Dehydromicrosclerodermin†B and Microsclerodermin†J: Total Synthesis and Structural Revision.

The total synthesis of dehydromicrosclerodermin B and microsclerodermin J is described. Efficient approaches to the unusual amino acids in the target molecules were developed on the basis of a Negishi coupling (for Trp-2-CO2 H) and Blaise reaction (for Pyrr). An incorrect assignment of the pyrrolidinone stereochemistry of both compounds was confirmed by synthesizing epimers of the proposed structures. The spectroscopic data of these epimers were in complete agreement with those for the naturally derived material.

Diverse saturated heterocycles from a hydroacylation/conjugate addition cascade.

Rhodium-catalyzed hydroacylation using alkynes substituted with pendant nucleophiles, delivers linear α,ß-unsaturated enone intermediates with excellent regioselectivity. These adducts are used to construct a broad range of diversely substituted, saturated O-, N- and S-heterocycles in a one-pot process. Judicious choice of cyclisation conditions enabled isolation of O-heterocycles with high levels of diastereoselectivity. A variety of derivatisation reactions are also performed, generating functionalised hydroacylation products. This sequence serves as a general approach for the synthesis of fully saturated heterocycles.

One-pot near-ambient temperature syntheses of aryl(difluoroenol) derivatives from trifluoroethanol.

Difluoroalkenylzinc reagents prepared from 1-(2'-methoxy-ethoxymethoxy)-2,2,2-trifluoroethane and 1-(N,N-diethylcarbamoyloxy)-2,2,2-trifluoroethane at ice bath temperatures underwent Negishi coupling with a range of aryl halides in a convenient one pot procedure. While significant differences between the enol acetal and carbamate reagents were revealed, the Negishi protocol compared very favourably with alternative coupling procedures in terms of overall yields from trifluoroethanol.

Amine-promoted synthesis of vinyl aziridines.

N-Unsubstituted vinyl aziridines were synthesized via an amine-promoted regioselective nucleophilic aziridination of alpha,beta,gamma,delta-unsaturated carbonyl compounds. The reaction is completely regioselective (>95: 5) for the alpha,beta-alkene and completely diastereoselective, affording the trans-vinyl aziridine in moderate-to-good yields.

Desymmetrization of meso-bisphosphates using copper catalysis and alkylzirconocene nucleophiles.

The desymmetrization of meso-compounds is a useful synthetic method, as illustrated by numerous applications of this strategy in natural product synthesis. Cu-catalyzed allylic desymmetrizations enable the enantioselective formation of carbon-carbon bonds, but these transformations are limited in substrate scope and by the use of highly reactive premade organometallic reagents at cryogenic temperatures. Here we show that diverse meso-bisphosphates in combination with alkylzirconium nucleophiles undergo highly regio-, diastereo- and enantio-selective Cu-catalyzed desymmetrization reactions. In addition, C2-symmetric chiral bisphosphates undergo stereospecific reactions and a racemic substrate undergoes a Cu-catalyzed kinetic resolution. The reaction tolerates functional groups incompatible with many common organometallic reagents and provides access to a broad range of functionalized carbo- and hetero-cyclic structures. The products bear up to three contiguous stereogenic centers, including quaternary centers and spirocyclic ring systems. We anticipate that the method will be a useful complement to existing catalytic enantioselective reactions.

Tethered aminohydroxylation: synthesis of the ß-amino acid of microsclerodermins A and B.

The utility of the tethered aminohydroxylation (TA) has been demonstrated by synthesis of the complex ß-amino acid residue of microsclerodermins A and B. The TA provided a regio- and stereoselective functionalization of a complex homoallylic alcohol. The route includes late-stage introduction of the aliphatic side chain via a cuprate addition and cross metathesis, a tactic designed to render the synthesis applicable to other microsclerodermins.

Natural product synthesis as a challenging test of newly developed methodology.

This article describes the examination of several synthetic methodologies that have been developed in our laboratories, by application to the construction of a variety of natural product targets.

Silenes in organic synthesis: a concise synthesis of (+/-) -epi-picropodophyllin.

A concise, seven step synthesis of the aryl tetralin lignan lactone epi-picropodophyllin from piperonal is described. The key steps are a silene diene Diels-Alder reaction and the Hosomi-Sakurai reaction of the resultant silacyclohexene.

Hosomi-Sakurai reactions of silacyclohexenes.

Silacyclic allyl silanes, derived from silene-diene Diels-Alder reactions, combine with acetals in the presence of Lewis acids to afford, following oxidation of the intermediate fluorosilane, either butane-1,4-diols or tetrahydronaphthalenes containing four contiguous chiral centres with moderate to good diastereoselectivity.