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Novel Aplysinopsin‐Type Alkaloids from Scleractinian Corals of the Family Dendrophylliidae of the Mediterranean and the Philippines. Configurational‐assignment criteria, stereospecific synthesis, and photoisomerization

1988; Wiley; Volume: 71; Issue: 4 Linguagem: Inglês

10.1002/hlca.19880710412

1522-2675

Graziano Guella, Ines Mancini, Helmut Zibrowius, Francesco Pietra,

Synthesis of Indole Derivatives

Abstract From the scleractinian coral Tubastraea sp. (Dendrophylliidae) collected at Palawan, Philippines, 3′‐deimino‐3′‐oxoaplysinopsin ( 4 ) and 6‐bromo‐3′‐deimino‐3′‐oxoaplysinopsin ( 6 ) are now isolated as 5:2 mixtures of ( E/Z ) stereoisomers. The 3′‐deimino‐2′,4′‐bis(demethyl)‐3′‐oxoaplysinopsin ( 7 ) and 6‐bromo‐3′‐demino‐2′,4‐bis(demethyl)‐3′‐oxoaplysinopsin ( 5 ) are isolated as 2:3 and 1:1 ( E/Z ) mixtures, respectively, from another dendrophylliid, Leptopsammia pruvoti , collected near Marseille, Mediterranean coast of France. Larger amounts of these and related compounds, needed for a full structural determination, are obtained by synthesis. Thus, condensations of indol‐3‐carboxaldehyde (9) or of its 6‐bromo derivative 14 with hydantoin (15) , 3‐methylhydantoin (11) , or 1,3dimethylhydantoin (10) give the prevalent natural aplysinopsins with high stereospecificity. The minor stereoisomers (Z)‐ 4 , (Z)‐ 6 , ( E )‐ 7 , and ( E )‐ 5 are obtained by ( E /Z) photoisomerization under UV light of the condensation mixtures. The configuration is assigned from larger HC(8)/C(5′) 1 H, 13 C couplings in the ( E ) than in the (Z) isomer, and, in the case of 4 and 6 , from NOE enhancement at MeN(2′) on irradiation at HC(8). The stereospecificity of the condensations is attributed to steric inhibition to planarity in the rate‐limiting transition states, due to N(2′)/HC(2) repulsion with (Z)‐ 4 and (Z)‐ 6 , or to C(5′)O/HC(2) repulsion with (E) –7 or (E) ‐ 5 . As the aplysinopsins undergo ( E/Z ) phostoisomerization also under the daylight conditions of the laboratory, their isomeric composition in nature can not be presently assessed.