ABSTRACT
A highly convergent, enantioselective total synthesis of brevetoxin A is reported. The development of a [X+2+X] Horner-Wadsworth-Emmons/cyclodehydration/reductive etherification convergent coupling strategy allowed a unified approach to the synthesis of two advanced tetracyclic fragments from four cyclic ether subunits. The Horner-Wittig coupling of the two tetracyclic fragments provided substrates that were explored for reductive etherification, the success of which delivered a late-stage tetraol intermediate. The tetraol was converted to the natural product through an expeditious selective oxidative process followed by methylenation.
Subject(s)
Ethers, Cyclic/chemical synthesis , Marine Toxins/chemical synthesis , Oxocins/chemical synthesis , Catalysis , Cyclization , Ethers, Cyclic/chemistry , Marine Toxins/chemistry , Molecular Structure , Oxocins/chemistry , StereoisomerismABSTRACT
Brevetoxin A is a decacyclic ladder toxin that possesses 5-, 6-, 7-, 8-, and 9-membered oxacycles, as well as 22 tetrahedral stereocenters. Herein, we describe a unified approach to the B, E, G, and J rings based upon a ring-closing metathesis strategy from the corresponding dienes. The enolate technologies developed in our laboratory allowed access to the precursor acyclic dienes for the B, E, and G medium-ring ethers. The strategies developed for the syntheses of these four monocycles ultimately provided multigram quantities of each of the rings, supporting our efforts toward the completion of a convergent synthesis of brevetoxin A.
Subject(s)
Ethers/chemical synthesis , Marine Toxins/chemical synthesis , Oxocins/chemical synthesis , Alkylation , Cyclization , Ethers/chemistry , Marine Toxins/chemistry , Molecular Structure , Oxocins/chemistry , Stereoisomerism , Structure-Activity RelationshipABSTRACT
Cationic gold(I) phosphite catalysts activate allenes for epoxide cascade reactions. The system is tolerant of numerous functional groups (sulfones, esters, ethers, sulfonamides) and proceeds at room temperature in dichloromethane. The cyclization pathway is sensitive to the substitution pattern of the epoxide and the backbone structure of the A-ring. It is capable of producing medium-ring ethers, fused 6-5 bicyclic, and linked pyran-furan structures. The resulting cycloisomers are reminiscent of structures found in numerous polyether natural products.
Subject(s)
Alkadienes/chemistry , Epoxy Compounds/chemistry , Gold/chemistry , Catalysis , Crystallography, X-Ray , Cyclization , Methylene Chloride/chemistry , Pyrans/chemistryABSTRACT
A total synthesis of brevetoxin A is reported. Two tetracyclic coupling partners, prepared from previously reported advanced fragments, were effectively united via a Horner-Wittig olefination. The resulting octacycle was progressed to substrates that were explored for reductive etherification, the success of which led to a penultimate tetraol intermediate. The tetraol was converted to the natural product through an expeditious selective oxidative process followed by methylenation.
Subject(s)
Marine Toxins/chemical synthesis , Oxocins/chemical synthesis , Aldehydes/chemistry , Alkenes/chemistry , Marine Toxins/chemistry , Oxidation-Reduction , Oxocins/chemistry , Substrate SpecificityABSTRACT
[reaction: see text] A stereoselective synthesis of the GHIJ fragment of brevetoxin A utilizing a convergent assembly strategy is described. Glycolate alkylation, ring-closing metathesis, and Hosomi-Sakurai reactions were central operations in the construction of the G ring and J ring subunits, which were united through a Horner-Wadsworth-Emmons coupling. Subsequent dehydrative cyclization produced an endocyclic enol ether that was further elaborated to the tetracyclic GHIJ fragment of brevetoxin A.
