Chagosensine: A Riddle Wrapped in a Mystery Inside an Enigma.

The marine macrolide chagosensine is supposedly distinguished by a ()-configured 1,3-chlorodiene contained within a highly strained 16-membered lactone ring, which also incorporates two -2,5-disubstituted tetrahydrofuran (THF) rings; this array is unique. After our initial synthesis campaign had shown that the originally proposed structure is incorrect, the published data set was critically revisited to identify potential mis-assignments. The "northern" THF ring and the -configured diol in the "southern" sector both seemed to be sites of concern, thus making it plausible that a panel of eight diastereomeric chagosensine-like compounds would allow the puzzle to be solved.

Total Synthesis of Putative Chagosensine.

The marine macrolide chagosensine is the only natural product known to date that embodies a Z,Z-configured chloro-1,3-diene unit. This distinguishing substructure was prepared by a sequence of palladium-catalyzed 1,2-distannation of an alkyne precursor, regioselective Stille cross-coupling at the terminus of the resulting bisstannyl alkene with an elaborated alkenyl iodide, followed by chloro-destannation of the remaining internal site. The preparation of the required substrates centered on cobalt-catalyzed oxidative cyclization reactions of hydroxylated olefin precursors, which allowed the 2,5-trans-disubstituted tetrahydrofuran rings, embedded into each building block, to be formed with excellent selectivity.

Structure and Reactivity of Half-Sandwich Rh(+3) and Ir(+3) Carbene Complexes. Catalytic Metathesis of Azobenzene Derivatives.

Traditional rhodium carbene chemistry relies on the controlled decomposition of diazo derivatives with [Rh(OAc)] or related dinuclear Rh(+2) complexes, whereas the use of other rhodium sources is much less developed. It is now shown that half-sandwich carbene species derived from [Cp*MX] (M = Rh, Ir; X = Cl, Br, I, Cp* = pentamethylcyclopentadienyl) also exhibit favorable application profiles. Interestingly, the anionic ligand X proved to be a critical determinant of reactivity in the case of cyclopropanation, epoxide formation and the previously unknown catalytic metathesis of azobenzene derivatives, whereas the nature of X does not play any significant role in -OH insertion reactions.

Stabilization of a Chiral Dirhodium Carbene by Encapsulation and a Discussion of the Stereochemical Implications.

For the first time, the stereochemical course of an asymmetric cyclopropanation can be discussed on the basis of experimental structural information on a pertinent chiral dirhodium carbene intermediate. Key to success was the formation of racemic single crystals of a heterochiral [Rh2 {(S*)-PTTL}4 {=C(Ar)COOMe}][Rh2 {(R*)-PTTL}4 ] (Ar=MeOC6 H4 ; PTTL=N-phthaloyl-tert-leucinate) capsule, which has been characterized by X-ray diffraction. NMR spectroscopic data confirm that the obtained structural portrait is also relevant in solution and provide additional information about the dynamics of this species.

Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis.

Owing to its tremendous preparative importance, rhodium carbene chemistry has been studied extensively during past decades. The invoked intermediates have, however, so far proved too reactive for direct inspection, and reliable experimental information has been extremely limited. A series of X-ray structures of pertinent intermediates of this type, together with supporting spectroscopic data, now closes this gap and provides a detailed picture of the constitution and conformation of such species.

Concise total syntheses of amphidinolides C and F.

The marine natural products amphidinolide C (1) and F (4) differ in their side chains but share a common macrolide core with a signature 1,4-diketone substructure. This particular motif inspired a synthesis plan predicating a late-stage formation of this non-consonant ("umpoled") pattern by a platinum-catalyzed transannular hydroalkoxylation of a cycloalkyne precursor. This key intermediate was assembled from three building blocks (29, 41 and 47 (or 65)) by Yamaguchi esterification, Stille cross-coupling and a macrocyclization by ring-closing alkyne metathesis (RCAM).