Rhenium(VII) catalysis of prins cyclization reactions

Article abstract of DOI:10.1021/ol8019204

(Equation Presented) The rhenium(VII) complex O₃ReOSiPh ₃ is a particularly effective catalyst for Prins cyclizations using aromatic and α,β -unsaturated aldehydes. The reaction conditions are mild, and the highly substituted 4-hydroxytetrahydropyran products are formed stereoselectively. Rhenium(VII) complexes appear to spontaneously form esters with alcohols and to directly activate electron-rich alcohols for solvolysis. Re₂O₇ and perrhenic acid are equally effective in catalyzing these cyclizations.

Full text of DOI:10.1021/ol8019204

ORGANIC
LETTERS
2008
Vol. 10, No. 21
4839-4842
Rhenium(VII) Catalysis of Prins
Cyclization Reactions
Kwanruthai Tadpetch and Scott D. Rychnovsky*
Department of Chemistry, 1102 Natural Sciences II, UniVersity of California,
IrVine, California 92697-2025
srychnoV@uci.edu
Received August 16, 2008
ABSTRACT
The rhenium(VII) complex O3ReOSiPh3 is a particularly effective catalyst for Prins cyclizations using aromatic and r,ꢀ-unsaturated aldehydes. The
reaction conditions are mild, and the highly substituted 4-hydroxytetrahydropyran products are formed stereoselectively. Rhenium(VII) complexes
appear to spontaneously form esters with alcohols and to directly activate electron-rich alcohols for solvolysis. Re2O7 and perrhenic acid are equally
effective in catalyzing these cyclizations.
Osborn reported that the Re(VII) complex O3ReOSiPh31 was
a highly effective catalyst for allylic alcohol isomerizations.2
Grubbs studied Osborn’s reaction and found that O3ReOSiPh3
isomerizes allylic alcohols stereospecifically.3 Both Osborn and
Grubbs invoked a concerted [3,3] rearrangement of allylic
perrhenate esters to explain this very facile and selective
isomerization (Figure 1).2,3 An alternative mechanism was
invoked to explain some of the peculiarities of the reaction.
Osborn proposed an ionic mechanism to explain the kinetics
for formation of Z-alkenes,2b and Grubbs invoked a similar
mechanism to explain the loss of optical purity with certain
electron-rich allylic alcohol substrates (Figure 1).3 The
proposed ionic mechanism attracted our attention because it
suggested that Re(VII) complexes might activate and sol-
volyze electron-rich alcohols. We report herein a Prins
cyclization initiated by Re(VII) catalysts4 based on this
concept that is very mild and shows unusual selectivity.
Figure 1. Grubbs-Osborn mechanism for stereoselective isomeriza-
tion of allylic alcohols and the alternative ionic mechanism that leads
to loss of selectivity.
Prins cyclization reactions between homoallylic alcohols and
aldehydes are generally promoted by strong Brønsted or Lewis
acids.5 If halide counterions are present, they are incorporated
(1) Schoop, T.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. G.
Organometallics 1993, 12, 571–574.
at the 4-position of the newly formed tetrahydropyran (THP)
ring.5 Oxygen atoms can be captured at the 4-position of the
THP ring,6although most acids lead to esters that must be
hydrolyzed to form 4-hydroxy THP products.7 Most Prins
cyclizations use superstoichiometric acid to promote the reaction
because the acid counterion is trapped in the product, thus
(2) (a) Bellemin-Laponnaz, S.; Gisie, H.; Pierre Le Nye, J.; Osborn, J. A.
Angew. Chem., Int. Ed. Engl. 1997, 36, 976–978. (b) Bellemin-Laponnaz,
S.; Le Ny, J. P.; Osborn, J. A. Tetrahedron Lett. 2000, 41, 1549–1552.
(3) (a) Morrill, C.; Grubbs, R. H. J. Am. Chem. Soc. 2005, 127, 2842–
2843. (b) Morrill, C.; Beutner, G. L.; Grubbs, R. H. J. Org. Chem. 2006,
71, 7813–7825.
10.1021/ol8019204 CCC: $40.75
Published on Web 09/25/2008
2008 American Chemical Society

consuming the acid.5 Re(VII) complex O3ReOSiPh3 is proposed
to catalyze the Prins cyclization as illustrated in Scheme 1.
Table 1. Screening Prins Cyclization Conditions with
O3ReOSiPh3 and Other Catalysts
Scheme 1
.
Proposed Mechanism for Activation and Prins
Cyclization Using O3ReOSiPh3
entry
solvent
time (h)
yield (%)
eq/ax
1a
2
3
4
5
CH2Cl2
CH2Cl2
CHCl3
EtOAc
MeNO2
hexanes
toluene
MeCN
36
3
54
63
67
33
40
62
38
0
1.5:1
1.9:1
2.7:1
4.5:1
6.0:1
15:1
4.4:1
NA
1:2.4
>10:1
48
21
24
48
48
48
72
20
6
7
8
9b
10c
CH2Cl2
CH2Cl2
37
43
Activation is achieved by forming perrhenate ester 4 from
hemiacetal 3. Solvolysis of ester 4 generates the intermediate
5 as a contact ion pair. Cyclization and trapping leads to
perrhenate ester 6. Ester exchange with Ph3SiOH would
regenerate the catalyst. The proposed Re(VII)-initiated reaction
has several potential advantages over traditional Prins cyclization
reactions. It avoids the use of strong acid, and the facile
transesterification of perrhenate esters with alcohols allows
turnover and makes the system catalytic. Additionally, the
alcohol product would be produced directly from the reaction
rather than requiring a subsequent hydrolysis step. These
potentially attractive features led us to explore this reaction.
a Reaction was carried out with 1 mol % of O3ReOSiPh3. b Reaction was
carried out with 30 mol % of VO(OPrn)3 instead of O3ReOSiPh3 and with 2
equivofthealdehyde.c Reactioncatalyzedwith5mol%ofH3[P(Mo3O10)4]·xH2O.
alcohol were selected with the same substituent to avoid
complications from oxonia-Cope induced side-chain ex-
changes.8 O3ReOSiPh3 catalyzed the Prins cyclization ef-
fectively in most solvents except acetonitrile. Although the
reaction was effective using 1 mol % of catalyst (entry 1), 5
mol % of catalyst was selected to screen conditions. Methylene
chloride, chloroform, and hexanes all gave yields over 60% on
stirring at room temperature (entries 2, 4, and 7). The reaction
times varied from several hours to several days. Vanadyl esters
were also investigated as catalysts,9 but they were less efficient.
For example, 30 mol % of tri-n-propyl vanadate (entry 9) gave
slightly more than 1 mol of product per mol of catalyst after
several days. Phosphomolybdic acid catalysis was evaluated for
comparison (entry 10) and gave a lower yield but with higher
equatorial selectivity.6g Two molybdenum oxo complexes were
screened as catalysts, but they were ineffective.10
Table 1 presents an initial exploration of the Re(VII)-
catalyzed Prins cyclization. The aldehyde and homoallylic
(4) For several other related applications of high-oxidation Re catalysts in
synthesis, see: (a) Ishihara, K.; Furuya, Y.; Yamamoto, H. Angew. Chem.,
Int. Ed. 2002, 41, 2983–2986. (b) Maeda, Y.; Nishimura, T.; Uemura, S.
Chem. Lett. 2005, 34, 790–791. (c) Sherry, B. D.; Radosevich, A. T.; Toste,
F. D. J. Am. Chem. Soc. 2003, 125, 6076–6077. (d) Sherry, B. D.; Loy,
R. N.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 4510–4511.
(5) (a) Pastor, I. M.; Yus, M. Curr. Org. Chem. 2007, 11, 925–957. (b)
Snider, B. B. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Heathcock, C. H., Eds.; Pergamon Press: New York, 1991; Vol. 2, pp
527-561. (c) Adams, D. R.; Bhatnagar, S. P. Synthesis 1977, 661–672. (d)
Overman, L. E.; Pennington, L. D. J. Org. Chem. 2003, 68, 7143–7157.
(6) (a) Hanschke, E. Chem. Ber. 1955, 88, 1053–1061. (b) Zhang, W.-
C.; Viswanathan, G. S.; Li, C.-J. Chem. Commun. 1999, 291–292. (c) Zhang,
W.-C.; Li, C.-J. Tetrahedron 2000, 56, 2403–2411. (d) Yadav, J. S.; Reddy,
B. V. S.; Kumar, G. M.; Murthy, C. V. S. R. Tetrahedron Lett. 2001, 42,
89–91. (e) Keh, C. C. K.; Namboodiri, V. V.; Varma, R. S.; Li, C.-J.
Tetrahedron Lett. 2002, 43, 4993–4996. (f) Hart, D. J.; Bennett, C. E. Org.
Lett. 2003, 5, 1499–1502. (g) Yadav, J. S.; Reddy, B. V. S.; Kumar,
G. G. K. S. N.; Aravind, S. Synthesis 2008, 395–400.
One surprising aspect of the oxo-metal-catalyzed reaction is
the isolation of axial THP product (7ax) as a significant
component of the product. Prins reactions normally give
predominantly equatorial 4-heteroatom products, and all previ-
ously reported 4-oxygen adducts showed high equatorial se-
lectivity.6,7 We previously reported that the axial selectivity
was a function of the lifetime and reactivity of the ion pair,
with very reactive nucleophiles, such as bromide anion,
leading to high axial selectivity.11 Typical oxygen nucleo-
(7) (a) Kay, I. T.; Williams, E. G. Tetrahedron Lett. 1983, 24, 5915–
5918. (b) Kay, I. T.; Bartholomew, D. Tetrahedron Lett. 1984, 25, 2035–
2038. (c) Jaber, J. J.; Mitsui, K.; Rychnovsky, S. D. J. Org. Chem. 2001,
66, 4679–4686. (d) Barry, C. S. J.; Crosby, S. R.; Harding, J. R.; Hughes,
R. A.; King, C. D.; Parker, G. D.; Willis, C. L. Org. Lett. 2003, 5, 2429–
2432. (e) Barry, C. S.; Elsworth, J. D.; Seden, P. T.; Bushby, N.; Harding,
J. R.; Alder, R. W.; Willis, C. L. Org. Lett. 2006, 8, 3319–3322.
(8) (a) Lolkema, L. D. M.; Semeyn, C.; Ashek, L.; Hiemstra, H.;
Speckamp, W. N. Tetrahedron 1994, 50, 7129–7140. (b) Rychnovsky, S. D.;
Marumoto, S.; Jaber, J. J. Org. Lett. 2001, 3, 3815–3818. (c) Alder, R. W.;
Harvey, J. N.; Oakley, M. T. J. Am. Chem. Soc. 2002, 124, 4960–4961. (d)
Crosby, S. R.; Harding, J. R.; King, C. D.; Parker, G. D.; Willis, C. L.
Org. Lett. 2002, 4, 577–580. (e) Barry, C. S.; Bushby, N.; Harding, J. R.;
Hughes, R. A.; Parker, G. D.; Roe, R.; Willis, C. L. Chem. Commun. 2005,
3727–3729. (f) Jasti, R.; Anderson, C. D.; Rychnovsky, S. D. J. Am. Chem.
Soc. 2005, 127, 9939–9945. (g) Jasti, R.; Rychnovsky, S. D. J. Am. Chem.
Soc. 2006, 128, 13640–13648.
-
philes, such as CF3CO2- or AcO·BF3 , have low nucleophi-
licity and lead to ca. 20:1 selectivity for the equatorial
product.8f The Re(VII) catalyst, while still favoring the
equatorial product, shows a much lower level of selectivity.
(9) Chabardes, P.; Kuntz, E.; Varagnat, J. Tetrahedron 1977, 33, 1775–
1783.
(10) MoO2(acac)2: (a) Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1981,
20, 660–664 MoO2(OSiPh3)2: (b) Huang, M.; DeKock, C. W. Inorg. Chem.
1993, 32, 2287–2291.
(11) (a) Jasti, R.; Vitale, J.; Rychnovsky, S. D. J. Am. Chem. Soc. 2004,
126, 9904–9905. (b) Miles, R. B.; Davis, C. E.; Coates, R. M. J. Org. Chem.
2006, 71, 1493–1501.
4840
Org. Lett., Vol. 10, No. 21, 2008

The vanadate ester example (entry 9) actually favors the axial
alcohol product. These experiments suggest that perrhenate
and other oxo-metal counterions are much more nucleophilic
than trifluoroacetate counterion in Prins reactions, and that
they may show unusual patterns of reactivity and selectivity.
On a practical note, the best selectivity for the equatorial
product was found using O3ReOSiPh3 in hexanes (entry 6).
Table 2. Prins Cyclization with Aromatic Aldehyde and
O3ReOSiPh3
Prins reactions with aromatic and aliphatic aldehydes behave
differently when catalyzed by O3ReOSiPh3. Scheme 2 presents
Scheme 2
.
Prins Reaction with Aliphatic and Aromatic
Aldehydes
the reaction between isobutyraldehyde and alcohol 1. The
expected product 8 is accompanied by side products 9 and 7
that arise from facile 2-oxonia-Cope rearrangements.8 The
outcome with an electron-rich aromatic aldehyde is very
different. Side-chain exchange products are much less prevalent,
and the expected Prins product was formed in 49% yield with
excellent equatorial alcohol selectivity.12,13 We believe that the
greater nucleophilicity of the perrhenate anion limits oxonia-
Cope rearrangement with aromatic aldehydes.14 The excellent
equatorial selectivity is consistent with previous observations,
where a more stable oxocarbenium ion leads to a less reactive
ion pair and better equatorial selectivity.
The reactivity with aromatic aldehydes was further investi-
gated (Table 2). Substituted alcohol 11-R and its enantiomer
11-S were used in this study.15 Both electron-rich and electron-
poor aromatic aldehydes gave good yields and selectivities for
the equatorial alcohol Prins product. In entries 2 and 4, the
a Enantiomeric series (11-S alcohol, 99% ee). b 97% ee by HPLC anaylsis.
c 10 mol % of O3ReOSiPh3. d Starting material was also recovered in 11%
yield.
(12) This result stands in contrast to our Prins reaction approach to
kendomycin, where an electron-rich aromatic ring led to side-chain exchange
products were demonstrated to have high optical purities.
Remarkably, the unprotected phenol in entry 3 worked very
well in the reaction. The very electron-rich phenol in entry 7
did give some Prins product, but only very slowly. The
corresponding sulfonyl-protected phenol gave an excellent yield
of equatorial THP product. The aldehydes in entries 7 and 8
were previously investigated in our approach to kendomycin,13
but the Re(VII)-catalyzed Prins cyclization conditions appear
to be both milder and more general. Electron-withdrawing
substituents (entries 9 and 10) lead to slightly lower yields than
with electron-rich aldehydes.
products as the major component in a TFA/DCM promoted reaction (ref 13)
.
(13) Bahnck, K. B.; Rychnovsky, S. D. Chem. Commun. 2006, 2388–
2390
.
(14) The aliphatic aldehyde oxonia-Cope rearrangement evident in Scheme
2 should be thermoneutral, whereas the aromatic oxonia-Cope should favor
the aromatic oxocarbenium ion. In theory, this effect might suppress oxonia-
Cope side reactions with aromatic aldehydes, but in our experience, it does
not, perhaps because of a slower Prins cyclization with aromatic aldehydes.
See ref 13 for an example.
(15) (a) Nokami, J.; Nomiyama, K.; Matsuda, S.; Imai, N.; Kataoka, K.
Angew. Chem., Int. Ed. 2003, 42, 1273–1276. (b) Nokami, J.; Ohga, M.;
Nakamoto, H.; Matsubara, T.; Hussain, I.; Kataoka, K. J. Am. Chem. Soc.
2001, 123, 9168–9169.
Org. Lett., Vol. 10, No. 21, 2008
4841

A common problem with aliphatic aldehydes is the generation
of side-chain exchange products, as shown in Scheme 2. The
Re(VII) conditions appear to minimize this exchange with
aromatic aldehydes. An indirect solution for selective Prins
cyclization of aliphatic aldehydes might be developed using
unsaturated aldehydes, followed by reduction or functionaliza-
tion of the alkene.16 A few representative unsaturated aldehydes
were investigated, and the results are shown in Scheme 3.17
Because of ambiguities in the mechanism, we decided to test
other Re(VII) catalysts, and the results are shown in Scheme
4. Solutions of 65-70% aqueous perrhenic acid18 are equally
Scheme 4. Comparison of Different Re(VII) Catalysts
Scheme 3. Prins Reaction with Unsaturated Aldehydes
effective as O3ReOSiPh3 in the Prins cyclization of alcohol 11-R
and p-anisaldehyde (19). Re2O7 is also effective for catalyzing
Prins reactions. In both cases, these commercially available
Re(VII) compounds lead to outcomes essentially indistinguish-
able from O3ReOSiPh3. Perrhenic acid is a strong acid with a
pKa ) -1.25,19 so an acid-catalyzed activation of hemiacetal
3 (Scheme 1) is certainly possible. An indirect test for perrhenate
ester formation was conducted by exploring the allylic alcohol
rearrangement of allylic alcohol 21.3 Both O3ReOSiPh3 and
perrhenic acid lead to efficient rearrangement to conjugated
allylic alcohol 22 under similar conditions, suggesting that
65-70% aqueous perrhenic acid is capable of forming perrhe-
nate esters under these conditions.18 Thus perrhenic acid may
be active through the same mechanism (Scheme 1) proposed
for O3ReOSiPh3.
All three aldehydes worked well in the reaction, although the
more complex aldehyde 17, prepared by a metathesis reaction
between crotonaldehyde and the corresponding terminal alkene,
was noticeably slower than the others. All of the products
showed very good selectivity for the equatorial alcohol THP
products. Unsaturated aldehydes will be useful for forming
complex THP fragments suitable for natural product synthesis.
Several details of the catalytic cycle are not clear at this time,
but a few control experiments have placed some limits on
possible mechanisms. The Prins reaction in Table 1 in DCM
returned only starting material when proton scavenger 2,6-di-
tert-butyl-4-methylpyridine (2,6-DTBMP) or 4 Å sieves were
added. Grubbs reported that hindered tertiary amines also halted
the allylic isomerization by O3ReOSiPh3.3 Addition of water
stopped the reaction.18 Added CSA had no effect on the
reaction. The basic path of the reaction presumably follows the
mechanism outlined in Scheme 1. One significant unanswered
question is whether the activation of the hemiacetal 3 is due to
perrhenate ester formation or simple acid-promoted solvolysis.
Re(VII) complexes are uniquely effective catalysts for the
Prins cyclization. These catalysts will be useful for Prins
cyclizations and will be of interest for the development of other
catalytic processes.
Acknowledgment. This work was supported by the National
Cancer Institute (CA-081635) and by a generous gift from the
Schering-Plough Research Institute. K.T. was supported by a
fellowship from the Royal Thai Government.
(16) Chan, K.-P.; Ling, Y. H.; Loh, T.-P. Chem. Commun. 2007, 939–
941.
(17) A Prins reaction between alcohol 1 and cinnamaldehyde led to the
expected product in 40% yield and the side-chain exchanged product 7 in 18%
yield. Further work needs to be done to make Prins reactions with terminal
alkenes practical.
Supporting Information Available: Characterization data
and experimental procedures for all compounds described are
included. This material is available free of charge via the Internet
at http://pubs.acs.org.
(18) The “wet” reaction was run in water-saturated DCM. At 27 °C, the
concentration of water would be 0.137 M (0.186 wt %): Stephenson, R. M.
J. Chem. Eng. Data 1992, 37, 80–95 With a catalyst concentration of 0.005
M, the molar ratio of water/Re(VII) would be ca. 27:1. For comparison, the
molar ratio of commercial 70 wt % perrhenic acid is about 6:1 water/Re(VII).
Presumably the difference in the water concentration led to the difference in
outcome.
OL8019204
(19) Bailey, N.; Carrington, A.; Lott, K. A. K.; Symons, M. C. R.
J. Chem. Soc. 1960, 290–297.
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