A hydrophobic cuticle consisting of waxes and the polyester cutin covers the aerial epidermis of all land plants, providing essential protection from desiccation and other stresses. We have determined the enzymatic basis of cutin polymerization through characterization of a tomato extracellular acyltransferase, CD1, and its substrate, 2-mono(10,16-dihydroxyhexadecanoyl)glycerol (2-MHG). CD1 has in vitro polyester synthesis activity and is required for cutin accumulation in vivo, indicating that it is a cutin synthase.

Summary The aerial epidermis of all land plants is covered with a hydrophobic cuticle that provides essential protection from desiccation, and so its evolution is believed to have been prerequisite for terrestrial colonization. A major structural component of apparently all plant cuticles is cutin, a polyester of hydroxy fatty acids. However, despite its ubiquity, the details of cutin polymeric structure and the mechanisms of its formation and remodeling are not well understood. We recently reported that cutin polymerization in tomato (Solanum lycopersicum) fruit occurs via transesterification of hydroxyacylglycerol precursors, catalyzed by the GDSL-motif lipase/hydrolase family protein (GDSL) Cutin Deficient 1 (CD1). Here we present additional biochemical characterization of CD1 and putative orthologs from Arabidopsis thaliana and the moss Physcomitrella patens, which represent a distinct clade of cutin synthases within the large GDSL super-family. We demonstrate that members of this ancient and conserved family of cutin synthase-like (CUS) proteins act as polyester synthases with negligible hydrolytic activity. Moreover, solution-state NMR analysis indicates that CD1 catalyzes the formation of primarily linear cutin oligomeric products in vitro. These results reveal a conserved mechanism of cutin polyester synthesis in land plants, and suggest that elaborations of the linear polymer, such as branching or cross-linking, may require additional, as yet unknown, factors.

Many site-selective palladium-catalyzed C-H functionalization methods require directing groups. We report here β-carboline amides as intrinsic directing groups for C(sp)-H functionalization. Various substrates including the natural product alangiobussinine and the marinacarboline core structure were functionalized using carboline-directed δ-C(sp)-H alkynylations. This transformation proceeds under mild conditions and is compatible with a wide variety of β-arylethamines. δ-Alkynylation of β-arylethamines via a six-membered palladacycle is favored over γ-C(sp)-H bond functionalization when both positions are accessible. The versatility of β-carboline amides as directing groups is evidenced by other δ-C(sp)-H functionalizations such as alkenylation, arylation, and C-N bond formation.

A general approach for the formation of five-membered saturated heterocycles by intramolecular C(sp(3))-H functionalization is reported. Using N-sulfonyltriazoles as Rh(II) azavinyl carbene equivalents, a wide variety of stereodefined cis-2,3-disubstituted tetrahydrofurans were obtained with good to excellent diastereoselectivity from readily available acyclic precursors. The reaction is shown to be amenable to gram scale, and judicious choice of reaction conditions allowed for stereodivergence, providing selective access to the trans diastereomer in good yield. The resulting products were shown to be valuable intermediates for the direct preparation of fused N-heterotricycles in one step by intramolecular C-H amination or Pictet-Spengler cyclization.

A coupling reaction between secondary propargyl amines and isothiocyanates in aqueous media is described. The reaction is high-yielding and affords cyclized products within 2-24 h. A functionalized ether lipid was synthesized in 8 steps, formulated as liposomes with POPC and conjugated to FITC under mild conditions using this method.