Olefin metathesis has become a mainstay in organic synthesis. 1 Cross-metathesis (CM), however, is largely underdeveloped compared to ring closing metathesis (RCM) and ring opening metathesis polymerization since CM does not have the entropic driving force of RCM and is complicated by self-metathesis. 2 Our group has a long-term interest in site-selective chemical modification of proteins in an effort to study and modulate their function. 3 Olefin metathesis is an attractive way to install these protein modifications through a stable carbon-carbon bond. Indeed, incorporation of olefins into proteins has been possible for nearly a decade, 4 but metathesis at such residues has not been realized. Despite recent reports of olefin metathesis in water, the current benchmark for homogeneous aqueous CM is the self-metathesis of simple unsaturated alcohols such as allyl alcohol. 5,6 The limited examples revealed to date highlight the challenges for aqueous CM and the gap in substrate complexity that must be bridged to carry out metathesis on protein surfaces.To determine the viability of CM on protein surfaces, simple amino acid models were investigated. Substrates were selected on the basis of potential incorporation into proteins. A reasonable starting point was homoallylglycine (Hag) since its in ViVo incorporation by methionine auxotrophic Escherichia coli is known. 4 Hoveyda-Grubbs second generation catalyst 1 7 was selected since it is phosphine free and therefore more likely to be compatible with protein disulfides than other conventional catalysts. A simple test metathesis with allyl alcohol 2 was carried out to assess the reactivity of Hag derivative 3. At the outset, we limited ourselves to temperatures generally compatible with proteins (e37°C ) and made no effort to exclude oxygen. Since 1 is not freely soluble in water, it was added as a solution in t BuOH. Unfortunately, despite repeated attempts, only starting material 3 was recovered (Table 1, Entry 1). We turned next to cysteine derivatives since incorporation into proteins should be possible by either chemical or genetic means if they proved reactive in CM. Remarkably, S-allylcysteine (Sac) derivative 4 underwent metathesis with allyl alcohol (Entry 2), affording the CM product in 56% isolated yield (74% based on recovered 4). This result was noteworthy given the number of instances where thioethers were detrimental to rutheniumbased metathesis catalysts. 8 The metathesis was also efficient with allyl homocysteine 5 and bisamide Sac derivative 6. Yet when the alkene was extended by one or two methylene units from the sulfur center, only allyl alcohol self-metathesis was observed along with recovered starting material (Entries 5 and 6). Other allylheteroatom substrates were screened, but allyl sulfides remained the most efficient metathesis substrates under the conditions employed (Entries 7-13).While the self-metathesis of allyl sulfides has been carried out in organic solvents, the efficiency relative to other heteroatom or hydrocarbon analogues was...

Tagged for thiolation: A novel glycoconjugation strategy utilizes a non‐natural olefin‐containing amino acid (homoallylglycine, Hag) as a “tag” for modification and a photoinitiated hydroglycothiolation reaction that is selective only for the Hag olefinic “tag”. Application of this method to a number of model proteins allowed complete and precise site‐selective glycosylation generating glycoconjugates that include, for example, virus‐like particles displaying up to 180 glycans at preselected positions (see scheme).

Tagged for thiolation: A novel glycoconjugation strategy utilizes a non‐natural olefin‐containing amino acid (homoallylglycine, Hag) as a “tag” for modification and a photoinitiated hydroglycothiolation reaction that is selective only for the Hag olefinic “tag”. Application of this method to a number of model proteins allowed complete and precise site‐selective glycosylation generating glycoconjugates that include, for example, virus‐like particles displaying up to 180 glycans at preselected positions (see scheme).

Nonenzymatic, chemical methods for the controlled cleavage of proteins at predictable sites in a site-specific manner are rare and of strong potential utility in clean, post-translational manipulation of protein structure for use in, for example, proteomics, sequencing, and tagged-protein production. Unprecedented photochemical, site-selective cleavage of a His-Trp (HW) motif in the GH1 family TIM-barrel proteins is observed upon exposure to 240-308 nm light to cleanly release N-terminal primary amide and C-terminal indolylenamide fragments. We also show that this photocleaveable motif can be transferred to fusion proteins for use in photoresponsive affinty purification. The presence of this motif in proteins found only in organisms that are not typically exposed to light raises the possibility of direct biological relevance for this new type of protein reaction.

Thioethers P 0430Allyl Sulfides Are Privileged Substrates in Aqueous Cross-Metathesis: Application to Site-Selective Protein Modification. -Allyl sulfides undergo efficient cross-metathesis in aqueous media with Hoveyda-Grubbs catalyst second generation. The high reactivity of allyl sulfides in cross-metathesis is investigated in the first examples of cross-metathesis on a protein surface. S-Allylcysteine can be incorporated chemically into the protein. Preliminary efforts to insert S-allylcysteine genetically into proteins are also reported. -(LIN, Y. A.; CHALKER, J. M.; FLOYD, N.; BERNARDES, G. J. L.; DAVIS*, B. G.; J. Am. Chem. Soc. 130 (2008) 30, 9642-9643; Chem. Res. Lab., Dep. Chem., Univ. Oxford, Oxford OX1 3TA, UK; Eng.) -S. Adam 49-049