5,8-Didehydroisoquinolinium ion, a para benzyne analogue, was generated in a Fourier transform ion cyclotron resonance mass spectrometer, and its reactivity toward various neutral reagents was examined. A direct comparison of the reaction kinetics of the para benzyne, a meta isomer, and analogous monoradicals, indicates that the para benzyne is a poorer electrophile but a more reactive radical than its meta isomer.

The cyclic dipeptide cyclo[(R)-His-(R)-Phe] (1) has been known since 1981 to catalyze the enantioselective formation of cyanohydrins from aldehydes and HCN. Although 1 has proved to be very effective in the production of optically active cyanohydrins, the precise structure of its catalytically active form remains unresolved. The reaction of 3-phenoxybenzaldehyde and HCN in the presence of 1 has also been shown to exhibit enantioselective autocatalysis:  the product (S)-3-phenoxymandelonitrile reacts with 1 to form a new, more enantioselective catalytic species. It is now demonstrated that this autocatalytic phenomenon is general and that, furthermore, it can be used to improve the enantioselectivity of cyanohydrin formation for several problematic substrates. Upon addition of a small (8 mol %) quantity of (S)-mandelonitrile or (S)-3-phenoxymandelonitrile to these reactions, the enantioselectivity of cyanohydrin formation was improved by as much as 20% ee. This effect has been ascribed to the formation of a complex between the added (S)-cyanohydrin and 1 that exhibits superior enantioselectivity to 1, either alone or complexed to the cyanohydrins of problematic substrates. A mathematical model has been developed, on the basis of a two-state equilibrium between 1 and a complex of 1 and cyanohydrin and used to explain the enantioselective autoinduction phenomenon in terms of five parameters:  rate constants for the production of (R)- and (S)-cyanohydrin by both 1 and its cyanohydrin complex and an association constant for the formation of a cyanohydrin complex by 1. Two versions of this model, based on monomeric and dimeric 1, have been evaluated in light of the available data. Examination of the results reveals that the complexes of 1 and many of the cyanohydrins studied are highly enantioselective catalysts but that the complexes of 1 and cyanohydrins are only weakly associated; moreover, the complexation of 1 with most cyanohydrins leaves the rate of cyanohydrin formation unchanged, though both autocatalysis and enantioselective poisoning have been observed as well.

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The novel amino acid residue (2R,3R,4S)-4-amido-7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE, 3), a constituent of the cyclic depsipeptides callipeltins A and D, and its (2S,3S,4S) diastereomer were synthesized from a protected L-ornithine derivative in 13 steps (15% overall yield), and its configurational assignment was reexamined by 1 H NMR.Callipeltin A (1, Figure 1), a cyclic depsidecapeptide isolated from the marine lithisda sponges Callipelta sp. 1 and Latruncula sp., 2 was shown to possess antifungal and anti-HIV activity and cytotoxicity against selected human carcinoma cell lines, 3 as well as powerful inhibition of the Na/Ca exchanger in guinea pig left atria. 4 The structure of 1 was determined by Minale and co-workers; 1 it contains a number of novel amino acids and a novel fatty acid, though the stereochemistry of the β-methoxytyrosine residue could not be determined. Current efforts in our laboratory are focused on the synthesis and study of 1 and related compounds.

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