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3.3.1 Retrosynthetic analysis

To achieve the main goals of our synthesis, our approach should ideally form the tetramic acid core of JBIR-22 (4) with the attached glutamic acid side chain early in the synthesis, as this is common to all members of the tetramic acid sub-family. This would be followed by incorporation of the divergent C3’-acyl moiety. This is in contrast to the synthetic approach most commonly used to access related tetramic acids such as equisetin (10), which generally form the decalin ring prior to formation of the tetramic acid core (Section 3.1.6). Therefore, to develop an efficient and practical route to this family of tetramic acids our synthetic strategy needed to address three main challenges:

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1.) Access to the different members of this family containing a decalin ring at the C3’ position would require a late-stage Diels-Alder reaction of the appropriate polyene chain or the incorporation of a pre-formed decalin moiety.

2.) Identification of a stable masked tetramic acid core onto which the polyene chain could be attached, thereby reducing the significant synthetic challenges associated with tetramic acids (Section 3.1.4).

3.) Development of a novel, asymmetric synthesis of the required 4,4-disubstituted glutamic acids.

The key step in our initial retrosynthetic strategy (Scheme 3.4) of JBIR-22 (4) was a late stage Diels- Alder cyclisation of the appropriate polyene derivative 25. By altering the polyene side chain, a range of different analogues with diverse ring systems such as zopfiellamide A (21) and Sch210972 (20) could be accessed (Figure 3.10). A range of tetramic acids with an uncyclised polyene side chain such as harzianic acid (19) could also be prepared (Figure 3.10).

Scheme 3.4. Retrosynthetic analysis of JBIR-22 (4).

This strategy required the identification of an advanced masked tetramic acid intermediate due to the considerable synthetic challenges associated with handling compounds containing the tetramic acid motif (Section 3.1.4). Inspiration for an appropriate masked tetramic acid came from Janda et al.’s non-enzymatic biosynthetic route to 3-acyltetramic acids from N-acylhomoserine lactones (AHL) (Section 3.1.5).179 Based on this transformation it was proposed that a biomimetic synthesis using 3- oxo-AHL derivative 26 which could undergo a late stage Lacey-Dieckmann cyclisation to reveal the desired tetramic acid 25 could be achieved (Scheme 3.4). It was envisioned that 26 could be

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obtained by an N-acylation of lactone 27 with the desired polyene side chain. Intramolecular cyclisation of the 4,4-disubstitued glutamic acid 28 (Scheme 3.4) would furnish 27, a key intermediate from which a divergent approach could lead to a large and diverse collection of tetramic acids (Scheme 3.4).

Harzianic acid (19) was identified as a being structurally related to 25, a precursor of JBIR-22 (4) in our retrosynthetic strategy. It was decided that it would therefore be a good initial target for development of this synthetic route (Scheme 3.4). The identification of a synthetic approach to harzianic acid (19) which could be readily adapted to access JBIR-22 (4) would provide the ideal validation of this approach as a general protocol for accessing the members of this tetramic acid sub- family. Furthermore, harzianic acid (19)’s anticancer and potent siderophoric properties make it an attractive and rewarding synthetic target in its own right (see Section 3.1.7).

3.3.2 A novel approach to 4,4-disubstituted amino acids

The first aim in our synthetic strategy was a concise and stereoselective synthesis of the 4,4- disubstituted glutamic acid 28. Although, there is no reported synthesis of the required 4-hydroxy-4-

isopropylglutamic acid (28), a chemical180 and an enzymatic route181,182 to the related 4-hydroxy-4- methylglutamic acid (29) have been described. Baldwin and Long180 devised a 5-step route to 4- hydroxy-4-methylglutamic acid (29) from Seebach’s tert butyl-substituted imidazolidinone 30183 via

cycloadduct 32 (Scheme 3.5). Whilst this approach did provide 29 in good yield and stereoselectivity, it was not ideal for our synthesis due to the length of the route and the cost of reagent 30. In addition, it would only provide access to the (S,S) and (R,R) diastereomers of 29 as it proceeds via

cycloadduct 32.

Scheme 3.5. Baldwin and Long’s synthesis of 4-hydroxy-4-methylglutamic acid (29).180 The cycloaddition reaction between the homochiral-derived nitrone 31 and the appropriate alkene afforded cycloadduct 32 which after subsequent elaboration provided 4-hydroxy-4-methylglutamic acid(29).

Instead, we focused on developing a novel synthesis of these 4,4-disubstituted amino acids. The simplicity and efficiency often obtained by an elegant biomimetic synthesis attracted our attention to a potentially biomimetic approach to these amino acids. Although the isolation of 4-hydroxy-4-

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(2S,4R) isomers of 4-hydroxy-4-methylglutamic acid (29) have been isolated from Pandanus veitchii184 and Ledenbergia roseo-aena185 respectively, with both diastereomers occurring in Phyllitis scolopendrium186. It is proposed that 4-hydroxy-4-methylglutamic acid (29) derives from a homo- aldol condensation of pyruvic acid (33), followed by transamination of the resulting 4-hydroxy-4- methyl-2-oxoglutaric acid (34)(Scheme 3.6).187

Scheme 3.6. Proposed biosynthesis of (2S,4R)-4-hydroxy-4-methylglutamic acid 29 from pyruvic acid 33 via an aldol condensation followed by a transamination.187

Based on this biosynthetic pathway our proposed synthetic strategy to access the protected 4- hydroxy-4-isopropylglutamic ethyl ester (35) would involve an asymmetric aldol reaction of ethyl pyruvate (36) and ethyl dimethylpyruvate (37) followed by a diastereoselective transamination (Scheme 3.7).

Scheme 3.7. Proposed biomimetic synthesis of protected 4-hydroxy-4-isopropylglutamic ethyl ester (35).

We believed that the transamination could be achieved with high selectivity using Ellman’s tert- butanesulfinamide 38. Since its introduction by Ellman in 1997 as a chiral ammonia equivalent,188

tert-butanesulfinamide 38 has proven to be a versatile chiral auxiliary which has found extensive use both in industry and academia.189 The condensation of tert-butanesulfinamide 38 with aldehydes and ketones yields the corresponding N-tert-butanesulfinyl aldimines and ketimines in good yield (Scheme 3.8). The tert-butanesulfinyl group activates these imines towards the addition of a range of nucleophiles and serves as a powerful chiral directing group to provide products with generally high diastereoselectivity.189 Subsequent removal of the tert-butanesulfinyl group under mild conditions cleanly provides the corresponding amines (Scheme 3.8).190

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Scheme 3.8. General sequence for the asymmetric synthesis of amines using tert-butanesulfinamide 38.189

In particular this method has become one of the most extensively used approaches for the asymmetric synthesis of amines in the development and production of drug candidates. We have successfully applied this methodology in the asymmetric synthesis of both enantiomers of the chiral amine MJ05 (see Chapter 5). Using this method, selective reduction of the imine formed by condensation of glutaric acid 39 and tert-butanesulfinamide 38 could provide both the (2S) or the (2R) diastereomers of 35 depending on the nature of the reducing agent used (Scheme 3.9).

Scheme 3.9. Proposed diastereoselective reductive amination of 39 via the N-tert-butanesulfinyl ketimine 40. (a) Condensation of 39 with (RS)-38. (b) Diastereoselective reduction of the N-tert-butanesulfinyl ketimine 40. (c) Acidic deprotection of the tert-butanesulfinyl group.

Access to the required glutaric acid 39 would require an asymmetric aldol reaction of ethyl pyruvate (36) and ethyl dimethylpyruvate (37). Due to the prominence of the aldol structural motif in numerous important natural and synthetic molecules, extensive research has been carried out to identify and optimise asymmetric conditions for the aldol reaction.191 This has resulted in the identification of numerous methods including the utilisation of chiral auxiliaries (e.g. Evan’s oxazolidinone192), organocatalysts (e.g. proline193) or transition metal-based catalysts such as chiral bisoxazoline copper(II) complexes which were used successfully by Gathergood et al.194 to catalyse a homo-aldol reaction of ethyl pyruvate.

Unfortunately, there has been limited development of a comprehensive and general methodology for the asymmetric cross aldol reaction involving the use of a chiral catalyst, due to the added complication of a competing homo-aldol condensation and the propensity of the product to participate in subsequent condensation reactions. These concerns limit the options for this reaction, resulting in the selection of a chiral auxiliary approach. Drawbacks to this approach include the need for a stoichiometric quantity of an expensive chiral reagent and the added steps required for its

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incorporation and removal. In an attempt to overcome these issues, we proposed using Ellman’s

tert-butanesulfinamide 38 as a chiral directing group for the aldol reaction. This would then be followed by a diastereoselective reduction to form the desired 4,4-disubstituted amino acid 35

(Scheme 3.10) in an analogous manner to that previously reported for the synthesis of simple 1,3- amino alcohols.195,196 The coupling of these two steps would improve the cost and efficiency of accessing the required amino acid 35.

Scheme 3.10. Retrosynthetic analysis of (2S,4S)-4-hydroxy-4-isopropylglutamic acid 35. Synthetic strategy would involve a (RS)-38 mediated asymmetric aldol condensation of 42 and 37 to furnish 40 which could then undergo a diastereoselective reduction to provide the desired amino acid 35.

In summary, our synthetic route to 35 would involve condensing (RS)-tert-butanesulfinamide 38 with ethyl pyruvate (36) to provide ketimine 42 (Scheme 3.10). An asymmetric aldol reaction between 42

and ethyl dimethylpyruvate (37) would provide the glutarate 40 which could be reduced selectively to form the desired 4-hydroxy-4-isopropylglutamate ethyl ester (35) after removal of the tert- butanesulfinyl group (Scheme 3.10).