Synthesis of -a-allokainic acid

The main lines of this approach were later embodied in an enantioselective synthesis of (—)-a-allokainic acid (Scheme 34) (179). The sole stereo center of die ene reaction starting material was derived from a glutamic acid derivative (132) to avoid loss of optical activity via double bond migration (see Scheme 33), the a acid function of kainic acid had to be reduced before the pyrolysis step... 

Pyrrolidines are attainable by [3 + 2] cycloaddition of azomethine ylides and olefins one such reaction is exemplified by a total synthesis of a-allokainic acid (Scheme 36) (182). 

An analogous result had previously been reported by Ito and co-work-ers in a racemic synthesis of a-allokainic acid.70 Unsaturated derivative 103 was reduced under heterogeneous hydrogenation conditions to give a 1 19 ratio of epimers 104 105 in an overall 90% yield, the C-2 methyl ester influencing the stereoselectivity (Scheme 44). 

The fundamental principles of asymmetric synthesis employed throughout also apply to ene reactions. Chiral additives, chiral auxiliaries, and the preparation of chirons all lead to good enantioselectivity when applied to the ene reactions. Most chiral ene reactions seem to involve addition of a chiral catalyst or the use of a chiral auxiliary. Oppolzer et al. utilized a chiral auxiliary in 713 to prepare 714 (90 % de) in a synthesis of (+)-a-allokainic acid.515 Yamamoto used the chiral aluminum catalyst 715 for the intermolecular ene reaction of 1,6-dichlorobenzaldehyde and 2-phenylthio-l-propene to give 716 in 96% yield (65 % ee).5i6 These catalysts are similar to those used in Section 11.9.B. A similar titanium catalyst (717) was used to couple methylenecyclohexane and methyl glyoxalate, giving 89% of 718 (98 % ee). The titanium catalyst... 

Pyrrolidines and Related Compounds.—a-Allokainic acid and a-kainic acid, products isolated from the marine algae Digenea simplex Ag., have aroused considerable interest not only for their anthelmintic properties but more importantly for their potent neurophysiological activity in mammals. The original synthesis of a-allokainic acid was long and gave a low overall yield, but now a short, convenient stereoselective synthesis using an intramolecular ene reaction has been described (Scheme 44). The ene reaction leads exclusively to the... 

Kraus, G.A. and Nagy, J.O. (1983) The total synthesis of a-allokainic acid. Tetrahedron Lett., 24, 3427-3430. 

Mooiweer, H.H., Hiemstra, H., and Speckamp, W. (1991) Total synthesis of ( )-a-allokainic acid via two allylsilane N-acyliminium ion reactions. Tetrahedron, 47, 3451-3462. 

Agami, C., Cases, M., and Couty, F. (1994) New access to kainoids via a tandem aza-Cope/Mannich reaction. Application to the enantioselective synthesis of (—)-a-allokainic acid. /. Otg. Chem., 59, 7937-7940. 

Scheme 43 Synthesis of (-)-a-kainic acid and (+)-a-allokainic acid...

The alkyl-cobalt addition-elimination (cobalt group transfer) sequence has been used by Baldwin and Li during the enantiospecific synthesis of (-)-a-kainic acid (58a) and (-)-a-allokainic acid (58b). These reactions proceed via the carbon-centered... 

Hanessian, S. and Ninkovic, S. (1996) Stereoselective synthesis of ( )-a-kainic acid and (-l-)-a-allokainic acid via trimethylstannyl-mediated radical carbocyclization and oxidative destannylation. /. Otg. Chem., 61, 5418-5424. 

Two types of ring contractions have been used in the synthesis of pyrrolidines. The older one, an analogous Favorskii process, is the key step of Honjo s a-allokainic acid synthesis. The general lines of this approach are given in Scheme 29 (173). 

This sequence was obviously not amenable to a synthesis of optically active a-allokainic acid given the fact that an aminomalonate group was necessary. After unfruitful assays with menthyl esters, the Swiss group was rewarded by the discovery that the phenylmenthyl group (180) brings sufficient asymmetry to the reaction intermediate to afford products with a high percentage of favorable diastereoisomer (Scheme 35) (181). 

Asymmetric ene reactions.5 A key step in an asymmetric synthesis of the amino diacid (+ )-a-allokainic acid (5) involves an intramolecular ene reaction of the (Z)-diene 2, in which the enophile is a (Z)-( —)-8-phenylmenthyl acrylate unit. In the presence of (C2H5)2A1C1, (Z)-2 cyclizes to the two pyrrolidines 3 and 4 in the ratio of... 

Previous work in Baldwin s group based around cobalt(I)-mediated cyclization had led to syntheses of (-)-a-kainic acid 2, (+)-allokainic acid 3,34 and acromelic acid A 5.35 A cobaloxime-mediated cyclization of 27 gave the separable pyrrolidines 28 and 29, suitable for conversion to (-)-a-kainic acid 2 and (+)-allokainic acid 3, respectively.34 In this instance, the required stereoisomer 28 for the preparation of (-)-a-kainic acid 2 predominated in a ratio of 28 29,1.7 1 (Scheme 6). Both 28 and 29 were carried through to the respective kainoids 2 and 3. In this case, asymmetry was introduced at a very early stage in the synthesis via a Sharpless asymmetric epoxidation. 

Oppolzer and cowoikers have developed an enantioselective version of this reaction which permits the synthesis of (+)- and (-)-a-allokainic acid from (76) and (77), respectively. - Treatment of (-)-8-phe-nylmenthyl ester (74) with EtAlCh at -78 C gives a 95 5 mixture of (76) and (77). Treatment of (-)-8-phenylmenthyl ester (75) with EtAlCh at -35 C gives an 11 89 mixture of (76) and (77). Oppolzer and Mirza used C labeling to establish that the hydrogen is transferred exclusively from the frons-methyl group of (69). This result precludes a stepwise mechanism with a free cationic intermediate which should transfer hydrogen equally from both methyl groups. 

The plane structure of kainic acid was determined by three independent research groups by three different methods, and resulted in identical chemical structure [5-7].The stereochemistry ofi-a-kainic acid and of L-a-allokainic acid was determined by the total synthesis of the respective alkaloids [8,9]. 

This very short synthesis of the neurotoxin (+)-a-allokainic acid demonstrates how the understanding of the transition state of a... 

The reaction has been extended to the synthesis of (-i-)-a-allokainic acid and provided a new entry to the isogeissoschizoid skeleton [170-173]. 

The ene reaction offers great possibilities in the synthesis of 3,4 disubstituted pyrrolidines or piperidines (775). An illustration of this reaction is a short and selective synthesis of allokainic acid by Oppolzer and Andres (176) (Scheme 31). In subsequent work, the influence of the double-bond geometry in the... 

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