Marine dolabellane diterpene

The first total synthesis of the marine dolabellane diterpene (+)-4,5-deoxy-neodolabelline (70) was accomplished by D. R. Williams et al. [58]. The trans-disubstituted dihydropyran moiety in key intermediate 69 was efficiently prepared from mixed acetal 66 by RCM with second-generation catalyst C and subsequent Lewis acid-catalyzed allylation of ethyl glycosides 67 with allylsi-lane 68 (Scheme 12) [59]. 

D.R. Williams and co-workers accomplished the first total synthesis of marine dolabellane diterpene (+)-4,5-deoxyneodolabelline. The Type I carbon-Ferrier reaction was utilized to assemble the key frans-2,6-disubstituted dihydropyran with complete stereoselectivity (a-anomer). The macrocyclization was carried out with a vanadium-based pinacoi coupiing. 

The first total synthesis of the marine dolabellane diterpene (+)-deoxyneodolabelline was achieved in the laboratory of D.R. WilliamsJ In the final step of the synthetic sequence, the oxidation of a secondary alcohol functionality of a 1,2-diol to the corresponding a-hydroxy ketone was required. Such 1,2-diols are known to be unstable under most oxidation conditions, and often glycol cleavage is observed. Indeed, when Dess-Martin and Ley oxidations were tried, the substrate suffered carbon-carbon bond cleavage. However, under the Swern oxidation conditions, the desired a-hydroxy ketone was isolated in a 65% yield. Interestingly, the substrate was a mixture of four inseparable diastereomeric diols (obtained in a McMurry reaction), which gave two easily separable ketone products, one of which was the natural product. 

Su, J., Zhong, Y., and Zeng, L. (1991a) Clavudiol A and davirolide A, two marine dolabellane diterpenes from the soft coral Clavularia viridis. J. Org. Chem., 56, 2337-2344. 

Besides sulphated PS, reports on anti-HSV compounds isolated from marine algae are recorded in the literature. Specimens of the brown alga Dictyota pfaffii from Atol das Rocas, northeast Brazil, afforded the rare dolabellane diterpene 10,18-diacetoxy-8-hydroxy-2,6-dolabella-diene and the new 10-acetoxy-8,18-dihydroxy-2,6-dolabella-diene [46]. These substances showed strong anti-HSV-1 activity in vitro. Several antiviral diterpenes were isolated from other brown algae of the Dictyota genus. 

Abstract The isolation, structure and total synthesis of members of four classes of diterpenes has been summarized. Dolabellanes, neodolabellanes, dolastanes and neodolas-tanes are structurally related bi- or tricyclic diterpenes. Dolabellanes belong to a continuously growing class of diterpenes being isolated from marine and terrestrial sources. The published work on isolation and synthesis since 1998 has been summarized. Neodolabellanes represent a scarce class of diterpenes that have been isolated exclusively... 

Costantino, V, Fattorusso, E., Mangoni, A., Di Rosa, M., lanaro, A., Aknin, M., and Gaydou, E.M. (1999b) A new cytotoxic diterpene with ihe dolabellane skeleton from the marine sponge SigmosceptrAla quadrilobata. Eur. ]. Org. Chem., 227—230. 

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