A new diterpene with two polymorphic modifications from a marine alga

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A new diterpene, β-dictalediol monoacetate (), has been isolated from a brown alga X-ray crystal structure studies of have revealed the existence of two polymorphic modifications: dimorph in a chair/boat/chair conformation, and dimorph in a
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  Tetrahedron Letters,Vol.28,No.52,pp 6699-6700,1987 oo4o-4039/87 $3.00 + .Oo Printed in Great Britain Peroamon Journals Ltd. A NEW DITERPENE WITH TWO POLYMORPHIC MODIFICATIONS FROM A MARINE ALGA ~~~~ lardy,+a c..sm ~~~~ ....sm n..- .-ll--rl- Eduai-do #ants b ,..,:A "Arm YUJL,T, nrro “4LLa‘ U” D. Cirilo Perez,b Ricardo Pere~,~ Jose L. Ravelo,b Matias L. R~dr~~5~~,*b u_-CI- +h 1'1Q1.L111( and Gayle K. Schultea aDepartment of Chemistry, Baker Laboratory, Cornell University, Ithaca, N.Y. 14853-1301, U.S.A. b Centro de Productos Naturales Orgdnicos Antonio Gonzalez, Universidad de La Laguna- C.S.I.C., Carretera de La Esperanza 2, 38206 La Laguna, Tenerife, Spain ABSTRACT: A new diterpene, B-dictalediol monoacetate (L), has been isolated -------- from a brown alga Dictyota sp. X-ray crystal structure studies of 1 have revealed the existence of two polymorphic modifications: dimorph la in a chair/boat/chair conformation, and dimorph lb in a chair/chair/chair con- -- formation. Extensive studies in our laboratory on the constituents of algae of the genus Dictyota (Chromophycota, Dictyotaceae) of the Canary Islands 1 have resulted in -------- the isolation of a number of diterpenes which have been fully characterized and for which a general biogenetic scheme has been proposed. Further studies of the same organism 2 have now resulted in the isolation and structure determination of B-dictalediol monoacetate (l), - a novel diterpene possessing an unusual ring system. Extraction of the fresh material gave a diethyl ether-soluble portion which, after chromatography, afforded 1 in 0.38% yield of the crude extract. - OH c ? @= B-Dictalediol monoacetate (1) has m.p. 136-138°C,[a]D-0.50 g ,$ M (c 1.2, CHC13).3 X-ray crystal structure studies of 1 have _ revealed the existence of two polymorphic modifications. 4 'C' 806 Crystals of la (needles) 5 are monoclinic, -- space group P21, H H 1 ~=8.423(2), b=12.947(3), c=19.613(2) ;, 8=98.72(2)O, z=4. - The structure was solved by direct methods, R=0.061 for 2706 reflections with F,>3o(F,) (left stereodiagram, Figure 1). Crystals of lb (plates)6 are orthorhombic, 5=16.0925(18) ;, space group P212121, 2=9.0505(4), 5=13.8160(5), z=4, R=0.037 for 1004 observed reflections with 1>30(1) (right stereodiagram, Figure 1). A comparative conformational study based on the crysta and molecular structures of both dimorphs has been carried out, and the Table 1 shows the Cremer's conformational parameters 7 for la and lb. The conformational -- -- parameters have been calculated for the six-membered rings beginning at atoms l 14, and 12 in a clockwise way for the needle crystalline modification (la), and -- the platelike crystal form (lb). Ring C is in the chair conformation with the -- methylene group on C20 and the gem-dimethyl group on Cg with identical relative orientation in both conformers. The conformation of B ring changes from the chair lb to the twist boat conformation in la and the conformation of ring A -- __* from the chair (lC4) in la to the chair c4Cl) in lb -- __* Therefore, in la the cis -- --- 6699  67 TAB L t fused rings A and B are folded back toward the rest of the malecule instead of being extended as they Dimorph tr are in lb (Figure 1). How?ver, since A B c crystal packing energies are gene- rally of the order of a few Kcal, a 19 11 169 both conformations would presumably 80 i?L 30 be present in solution in equili- 0.52 0.14 0.54 brium with perhaps even other arrangements. 4 5 7C14 =cg Acknowledgment. This work was sup- ported by grants from the Spanish C.A.1.C.Y T. (to J.D.M. and M.L.R.) and the New York State Sea Grant and N.I.H. (to J.C.). E.M. thanks the A. I.E.T. I. Foundation (Madrid) for a fellowship. FIGURE 1 References and Footnotes. _--_--_--__------------- 1. a) A.G. Gonzalez, J.D. Martin, B. Gonzalez, J.L. Ravelo, C. Perez, S. Rafii, and J. Clardy, J. Chem. Sot. Chem. Commun., (1984) 669; b) J. Clardy, G.Van Duyne, A.G. Gonzalez, E. Manta, J.D. Martin, C. PBrez, J.L. Ravelo, and G.K. Schulte, Ibid., (1987) 767. 2. The Dictyota sp. was collected off Giiimar, Tenerife, Canary Islands, during -----_----- the summer of 1983. 3. Satisfactory elemental analyses for C, and H. Spectral data: m/z 348.2665 _- (talc. for C22H3603 348.2664); vmax (KB~) 3500,1715,1640,1460,1370,1030,890, and 810 cm-l; 1~ n.m.r.b(CDC13) 0.90,0.94 (d, J 7Hz, 3H each), 1.19,1.31,2.09 (s, 3H each), 4.87,4.92 (s, IH each), 5.05 (dd, J 9, 4H2, 1H); 13C n.m.r.(CDC13) 5xMe: 6 26.8,21.6,21.4,21.1, and 21.1; 7xCH2:6108.1,37.5,36.4,31.8,26.3,24.0, and 17.2; 6xCH:6 74.0,51.2,48.8,43.4,35.3, and 29.1; 4xquaternary C:6170.9, 151.5,66.0, and 37.8 (assigned using off-resonance and selective proton-noise decoupling, 4. The atomic co-ordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW. Any request should be accompanied by the full literature citation for this communication. 5. J. Clardy, author to whom correspondence regarding the X-ray structure of la (Figure 1) should be addressed. 6. x-L. Rodriguez, author to whom correspondence regarding the X-ray structure of lb (Figure 1) should be addressed. 7. D. Cremer and J.A. Pople, J. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA m Chem. Sot. 97 (1975) 1354. 8. J. Bernstein and A.T. Hagler, J. Am. Chem. S&. 100 (1978) 673. (Received in UK 3 November 1987)
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