Two analogs of blestriarene C (4,4'-dimethoxy-1,1'-biphenanthrene-2,2',7,7'-tetraol) bearing no 7,7'-dihydroxy (3) and 4,4'-dimethoxy groups were prepared. Unlike blestriarene C (1), compounds and , as well as 1,1'-biphenanthrene-2,2'-diol (5), do not racemize under fluorescent lamp illumination. Cyclic voltammetry analysis reveals that compound has a lower half-wave potential (E(1/2)) than compounds , suggesting that a redox cycle is involved in the racemization. Compound racemizes by absorbing UV light corresponding to the (1) L(b) band. During the reaction, no side products are observed. The racemization is significantly inhibited under nitrogen. Based on these observations, we propose a feasible mechanism for the easy racemization of compound , which is mediated by a cation radical generated in situ by a reversible photo-induced oxygen oxidation.
Here, we report a synthesis of the lower half C 21 −C 40 fragment of the shellfish toxin, azaspiracid-1. The C 28 −C 40 fragment was synthesized by a coupling between the C 28 −C 35 epoxide and the C 36 −C 40 dithioacetal anion, followed by the HI-ring spiroaminal formation. An aldehyde corresponding to the C 28 −C 40 fragment was then coupled with the C 21 −C 27 allylic stannane by using InCl 3 . Finally, the FG-ring was constructed by HF‚pyridine to accomplish the synthesis of the suitably protected C 21 −C 40 fragment.
Here, we report a synthesis of a differentially protected, open-chain C21-C40 fragment of azaspiracid-1, corresponding to the lower half EFGHI-ring domain. The synthesis features modular coupling of three advanced intermediates, aiming for diverted analogue synthesis. A new method for construction of E-ring moiety amenable to the diverted synthesis is also reported. Azaspiracid-1 (AZA-1, 1) is a marine toxin responsible for azaspiracid poisoning prevailed at a coastal region in Europe since 1995 (Figure 1). The toxin was first isolated from contaminated mussels (Mytilus edulis) from Killary Harbor, Ireland, and characterized by a group led by Yasumoto and Satake in 1998, 1 and the structure was synthetically settled by a Nicolaou group in 2004. 2 Ten AZA congeners have been determined thereafter, 3 and the other congeners have been proposed. 4 The structure of 1 is quite unique; 1) a C6-C17 bisspiroketal fused to a C17-C20 tetrahydrofuran, and 2) an unusual C33-C40 azaspiro ring fused with C28-C40 2,9-dioxabicyclo[3.3.1]nonane. The lethality of 1 against mice was found to be highly potent (LD 50 = 0.2 mg/kg), 5 although the mechanism for the biological action of 1, however, still remains unsolved. Recently, AZAs have received increasing attention because of the wider geographical occurrence; AZAs are now detected also in brown crabs from coast of Norway and Sweden, 6 and in several kinds of shellfishes from Portuguese coast. 7 In order to study the biological function of AZAs precisely, highly pure analogues and/or partial structure are required. 2,8 We have been studying a synthesis of 1, and the synthesis of the lower half domain has been recently accomplished. 9,10 In this letter, we report a second-generation synthesis of a differentially protected, open-chain fragment corresponding to the lower half of 1, amenable to diverted synthesis of various AZA analogues. 11 In addition, a new synthetic approach toward E-ring moiety has been newly developed here.
Synthesis of Open-Chain C21-C40 Fragment of Azaspiracid-1. -The synthesis of a key fragment (I) is described which is needed for the total synthesis of azaspiracid-1. -(OIKAWA*, M.; IWAYAMA, T.; SASAKI, M.; Heterocycles 78 (2009) 3, 609-615; Lab. Biostruct. Chem., Grad. Sch. Life Sci., Tohoku Univ., Aoba, Sendai 981, Japan; Eng.) -K. Schneider 28-197
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