Synthesis of Open-Chain C21-C40 Fragment of Azaspiracid-1

Abstract

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.