Extension of ring switching strategy to the glutamate antagonist 2-(pyrimidin-2,4-dione-5-ylmethyl)-(2S)-glycine and related compounds with two chiral centres

Article abstract of DOI:10.1016/S0040-4039(00)01800-1

2-(Pyrimidin-2,4-dione-5-ylmethyl)-(2S)-glycine 8 has been prepared by treatment of the pyroglutamate urea 12 with mild base, followed by deprotection in a modification of our ring switching approach to the synthesis of glutamate antagonists. The product is an isomer of the natural product willardiine 7. Use of this two step strategy has allowed us to synthesise L-alanine derivatives, which are β-substituted by a reduced pyrimidinedione containing a second chiral centre. There is little difference between the diastereoisomers of one of these compounds as antagonists at metabotropic glutamate receptors. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01800-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 10153–10158
Extension of ring switching strategy to the glutamate
antagonist 2-(pyrimidin-2,4-dione-5-ylmethyl)-(2S)-glycine
and related compounds with two chiral centres
Andrew Dinsmore†, Paul M. Doyle‡, Peter B. Hitchcock and Douglas W. Young*
Sussex Centre for Biomolecular Design and Drug Development, University of Sussex, Falmer,
Brighton BN1 9QJ, UK
Received 6 September 2000; accepted 11 October 2000
Abstract
2-(Pyrimidin-2,4-dione-5-ylmethyl)-(2S)-glycine 8 has been prepared by treatment of the pyroglutamate
urea 12 with mild base, followed by deprotection in a modification of our ring switching approach to the
synthesis of glutamate antagonists. The product is an isomer of the natural product willardiine 7. Use of
this two step strategy has allowed us to synthesise -alanine derivatives, which are b-substituted by a
L
reduced pyrimidinedione containing a second chiral centre. There is little difference between the
diastereoisomers of one of these compounds as antagonists at metabotropic glutamate receptors. © 2000
Elsevier Science Ltd. All rights reserved.
L
-Alanine derivatives substituted at the b-carbon with a heterocyclic ring are of interest
because of their potential for interaction with excitatory glutamate receptors in the central
nervous system and because of their involvement in many other biological processes. We have
recently devised a novel ‘ring switching’ strategy to allow for the versatile synthesis of
homochiral compounds with structures typical of glutamate agonists and antagonists.1–3 In this
synthesis, reaction of a protected 4-formylpyroglutamate ester 3 with a bisnucleophile gave rise,
on deprotection, to a variety of homochiral heterocyclic amino acids such as 4, 5 or 6, as shown
in Scheme 1.
* Corresponding author.
†
Present address: Department of Chemistry, University of Witwatersrand, PO WITS 2050, South Africa.
Present address: BioFocus plc, 130 Abbott Drive, Sittingbourne Research Centre, Sittingbourne, Kent ME9 8AZ,
‡
UK.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01800-1

10154
Scheme 1.
The reported biological activity of the natural product willardiine 74,5 made the synthesis of
isomeric pyrimidinediones of interest. 2-(Pyrimidin-2,4-dione-5-ylmethyl)-(2S)-glycine
8
has been erroneously reported in the secondary literature6 as the structure of a naturally
occurring compound present in pea seedlings. We showed2,3 that reaction of an enol ether of
3 with formamidine, benzamidine, acetamidine and guanidine gave rise, after deprotection,
1
to the pyrimidinones 5 (R=H, Ph, Me and NH2). The H NMR spectra of these compounds
showed unusual conformational isomerism,3 and a single crystal X-ray structure reported
here7 (Fig. 1(a)) has confirmed the structure of the dihydrochloride hydrate of 5 (R=H).
However, all attempts to prepare the pyrimidinedione 8 and related compounds by reacting
the poorer nucleophiles urea and thiourea with the pyroglutamate aldehyde 3 have proved
ineffective.2,3
Figure 1. X-Ray structures of (a) 5 (R=H)·2HCl·H2O; (b) 14 and (c) 19 (R=R1=Me, X=S)

10155
An alternative synthetic approach to pyrimidinedione analogues of willardiine was required.
Arguing that, if a bond could be made between the carbon atom of the aldehyde and a nitrogen
atom from the poor nucleophile, then the subsequent ‘ring switching’ process might be possible,
we prepared the secondary enaminone 9 (R=CH3), umax 315 nm, by treating aldehyde 3
(R=CH2Ph) with excess methylamine at room temperature for 15 min. The enaminone was
entirely the E-isomer 9 (R=Me) in the crystalline state but formed a mixture of E- and
Z-isomers on standing in solution. The signal for the NH proton at l ca. 7.8 ppm for the
Z-isomer was over 3 ppm downfield from the corresponding proton (l 4.42 ppm) in the
E-isomer, consistent with the hydrogen bonding expected in the former isomer. The enaminone
9 (R=Me) reacted with chlorosulfonyl isocyanate to afford the urea 10 (R=CH3) as a glass,
[h]D +11 (c 0.6, CHCl3), umax 274 nm§ in 88% yield. Attempts to cause this compound to undergo
‘ring switching’ by thermolysis failed completely and heating to reflux in K2CO3/EtOH seemed
merely to remove the protecting groups (Scheme 2).
Scheme 2.
Because failure of the urea 10 (R=CH3) to undergo ‘ring switching’ might be due to it being
fixed as the E-isomer, the corresponding urea 10 (R=H) was prepared from the primary
enaminone 9 (R=H). On treatment with base, this should be capable of yielding the Z-isomer
required for cyclisation. The primary enaminone 9 (R=H), mp 82–85°C, [h]D −33 (c 1, CHCl3),§
was prepared in 68% yield by reacting the unstable aldehyde 3 (R=CH2Ph) with NH4OAc/
1
,
AcOH in the presence of 3 A molecular sieves. The H NMR spectrum at room temperature
showed the olefinic proton at l 6.67 ppm, coupled to the NH2 protons, which were not apparent
in the spectrum at this temperature but were evident at 60°C as a broad signal at l 6.16 ppm,
and at −50°C as individual absorptions due to the NH protons of the Z-isomer. Reaction of the
enaminone 9 (R=H) with chlorosulfonyl isocyanate gave the urea 10 (R=H), mp 199–200°C,
[h]D +17 (c 0.8, CHCl3)§ in 67% yield. This was heated to reflux in K2CO3/EtOH, but again
deprotection appeared to be the only reaction.
§ These compounds had the expected analytical and spectroscopic properties.

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Since instability of the protecting groups seemed to be a problem, we prepared the tert-
butoxycarbonyl tert-butyl ester 11§ in 40% overall yield, by first reacting the corresponding
unfunctionalised diprotected pyroglutamate8 with lithium hexamethyldisilazide, followed by
treatment with methyl formate to yield the intermediate aldehyde as an oil and then reacting this
§
,
with NH4OAc/AcOH and 3 A molecular sieves. The primary enaminone 11 had similar
spectroscopic properties at room temperature, 60 and −50°C to those exhibited by 10 (R=H).
Reaction with chlorosulfonylisocyanate and flash chromatography of the product on silica gel
gave the Z-isomer 14,§ mp 183–185°C, [h]D −31.6 (c 0.6, MeOH) in 18% yield, and the E-isomer
12,§ mp 175–177°C, [h]D +13 (c 0.5, MeOH) in 22% yield. The stereochemistry and structure of
the isomers was indicated by NOE experiments. Irradiation of the olefinic proton, H-6, at 7.24
ppm in the spectrum of the Z-isomer 14 caused a 1.98% NOE in one of the absorptions due to
H-3, whereas irradiation at the exchangeable NH signal at l 8.99 ppm in the spectrum of the
E-isomer 12 caused small enhancements in both protons H-3. The NH signal in the Z-isomer
14 was 0.7 ppm downfield from that in the E-isomer 12, indicating hydrogen bonding in 14. The
stereochemistry was confirmed by single-crystal X-ray structure analysis of the Z-isomer 14,9
,
shown in Fig. 1(b), where the distance between the lactam carbonyl group and NH was 1.98 A.
When the E-urea 12 was heated at reflux in ethanol containing 1 equiv. of K2CO3, ‘ring
switching’ was finally accomplished and the protected pyrimidin-2,4-dione 15§ was obtained in
57% yield. Deprotection of the pyrimidine-2,4-dione using hydrochloric acid gave the amino
acid 8§ in quantitative yield, mp 245°C (decomp.), [h]D −11.6 (c 1.2, H2O).
All of the compounds which we have prepared to date with potential for interaction with
glutamate receptors have been in the -series and have contained but one asymmetric centre. It
L
was of interest to see whether compounds containing a second asymmetric centre in the
heterocyclic ring might be biologically active and whether biological activity might be related to
stereochemistry. We therefore reduced the secondary enaminone 9 (R=Me) with sodium
cyanoborohydride in methanol at pH 3–4.6 using screened methyl orange and 1 M HCl to
maintain pH as shown in Scheme 3. The diastereoisomeric amines 16 (R=Me) were obtained as
an inseparable mixture. Reaction with phenylisocyanate gave the corresponding ureas 17
(R=Me, R1=Ph, X=O) and 18 (R=Me, R1=Ph, X=O) in 90% yield and these could be
Scheme 3.

10157
separated chromatographically and assigned stereochemistry using NOE experiments. The
ratio of trans 17 to cis 18 isomers was 7:3. Although treatment of the major isomer 17
(R=Me, R1=Ph, X=O) with sodium hydride in THF caused ‘ring switching’ in 40% yield,
this was accompanied by epimerisation to give a mixture of 19 (R=Me, R1=Ph, X=O) and
20 (R=Me, R1=Ph, X=O) which could be separated chromatographically. Thermal rear-
rangement could be achieved without epimerisation, albeit in only 20% yield, but this allowed
the stereochemistry of the products to be assigned. Deprotection by hydrogenolysis gave the
free amino acids 21 (R=Me, R1=Ph, X=O) and 22 (R=Me, R1=Ph, X=O) and these were
found to be equally weakly antagonistic to the action of the metabotropic agonist trans-
aminocyclopentanedicarboxylic acid (ACDP) in Purkinje rat cells.10
Reaction of the amines 16 (R=Me) with KCNO/HOAc gave the ureas 17 (R=Me, R1=H,
X=O) and 18 (R=Me, R1=H, X=O) in 86% yield as a pure mixture of diastereoisomers,
which could be rearranged to the protected heterocyclic amino acids 19 (R=Me, R1=H, X=O)
and 20 (R=Me, R1=H, X=O) using NaH in 90% yield. These could be deprotected by
hydrogenolysis, giving the amino acids 21 (R=Me, R1=H, X=O) and 22 (R=Me, R1=H,
X=O) in 90% yield. Reaction of the amines 16 (R=Me) with methylisothiocyanate in CH2Cl2
gave an inseparable 7:3 mixture of the diastereoisomeric thioureas (17, R=R1=Me, X=S) and
18 (R=R1=Me, X=S) in 72% yield. Thermal rearrangement gave the ‘ring switched’ products
19 (R=R1=Me, X=S) and 20 (R=R1=Me, X=S) in 75% yield in unchanged ratio, and these
were separated chromatographically. Single-crystal X-ray structural analysis (Fig. 1(c))11 con-
firmed that the major isomer was the (2S,4S)-isomer 19 (R=R1=Me, X=S). All attempts to
deprotect the thiopyrimidones to obtain the corresponding amino acids were unsuccessful.
Reduction of the primary enaminone 11, reaction with KCNO/HOAc, ring switching and
deprotection has also allowed us to access 21 (R=R1=H, X=O) and 22 (R=R1=H, X=O).
Acknowledgements
We thank the E.P.S.R.C. and GlaxoWellcome for a CASE studentship (to A.D.).
References
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7. Crystal data for 5 (R=H)·2HCl·H2O: C7H13Cl2N3O4, orthorhombic, P212121 (No. 19), a=5.9010(10), b=
,
11.953(2), c=16.801(6) A, Z=4, R1=0.033 for 1498 reflections with I>2|(I). The atomic coordinates are
available on request from The Director, Cambridge Crystallography Data Centre (CCDC), University Chemical
Laboratory, Lensfield Road, Cambridge CB2 1EW, UK.
8. August, R. A.; Khan, J. A.; Moody, C. M.; Young, D. W. J. Chem. Soc., Perkin Trans. 1 1996, 507–514.
9. Crystal data for 14: C16H25N3O6, orthorhombic, P212121 (No. 19), a=9.644(3), b=18.766(4), c=21.049 (5) A,
,
Z=8, R1=0.086 for 2664 reflections with I>2|(I). The atomic coordinates are available on request from The
Director, Cambridge Crystallography Data Centre (address, Ref. 6).
.

10158
10. The assay of East, S. J.; Garthwaite, J. Eur. J. Pharmacol. 1992, 219, 395–400 was performed by Dr. A. Batchelor
of the Wellcome foundation. 100 mm of compound 21 (R=Ph, X=O) elicited 85% diminution in the response
of 50 mm ACDP, whereas 100 mm of compound 22 (R=Ph, X=O) elicited 76% diminution in this response.
11. Crystal data for 19 (R=Me, X=S): C24H27N3O5S, orthorhombic, P212121 (No. 19), a=11.315(3), b=10.348(4),
,
c=23.186(6) A, Z=4, R1=0.130 for 3371 reflections with I>2|(I). The atomic coordinates are available from
The Director, Cambridge Crystallography Data Centre (address, Ref. 6).
.
.