp 93-94 °C, 1H NMR -0.10 (3H, s,
SiCH3), 0.00 (3H, s, SiCH3), 0.62 (9H, s, t-Bu), 1.70 (1H, m,
H4), 1.82 (1H, m, H4'), 2.42 (3H, s, ArCH3), 2.44 (3H, s,
COCH3), 3.29 (1H, ddd, J = 2.6, 3.5, 6 Hz, H5), 3.70 (1H, t,
J = 3.5 Hz, H5'), 3.92 (1H, d, J = 1.7 Hz, H3), 4.4 (1H, s, H2),
6.34 (1H, s, =CH), 6.41 (1H, s, =CH), 7.25 (2H, d, J = 8 Hz,
(13) Carretero, J.C., Arrayás, P., Storch de Gracia, I., Tetrahedron
Lett. 1996, 37, 3379.
(14) (12t) and (12c) are diastereoisomeric, but interconversion can
be expected to take place via a - - mechanism.
(15) Krafft, M.E., Juliano, C., J. Org .Chem. 1992, 57, 5106.
(16) Michael additions: Bates, R.W., Kongsaeree, P., Synlett 1999,
1307.
Ar), 7.75 (2H, d, J = 8 Hz, Ar); 13
C 199.4, 147.3, 143.3,
133.4, 129.6, 128.4, 127.8, 76.1, 68.3, 45.9, 31.7, 26.4, 25.7,
21.4, 17.5, -4.9, -5.5; /cm-1: 2928, 1687, 1599, 1 101, 834;
m/z: 366 (100, M+-t-Bu), 291 (8), 210 (32); Found C 59.53%,
H 7.88, N 3.32; Calcd. for C21H33NSO4Si: C 59.54, H 7.85, N
3.31.
Article Identifier:
1437-2096,E;2001,0,04,0532,0534,ftx,en;Y00901ST.pdf
Synlett 2001, No. 4, 532–534 ISSN 0936-5214 © Thieme Stuttgart · New York