for the mass spectra (recorded on an AEI MS902 instrument, operated at 70 eV).(19) E. Le Goff and R. B. LaCount, Tetrahedron Lett., 2787 (1965).(20) G. M. Badger, "Aromatic Character and Aromaticity," Cambridge University Press, Cambridge, 1969, p 61. (21) We thank Dr.

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We have exploited the concept of multivalency in the context of DNA recognition, using novel chemistry to synthesize a new type of bis-intercalator with unusual sequence-selectivity. Bis-intercalation has been observed previously, but design principles for de novo construction of such molecules are not known. Our compounds feature two aromatic moieties projecting from a rigid, polynorbornane-based scaffold. The length and character of the backbone as well as the identity of the intercalators were varied, resulting in mono- or divalent recognition of the double helix with varying affinity. Our lead compound proved to be a moderately sequence-selective bis-intercalator with an unwinding angle of 27 degrees and a binding constant of about 8 microM. 9-aminoacridine rings were preferred over acridine carboxamides or naphthalimides, and a rigid [3]-polynorbornane scaffold was superior to a [5]-polynorbornane. The flexibility of the linker connecting the rings to the scaffold, although less influential, could affect the strength and character of the DNA binding.

Molecular Scaffolds formed by the linking of bicyclo[2.2.1]heptane (norbornane) subunits by sharing a common C 2bridge have great appeal to the molecular architect since modelling studies have shown that the syn-facially fused members have a curved topology. Furthermore, this curvature can be modified by replacement of the methylene bridge by heteroatom bridges (O, S, NR), and made rod-like by introducing a σ-bond between adjoining methylene bridges in sesquinorbornane subunits. This review presents synthetic methodology which allows the stereocontrolled construction [a] ). His research interests have focused on synthetic methodology involving cycloaddition chemistry (isobenzo systems) and photochemistry (small ring synthesis), with occasional forays into natural product synthesis (daunomycin, nucleosides, protoberberine). At the ANU, he set up a Centre for Forensic Science specialising in fingerprint detection, and has been the driving force behind the creation of the Centre for Molecular Architecture at CQU. He has published more than 250 research papers and was awarded a senior ARC Research Fellowship (1992Ϫ1996) and the RACI Organic Medal in 1996.MICROREVIEWS: This feature introduces the readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included.

The rigid bis-porphyrinic host 2 forms 1:1 complexes with rigid 5,15-bis(4-pyridyl)porphyrins and both 1:1 and 2:1 complexes with tetrapyridylporphyrin 4, with the latter complex providing a new example of a self-assembled system containing an enclosed molecular environment.

Aziridinocyclobutenes react with electron-deficient or ringstrained alkenes to produce 7-azanorbornenes in a novel 1,3-dipolar cycloaddition reaction suitable for BLOCK assembly protocols. Benzo-7-azanorbornadiene and 7-heterobridged analogues react stereoselectively to produce compounds with syn-facial orientation of their bridges. 7-Azanorbornanes have received much interest following the discovery of the analgesic properties of epibatidine 1 (Scheme 1) and several specialist syntheses of this alkaloid have appeared in the recent literature. 4 Higher norbornalogues of 7-azanorbornanes, on the other hand, are uncommon and one of the rare examples is the doubly Nbridged adduct 2 reported by Sasaki et al. as the minor product (21%) in the cycloaddition of N-t Boc-isoindole with N-t Boc-7-azabenzonorbornadiene. 5 Scheme 1Following the success of our recently described ACE reaction (Scheme 2, equation 1) for the synthesis of 7-oxanorbornenes, 6 we reasoned that the development of an aziridine equivalent (aza-ACE reaction, Scheme 2, equation 2) held much potential for the synthesis of N-bridged alicyclics, especially 7-azanorbornanes. This is the subject of the present letter where we demonstrate the versatility of this new BLOCK reaction by the synthesis of the first examples of syn-facial polynorbornane systems containing multiple N-bridges. 7 Scheme 2The aziridinocyclobutane 8 required to test the potential of the aza-ACE coupling reaction was prepared as outlined in Scheme 3. Conversion of the benzonorbornadiene 3 8 to the cyclobutene-1,2-diester 4 followed the established method of Mitsudo and co-workers, which employs the ruthenium-catalysed addition of dimethyl acetylene dicarboxylate 4 (DMAD). 9 Conversion of cyclobutene 5 to the triazoline 7 10 was achieved by thermal addition of benzyl azide 6 and elimination of dinitrogen to produce aziridine 8 10 was conducted photochemically (Hanovia 450 watt Hg lamp, quartz, benzene, RT). The synthesis of aziridinocyclobutane 8 was achieved in 40% overall yield for the three step process from the readily available benzonorbornadiene 3.The reactivity of the N-benzyl aziridinocyclobutane 8 towards 1,3-dipolar cycloaddition was assessed initially by reaction with excess DMAD 4 (benzene at reflux) which produced the 1:1-adduct 10. 10 The structure of 10 was supported by NMR data and presumed to arise via attack of the DMAD onto the α-face of the 1,3-dipolar intermediate 9, formed by ring-opening of the aziridine (Scheme 3). Support for this proposal is provided by the formation of a yellow colour when aziridine 8 is heated alone in toluene. 11 Colour formation is lost on cooling and regenerated on heating, indicating the reversibility of the ring-opening process; further, the colour is discharged immediately on the addition of dipolarophiles like dimethyl fumarate. Scheme 3High stereoselectivity is observed in the reaction of the aziridine 7 with ethylenic dienophiles, eg maleic anhydride 11 forms a single product, shown to be the exo-fused adduct 12 10 by the existence of ...