since the beginning of mankind , organisms 
 producing natural products 
 have provided a reservoir of therapeutic remedies and medicines for 
 various diseases .
a subset of these drugs 
 has been classified as antitumor antibiotics based on their ability 
 to  block cell growth by interfering with dna , the genetic 
 material in cells  .
key general 
 features of an antitumor antibiotic include interference with dna 
 synthesis , membrane transport , and production of reactive oxygen species .
one of the most notable examples of an antitumor 
 antibiotic is mitomycin c , a microbial metabolite that is used currently 
 for the treatment of breast and bladder cancer . among other antitumor antibiotics , daunorubicin and its 
 semisynthetic derivative doxorubicin represent chemotherapeutic leukemia 
 agents in clinical settings .
the 
 search for new antitumor antibiotics led to the discovery of 
 chlorothricin ( 1 ) , a complex polyketide produced by various streptomyces strains .
its intriguing 
 chemical structure and bioactivity defined a new family of microbial 
 metabolites , commonly referred to as spirotetronate polyketides .
this 
 family is identified by the presence of a cyclohexene ring spiro - linked 
 to a tetronic acid moiety ( figure 1 , fragment 
 a ) that is embedded in a macrocycle ( figure 1 , fragment b ) . in several cases , the structure also contains a trans - decalin ring ( figure 1 , fragment 
 c ) and is decorated by various deoxy oligosaccharides ( figure 1 , fragment d ) . in terms of biological profile ,
spirotetronate 
 polyketides exhibit potent antibacterial and antitumor activities 
 and a documented value as tools in the elucidation of new biological 
 pathways . as such
, they represent highly promising leads in drug discovery . 
 to appreciate their untapped potential , in this review
we group the 
 known spirotetronates based on common structural elements and biosynthetic 
 considerations .
recently sssmuth and co - workers proposed a classification 
 of tetronates based on two main categories : the linear tetronates 
 and the spirotetronates . on the basis 
 of biosynthetic considerations
, the latter subgroup can be divided 
 into two classes : class i ( generic structure 4 ) , which 
 contains the spirotetronate moiety within a varying size macrocycle , 
 and class ii ( generic structure 5 ) , which additionally 
 contains a decalin moiety ( figure 2 ) .
representative 
 members of the class i spirotetronates in order of increasing macrocyclic 
 length are abyssomicin c ( 6 ) ( containing a c11 macrocycle ) , okilactomycin d ( 7 ) ( containing a c13 macrocycle ) , and spirohexenolide a / b ( 8/9 ) ( containing a c15 macrocycle ) . 

representative members of the class ii spirotetronates include maklamicin 
 ( 11 ) ( containing a c11 macrocycle ) , tetronolide ( 12 ) ( the aglycon of tetrocarcin a containing a c13 macrocycle ) , and chlorothricolide ( 2 ) ( the aglycon of chlorothricin containing a c13 macrolactone ) . in this class
is also included versipelostatin aglycone 
 ( 13 ) , which contains the largest c17 macrocyclic 
 motif isolated to date .
quartromicins 10 , unusual spirotetronate polyketides containing four spirotetronate 
 subunits , lie outside these two classes due to their peculiar structure and unique biosynthesis .
the above classification stems from a common biosynthetic 
 pathway that accounts for construction of these compounds .
in general , the biosynthesis of spirotetronates occurs through 
 condensation of acetic acid units via the type i polyketide synthase 
 pathway ( scheme 1 ) . as shown in the biosynthesis of abyssomicin c and okilactomycin , construction 
 of the class i spirotetronates proceeds by elongating their carbon 
 chain via incorporation of propanoyl and/or acetyl units ( 14/15 ) to the acyl carrier protein ( acp ) .
incorporation of 
 a glyceryl unit , via coa intermediate 17 , forms tetronate 18 likely via a 
 claisen condensation followed by lactonization . the precise mechanism 
 for the elimination of the c-5 hydroxy group
was recently elucidated 
 by the sun and leadlay groups and shown to proceed via acetylation 
 and subsequent elimination , thereby forming dienophile 19 . an intramolecular diels 
the resulting substrates subsequently undergo peripheral oxidations 
 to produce the final structures of the natural products .
the biosynthetic pathways of the class 
 ii polyketide spirotetronates 
 have been elucidated for chlorothricin , tetrocarcin a , kijanimicin , and versipelostatin .
following chain elongation , the diene and dienophile groups of 20 undergo an imda to construct the characteristic decalin 
 moiety of 21 .
glyceryl coa ( 17 ) is then inserted to generate tetronate 22 , which following a second imda gives rise to the aglycones 
 of the class ii spirotetronates ( figure 2 ) .
oxidations and/or glycosylations 
 at the periphery of the aglycone 
 lead to various natural products of the class ii spirotetronates . 

for instance , chlorothricolide ( 2 ) , the aglycone of chlorothricin , 
 contains an acyl - oxy tetronic acid moiety .
this functionality ( i.e. , 
 oxygenation at the c-2 position ) is proposed to result from a baeyer 
a similar biosynthetic scenario can be proposed for the construction 
 of pa-46101 a and b ( see structures 57/58 ) .
another interesting example of post - translational 
 modification is found in the structure of tetronolide ( 12 ) , the aglycone of tetrocarcin a ( see structure 47 ) . 

this functionality was proposed to result 
 from oxidation at c-32 to the corresponding aldehyde followed by double - bond 
 migration to c-22c-23 and further allylic oxidation at c-21 .
several class ii spirotetronates are 
 subject to glycosylation mostly 
 with 2-deoxycarbohydrates such as d - tetronitrose ( 26 , d - kijanose ) , amicetose ( 27 ) , and digitoxose 
 ( 28 ) .
these carbohydrates are proposed to arise from 
 thymidine diphosphate ( tdp)-6-deoxy-4-keto - d - glucose ( 24 ) , which , in turn , is available from d - glucose-1-phosphate 
 ( 23 ) ( scheme 2 ) .
biosynthesis 
 of the uncommon tetronitrose is proposed to occur from 25 via aminotransferase and methylation , while the precise mechanism 
 for the carbamate biosynthesis still remains elusive .
the majority of spirotetronates have been subjected 
 to biological 
 assays that aim to define their bioactivity as antibiotic and/or anticancer 
 leads as well as compounds that regulate metabolism . with this in 
 mind , we have grouped these molecules in three major classes that 
 describe the commonality of their bioactivities .
isolated 
 from a marine verrucosispora , abyssomicin c ( 6 ) and its atropisomer 
 ( 29 ) ( figure 3 ) are the first 
 known natural products to block para - aminobenzoic 
 acid ( paba , 41 ) biosynthesis.paba is a biosynthetic precursor of folic acid 
 ( vitamin b9 ) , and as such , it is essential for dna synthesis / repair 
 and cell survival ( scheme 3 ) . on the other hand , lack of folic acid
importantly , blocking the paba pathway is detrimental to bacteria but inconsequential 
 to humans since the latter can not produce folic acid but only absorb 
 it through their diet .
studies on the 
 effect of the abyssomicin motif in paba biosynthesis 
 have been performed with atrop - abyssomicin ( 29 ) and are highlighted in scheme 3 .
amino-4-deoxychorismate ( adc ) synthase , a heterodimeric protein , 
 catalyzes the biosynthesis of amino-4-deoxychorismate ( 40 ) , a synthetic precursor of paba . compound 29 was found to covalently react with the cys-263 of the pabb 
 subunit of adc synthase at the c-9 enone center .
the transiently formed 
 c-8 nucleophile then reacts with the spirotetronate subunit at the 
 c-16 center to form compound 39 , thus irreversibly binding 
 to adc synthase .
several natural products of the abyssomicin family have been 
 tested 
 for their ability to inhibit paba biosynthesis ( scheme 3 ) . among them , only abyssomicin c ( 6 ) , atrop - abyssomicin c ( 29 ) , and abyssomicin 
 j ( 31 ) have shown promising bioactivities .
specifically , 6 and 29 potently inhibit proliferation of methicillin - resistant staphylococcus aureus at mic values of 5.2 and 3.5 g / ml , 
 respectively .
similar cytotoxicities have been 
 reported against various tuberculosis - related mycobacteria . on the other hand ,
abyssomicin d ( 32 ) and related analogues 
 lacking the c-7c-9 enone motif are inactive , attesting to 
 the biological significance of this functionality .
this increased potency has been attributed to 
 an increased conjugation between the c-7 carbonyl group and the c-8c-9 
 alkene that renders 29 a stronger michael acceptor than 6 .
the bioactivity of abyssomicin 
 j ( 31 ) , a thioether dimer of the abyssomicin scaffold , 
 can be explained by considering that oxidation of the sulfur accelerates 
 a retro - michael addition , producing the c-7c-9 enone functionality 
 in situ .
in fact , it has been suggested that 31 is a 
 prodrug of 6 , and as such , it represents a more attractive 
 drug candidate .
isolated 
 from various micromonospora bacteria , kijanimicin 
 ( 43 ) and lobophorin b ( 42 ) are 
 structurally defined by a c13 macrocycle ( referred to as 
 kijanolide , 44 ) in which the c-9 and c-17 hydroxy groups 
 have been glycosylated ( figure 4 ) .
most members 
 of this group show potent activity against gram - positive bacteria as well as cytotoxicity against various cancer cell lines .
in addition , kijanimicin was 
 shown to exhibit robust anticancer and 
 antimalarial activities in mouse models
. 
 moreover , fenical et al . reported promising anti - inflammatory activities 
 of lobophorins in a mouse ear edema model .
interestingly , this is 
 the first report on the untapped potential of spirotetronates as small - molecule 
 leads against inflammation .
a similar 
 framework is in the structures of pyrrolosporin a ( 45 , c-9-glycosylation ) and mm46115 ( 46 , c-17 glycosylation ) .
the 
 glycopyranose motif of pyrrolosporin is also found in the structures 
 of decatromicin a / b and nai414-a / b , which also exhibit similar antibiotic activity 
 against various strains of gram - positive bacteria .
in addition to 
 its potent antibiotic activities mm46115 was found to exhibit promising 
 antiviral activities . along these lines , 
 the structurally unrelated quartromicins 9(14a,14c )
were shown to display potent bioactivity against herpes simplex 
 virus ( hsv ) and human immunodeficiency virus ( hiv ) at low m 
 concentration .
recent studies indicate 
 that kijanimicin binds to the tetr family 
 of transcriptional regulators that control 
 expression of various cytoplasmic proteins in prokaryotes .
this binding 
 leads to ( a ) c-9-deglycosylation of kijanimicin , which results in 
 loss of activity , and ( b ) overexpression of the receptor , thus increasing 
 antibiotic resistance .
the structurally 
 related saccharocarcins are subject to 
 a similar mechanism of deactivation and antibiotic resistance .
reported 
 the isolation of spirohexenolides a ( 8) and b ( 9 ) and their potent cytotoxicities 
 against various cancer cell lines ( figure 2 ) .
subsequent immunoaffinity - fluorescent 
 labeling studies indicated that 8 targets human macrophage 
 migration inhibitory factor ( hmif ) .
this 
 interaction reduces the phosphorylation levels of pi3k / akt , ultimately 
 leading to a reduction of tumor cell growth ( figure 6 ) .
conjugation of spirohexenolide 
 a with fluorescent tags showed localization in the lysosome of hct-116 
 cells , suggesting that spirohexenolides interfere with cellular endocytosis 
 of hmif .
isolated from 
 various micromonospora bacteria , tetrocarcin a ( 47 , also known as antlermicin a and ac6d ) represents the archetype of the tetronolide family of natural 
 products that also includes ac6h ( 48 ) and arisostatins a ( 49 ) and b ( 50 ) ( figure 7 ) .
the antibiotic potential of these spirotetronates against several 
 gram - positive bacteria has been reported .
animal studies 
 have shown that 47 is about 4 times more potent than 
 the commonly used antibiotic diminazene .
although 47 has 
 a narrow safety margin , it can be used in combination with diminazene , 
 providing a beneficial synergistic effect .
various reports on the potential anticancer profile 
 of tetrocarcin 
 a and related family members have been published .
initial studies 
 showed tumor reduction in a mouse sarcoma model upon administration 
 of 10 mg / kg of 47 over a period of 6 days .
similar treatment 
 in a mouse leukemia p388 model led to an increased life expectancy .
comparable studies in b16 mouse melanoma showed that the life expectancy 
 more than doubled at a single dose of 27 mg / kg of 47 .
moreover , ac6h 48 exhibited cytotoxicity 
 against p388 leukemia and b16 melanoma cells at 6.25 and 25 g / ml , 
 respectively .
ac6h also showed a moderate 
 increase in the life expectancy of a p388 leukemia mouse model albeit 
 less active than tetrocarcin a. studies 
 in u937 cells indicated that arisostatin a ( 49 ) is equipotent 
 to tetrocarcin a , while arisostatin b ( 50 ) was 10-fold 
 less active .
arisostatin a was also found to be active in various 
 breast and lung cancer cell lines at low micromolar concentrations .
mode - of - action studies in hela cells showed 
 that tetrocarcin a 
 ( 47 ) potently inhibits bcl-2 , an important antiapoptotic 
 protein that is often overexpressed in cancer cells ( figure 6 ) .
although there is 
 no evidence of direct binding to bcl-2 , the phenotypical response 
 induced by 47 is very promising and suggests that this compound represents an important and unexplored 
 lead against cancer .
studies in 
 lymphoma cells showed that 47 induces a 
 stress response of the endoplasmic reticulum ( er ) , resulting in upregulation 
 of the heat shock protein hsp70 , ultimately triggering cell apoptosis 
 ( figure 6 ) .
studies 
 in breast cancer cells have suggested an alternative mechanism of 
 action of 47 that proceeds by inhibiting phosphorylation 
 of the pi3k / akt signaling cascade .
although 
 the main cellular target of tetrocarcin a is still under investigation , 
 preclinical studies have demonstrated its potential as a drug against 
 chemoresistant cancers .
in fact , 47 was reported to be 
 more effective than paclitaxel at inducing cell apoptosis in breast 
 cancer cells .
arisostatin a ( 49 ) was found to induce cell apoptosis 
 by generating reactive oxygen species ( ros ) , altering mitochondrial 
 transmembrane potential , and releasing cytochrome c ( cyt c ) in amc - hn-4 cells ( figure 6 ) , ultimately 
 leading to activation of caspase-3 and induction of apoptosis .
however , 
 bcl-2 activation was not altered by arisostatin a , indicating a different 
 mode of action from that of 47 .
screening the potential anticancer and antimicrobial activities 
 of naturally occurring tetrocarcins has produced the main structure 
these studies have led to 
 the following observations : ( a ) the number of carbohydrate units ( digitoxose 
 and amicetose ) attached at the c-9 center of tetrocarcin a is proportional 
 to its antimicrobial activity ; ( b ) c-21 acetylation and c-9 glycosylation of tetrocarcin a did 
 not significantly affect bcl-2 activation .
the results suggest that the attachments of amicetose ( 27 ) and digitoxose ( 28 ) at the c-9 position of tetrocarcin 
 a enhance its antibacterial profile but have no significant effect 
 on its anticancer potential .
versipelostatin a ( 51 ) was isolated from a strain 
 of streptomyces versipellis ( figure 8) .
biological 
 studies showed that 51 is the first known molecule to 
 inhibit gene expression of grp78 . together with its isoform grp94
, 
 these heat shock proteins are induced by stress responses in the endoplasmic 
 reticulum and are essential for cancer cell survival .
in addition to its role in cancer , er stress is considered to play 
 a major role in the pathogenesis of various cns diseases , such as 
 alzheimer s and parkinson s disease .
recent studies have shown that versipelostatin 
 a ( 51 ) inhibits heat shock proteins and unfolded protein 
 response ( upr ) 
 under glucose deprivation conditions .
as such , it appears to operate via a different mechanism as compared 
 to that of rapamycin , an fda - approved immunosuppressive drug that 
 activates grp78 independently of glucose availability .
thus , versipelostatin may offer a significant 
 advantage due to its selective effect in hypoglycemic cells .
although there is no information for direct 
 binding of 51 to a protein target , its effect on the 
 upr signaling pathway offers a novel tool to understand er - induced 
 stress and pharmacologically regulate related illnesses .
sar studies on this family have been limited to the bioactivities 
 of naturally occurring versipelostatins .
the results show that versipelostatins a ( 51 ) , e ( 52 ) , and f ( 53 ) are the only biologically active 
 compounds , inhibiting grp78 expression at low micromolar ic50 values .
interestingly , 53 was found to be 10 times more potent than 51 in grp78 
 expression with an ic50 of 0.3 m .
the data attest to the significance of the glycosylation 
 motif to the grp78 expression and bioactivity .
in addition to these studies , takahashi et al . demonstrated 
 the importance of the l - oleandrose sugar for the bioactivity , 
 and changes in the oxidation state of c-7 had no effect on biological 
 activity .
okilactomycin ( 54 ) was isolated from streptomyces griseoflavus and is noted for its potent antitumor 
 activity against ehrlich ascites carcinoma in vivo at 2.5 mg / kg with 
 a t / c of 145.7% for mice survival . in 
 addition , 54 exhibited in vitro activity against p388 
 and l1210 leukemia cells , with ic50 values of 89 and 216 
 nm , respectively .
recently , okilactomycin 
 was shown to inhibit rrna protein synthesis at low m concentrations , suggesting potential applications as an antibacterial 
 agent .
although other natural okilactomycins 
 were found to be inactive , the related 
 chrolactomycin ( 55 ) was reported to exhibit antibacterial 
 and anticancer activity at a low m concentration .
it has been reported that 55 inhibits 
 telomerase activity , thus blocking the ability of cancer dna to replicate .
the most recently isolated 6-hydroxy chrolactomycin 
 was less active than 55 against gram - positive bacteria .
the potent antibiotic 
 properties of pa-46101a ( 57 ) and b ( 58 ) 
 have been reported .
recent efforts by 
 igarashi and co - workers led to the isolation of maklamicin ( 11 ) and nomimicin ( 59 ) , which contain the smallest macrocyclic 
 ring of the class ii spirotetronates .
both compounds display potent 
 activity against many gram - positive bacteria , while 11 also exhibits moderate antitumor activity against hela and mcf7 
 breast cancer cells .
chlorothricin ( 1 ) was shown 
 to inhibit the activity of pyruvate carboxylase , a key enzyme that converts pyruvate to oxaloacetate , thus 
 allowing consumption of glucose through the krebs cycle ( figure 11 ) .
inhibition of pyruvate carboxylase leads to 
 an increase of pyruvate concentration in liver , which through gluconeogenesis 
 accounts for accumulation of glucose , ultimately leading to diabetes .
moreover , an inhibitory effect of 1 on malate dehydrogenase , an enzyme that oxidizes malate to oxaloacetate 
 in the krebs cycle , has also been reported .
it should be noted , however , that the direct cellular target of 1 is under debate and may involve interaction with components 
 in the cell membrane that may account for the observed downstream 
 effects .
although the potential 
 anticancer properties of chlorothricin ( 1 ) have not been 
 investigated , c-31 hydroxychlorothricin ( figure 1 ) was shown to exhibit antitumor activity at 40 
 mg / kg against implanted ehrlich carcinoma cells in mice with an ld50 of 295 mg / kg .
c-28 methyl ester 
 of chlorothricolide ( 2 ) , the aglycone of 1 ( figure 1 ) , 
 also inhibits pyruvate carboxylase albeit at higher concentrations 
 than 1 , suggesting that glycosylation enhances biological 
 activity .
efforts to discover 
 new gastric atp - ase inhibitors led to 
 the isolation of a88696f ( 61 ) and its 
 dehydroxylated counterpart a88696c ( 60 ) .
hydroxylation at c-3 was found to enhance the biological 
 activities since 61 was the most active , with an ic50 at 0.5 m , while 60 was considered inactive .
isolated from a streptomyces species , tetronothiodin 
 ( 62 ) was shown to inhibit brain - type cholecystokinin 
 ( cck)-b receptor in rat cerebral cortex with an ic50 value 
 of 3.6 nm .
it is worth noting that cck 
 receptors are structurally similar to gastrin and are used throughout 
 the central nervous system ( cns ) and gastric tract .
interestingly , 62 has 27  000 times 
 higher affinity for cck - b over cck - a in rat models .
thus , in addition to its pharmaceutical promise , 62 could be used as a tool to study the cck - b / cck - a signaling 
 pathway .
chlorothricin ( 1 ) was shown 
 to inhibit the activity of pyruvate carboxylase , a key enzyme that converts pyruvate to oxaloacetate , thus 
 allowing consumption of glucose through the krebs cycle ( figure 11 ) .
inhibition of pyruvate carboxylase leads to 
 an increase of pyruvate concentration in liver , which through gluconeogenesis 
 accounts for accumulation of glucose , ultimately leading to diabetes .
moreover , an inhibitory effect of 1 on malate dehydrogenase , an enzyme that oxidizes malate to oxaloacetate 
 in the krebs cycle , has also been reported .
it should be noted , however , that the direct cellular target of 1 is under debate and may involve interaction with components 
 in the cell membrane that may account for the observed downstream 
 effects .
although the potential 
 anticancer properties of chlorothricin ( 1 ) have not been 
 investigated , c-31 hydroxychlorothricin ( figure 1 ) was shown to exhibit antitumor activity at 40 
 mg / kg against implanted ehrlich carcinoma cells in mice with an ld50 of 295 mg / kg .
c-28 methyl ester 
 of chlorothricolide ( 2 ) , the aglycone of 1 ( figure 1 ) , 
 also inhibits pyruvate carboxylase albeit at higher concentrations 
 than 1 , suggesting that glycosylation enhances biological 
 activity .
efforts to discover 
 new gastric atp - ase inhibitors led to 
 the isolation of a88696f ( 61 ) and its 
 dehydroxylated counterpart a88696c ( 60 ) .
hydroxylation at c-3 was found to enhance the biological 
 activities since 61 was the most active , with an ic50 at 0.5 m , while 60 was considered inactive .
isolated from a streptomyces species , tetronothiodin 
 ( 62 ) was shown to inhibit brain - type cholecystokinin 
 ( cck)-b receptor in rat cerebral cortex with an ic50 value 
 of 3.6 nm .
it is worth noting that cck 
 receptors are structurally similar to gastrin and are used throughout 
 the central nervous system ( cns ) and gastric tract .
interestingly , 62 has 27  000 times 
 higher affinity for cck - b over cck - a in rat models .
thus , in addition to its pharmaceutical promise , 62 could be used as a tool to study the cck - b / cck - a signaling 
 pathway .
in this part of the review , 
 we highlight the key steps toward the 
 synthesis of selected spirotetronates . when possible , we compare the 
 various strategies in terms of overall efficiency . abyssomicin c ( 6 ) and its atropisomer 29 contain a rigid oxabicyclo 
 [ 2.2.2 ] octane substructure that encapsulates the spirotetronate moiety . 
 to date , three chemical syntheses of 6 and 29 have been reported .
sorensen s group used a biomimetic imda 
 to construct spirotetronate moiety 65 from diene 63 .
c-11c-12 epoxidation of 65 followed 
 by c-16 intramolecular enol epoxide opening produced a 1:1 mixture 
 of abyssomicin c ( 6 ) and atrop - abyssomicin 
 ( 29 ) .
the nicolaou 
 group s synthesis of abyssomicin c is highlighted by an intermolecular 
 diels  alder cycloaddition that furnishes cyclohexene 67 with the desired stereochemistry ( scheme 4 ) .
a ring - closing metathesis 
 ( rcm ) was used to generate the macrocyclic skeleton of 6 from diene 68 .
interestingly , the authors showed that 
 treatment of 29 with lithium selectride led to formation 
 of abyssomicin d ( 32 ) .
interestingly , this finding supports 
 the notion that abyssomicin c ( 6 ) is a biosynthetic progenitor 
 of 32 and further validates the proposed mechanism of 
 abyssomicin c deactivation as presented in scheme 3 .
more recently , the groups 
 of bihelovic and saicic reported a synthesis 
 of 29 .
interestingly , this strategy produces exclusively atrop - abyssomycin c. it is likely that the 
 restricted rotation around the c-2 and c-3 centers , due to the sp hybridization , affects the formation of the two isomers . 
 in support of this hypothesis ,
the nicolaou group has shown that 29 can be converted to 6 by protonating the c-16 
 oxygen under mild acidic conditions .
other 
 studies toward the abyssomicin scaffold have been reported in addition 
 to the mentioned total syntheses .
ferrier 
 union / rearrangement of 72(97 ) that yielded the 2,6-cis - tetrahydropyranone ring 73 .
grubbs 
 second - generation catalyst was used to construct the 13-membered macrocycle 
 of 54 .
key 
 to this approach was a prins - type fragment assembly between cyclohexene 75 and -keto - ester 74 that formed the 2,6-cis - tetrahydropyranone 
 ring of 76 . similarly to the smith approach , an intramolecular 
 ring - closing metathesis using grubbs second - generation catalyst constructed 
 the macrocycle .
additional synthetic 
 studies toward okilactomycin have been reported by the yoshii and paquette groups .
key to this strategy was an imda cycloaddition 
 that formed spirotetronate 80 from precursor 79 .
the overall synthesis proceeds in 13 linear steps ( 17 total steps ) 
 and 17% yield .
the burkart group reported a strategy toward spirohexenolides 
 based on an intermolecular diels  alder cycloaddition ( scheme 7 ) .
a ring - closing 
 julia  kocienski coupling was applied for the synthesis of macrocycle 82 .
although the projected intramolecular hemiacetalation 
 to 84 failed due to an oxidative rearrangement of 82 to 83 , the overall strategy has successfully 
 installed the major skeletal features of spirohexenolides .
structurally 
 tetronothiodin is highlighted by 
 an -acyl tetronic acid moiety and tetrahydrothiophene moiety . 

page et al . have reported a synthesis of the spirotetronate subunit 
 isomer 87 using a diels  alder reaction with propenal 
 and the hydroxyl diene 85 to install the desired stereochemistry 
 of 86 ( scheme 8) .
several strategies 
 have been employed for the synthesis of tetronolide ( 12 ) , the aglycone of tetrocarcin a ( 47 ) , kijanolide ( 44 ) , the aglycone of kijanimicin ( 42 ) , and chlorothricolide 
 ( 2 ) , the aglycone of chlorothricin ( 1 ) . 

tetronolide has been synthesized by yoshii and boeckman , while an improved formal 
 synthesis has also been reported by roush . in general , these strategies rely upon independently constructing 
 the spirotetronate and decalin moieties and then connecting them to 
 form the c13 macrocycle .
a remarkable synthesis of chlorothricolide 
 ( 2 ) was reported by the roush group .
the yoshii and roush syntheses of the decalin 
 moiety 92 , common to both tetronolide and kijanolide , 
 are summarized in scheme 9 .
in yoshii s 
 approach a horner  wadsworth  emmons 
 ( hwe ) olefination between 88 and 89 was 
 used to construct polyene 90 , which underwent an imda 
 reaction to produce decalin 92 .
the roush group implemented a suzuki coupling between 93 and 94 to form polyene 91 , which , following 
 further functionalizations , gave rise to decalin 92 via 
 an imda cycloaddition .
a synthetic approach toward spirotetronate 100 has 
 been reported by yoshii and subsequently 
 optimized by roush .
this approach is based on constructing 
 triene 97 via a hwe olefination between 95 and 96 .
an intermolecular diels  alder of diene 97 and chiral dienophile 98 , followed by oxidative 
 functionalization and double - bond migration , yielded enal 99 .
coupling of lithiated spirotetronate 100 with aldehyde 92 followed by subsequent functionalizations yielded sulfone 101 , which , under julia coupling conditions , gave rise to 
 the 13-membered macrocycle of 12 .
boeckman s group synthesis of 12 is highlighted 
 by a tandem ketene - trapping [ 4 + 2 ] cycloaddition of diene 103 and alcohol 102 to form spirotetronate subunit 104 .
overall , this approach 
 significantly reduces the number of steps required for completion 
 of the tetronolide synthesis .
various synthetic 
 studies toward kijanolide ( 44 ) have 
 been reported by the groups of marshall , yoshii , and roush .
application of a julia coupling to the synthesis 
 of 28,29-bisnor-(+)-kijanolide has been reported by the yoshii group . a tandem intra / intermolecular 
 diels  alder reaction between 
 polyene 105 and chiral dienophile 98 was 
 implemented for the synthesis of chlorothricolide ( 2 ) 
 ( scheme 11 ) .
the reaction gave the desired 
 cycloadduct in 40% yield together with partially reacted decalin 107 .
upon treatment with dienophile 98 , 107 was converted to the desired product 106 in 
 58% yield .
construction of the spirotetronate 
 unit followed by coupling with the allyl ester completed the synthesis 
 of 2 .
a late - stage imda reaction 
 was used by yoshii s group for 
 the synthesis of ( )-24-o - methylchlorothricolide 
 ( scheme 12 ) .
although the selectivity of the 
 imda reaction was moderate , the overall strategy represents a noteworthy 
 bioinspired approach toward these compounds .
the groups of marshall , ireland , snider , schmidt , and meyers have also reported studies toward the synthesis 
 of 2 .
numerous synthetic studies have been reported 
 toward the total synthesis of versipelostatin a ( 51 ) , 
 but to date its total synthesis has not been completed .
a synthesis of the versipelostatin ( 51 ) trisaccharide 114 is shown in scheme 13 .
the 
 resulting disaccharide 112 was deprotected and coupled 
 with l - oleandrosyl imidate 113 to produce 114 ( scheme 13 ) .
further functionalization 
 of glycosyl 114 and schmidt glycosylation with acetyl 
 c-7c-9c-37 versipelostatin aglycone 13 ( figure 2 ) yielded a versipelostatin derivative 
 used for biological studies .
on the basis of nmr and biological consideration , 
 the oleandrose sugar was structurally reassigned from d to l. an alternate strategy used was adding each sugar individually 
 to the versipelostatin aglycone , thus elongating the glycosyl chain .
a stereocontrolled 
 diels  alder reaction has been implemented by the roush group 
 for the synthesis of the quartromicin spirotetronate unit . in addition , this group reported a strategy of connecting subunits 115 and 116 together using lithium halogen exchange 
 and cecl3 coupling .
group offered an alternative strategy of constructing the spirotetronate 
 subunits using rcm , but to date no total syntheses of quartromicins 
 have been completed .
the discovery of penicillin 
 revolutionized pharmaceutical research 
 by demonstrating , for the first time , that microorganisms can produce 
 secondary metabolites of value to medicine .
since then , cultured microorganisms 
 have been recognized as prolific producers of secondary metabolites 
 that are used either directly as drugs or have inspired the design 
 of drugs . on the other hand ,
the intricate 
 structures of these compounds represent exceptional tools to explore 
 new biological pathways and unknown mechanisms of action .
these qualities , 
 although scattered , are observed in the family of spirotetronate polyketides 
 and provide evidence for their significant but still untapped pharmacological 
 value .
more than 40 years after the discovery of chlorothricin , 
 the spirotetronate 
 family has grown to include over 70 macrocycles of various sizes that , 
 in certain cases , are decorated with carbohydrate side chains . in 
 addition to their potent antitumor and antibiotic activities ,
for example , versipelostatin was found to induce 
 potent and selective cytotoxicity in glucose - deprived tumor cells .
moreover , abyssomicin c was found to be the 
 first natural product to block paba biosynthesis , 
 a pathway essential to bacteria but insignificant to humans .
impressive synthetic and chemical biology efforts 
 were combined to decipher the mode - of - action of abyssomicins at the 
 molecular level .
this underscores the enormous 
 significance of the spirotetronate polyketide family to biology in 
 addition to their pharmacological potential .
several studies 
 have documented the significance of the carbohydrate 
 chains for the observed antibiotic activity of spirotetronates . however , with the exception of abyssomicins , there is no clear understanding 
 of the biological significance of the spirotetronate aglycone core . 
 at present ,
chemical strategies developed toward the synthesis of 
 spirotetronates have uncovered the value of certain key reactions , 
 such as diels  alder cycloaddition , ring -closing metathesis , 
 and julia olefination .
nonetheless , the vast majority of these strategies 
 have not yielded sufficient amounts of compound for a methodical structure  activity 
 relationship study , thereby hampering rational drug design .
it is 
 evident that a methodical fragment - based approach to this structure , 
 in combination with chemical biology studies , will be highly beneficial , 
 as it could reveal the role of the spirotetronate motif , the effect 
 of the macrocyclic size , and the role of the decalin system . in turn
, 
 this effort would allow a detailed evaluation and optimization of 
 the spirotetronate pharmacophore .
in addition to a dearly needed scalable 
 synthesis , advances in microbial biosynthesis should offer a potential solution to large - scale 
 production or semisynthesis of a lead candidate .
a combination of 
 these efforts should unveil the pharmacological value of spirotetronates 
 and would have significant impact in current efforts toward personalized 
 medicine .
 the discovery of chlorothricin ( 1 ) defined a new family 
 of microbial metabolites with potent antitumor antibiotic properties 
 collectively referred to as spirotetronate polyketides . 
 these microbial 
 metabolites are structurally distinguished by the presence of a spirotetronate 
 motif embedded within a macrocyclic core . 
 glycosylation at the periphery 
 of this core contributes to the structural complexity and bioactivity 
 of this motif . 
 the spirotetronate family displays impressive chemical 
 structures , potent bioactivities , and significant pharmacological 
 potential . 
 this review groups the family members based on structural 
 and biosynthetic considerations and summarizes synthetic and biological 
 studies that aim to elucidate their mode of action and explore their 
 pharmacological potential . 