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S y n t h e s i s o f C o n s t r a i n e d b -A m i n o E s t e r s f r o m N o r b o r n e n e sAbstract: Synthesis of a number of novel, conformationally rigid b-amino esters has been achieved via a tandem olefin metathesis reaction. The starting materials are readily accessible from the Diels-Alder adduct between cyclopentadiene and maleic anhydride.The ring-opening/ring-closing metathesis reaction of norbornene substrates has attracted much attention in the recent literature, due to the inherent ring strain present in such bridged alkenes. 1 Thus, the reaction has been utilized to excellent effect in a number of complex synthesis of both natural and non-natural products. 2 Application of this methodology for synthesis and reactions of amino acids has also seen recent interest, with a notable recent report from Winkler et al. who described a series of tandem metathesis reactions on b-amino acids based on the oxanorbornene core. 3 We disclose herein our efforts in this area which demonstrate that metathesis can be a powerful tool for the conversion of readily accessible b-amino acids based on the norbornene skeleton into novel, highly constrained b-bicyclic amino acids.The initial starting point for the synthesis was to be endob-amino ester 1, which was readily prepared, as shown in Scheme 1. 4 Cyclopentadiene and maleic anhydride were reacted to give the expected Diels-Alder adduct, the endo-product was obtained via a single recrystallization. Ring-opening of the anhydride was easily achieved following the method reported by Sabitha to give the monoester. 5 Curtius rearrangement of the monoester under standard conditions gave the desired amino ester, which was immediately protected as the corresponding toluenesulfonamide in order to prevent further reaction and to aid purification. 6Alkylation of the nitrogen, with either allyl bromide or propargyl bromide in the presence of potassium carbonate, lead to the corresponding allyl-and propargyl-substituted norbornenes 2 and 3 (Scheme 2).Conversion of these norbonenyl b-amino esters was initially attempted using the first generation Grubbs catalyst, since reports from the groups of Grubbs and Bolm had demonstrated that the reaction was feasible on similar substrates. 7 Norbornenes 2 and 3 were dissolved in dichloromethane and ethene was added, historically believed to reduce ring-opening polymerization of the norbornene. 8 Grubbs I catalyst (10 mol%) was added and the extent of reaction monitored by TLC, even after an Scheme 1 Synthesis of protected b-amino ester 1

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In this report we describe the ring-rearrangement metathesis of 2-aminonorbornene derivatives. An efficient rutheniumcatalysed metathesis reaction occurs with a wide range of pendent alkenes and alkynes to generate bicyclic amines and amides.

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