Inter- and intramolecular reactions of 1-deoxy-1-thio-1,6-anhydrosugars with α-diazoesters: synthesis of the tagetitoxin core by photochemical ylide rearrangement.

The one-carbon ring expansion of 1-deoxy-1-thio-1,6-anhydrosugars, mediated by metal carbenes and proceeding through the intermediacy of sulfur ylides, has been proposed as a route for the synthesis of the tagetitoxin skeleton. Intermolecular reactions of such thioanhydrosugars with diazoesters afford a range of undesired products derived from the initially formed ylide, whereas use of an intramolecular process generates stable ylides which can be converted to the tagetitoxin skeleton by photo-Stevens rearrangement. Computational studies using density functional theory indicate that the photochemical rearrangement likely proceeds through a homolysis-recombination pathway.

Concise synthesis of bicyclic aminals and their evaluation as precursors to the sarain core.

A number of bicyclic aminals have been prepared in a stereospecific manner as potential precursors to the core structure of the sarain alkaloids. Rearrangement of these aminals to new diazabicyclic and diazatricyclic systems has been observed under various conditions.

Atroposelective synthesis of axially chiral biaryl compounds.

A rotationally hindered and thus stereogenic biaryl axis is the structurally and stereochemically decisive element of a steadily growing number of natural products, chiral auxiliaries, and catalysts. Thus, it is not surprising that significant advances have been made in the asymmetric synthesis of axially chiral biaryl compounds over the past decade. In addition to the classic approach (direct stereoselective aryl-aryl coupling), innovative concepts have been developed in which the asymmetric information is introduced into a preformed, but achiral-that is, symmetric or configurationally labile-biaryl compound, or in which an aryl--C single bond is stereoselectively transformed into an axis. This Review classifies these strategies according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed.

Asymmetric total synthesis of sperabillins B and D via lithium amide conjugate addition.

Diastereoselective conjugate addition of homochiral lithium (R)-N-allyl-N-alpha-methylbenzylamide to methyl (2E,5E)-hepatadienoate, followed by protecting group manipulation and subsequent iodocyclocarbamation allows a concise route to the core fragment, methyl (3R,5R,6R)-3,6-diamino-5-hydroxyheptanoate, of sperabillins B and D. Differentiation between the C-3 and C-6 primary amino groups of this core amino acid was readily achieved by treatment with acetone, giving the 5,6-isopropylidene and C-3-imine protected diamine, with subsequent regioselective acylation of the C-6-nitrogen facilitating the total synthesis of sperabillin D in 10.8% overall yield, and the first asymmetric synthesis of sperabillin B in 5.8% overall yield.