The structures and relative stereochemistry of nine new briaran -based diterpenes from a Briareum species have been deduced from 2-D n.m.r. experiments at high field, and nuclear Overhauser effect experiments. The new compounds are (1R*2R*,3R*,5Z,7S*,8S*,9S*,10S*,11R*,-12S*,14S*,17R*)-2,3,14-triacetoxy-8,17:11,12-bisepoxy-9-hydroxybriar-5-en-18-one (5), (1R* ,2R* ,3R* ,5Z*,7S*,8S* ,9S*,10S*,11R*,12S*, 14S*, 17R*)-3,14-diacetoxy-2-butyryloxy-8,17:11,12-bisepoxy-9-hydroxybriar-5-en-18-one (6),(1R*,2R*,3R*,5Z,7S*,8S*,9S*,10S*,11R*,12S*,14S)-2,3,14-triacetoxy-11,12-epoxybriara-5,8(17)-dien-18-one(17),(1S*,2S*,4R*,5Z,7S*,8S*,9S*,10S*,2,4,9.12-tetraacetoxy-8,17-epoxy-11-hydroxybriara-5,13-dien-18-one (8),(1S*, 2S*,4R*, 5Z,7S*,8S*,9S*,10S*, 11S*, 12R*,13Z,17R*)-2,4,9-triacetoxy-8,17-epoxy-11,12-dihydroxybriara-5,13-dien-18-one (9), (1S *,2S*,4R*,5Z,7S*,8S*,9S*,10S*, 11S *,12R*,13Z,17R*)-2,4,9,12-tetraacetoxy-8,17-epoxy-11-hydroxybriara-5,13-dien-18-one(10),(1Si,2S*,5Z,7S',8S*,-9S*,10S*,11R*,12R*,13Z,17R*)-2,12-diacetoxy-8,17-epoxy-9-hydroxybriara-5,13-dien-18-one(ll), (1R*,2R*,3Si,5Z,7S* ,8S* ,9S* ,10S* ,11Z,14S*,17R*)-2,14-diacetoxy-3-butyryloxy-8,17-epoxy-9-hydroxybriara-5,11-dien-18-one (12) and (1R *,2R*,3S*,5Z,7S*,8S*,9S*,10S*,11Z,-14S',17R*)-2,3,14-triacetoxy-8,17-epoxy-9-hydroxybriara-5,11-dien-18-one (13). In addition to the nine new briaran derivatives, the same coral afforded two cladiellane derivatives (1R*,4R*,5S*,6R*,8R*,12R*,13R*,14R*)-cladiellane-4,8,12-triol (14) and its 4-acetoxy derivative (15). Aknowncembranoid diterpene (1R *,4R*,ZE,7E,l1E)-cembra-2,7,11-trien-4-ol(21) wasalso isolated together with an unprecedented perhydrophenanthrene-based diterpene (20): (1R *, 1aR*,4S*,4aR*,5aR*,8aR*)-4-isopropyl-l,8a-dimethyl-5-methylenetetradecahydrophenanthren-1-ol. The extraordinary biosynthetic ability of soft corals from the genus Briareum is once more revealed.

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Four previously documented ligand design strategies for achieving Ag(I) discrimination have been applied to the design of a new N-benzylated N 2 S 3 -donor macrocycle; the latter shows high selectivity for Ag(I) over Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) in log K and bulk membrane transport studies.We have previously reported the synthesis of an extended series of 16-to 19-membered, dibenzo-substituted macrocycles incorporating oxygen, nitrogen and/or sulfur heteroatoms.1 These rings were synthesised as part of an investigation of structurefunction relationships underlying discrimination behaviour within the above seven metal-ion series. Several examples of such discrimination are documented; 2-4 for example, step-wise 'tuning' of the donor set present within 17-membered rings of type 1 resulted in >10 9 discrimination for Ag(I) over Pb(II) 4 -metals that are found together in nature. However discrimination for Ag(I) over Cu(II), 5 while less spectacular, was still ∼10 4 .In our previous (wide-ranging) studies involving ligands of type 1 four design elements that influence the discrimination for Ag(I) over the above transition and post-transition ions are documented: (i) the macrocyclic ring size, (ii) the macrocyclic donor set, (iii) macrocyclic donor atom sequence and (iv) the nature of any donor atom substituents present. We have now applied the 'lessons learnt' in the these studies to the design and synthesis of the new macrocyclic ligand 2 which was thus anticipated to yield enhanced discrimination for Ag(I) over the other six transition and posttransition ions mentioned above.In the previous studies, as expected from a consideration of HSAB theory, 6 it was confirmed that progressive substitution of thioether sulfur atoms for oxygen in an O 3 N 2 -donor ring of type 1 led to increased silver ion discrimination. A second general observation was that both the strength of binding as well as silver ion discrimination tended to peak for the 17-membered ring system (which forms three five-membered and two six-membered chelate rings when all donors bind to a central metal ion). In this context it is noted that the X-ray structures of the 1 : 1 (Ag + : L) complexes, where L = 1 with X = S, Y = O, 4 X = S, Y = NH 7 and X = S, Y = S 8 (all 17-membered rings) confirm coordination of all five macrocyclic ring donors to silver in each case (with no other ligands present in the coordination sphere). A third observation, was that the presence of a NHCH 2 CH 2 XCH 2 CH 2 NH (X = O or S) donor atom sequence, rather than the corresponding sequence with X = NH, also promotes enhanced silver discrimination; even though the absolute log K values tend to be smaller when X = O. Finally, in a number of studies by us and others, 9 it has been well documented that N-benzylation of a secondary amine donor in a variety of aza macrocyclic systems leads to enhanced discrimination for silver over a range of transition and post-transition metal ions-behaviour we have previously termed 'selective detuning'.10 For example, it h...

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The synthesis and characterization of new mixed-donor macrocyclic ligands incorporating nitrogen, sulfur and/or oxygen heteroatoms are described. The new 17- or 18-membered macrocyclic rings contain unsymmetrical arrangements of their heteroatoms in contrast to related, previously reported rings in which the donor sets are arranged symmetrically. The X-ray structures of the 17-membered rings incorporating N4O- and N4S-donor sets are presented.