The preparation of simplified scyphostatin analogues using a tethered aminohydroxylation (TA) strategy

Article abstract of DOI:10.1016/j.tetlet.2004.07.012

The first tethered aminohydroxylation reaction employing a tertiary alcohol is reported as part of a route to prepare analogues of the naturally occurring sphingomyelinase inhibitor, scyphostatin. The tethered aminohydroxylation of 1-allylcyclohexanol produces the β-amino alcohol product, in protected form, with the required regiochemistry. Two approaches to the installation of the lipophilic side chain are described and the successful route used to prepare five novel scyphostatin analogues, one containing the natural lower side chain.

Full text of DOI:10.1016/j.tetlet.2004.07.012

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6661–6664
The preparation of simplified scyphostatin analogues using
a tethered aminohydroxylation (TA) strategy
Martin N. Kenworthy, Graeme D. McAllister and Richard J. K. Taylor*
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Received 12 March 2004; accepted 2 July 2004
Available online 22 July 2004
Abstract—The first tethered aminohydroxylation reaction employing a tertiary alcohol is reported as part of a route to prepare ana-
logues of the naturally occurring sphingomyelinase inhibitor, scyphostatin. The tethered aminohydroxylation of 1-allylcyclohexanol
produces the b-amino alcohol product, in protected form, with the required regiochemistry. Two approaches to the installation of
the lipophilic side chain are described and the successful route used to prepare five novel scyphostatin analogues, one containing the
natural lower side chain.
Ó 2004 Elsevier Ltd. All rights reserved.
Scyphostatin (1) was isolated in 1997 by Ogita and
co-workers from a mycelial extract of Dasyscyphus moll-
O
issimus SANK-13892 and its gross structure determined
by extensive spectroscopic and derivatisation studies.1
Unambiguous assignment of the stereochemistry of the
side-chain methyl groups, together with the total synthe-
sis of the side-chain acid, was published by Hoye and
Tennakoon2 in 2000, and an improved synthesis was re-
cently published by our group.3 Scyphostatin was found
to have potent inhibitory activity of neutral sphingo-
myelinase (N-SMase), and was also found to possess a
good level of acidic sphingomyelinase (A-SMase) inhibi-
tion. It has been proposed that this activity could assist
in the therapy of inflammation and autoimmune
diseases.1
O
HO
HN
OH
O
Scyphost at in 1
O
O
HO
HO
HN
OH
O
2
3
The unusual enzyme inhibitory properties of scyphosta-
tin, together with its challenging structure, have gener-
ated considerable interest from synthetic and medicinal
chemists. Initial synthetic approaches to the total
synthesis of this natural product concentrated on the
construction of the highly functionalised epoxy-cyclo-
hexenone core. Thus, wide-ranging approaches have
been employed to set up the absolute and relative chem-
istry of the epoxy-enone system; these include Ôchiral-
poolÕ approaches by the groups led by Gurjar 4a and Ka-
toh,4b,c an intramolecular bromo-etherification ap-
proach by Fujioka et al.4d and an oxidative approach
by Ohkata and co-workers.4e Our efforts in this area
have led to two syntheses of the allyl-substituted core
2, via an organometallic route5a and a chiral pool ap-
proach commencing from (À)-quinic acid.5b In addition
to synthetic approaches to the natural product, Giannis
and co-workers have pioneered efforts to synthesise
novel scyphostatin analogues in search of selective
inhibitors of N-SMase.6 A range of compounds were
prepared and several were found to exhibit selective irre-
versible inhibition of N-SMase. However, selective,
reversible inhibitors of N-SMase are still required.
Keywords: Scyphostatin analogues; Tethered aminohydroxylation;
Sphingomyelinase inhibitors.
*
Corresponding author. Tel.: +44-1904-432606; fax: +44-1904-4345-
23; e-mail: rjkt1@york.ac.uk
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.012

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M. N. Kenworthy et al. / Tetrahedron Letters 45 (2004) 6661–6664
In this paper we report the development of methodology
to access lower side-chain analogues of scyphostatin,
including 3, which possesses the complete scyphostatin
lower side chain. This methodology should aid on-going
structure–activity studies, and could provide an end-
game strategy for scyphostatin itself. Our aim (Scheme
1) was to use the Sharpless asymmetric aminohydroxyl-
ation (AA)7 to deliver the required b-amino alcohol
directly from an allyl-substituted cyclohexanol 4. Unfor-
tunately, AA reactions on terminal mono-substituted
alkenes have been found8 to deliver the terminal amino
group, in this case presumably giving product 5 (Scheme
1). Recently, an alternative regio-controlled tethered
aminohydroxylation reaction (TA) of allylic and
homo-allylic alcohols has been developed by Donohoe
et al.9 by employing an internally-disposed carbamate.
In this way the nitrogen source is delivered in an intra-
molecular manner, providing the required regioselectiv-
ity. We, therefore, considered using the TA methodol-
ogy on 4 as is shown in Scheme 1, providing the cyclic
carbamate 6. Protection of the primary alcohol and
installation of the unsaturated amide would deliver N-
acyl carbamate 7. We then hoped to use the electron-
withdrawing nature of the N-acyl group to cleave the
cyclic carbamate under mild, basic conditions10 to pro-
duce the required system 8. In this paper we report the
eventual success of a modified version of this strategy,
installing the b-amino alcohol functionality and hydro-
phobic side chain, starting from the parent allylic cyclo-
hexanol system 4, R=H. We then describe the
utilisation of this model system to prepare several novel
scyphostatin analogues.
Initial studies aimed at establishing the feasibility of this
approach are shown in Scheme 2. The known homo-al-
lylic alcohol 9,11 which was prepared in quantative yield
from cyclohexanone, was treated with trichloroacetyl
isocyanate. The carbamate 10 was then obtained by met-
hanolysis of the intermediate trichloroacetylcarbamate
in excellent overall yield. Treatment of 10 under the
standard TA conditions9 led to formation of cyclic
carbamate 11 in 40% yield, with recovery of 45% of
the starting material. This level of conversion is compa-
rable to many examples reported by Donohoe et al.9 It
should be noted that this is the first report of the TA
reaction employing a tertiary alcohol, further expanding
the range of substrates for this process.
On acquiring cyclic carbamate 11, we examined the pos-
sibility of installing the unsaturated amide side chain
and subsequent cleavage of the N-acyl carbamate
(Scheme 3). Protection of the primary alcohol was per-
formed with TBDPSCl delivering the silyl ether 12 in
good yield. Deprotonation of the nitrogen with n-butyl-
lithium at À78°C in THF, followed by treatment with
the mixed pivaloyl anhydride of crotonic acid gave the
N-acyl carbamate 13 in good yield. We had hoped at this
point to be able to cleave the cyclic carbamate employ-
ing mild basic conditions. To this end, the N-acyl carba-
mate was treated with LiOH or NaOH in THF at
0°C,10a but disappointingly conversion into amide 14
was not observed, and by-products owing to silyl group
cleavage were identified after extended reaction times.
TBDPSCl
NEt
3
O
DMAP
O
R
R
R
O
O
N
H
O
N
H
CH Cl
86%
2
2
i. prot ect
ii. acylat e
OH
O
OTBDPS
TA
12
O
11
HO
O
N
O
N
H
OPG
OH
-
7
6
4
Piv =
OH
i. nBuLi
THF
-78ÞC
R
O
ii.
O
72%
C( CH3)3
Sharpless
AA
OPiv
R
R
LiOH
or
NaOH
-correct
-wrong
regioisomer
regioisomer
-side-chain
incorporat ed
HO
HO
HO
HO
O
HN
X
HN
O
N
NHR
OPG
R
O
aq. THF
OTBDPS
OTBDPS
O
O
5
8
14
13
Scheme 1. Synthetic plan.
Scheme 3. Attempted carbamate cleavage.
n
Pr OH:H O
2
i.
4% K OsO H O
O
2
4. 2
11, 40%
(+ 45%
recovered
10)
i
5% Et N Pr
Cl C NCO
2
3
O
O
HO
ii. MeOH, K CO
NaOH
2
3
O
NH
O
N
H
t
2
92%
BuOCl
OH
10
9
Scheme 2. Tethered aminohydroxylation (TA) reaction.

M. N. Kenworthy et al. / Tetrahedron Letters 45 (2004) 6661–6664
6663
H O-MeOH
2
8:1
LiOH
Ph P( O) OC( O) R
2
O
HO
HN
HO
Ref lux
84%
NEt
THF
3
O
N
H
H N
2
OH
OH
OH
15
11
16a 70%
O
O
O
16b 72%
16c 89%
16d 74%
O
O
3 83%
Scheme 4. Synthesis of novel scyphostatin analogues.
Due to the failure of our first approach described in
Scheme 3, we investigated an alternative strategy for
the transformation of cyclic carbamate 11 into scyphost-
atin analogues. To this end, the direct hydrolysis of the
cyclic carbamate 11 was explored. We eventually found
that this could be achieved by heating 11 in aqueous
methanol with LiOH,12 delivering aminodiol 15 in 84%
yield. Direct amide formation from aminodiol 15 was
then carried out using the diphenylphosphinic chlo-
ride—derived anhydrides of various acids to give the
analogues 16a–d and 3 in good yield, with no protection
needed during the procedure. In this sequence, we used
commercially available crotonic acid, sorbic acid and
palmitic acid, along with octatrienoic acid13 and the nat-
ural scyphostatin side chain recently synthesised by our
group.3 In this way, a wide variety of side-chain struc-
tural analogues are available, from mono-unsaturated
16a to the more complex 16d and 3.14 It should be noted
that compound 3 is the first scyphostatin analogue
known, which possesses the complete southern hemi-
sphere functionality, that is, the aminodiol unit
acylated with the enantiomerically pure, natural lipo-
philic acid. These analogues will be useful in future
biological tests to understand further the effect of
structure on inhibitory activity (Scheme 4).
Acknowledgements
We gratefully acknowledge Dr. S. Handa (University of
Leicester) for unpublished experimental details. We
would also like to thank Elsevier (M.N.K.) and the Uni-
versity of York (G.D.M) for postdoctoral support.
References and notes
1. Tanaka, M.; Nara, F.; Suzuki-Konagai, K.; Hosoya, T.;
Ogita, T. J. Am. Chem. Soc. 1997, 119, 7871–7872.
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3. McAllister, G. D.; Taylor, R. J. K. Tetrahedron Lett. 2004,
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2000, 41, 7651–7655; (c) Izuhara, T.; Katoh, T. Org. Lett.
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Takagi, R.; Miyanaga, W.; Tamura, Y.; Ohkata, K. Chem.
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3237–3239; (b) Murray, L. M.; OÕBrien, P.; Taylor, R. J.
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2001, 137–140; (c) Arenz, C.; Gartner, M.; Wacholowski,
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Chem. 1999, 64, 9613–9624.
In conclusion, we have applied the TA reaction of a
homo-allylic tertiary alcohol 9 to the synthesis of a model
system of the core of scyphostatin (1). In this way we
were able to obtain the correct b-amino alcohol regio-
chemistry for the natural compound. We then used this
system to investigate a strategy for incorporation of the
unsaturated amide side chain into a late-stage synthetic
intermediate. Also, using our model system, we synthe-
sised five novel scyphostatin analogues 16a–d and 3,
which could give insights into the mode of action of scyp-
hostatin in future biological tests. We are currently
extending this methodology to evaluate its use as an
end-game strategy for the synthesis of scyphostatin itself.
9. (a) Donohoe, T. J.; Johnson, P. D.; Helliwell, M.; Keenan,
M. Chem. Commun. 2001, 2078–2079; (b) Donohoe, T. J.;
Johnson, P. D.; Cowley, A.; Keenan, M. J. Am. Chem.
Soc. 2002, 124, 12934–12935; (c) Donohoe, T. J.; Johnson,
P. D.; Pye, R. J. Org. Biomol. Chem. 2003, 1, 2025–2028.

6664
M. N. Kenworthy et al. / Tetrahedron Letters 45 (2004) 6661–6664
10. For cleavage of cyclic N-acyl carbamates see: (a) Evans,
D. A.; Britton, T. C.; Allman, J. A. Tetrahedron Lett.
1987, 28, 6141–6144; (b) Kende, A. S.; Koch, K.; Dorey,
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14. Example characterisation 16b, white powder (Et2O), mp
120–122°C. mmax (film)/cmÀ1 3365, 3285 (OH and NH),
2924, 2853 (CH), 1655, 1625, 1600 (amide and conjugated
alkenes). dH (400MHz, CD3OD) 7.10 (1H, dd, J 10.5,
15.5), 6.22 (1H, m), 6.12 (1H, dq, J 15.5, 6.5), 5.90 (1H, d,
J 15.5), 4.16 (1H, m), 3.51 (2H, d, J 9), 1.84 (3H, d, J 6.5),
1.73–1.24 (12H, m). dC (100MHz; CD3OD) 168.7, 142.2,
138.7, 131.1, 123.0, 71.8, 66.3, 1C signal under CD3OD,
44.5, 39.3, 37.8, 26.9, 23.3, 23.2, 18.6. (Found: MH+,
268.1905. C15H25NO3 requires MH+, 268.1913).