Synthesis of Tetramates

Amino esters 52b-e were reacted with keteneylidenetriphenylphosphorane 1a to generate pyrrol-2-one derivatives, tetramates 69a-d:

NH₂

This reaction can be referred to as a domino reaction^[77] where a) the amine group of 52 adds across the C^α=C^β of 1a forming acyl ylide 67 and b) the ylide and ester

functionalities of 67 react via an intramolecular Wittig olefination (IMWO) generating an unstable 4-membered oxaphosphetane intermediate 68, which decomposes to the stable tetramate 69 and triphenylphosphine oxide. Reaction of the amino ester and ylide is initiated by the addition of a catalytic amount of benzoic acid which protonates ylide 1a at C^α, rendering C^β more reactive towards nucleophiles, while the intramolecular Wittig reaction is promoted by heating. Both steps occur as part of a 'one-pot' reaction under the conditions shown (Scheme 28). Formation of the 5-membered heterocycle was clearly indicated by the disappearance of the ester peak of 52 at around 1740cm^-1 in the IR spectrum and the appearance of a peak around 1680cm^-1, characteristic of tetramates 69.

Derivatives of L-isoleucine 52c and 52d reacted with ylide 1a generating the corresponding tetramates as mixtures of diastereoisomers. Taking advantage of the presence of a second chiral centre at the 5-sec-butyl group of 69b and 69c, ¹H-NMR spectroscopy was used to measure the extent of epimerisation at C-5. The ¹H-NMR spectrum of 69b showed two multiplet signals for 5-H at 4.0 and 4.1 ppm, the integrations for which revealed the diastereoisomeric ratio to be 1:4.25. The presence of two clear singlet signals at 5.35 and 5.39 ppm for N-H further consolidated this ratio.

The ¹H-NMR of 69c (Fig.6) shows two clear doublet signals at 4.08 and 4.15 ppm for 5-H of the diastereoisomers, in a 1:1 ratio. This ratio is also supported by the integrations of the two clear signals at 6.48 and 6.57 ppm for N-H.

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

7.25 6.57 6.48 4.15 4.14 4.08 4.08 0.980.960.95 0.870.86 0.74 0.73

4.15 4.10

4.15 4.14 4.08 4.08

1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65

0.98 0.96 0.95 0.94 0.89 0.87 0.86 0.74 0.73

N

H O

BnO

69c

5.04 1.00 1.022.04 1.02 1.27 2.10 2.97 1.44

0.29 0.15 0.14

Fig.6 ¹H-NMR spectrum of tetramate 69c

The absence of a second chiral centre in tetramates 69a and 69d prevented the detection of racemisation using ¹H-NMR spectroscopy.

Problems were encountered in the purification of tetramate 69c from the triphenylphosphine oxide side product, this is a well-documented setback when dealing with reactions of ylides^[78]. When applied to TLC plates, the crude mixture was difficult to separate, even with a variety of solvent systems. Both 69c and phosphine oxide travelled up the plate together appearing as an inseparable streak (Fig.7), even when well diluted.

Solvent system Tetramate69c Rf values

Ph3PO Rf values

1) Et2O 0.11 0.17

2) EtOAc 0.22 0.64

3) 1:1 Hex, EtOAc 0.08 0.1

4) 1:1 Et2O, EtOAc 0.35 0.42

5) 1:19 MeOH, EtOAc 0.62 0.68

6) 3:1:1 EtOAc, Et2O, Hex 0.26 0.28 Table 2 Rf values of 69c and Ph3PO with a variety of solvent systems

cm cm

2 4

PO PO

Fig.7 TLC plates for solvent systems 2 and 4 (Table 2); PO = pure Ph3PO, cm = crude 69c and PO mixture

The best separation was achieved with ethyl acetate as the developing solution, but when applied to column chromatography, the phosphine oxide was not completely removed.

The size of the glass column, the length of the silica plug and the method of application were all varied, but to no avail. The tetramate-phosphine oxide mixture was also applied to a preparative TLC plate and developed using ethyl acetate, but this failed to remove all of the phosphine oxide.

Extraction of either the desired product 69c or the phosphine oxide was attempted with various solvents, both cold and warm, but separation was not achieved due to the 'sticky' unmanageable consistency of the crude product. However, repetitive washing with a hexane-ethyl acetate mixture, followed by numerous recrystallisations, yielded pure 69c.

Poor yields were obtained using this method of purification and it would not be suitable for larger scale preparations of tetramates.

An 'acid-base' work-up was also tested in the hope that protonation of the tetramate nitrogen of 69c would allow easy extraction of the phosphine oxide. This was tested using a homogeneous system with various solvents, 2M solution of HCl in Et2O and a non-aqueous base, but the salt of 69c did not precipitate out of solution. A heterogeneous system was also tested, but again the consistency of the 69c-Ph3PO crude mixture proved problematic. Ph3PO is soluble in DCM but this solvent could not be used due to the slight solubility of the tetramate ammonium salt.

Hot extraction of the oxide from the crude mixture using a 'soxhlet apparatus' was also considered but the tetramate proved partially soluble in all hot solvents tested.

Attention was then turned to a polymer supported scavenger which is believed to completely remove alcohols, thiols, phosphines and phosphine oxide from reaction mixtures^[79,80]. The scavenger is Poly(ethylene glycol) derivative 72a and is synthesised by the nucleophilic substitution of Poly(ethylene glycol)-monomethylether 350 (PEG-MME 350) 70 on 2,4,6-trichloro[1,3,5]triazine (cyanuric chloride or TCT) 71. The reaction was carried out in benzene with butyl lithium^[80] as the base to activate 70 for reaction:

MeO(CH₂CH₂O)_nCH₂CH₂OH

(MeO-PEG-OH) N N

N Cl Cl

Cl

N N

N C

O

Cl

BuLi, C₆H₆, 1 h, r.t.

70; n = 350

71 72a

+ ^MeO-PEG

51%

l

Scheme 29

The reaction was also carried out in chloroform in the presence of DIPEA (diisopropylethyl amine or Hünigs base; (i-Pr)2NEt), as described by Taddei and Falchi^[79], but was difficult to optimise, even when the quantities of TCT 71 and DIPEA were varied:

Expt.

Table 3 Conditions applied to reaction in Scheme 29. All reactions were carried out at r.t. See abbreviations for details.

No more than a 20% yield was achieved using the quantities and conditions in experiment 3. Although, when DIPEA was substituted with butyl lithium^[80] and used in approx. equimolar amounts with a large excess of TCT 71 in pre-dried solvents, the reaction was much more successful (51%). It was crucial to the success of the reaction that the solvents used during the reaction and subsequent work-up were pre-distilled over drying agents directly before use and cyanuric chloride 71 was recrystallised a number of times, also directly before use.

The monosubstitution of PEG-DCT 72a was only confirmed when derivatised with excess benzylamine, producing the trisubstituted derivative 72b^[79]:

N N

It was possible to record a ¹H-NMR spectrum of 72b and the relative integrations of the signals for the methylene groups of the benzylamine residues and the terminal methoxy peak revealed a 4:3 ratio, which proved the substitution of only one chlorine atom of 71.

PEG-DCT 72a is valuable as a phosphine oxide scavenger because of its solubility, unlike many polymer supports, in many organic solvents such as acetonitrile, benzene and toluene, and it removes nucleophilic reagents or by-products simply by precipitation from the reaction mixture. It is not known how exactly this scavenger system works but it has been suggested that it forms species such as 72c or 72d^[80]: