olefin metathesis is 
 a highly versatile tool for the generation 
 of carbon  carbon bonds , and a variety of applications have 
 evolved around its implementation .
the broad utility of olefin metathesis is a consequence of the exceptional 
 selectivity , activity , and functional - group compatibility of select 
 metathesis catalysts , highlighted by carbon  carbon bond formation 
 on a variety of complex substrates including small molecules , natural products , organic and inorganic materials , and even proteins .
the use of metathesis in biological 
 applications is an emerging field of research , in part , because of 
 advances in the genetic and chemical incorporation 
 of alkene - containing amino acids into peptides and proteins .
carbon 
 bonds with high fidelity for applications in peptide stapling , as surrogates of hydrogen bonding , and as methods for modifying 
 peptides and proteins used to mimic physiologically relevant post - translational 
 modifications .
the application of metathesis 
 for stabilizing peptide secondary structure and for performing selective 
 protein modifications has implications for imparting greater metabolic 
 stability , cellular permeability , and higher binding affinity toward 
 biological targets .
peptides 
 used as inhibitors of hiv fusion and 
 assembly , as modulators of signaling pathways involved in cancer , and as selective activators of enzymes involved in diabetes . despite the tremendous success of metathesis in peptide and 
 peptidomimetic 
 research , the ability to control olefin geometry in the product has 
 been met with limited success .
most metathesis catalysts exhibit 
 minimal kinetic selectivity , and thus , the product distribution reflects 
 the thermodynamic stability of each olefin isomer . in many cases , a mixture of e and z isomers
this imposes 
 challenges for examining the influence of alkene geometry on the stability 
 and activity of diverse compounds . in pursuit of catalysts with greater 
 control over olefin products
, we uncovered a series of cyclometalated 
 ruthenium catalysts that could achieve high conversions with exquisite z selectivity ( figure 1 ) .
the origin of z selectivity for cyclometalated 
 ruthenium catalysts involves approach of the olefin from a side - bound 
 position ( i.e. , cis to the nhc ligand and trans to the chelating adamantyl ) 
 and is favored through a combination of steric and electronic effects 
 imposed by the nhc ligand . although catalysts 1 and 2 demonstrate excellent selectivity in 
 olefin metathesis , their activity on complex substrates , including 
 peptides , remained unexplored . to this end
, we sought to initiate 
 a comprehensive evaluation of z - selective metathesis 
 on peptides using newly developed cyclometalated ruthenium catalysts . 
 through the combined efforts of homodimerization , cross metathesis 
 and ring - closing metathesis ,
we have developed guidelines for assessing 
 the influence of amino acids and peptides on catalyst activity and 
 selectivity .
these principles were applied for carrying out z - selective metathesis on challenging substrates including 
 peptides that comprise parallel -sheets and on stapling of 
 -helical peptides .
our goal 
 for expanding the utility 
 of z - selective ruthenium catalysts was to examine 
 the influence of amino acids on the selectivity and activity of catalysts 1 and 2 .
catalysts bearing n - adamantyl substituents and bidentate nitrato ligands were found 
 to be critical for achieving high z selectivity , and we set out to determine whether such catalysts 
 could be applied to substrates bearing multiple functionalities and 
 with varying steric profiles
. to benchmark the reactivities of catalysts 1 and 2 , we chose to investigate the homodimerization 
 of protected amino acids modified with homoallyl functionality ( table 1 ) . our initial studies 
 focused on the use of alanine 3 , as we anticipated that 
 amino acids bearing unhindered and aliphatic side chains would provide 
 an ideal platform for comparative studies .
we began with a catalyst 
 loading of 2.5 mol % in tetrahydrofuran ( thf ) at 40 c , and this 
 afforded the homodimerization product 4 in 53% yield 
 after 4 h using catalyst 1 and 58% yield in the presence 
 of catalyst 2 ( entry 1 ) .
gratifyingly , the z selectivity 
 remained high ( 90% ) throughout the course of the reaction .
catalyst loadings of 5 mol % afforded product 4 in 65% yield with 92% z selectivity in 
 the presence of catalyst 2 ( entry 2 ) .
increasing the 
 concentration led to modest improvements in yields while maintaining 
 high z selectivity ( entries 3 and 4 ) . ultimately , 
 we found that using a catalyst loading of 7.5 mol % afforded product 4 in 76% yield with 94% z selectivity ( entry 
 6 ) , and these conditions proved to be optimal among the various reaction 
 conditions explored .
we next examined the solvent dependence of the 
 activities of catalysts 1 and 2 in homodimerization , 
 as this is an important consideration in view of the solubility profiles 
 of many peptidic substrates ( table 2 ) .
several 
 polar and nonpolar solvents were investigated reflecting those most 
 often used in peptide synthesis . coordinating solvents
[ e.g. , acetonitrile 
 ( mecn ) ] were less active in promoting z - selective 
 metathesis as compared to noncoordinating solvents [ e.g. , dicholorethane 
 ( dce ) ] ( entries 2 and 3 ) .
polar solvents including dimethylformamide 
 ( dmf ) , dimethyl sulfoxide ( dmso ) , and n - methylpyrrolidone 
 ( nmp ) were tolerated by catalysts 1 and 2 , affording product 4 in yields ranging from 55% to 
 67% and 90% z selectivity ( entries 46 ) .
protic 
 solvents including methanol ( meoh ) , ethanol ( etoh ) , and aqueous tert - butanol ( t - buoh ) mixtures yielded 
 highly enriched z - olefin products ( entries 79 ) . other polar solvents including nitromethane 
 ( meno2 ) resulted in low conversions ( entry 10 ) , presumably 
 by activating decomposition pathways of the cyclometalated ruthenium 
 catalysts .
it is worth noting that , across 
 the variety of reaction conditions explored , the z selectivity remained consistently high .
determined by h or c nmr 
 spectroscopy . determined by h or c nmr 
 spectroscopy . to probe the 
 activities of catalysts 1 and 2 further , 
 we investigated the homodimerization of other canonical 
 amino acids ( table 3 ) .
boc - protected amino 
 acids bearing aliphatic or aromatic side chains were active in z - selective metathesis ( entries 14 ) .
branched side 
 chains from amino acids including valine ( 5a ) , isoleucine 
 ( 5b ) , and leucine ( 5c ) afforded products 6a  c in yields approaching 74% with 94% z selectivity ( entries 13 ) .
aromatic side chains 
 from amino acids including phenylalanine 5d afforded 
 product 6d in 75% yield and 93% z selectivity 
 in the presence of catalyst 2 ( entry 4 ) .
exceptions include 
 glycine and proline , as these are inactive in homodimerization ( entries 5 and 
 6 ) .
homodimerization 
 of tryptophan afforded product 6 g in 64% yield with 90% z selectivity ( entry 7 ) .
in contrast , histidine was deemed 
 incompatible with catalysts 1 and 2 ( entry 
 8) .
unprotected alcohols from the side 
 chains of serine 5i or threonine 5j were 
 tolerated by catalysts 1 and 2 but required 
 protection to reach acceptable conversions , providing products 6i and 6j in 72% and 73% yields , respectively 
 ( entries 9 and 10 ) .
homodimerization of tyrosine afforded product 6k in 64% yield and 87% z selectivity in 
 the presence of catalyst 1 and 68% yield and 90% z selectivity using catalyst 2 ( entry 11 ) . 

in contrast to alcohols , side chains bearing thiols ( i.e. , cysteine ) 
 or thioethers ( i.e. , methionine 5l ) generally led to 
 catalyst deactivation ( entry 12 ) .
protecting 
 the side chain of cysteine could overcome catalyst inactivity and 
 afford product 6 m in 55% yield and 87% z selectivity ( entry 13 ) .
polar side chains bearing carboxylate ( 5n , o ) , carboxamide ( 5p , q ) , or amine ( 5r ) functionality required protection prior 
 to homodimerization . in these cases , yields from 70% to 80% could 
 be achieved with high z selectivity ( entries 1418 ) . 

side chains bearing a protected guanidinium functionality ( i.e. , arginine 5s ) were less active in homodimerization , affording product 6s in 34% and 33% yields using catalysts 1 and 2 , respectively .
these findings reveal that the identity of 
 the amino acid side chain can profoundly influence the activity of 
 cyclometalated ruthenium catalysts .
determined by h or c nmr 
 spectroscopy . side chain 
 protected with a trityl 
 group .
emboldened by the success of z - selective homodimerization of amino acids , we next evaluated the 
 effects of varying the metathesis partner to achieve z - selective cross metathesis ( cm ) ( table 4 ) . 

for our studies , we chose cross partners that showed variable activity 
 in homodimerization . in this way
, we could begin to classify substrates 
 on the basis of their propensity for achieving productive cm .
we explored cm using homoallyl - modified alanine 3 and the similarly reactive substrate valine 5a as the cross partner .
cross metathesis in the presence of an equimolar 
 ratio of 3 to 5a afforded the desired heterocross 
 product 7 in 41% yield with 90% z selectivity 
 ( entry 1 ) . increasing the stoichiometry of 5a relative 
 to 3 led to a modest increase in yield , providing product 7 in 48% yield with 91% z selectivity ( entry 
 2 ) .
further increases in 5a afforded similar trends , 
 with yields approaching 60% and 90% z selectivity 
 ( entries 3 and 4 ) . to demonstrate the versatility of this method , 
 we explored conditions in which 5a was used as the limiting 
 reagent ( entries 57 ) . under these conditions , an incremental 
 increase in the yield of product 7
was observed upon 
 increasing the ratio of 3 to 5a . as in the 
 case of excess 5a
, modest improvements in the yield of 7 were achieved with increasing equivalents of 3 , demonstrating the statistical nature of cm using similar reactive 
 substrates .
given the modest conversions 
 to product and high z selectivities , we can conclude 
 that the products of cm are sparingly consumable using catalysts 1 and 2 , ensuring that the z selectivity can remain high throughout the course of the reaction .
we next evaluated whether amino 
 acids of differing reactivity profiles 
 could be used for selective cm ( table 5 ) . in 
 choosing the requisite cross partners , we focused on substrate 3 and homoallyl - modified arginine 5s , as they 
 both showed relatively low reactivity in homodimerization .
we began 
 our studies using an equimolar ratio of 3 and 5s , and this afforded 8 in 46% yield and 93% z selectivity ( entry 1 ) . increasing the ratio of 5s to 3 led to slightly diminished yields and z selectivities ( entries 24 ) .
by contrast , reversing the order such that 3 was in 
 excess of 5s afforded 8 in yields of 47% 
 and 58% using catalysts 1 and 2 , respectively 
 ( entry 5 ) . further increasing the ratio of 3 to 5s could achieve the heterocross product 8 in 
 improved yields and high z selectivities ( entries 
 6 and 7 ) .
taken together , these findings reveal that the intrinsic 
 reactivity differences between homoallyl - modified amino acids can 
 be used for productive z - selective cross metathesis .
the unique reactivity 
 profiles of canonical amino acids in z - selective 
 homodimerization and cm prompted investigation of a subset of non - natural 
 amino acids that have shown promise in peptide and protein modification 
 using olefin metathesis .
allyl - protected amino acids including serine , 
 cysteine , and selenocysteine have been shown to enhance the rate of 
 metathesis when incorporated into peptides and proteins .
studies by davis and co - workers ascribed the unique chemical reactivity 
 of such amino acids through a chelation - assisted mechanism whereby 
 precoordination of the heteroatom to ruthenium increases the effective 
 concentrations of the alkylidene and alkene without detrimental chelation . moreover , softer nucleophiles such as sulfur 
 and selenium were found to have an activating effect relative to oxygen 
 for enhancing the rate of cm .
these findings are intriguing considering 
 that sulfur can have a deactivating effect on olefin metathesis , and our results suggest that sulfur - containing amino acids ( e.g. , 
 compound 5l ) lead to catalyst inactivity .
nonetheless , 
 a wealth of information suggests that heteroatoms can modulate metathesis 
 activity ; however , this phenomenon was unexplored using z - selective cyclometalated catalysts . to investigate 
 the influence of allylic heteroatoms on the activities of catalysts 1 and 2
, we examined a series of allyl - protected 
 amino acids in homodimerization ( table 6 ) . 

we chose allylglycine ( 9a ) and homoallylglycine ( 9b ) , as well as allyl - protected serine ( 9c ) and 
 cysteine ( 9d ) , for our studies . in this sense
, 9a  d would reveal both the role of sterics 
 ( i.e. , comparison of 9a and 9b ) and the 
 effect of chelation by heteroatoms ( i.e. , comparison of 9b to 9c , d ) in facilitating metathesis . for 
 these tests , we investigated homodimerization of 9a and 
 compared its reactivity relative to that of substrate 9b .
conversion of 9a to the dimerized product 10a was low using catalyst 1 or 2 , occurring 
 in 45% yield and 90% z selectivity ( entry 1 ) .
by 
 comparison , dimerization of homoallylglycine 9b afforded 
 product 10b in 59% yield and 90% z selectivity 
 ( entry 2 ) .
this corroborates our earlier findings that the steric 
 environment around the alkene can influence the efficiency of z - selective metathesis .
allyl - protected serine 9c afforded product 10c in 67% yield with 92% z selectivity ( entry 3 ) . 
 by comparison ,
cysteine 9d was more active in homodimerization , 
 leading to 71% yield and 93% z selectivity in the 
 presence of catalyst 2 ( entry 4 ) .
these results suggest 
 that allylic heteroatoms can facilitate z - selective 
 metathesis in the presence of cyclometalated ruthenium catalysts . determined 
 by h or c nmr spectroscopy . the insights garnered from the homodimerization 
 of substrates 9a  d revealed that 
 the identity of the 
 alkene can influence the activities of catalysts 1 and 2 in z - selective metathesis .
although these 
 studies provide important insight for assessing the intrinsic reactivity 
 of allyl - modified amino acids with cyclometalated ruthenium catalysts , 
 the general utility of such catalysts could be further illuminated 
 by their use in cm . as such , we examined catalysts 1 and 2 in cm using allyl - modified amino acids 9a  d ( table 7 ) . to assess the relative 
 activities of substrates 9a  d in 
 cm , we chose allyl acetate 11 as the cross partner , as 
 we previously showed that 11 is highly active in z - selective cm .
we explored 
 a variety of conditions to test the generality of cm including the 
 use of solvents that are compatible with native peptides and proteins . 
 for our initial studies , we examined cm between allylglycine 9a and 11 under our previously optimized conditions , 
 and this afforded the heterocross product 12a in 42% 
 yield and 90% z selectivity .
we next explored cm 
 under aqueous conditions , including the use of additives shown to 
 enhance the efficiency of cm on peptides and proteins . in the presence of aqueous tert - butanol
, the heterocross product 12a was achieved in 
 38% yield with 90% z selectivity ( entry 2 ) .
inclusion 
 of salts such as licl or mgcl2 , which has been shown to 
 be beneficial for enhancing methathesis on peptides , afforded product 12a in 31% yield but with diminished z selectivity ( entries 3 and 4 ) .
these trends were also 
 observed using homoallylglycine 9b as the cross partner 
 ( entries 58 ) . to investigate whether amino acids bearing allylic 
 heteroatoms influence the efficiency of z - selective 
 cm , we exposed allylserine 9c and allylcysteine 9d to similar reaction conditions .
synthesis of the heterocross 
 product was improved , affording 12c in 63% yield with 
 92% z selectivity using catalyst 2 ( entry 
 9 ) .
the use of aqueous conditions ( entry 10 ) or inclusion of additives 
 ( entries 11 and 12 ) led to slightly diminished yields and z selectivity . by comparison , the use of allylcysteine 9d as the cross partner afforded the desired product 12d in 60% yield with 90% z selectivity ( entry 
 13 ) . as observed with substrates 9a  c , a decrease in catalyst activity occurred under aqueous conditions 
 and in the presence of salt additives ( entries 1416 ) .
collectively , these results suggest that the 
 activities of catalysts 1 and 2 are highly 
 dependent on the reaction conditions . in general , the trends observed 
 in cm with 9a  d parallel those observed 
 using nonchelated ruthenium catalysts ; however , the use of salts as additives appears to have a deactivating 
 effect on the activity of cyclometalated ruthenium catalysts .
these 
 results lend support to the importance of the chelating ligand ( i.e. , 
 bidentate versus mondentate ligand coordination ) in the activities 
 and selectivities of catalysts 1 and 2 . determined 
 by h or c nmr spectroscopy .
reaction conditions : 1 mmol of 9a  d in 1:1 h2o / t - buoh .
reaction conditions : 1 mmol of 9a  d in 1:1 h2o / t - buoh + 2 mm licl .
reaction 
 conditions : 1 mmol of 9a  d in 1:1 
 h2o / t - buoh + 2 mm mgcl2 .
investigation of both canonical and noncanonical 
 amino acids in 
 homodimerization and cross metathesis revealed that the choice of 
 cross partner is critical to the success of ruthenium - catalyzed z - selective metathesis . in this
regard , amino acid side 
 chains bearing aliphatic , aromatic , or protected polar functionalities 
 were highly active in metathesis .
those amino acids that generally 
 lead to lower conversion ( i.e. , arginine ) could undergo selective 
 cross metathesis in the presence of a more highly reactive substrate . 

moreover , the steric environments around the olefin and allylic heteroatoms 
 were shown to impact the efficiency of z - selective 
 cm . whereas these observations apply generally to amino acids that 
 are commonly used in olefin metathesis , we wanted to investigate whether 
 cyclometalated ruthenium catalysts could be used in cm on more complex 
 substrates , including peptides . in choosing the requisite cross partners , 
 we took inspiration from peptides known to adopt defined -sheet 
 secondary structures when tethered through a turn - promoting moiety 
 or as part of a macrocycle .
such structures hold promise 
 in applications ranging from supramolecular chemistry to biology and offer challenging substrates for catalysts 1 and 2 .
a wealth of information regarding 
 the use of peptides and small molecules as -sheet mimics has 
 revealed that both hydrogen bonding and amino acid side - chain pairing 
 preferences can be used to dictate the stability of -sheet 
 formation . to this end , we synthesized peptides 13 and 14 , which represent typical sequences found in parallel -sheets 
 and whose structures are known to rely on the sequence of amino acids 
 ( scheme 1 ) . for 
 our studies , we used an excess of peptide 13 , as we had 
 shown that less reactive amino acids , including arginine , could be 
 used for selective cross metathesis in the presence of more highly 
 active amino acids . under these conditions ,
conversion reached 60% 
 for the desired peptide 15 with greater than 90% z selectivity .
attempts to 
 improve the yield of the heterocross product by conducting cm in solvents 
 that promote hydrogen bonding and thereby preorganize the peptides 
 to facilitate metathesis led to similar conversions .
these results attest to the functional - group 
 tolerability of cyclometalated ruthenium catalysts and point to further 
 strategies aimed at accessing complex olefinic substrates bearing 
 multiple functionalities .
determined by analytical high - performance 
 liquid chromatography mass spectrometry ( hplc - ms ) .
our results regarding the activity of cyclometalated 
 ruthenium catalysts in cross metathesis of amino acids and peptides 
 revealed that the kinetic selectivities of catalysts 1 and 2 can be used to synthesize peptides highly enriched 
 in z - olefins .
although these studies provide a framework 
 for promoting metathesis through judicious choice of the amino acid 
 sequence , we observed that conversions in cm were optimal at relatively 
 high concentrations ( 0.30.4 m ) , which , for many substrates , 
 might be a limitation .
we sought a more general strategy such that 
 a variety of reaction conditions could be used to promote z - selective metathesis on a diverse collection of peptides . 
 to this end , we investigated ring - closing metathesis ( rcm ) of olefinic 
 amino acids for the synthesis of  stapled  peptides . 

such peptides hold promise as novel therapeutics by virtue of their 
 enhanced -helicity ; proteolytic stability ; and ability to target intracellular proteins involved in cancer , infectious disease , and metabolism . as 
 such
, we set about to broaden the available catalysts used to synthesize 
 this important class of peptides under conditions that would be amenable 
 to comprehensive screening of catalyst activity in the presence of 
 varying peptide sequences .
traditional methods for the synthesis 
 of stapled peptides by rcm have relied on the use of o - allyl serine or c-tetrasubstituted amino acids to install macrocyclic cross - links into 
 synthetic peptides .
most strategies incorporate non - natural amino 
 acids at positions spanning one ( i , i + 4 ) or two ( i ,
i + 7 ) turns of 
 a helix that serve to preorganize the reactive side chains on the 
 same helical face .
a wealth of knowledge derived from computational and experimental approaches has illuminated the minimal constraints necessary for 
 achieving rcm on peptides using first- and second - generation or grubbs 
an unmet challenge in the synthesis 
 of stapled peptides has been the ability to control olefin geometry 
 in the product , as the use of noncyclometalated ruthenium catalysts 
 typically gives rise to both e and z isomers that are often inseparable .
this imposes challenges for 
 examining the role of olefin geometry in the stability and biological 
 activity of stapled peptides , which , to date , has not been thoroughly 
 explored . to this end , we employed catalysts 1 and 2 in z - selective rcm for stapling -helical 
 peptides that encompass the vast majority of peptides used for biological 
 studies .
as part of our ongoing effort to expand the utility 
 of catalysts 1 and 2 , we chose to conduct 
 rcm on resin - supported 
 peptides .
this would streamline the synthesis of peptides and offer 
 a modular platform to test the activities of cyclometalated ruthenium 
 catalysts .
our goal was to compare the activities of z - selective catalysts with those of noncyclometalated ruthenium catalysts 
 in rcm , and we focused our efforts on peptides with known biological 
 activity .
the sequence we chose is derived from an -helical peptide known to target the bcl-2 family 
 of proteins involved in the regulation of apoptosis and whose activity 
 is modulated by constraining the peptide through hydrocarbon stapling 
 ( table 8) .
the 
 chemical features of peptide 16 consist of two stereochemically 
 defined ,-disubstituted olefinic amino acids separated by one turn of a helix ( i.e. , olefins 
 positioned at residues i and i + 
 4 ) that , upon ring closure , would generate a 21-membered macrocycle .
loading capacities for resin : wang , 
 0.5 mmol / g ; tentagel , 0.25 mmol / g ; mbha , 0.5 mmol / g .
amino acids were protected prior 
 to rcm . determined by analytical 
 hplc - ms .
the stabilities 
 and activities of catalysts 1 and 2 in the 
 presence of resins were unexplored , and we examined 
 commonly used resins for solid - phase peptide synthesis that varied 
 based on composition and loading capacity . throughout our studies , 
 we performed rcm in solvents that promote -helicity [ e.g. , 
 dicholorethane ( dce ) ] at concentrations that favor macrocyclic ring 
 closure .
initial screening revealed that the choice of resin influenced 
 the activities of catalysts 1 and 2 in rcm . 

conversions to the desired rcm product 17 were typically 
 low on wang resin using 10 mol % catalyst at room temperature for 
 2 h ( entry 1 ) .
resins bearing hydrophilic linkers proved beneficial , 
 affording conversions approaching 40% under the same reaction conditions 
 ( entry 2 ) .
the use of mbha resin led to 60% conversion ( entry 3 ) , 
 and we focused further optimization using this resin .
we next explored 
 the effects of reaction time and catalyst loading on rcm . prolonging 

the reaction led to modest improvements , generating product 17 in 70% conversion with greater than 90% z selectivity ( entry 4 ) , and subjecting the resin - bound peptide to 
 successive rounds of catalyst resulted 
 in conversions of 75% using two cycles of catalyst addition ( entry 
 5 ) .
increasing the temperature to 40 c afforded 17 in 80% yield and with greater than 90% z selectivity 
 ( entry 6 ) . to probe the generality 
 of the method , we investigated peptides 
 bearing olefinic amino acids spanning two turns ( i , i + 7 ) of a helix with varying amino acid sequences 
 ( scheme 2 ) .
to 
 span the distance of two helical turns , we modified the n - terminal 
 olefinic amino acid by increasing the tether length ( from five to 
 eight carbon atoms ) and inverting the stereochemical configuration 
 ( from s to r ) , both of which were 
 predicted to facilitate rcm . under our optimized conditions , conversions of 85% for the desired 
 rcm product 19 could be achieved after two cycles of 
 catalyst addition with greater than 90% z selectivity . 

these results demonstrate that cyclometalated ruthenium catalysts 
 can promote z - selective rcm on solid support for 
 the synthesis of stapled peptides bearing all hydrocarbon cross - links . determined by analytical hplc - ms .
we earlier demonstrated that aib - rich 
 peptides bearing i , i + 3 l - serine o - allyl residues afford highly e - selective rcm products , in studies 
 motivated , in part , by a theoretical 
 prediction that suggested an rcm - derived 18-membered ring using these 
 side chains would serve as a minimal constraint for the 310-helix .
we were curious to see whether 
 a z - selective catalyst could overcome any substrate 
 bias favoring the e - olefin geometry . to this end
, 
 we studied the rcm conversions of pentapeptide boc - l - ser(al)-aib - aib - l - ser(al)-aib - ome ( 20 ) and heptapeptide boc - l - val - l - ser(al)-l - leu - aib - l - ser(al)-l - val - l - leu - ome ( 24 ) to macrocycles 21 and 25 , respectively , using second - generation 
 catalysts 22 and 23 and z - selective catalyst 1 ( table 9 ) .
as expected from our earlier studies , peptide 20 readily 
 cyclized to the e macrocycle in the presence of 10 
 mol % 22 or 23 in dce held at 45 c 
 for 10 h ( entries 1 and 2 ) . under similar reaction conditions , no 
 macrocyclization was observed with catalyst 1 , even with 
 a 3-fold increase in catalyst loading and extended reaction time ( entry 
 3 ) . the same behavior was observed for heptapeptide 24 , although , in this case , the z macrocycle did form 
 with 22 or 23 to the extent of ca . 8% ( entries 
 4 and 5 ) . peptide 24 rapidly cyclized with the second - generation 
 catalysts in refluxing dichloromethane , but it was unreactive toward 
 catalyst 1 under identical conditions ( entry 6 ) . whereas 
 most peptide side - chain rcm reactions produce e / z mixtures , the minimal i , i + 3 cross - link in these aib - containing systems seems reluctant to 
 form the z - olefin , probably a consequence of the 
 conformational restrictions imposed by the c-tetrasubstituted 
 -amino residues .
these results 
 demonstrate that the i + 1 and i + 2 aib residues play a role in controlling the e - selective rcm behavior with catalysts 22 and 23 and this conformational bias can not be overcome by z - selective catalysts .
these studies highlight the importance 
 of peptide sequence and conformation in controlling olefin geometry 
 in the macrocyclic product and point to further strategies aimed at 
 understanding how these parameters affect the efficiency of z - selective rcm .
in 
 summary , we have reported the first examples of z - selective metathesis on peptides using cyclometalated ruthenium 
 catalysts . by examining a broad range of canonical and noncanonical 
 amino acids in cross metathesis , homodimerization , and ring - closing 
 metathesis ,
we have gleaned important criteria for achieving high 
 conversion while maintaining excellent z selectivity . 

the following insights based on our results are summarized : the side - chain identity of an amino acid can dictate the activities 
 of catalysts 1 and 2 in cross metathesis 
 and homodimerization . in general , amino acids bearing aliphatic or 
 aromatic side chains
( e.g. , alanine , leucine , and phenylalanine ) are 
 highly active in metathesis , with yields approaching 85% and 94% z selectivity .
sterically hindered side chains 
 ( e.g. , valine or isoleucine ) and amino acids bearing bulky protecting 
 groups lead to lower conversions but without degradation of z selectivity .
amino acids bearing carboxylate functionalities 
 ( i.e. , glutamic acid and aspartic acid ) require protection , as substrates 
 bearing acidic functionalities can lead to catalyst decomposition 
 and diminished z selectivity .
amino acids bearing 
 thiols or thioethers generally deactivate cyclometalated ruthenium 
 catalysts ; however , in some cases , the use of protecting groups can 
 lead to productive turnovers .
side chains bearing hydroxyl groups 
 ( e.g. , serine or threonine ) are generally tolerated by catalysts 1 and 2 , whereas amino acids bearing heterocycles 
 have variable activity .
tryptophan is active in z - selective homodimerization and cross metathesis , whereas histidine 
 is generally inactive .
polar side chains bearing carboxamide ( i.e. , 
 glutamine or asparagine ) or guanidinium ( i.e. , arginine ) functionalities 
 are generally intolerant of cyclometalated ruthenium catalysts
. protection 
 of these side chains can restore catalyst activity , leading to products 
 highly enriched in z - olefins .
cross metathesis 
 and homodimerzation of amino acids and peptides 
 using z - selective ruthenium catalysts can be performed 
 in a variety of solvents , provided that the acidity of the reaction 
 medium is kept minimal .
the use of solvents such as mecn , dmso , and 
 dmf generally leads to lower conversions than the use of noncoordinating 
 solvents ( e.g. , dce ) .
the use of protic solvents ( e.g. , meoh , etoh , 
 or h2o ) can lead to products enriched in z - olefins .
prolonged reaction times in protic solvents can , in some 
 cases , lead to decomposition of catalysts 1 and 2 .
amino acids bearing allylic or homoallylic functionality 
 are active 
 in z - selective metathesis . in general , higher conversions 
 in homodimerization and cm can be achieved using homoallylic functionalities , 
 particularly for sterically hindered substrates .
noncanonical amino 
 acids containing allylic heteroatoms including those that could be 
 incorporated into peptides and proteins are active in z - selective cross metathesis and follow trends similar to those of 
 noncyclometalated ruthenium catalysts .
the use of aqueous conditions 
 in the presence of salts as additives appears to diminish the activities 
 of catalysts 1 and 2 .
cyclometalated 
 ruthenium catalysts can be used to synthesize stapled 
 peptides bearing hydrocarbon olefinic cross - links positioned at residues i , i + 4 or i , i + 7 . to probe the limits of these catalysts in peptide 
 stapling , we exposed z - selective catalysts to aib - rich 
 peptides bearing o - allyl serine cross - links positioned 
 at residues
i and i + 3 that predominantly 
 give rise to highly e - selective macrocyclic products . 
 in these cases , catalysts such as
1 failed to undergo z - selective rcm , suggesting that conformational restrictions 
 imposed by substrates such as 20 can influence the activities 
 of cyclometalated ruthenium catalysts 1 and 2 in rcm .
notably , compounds 15 , 17 , and 19 represent the most complex substrates synthesized 
 by cyclometalated 
 ruthenium catalysts , and this bodes well for further studies aimed 
 at applying z - selective metathesis on substrates 
 bearing multiple functionalities .
it is envisioned that the studies 
 reported herein will serve as a guideline in choosing appropriate 
 alkene cross partners or promoting rcm on peptides .
we anticipate 
 that cyclometalated ruthenium catalysts could access new structures 
 and provide insight into the role of alkene geometry in the biological 
 activities of stapled peptides .
moreover , installation of z - alkenes into peptides and proteins could allow sites for 
 further modifications .
progress in the design and development of stereoselective 
 ruthenium catalysts will continue to broaden the application of olefin 
 metathesis in natural - product synthesis and in biology .
 olefin metathesis has emerged as 
 a promising strategy for modulating 
 the stability and activity of biologically relevant compounds ; however , 
 the ability to control olefin geometry in the product remains a challenge . 
 
 recent advances in the design of cyclometalated ruthenium catalysts 
 has led to new strategies for achieving such control with high fidelity 
 and z selectivity , but the scope and limitations 
 of these catalysts on substrates bearing multiple functionalities , 
 including peptides , remained unexplored . herein , we report an assessment 
 of various factors that contribute to both productive and nonproductive z - selective metathesis on peptides . 
 the influence of sterics , 
 side - chain identity , and preorganization through peptide secondary 
 structure are explored by homodimerization , cross metathesis , and 
 ring - closing metathesis . 
 our results indicate that the amino acid 
 side chain and identity of the olefin profoundly influence the activity 
 of cyclometalated ruthenium catalysts in z - selective 
 metathesis . 
 the criteria set forth for achieving high conversion and z selectivity are highlighted by cross metathesis and ring - closing 
 metathesis on diverse peptide substrates . 
 the principles outlined 
 in this report are important not only for expanding the scope of z - selective olefin metathesis to peptides but also for applying 
 stereoselective olefin metathesis in general synthetic endeavors . 
Introduction
Results and Discussion
Conclusions