Synthesis of enantiomerically pure quaternary glutamic acids via chemo-and diastereoselective alkylation of (5 S )-3-([2– methoxycarbonyl]ethyl)-5-phenyl-5,6-dihydro-2 H -1,4-oxazin-2-one [i]

The imine moiety of (5S)-3-([2-methoxycarbonyl]ethyl)-5-phenyl-5,6-dihydro-2H-1,4-oxazin-2-one 2 undergoes highly diastereocontrolled reduction and Lewis acid–mediated chemo-and diastereoselective nucleophilic addition of Grignard reagents. Subsequent dismantling of the cyclic template permits ready access to enantiomerically pure glutamic acid analogues


Introduction
We have previously reported the formation of a-substituted amino acid derivatives, via an approach in which enantiomerically pure 3-substituted 5-phenyl-5,6-dihydro-2H-1,4-oxazin-2-one 1a, prepared by cyclisation of (2S)-phenylglycinol with α-keto ester derivatives, underwent diastereocontrolled nucleophilic attack by Grignard reagents using Lewis acid activation to facilitate chemoselection between the imine and lactone functionalities. 2 The parent 3,4-dehydrooxazinone 1b, lacking 3-substitution, cannot be prepared by the direct approach but may be obtained via one-pot bromination-dehydrobromination of 5-phenyl-3,4,5,6-tetrahydro-2H-1,4-oxazin-2-one, 2 and has also been prepared by oxidative rearrangement of 2-methyl-4-phenyl-4,5-dihydrooxazole.2Although 1b is readily isomerised to 5-phenyl-3,6-dihydro-2H-1,4-oxazin-2-one in the presence of mild base, with concomitant loss of chirality, we were again able to achieve chemo-and diastereocontrolled imine alkylation using Grignard reagents after low temperature pre-complexation with boron trifluoride etherate, and to convert these adducts subsequently to enantiomerically pure a-amino acids (Scheme 1). 2 The chemoselectivity demonstrated by substrates 1 in our studies is complementary to that observed by Molinski on related substrates in the absence of Lewis acid, when phenylmagnesium bromide was shown to react via initial attack at the lactone of the 5,6-dihydro-2H-1,4-oxazin-2-one system. 3   During a recent programme, we required access to a series of a-substituted D-glutamic acid derivatives for structure-activity relationship studies.Glutamic acid plays a key rôle in the neuronal biochemistry of the mammalian central nervous system, being the main excitatory transmitter.It excites virtually all central neurones by activating the kainate, α-amino-3hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) ionotropic receptor subclasses, mediating intercellular signal transmission. [i][ii] However, excessive release of glutamate leads to neuronal damage, which has been linked with a variety of neurodegenerative disorders.To allow a greater insight into the structural and physiological characterisation of these receptors and increase understanding of conditions such as Alzheimer's and Parkinson's diseases, the synthesis of unnatural glutamate analogues has received much attention. [iii] Our alkylation approach appeared to promise a convenient and direct means of access to the desired quaternary glutamic acid analogues.However, the requisite precursor, (5S)-3-([2-methoxycarbonyl]ethyl)-5-phenyl -5,6-dihydro-2H-1,4oxazin-2-one 2, possesses a side chain ester and this caused concern as to whether the chemoselective imine alkylation previously observed with substrates 1 could be achieved.

O
Reaction of (2S)-phenylglycinol with dimethyl 2-oxoglutarate in 2,2,2-trifluoroethanol at reflux in the presence of activated 3Å molecular sieves according to our previous protocol1 afforded (5S) -3-[2-(methoxycarbonyl)ethyl]-5-phenyl-5,6-dihydro-2H-1,4-oxazin-2-one 2 in 60% purified yield. [iv] To assess the degree of diastereocontrol possible with this substrate without the complication of chemoselectivity associated with alkylation studies, hydrogenation of the imine function was effected under one atmosphere of hydrogen over Adams' catalyst.In contrast to our previously observed moderate diastereoselection in the hydrogenation of 3-ethyl-5-phenyl-5,6-dihydro-2H-1,4-oxazin-2-one, ii[v] hydrogenation furnished the 3,5-syn-disubstituted tetra-Hydrooxazinone 3 in higher than 94% d.e. according to integration of the proton NMR spectrum of the crude reaction mixture, the pure major diastereomer being isolated in 89% recrystallised yield.Subsequent degradation of the cyclic template using our standard conditions [H2/Pd(OH)2-C, aq MeOH, TFA2] furnished a two component mixture shown to consist of glutamic acid and pyroglutamic acid.However, heating this mixture in 5M HCl (conditions known not to cause racemisation [vi] ) afforded pure D-glutamic acid 4 in 97% isolated yield {[α]D 23 = -22.6 (c 0.31, H2O); lit for Lenantiomer [vii] [α]D 22 = +24.4(c 6.0, H2O)} (Scheme 2).This overall conversion can be considered to constitute a biomimetic pathway, since L-glutamic acid is synthesised in vivo by reductive amination of 2-oxoglutarate in the presence of glutamate dehydrogenase.Having established effective diastereocontrol in hydrogenation of 2 we turned our attention to alkylation of the cyclic imine whereupon initial efforts were disappointing.A series of conditions in a range of solvents using varying equivalents of Lewis acids, including those conditions that had previously proven successful, resulted in complex mixtures or recovered starting material.Aware that complexation of up to three equivalents of Lewis acid would lead to a sterically very encumbered substrate, we turned our attention to the use of aromatic solvents in an attempt to accelerate reaction.Accordingly, precomplexation of 2 with 3 equivalents of boron trifluoride etherate in toluene at -78 o C, followed by the addition of 1.2 equivalents of benzylmagnesium bromide and usual work-up after 5 hours furnished a single adduct in 74% recrystallised yield (Scheme 3).Spectroscopic and X-ray crystallographic analysis8 (Figure) confirmed its structure as that of the desired adduct 5a.Close scrutiny of the crude reaction mixture showed this to be the only diastereoisomer present, the rationale for the diastereocontrol being in accordance with our prediction based upon axial approach of the nucleophile to a conformationally locked substrate from the opposite face to the phenyl substituent. 2 Applying this modified protocol more generally always led to excellent chemoselectivity, but it was observed that increasing steric bulk at the α-position of the Grignard reagent led to a decrease in diastereoselectivity. Thus, whereas methyl 5b and ethyl 5c (Figure ) adducts were obtained diastereoisomerically pure (40% and 74% purified yields respectively), the diastereomeric isobutyl adducts 5d, 6d were obtained a ratio of 10.3 : 1.0, with the major isomer being isolated in recrystallised 43% yield.Likewise, the isopropyl adducts 5e, 6e were obtained as a 20.6 : 1.0 mixture of diastereoisomers (major isomer 37% isolated yield) and the tert-butyl adducts 5f, 6f were obtained in a 3.0 : 1.0 ratio (major isomer 39% isolated yield) (Table ).

Figure 1
This reduction in diastereocontrol is presumably a reflection of the greater tendency for the bulkier nucleophiles to approach 2 via an equatorial trajectory.Notwithstanding this fall of in stereocontrol, the pure major adducts 5a-e could be simply isolated from the reaction mixture by chromatography and subsequent hydrogenolytic degradation of the tetrahydrooxazinone templates proceeded uneventfully, for the main part, using the two step procedure described above to furnish the D-α-substituted glutamic acids as their hydrochloride salts 7a -7e in excellent yields.However, it proved impossible to isolate the t-butyl derivative 7f free of its corresponding anhydride (Scheme 3) iii, iv Scheme 3. Reagents and conditions: i, BF3Bt2O (3 equiv.).toluene, -78 ˚C, 2h; ii, RMgX (1,5 equiv.),-78 ˚C, 4h; iii, H2 (5 bar), Pd(OH)2 /-C, TFA, aq.MeOH, r.t.; iv, 5MHCl, d.
In summary, this three step procedure permits ready access to a wide range of diastereoisomerically and enantiomerically pure α-substituted glutamic acid derivatives via chemo and diastereocontrolled functionalisation of the glutamic acid a-cation synthetic equivalent 2 Experimental Section  CHCl3).

General method for preparation of glutamic acids
To a solution of the requisite tetrahydro-oxazinone (0.50 mmol, 1 equiv.) in methanol (5 mL) within a Fischer-Porter bottle was added Pearlman's catalyst (1 equiv.by mass), trifluoroacetic acid (0.1 mL) and water (0.5 mL).The solution was degassed, the bottle was pressurised with hydrogen to 5 bar and the reaction mixture stirred rapidly for 24 h at room temperature.After depressurisation, the suspension was filtered through Celite ® and the solvent evaporated in vacuo to afford the crude amino acid.This was dissolved in aqueous hydrochloric acid (5 M, 3 mL) and the solution heated at reflux for 2 h.The reaction mixture was allowed to cool and the solvent evaporated in vacuo to afford the amino acid hydrochloride salt.