An update on the synthesis and reactivity of spiro-fused β-lactams

β-Lactam ring-containing compounds play a pivotal role in drug design and synthetic chemistry. Spirocyclic β-lactams, representing an important β-lactam subclass, have recently attracted considerable interest with respect to new synthetic methodologies and pharmacological applications. The aim of this manuscript is to review the recent progress made in this field, covering publications disseminated between 2011 to 2018 concerning the synthesis and application of spirocyclic β-lactams. In the first part, new approaches to the synthesis of spirocyclic β-lactams, including Staudinger synthesis, cyclization and transformation reactions, will be presented. The reactivity and biological properties of spiro-β-lactams will be described in the second and third part, respectively.


Introduction
The β-lactam (azetidin-2-one) ring is one of the most significant azaheterocycles studied from both a synthetic and medicinal chemistry point of view in the last decades.Its prominence is due to the presence of the β-lactam core in many antibiotics (e.g., penicillins, carbapenems, monobactams and sulbactams) used in the fight against pathogenic bacteria.3][4][5] In the light of these excellent applications, much attention has been devoted to development of novel and more effective β-lactam compounds and the evaluation of their biological and synthetic potential. 1,6On the other hand, the overuse of β-lactam antibiotics leads to an increasing number of resistant bacterial strains.][9] Spirocyclic β-lactams, which contain a (small) ring spiro-fused at the C3 or C4 position of a β-lactam core (Figure 1), have recently been found to be eligible candidates for drug discovery.They have, for example, been exploited as antibacterial agents, antiviral agents and cholesterol absorption inhibitors.In that respect, structure-activity relationship studies have identified 3-spiro-cyclohexyl-β-lactams SCH 54016 A and SCH 58053 B as two potential cholesterol absorption inhibitors. 10In addition, spiro-β-lactams have been used as βturn mimetics in peptidomimetic chemistry.It has been found that spiro-β-lactams also act as poliovirus and human rhinovirus 3C-proteinases inhibitors.Thus, the search for new spiro-β-lactam derivatives comprises a relevant research topic. 10In addition, spiro-β-lactams have been used as starting materials to prepare α-and β-amino acids, alkaloids, heterocycles, taxoids 11 and other relevant compounds. 12][13] As an extension of the review article by De Kimpe in 2011, this review is constructed in a similar way, including synthetic approaches, reactivity, and biological activities of spiro-fused azetidin-2-ones, spanning publications from 2011 to 2018. 13gure 1.General representation of C3 and C4 spiro-fused azetidin-2-ones.

Cyclocondensation reactions
The Staudinger [2+2]-cyclocondensation of ketenes and imines is the most widely used method for the synthesis of the β-lactam ring skeleton.Although, according to IUPAC guidelines, the term 'cycloaddition' can be used for this two-step process via zwitterionic intermediates, this method is mostly referred to as a cyclocondensation reaction.Spiro-fused β-lactams can also be prepared in this way employing various types of ketenes and imines.Depending on the ketenes and imines applied, two different types of spiro-fused azetidin-2-ones can be synthesized.More specifically, the reaction of an acyclic ketene with a cyclic imine will furnish C4 spiro-fused azetidin-2-ones, whereas the reaction of a cyclic ketene with an acyclic imine will afford C3 spiro-fused azetidin-2-ones.

Scheme 2
8][19][20] For example, in the course of a screening study for β-lactam compounds possessing antibacterial and antifungal activities, Shah and co-workers successfully synthesized a number of novel spiro[azetidine-2,3 ' -indole]-2 ' ,4(1 ' H)-diones 10 and 12 by means of the Staudinger synthesis (Scheme 3).The preparation of imines 9 and 11 was performed by reaction between either dibromoisatin 8, which was obtained by treatment of isatin 7 with bromine in methanol, or isatin 7 with different primary aromatic amines in absolute ethanol in the presence of glacial acetic acid.Subsequently, treatment of imines 9 and 11 with chloroacetyl chloride in the presence of a tertiary base at 80 °C to reflux temperature led to the target spiro-βlactams 10 and 12.The antibacterial and antifungal activities of these spiro β-lactams were evaluated. 21

Scheme 3
In addition, the cyclocondensation of isatin-3-arylimine with various diazoketo esters catalyzed by rhodium(II) also gave rise to spiro(oxindolyl)-β-lactams. 22Recently, the Kandile group has reported the generation of bis-spiroazetidinone derivatives by the reaction of bis-Schiff bases, derived from 5-substituted isatins, with chloroacetyl chloride. 23Concurrently, Xu and co-workers have disclosed a direct asymmetric [2+2]-annulation reaction of simple aliphatic aldehydes with isatin-derived ketimines via N-heterocyclic carbene (NHC) catalysis furnishing enantioenriched spiroindole β-lactams. 24he synthesis of spiro-fused seleno-β-lactams has been studied by using Z/E isomers of α-seleniumsubstituted exocyclic imines 13 with various types of acyl chlorides, such as methoxy-, chloro-, propionyl-, phenyl-, cyclohexyl-, phenoxy-, and p-chlorophenoxyacetyl chloride under optimized Staudinger synthesis conditions (Scheme 4).The reaction of exocyclic imines 13 with acetyl chlorides 14 gave rise to spiro-β-lactams 15 in excellent yields as mixtures of stereoisomers.The diastereomeric ratio (d.r.) of the resulting spiro-βlactams 15 was calculated by 1 H NMR spectral analysis.It should be noted that the stereochemical outcome of β-lactam formation under Staudinger reaction conditions can be explained by the electronic effects of the substituents on the ketenes, which were generated from the corresponding acid chlorides. 25

Scheme 4
The synthesis of new spiro-fused azetidinone-androgen derivative 19 began with the conversion of 17b-[(tert-butyldimethylsilyl)oxy]androst-4-en-3-one 16 to Schiff's base 17 using boric acid as a catalyst (Scheme 5).The treatment of compound 17 with dihydrotestosterone afforded thiourea 18.The later compound 18 was further converted into the desired bis-spiro steroidal β-lactam 19 by treatment with chloroacetyl chloride in the presence of triethylamine.The Staudinger-based synthesis of androgen derivative 19 constitutes a straightforward procedure in comparison with other methodologies applied in the synthesis of steroid derivatives. 26

Scheme 6 Scheme 7
Imines 27, derived from 8-azabicyclo[3.2.1]octan-3-ones, have been shown to be interesting starting materials for the synthesis of a new class of β-lactams 28 via Staudinger synthesis with phenoxyacetyl chloride (Scheme 8).It is noteworthy that the stereochemical outcome of the obtained spiro-β-lactams (as single diastereoisomers) was opposite to the expected products of a [2+2]-cycloaddition reaction, which should have taken place from the exo face of compound 27. 29he Reddy group 30 has reported on the synthesis and biological evaluation of a novel series of spiroazetidin-2-ones 33 starting from 3-chloro-4-fluoroaniline 29 (Scheme 9).Firstly, the benzothiazole 30 was synthesized from 3-chloro-4-fluoroaniline 29 and further condensed with 5-methyl-2-phenylpyrazol-3-one to yield the imine 31, which was cyclized with chloroacetyl choride in triethylamine to obtain azetidin-2-one 32.Finally, the resulting azetidin-2-one 32 was further condensed with different primary and secondary amines, leading to the desired spiro-fused β-lactams 33.These β-lactams 33 exhibited moderate to significant activities in anti-inflammatory, anti-diabetic, anti-oxidant and anti-microbial tests.In an analogous approach, spiro-[chloroazetidinethiazolopyrimidine] derivatives were obtained by using the corresponding thiazolopyrimidine imines. 31

Scheme 10
The substituted norbornane carboxylic acids 38 have been used for the formation of the corresponding bicyclic norbornane-derived ketenes by a two-step procedure.The cycloaddition of the so-formed ketenes with imine 39 led to the generation of diastereomeric norbornane C3-spiro-β-lactams 40, 41, 42, 43 (Scheme 11, Table 2).It is noteworthy that the stereochemical outcome of spiro-β-lactams 40-43 is influenced by the presence of encumbering groups on the cyclic ketenes.In fact, a better selectivity was obtained when methyl groups were present near to the carbon bearing the carboxylic functional group.The ratio of diastereoisomers was determined by integration of the H-4 proton in the 1 H-NMR spectra of the crude reaction mixtures. 45heme 11 To study the synthesis of spiro-β-lactam 46 on a larger scale, the reaction of in situ generated ketene from compound 44 and imine 45 was conducted in both batch and flow mode (Scheme 12).The reaction outcome showed that the Staudinger synthesis in a continuous flow process was deemed a safe method for this highly exothermic reaction.However, the precipitation of generated salts quickly blocked a glass chip and made the operation technically impossible.46

Scheme 12
Recently, the Mykhailiuk group 47,48 has reported on a new approach for the synthesis of 3-substituted piperidinyl spiro-β-lactam 51 starting from acid 47 and aldehyde 49 (Scheme 13).In this synthetic approach, acid 47 was firstly converted into the corresponding acid chloride and subsequently into ketene 48 by the sequential treatment with SOCl 2 and NEt(iPr) 2 .Aldehyde 49 was converted into imine 50 upon treatment with LiN(TMS) 2 .In analogy, a diverse number of spiro-β-lactams attached with 4-, 5-, 6-, 7-membered rings at the C-3 position were prepared in moderate to good yields.Subsequently, the reductive removal of the carbonyl group of spiro-β-lactam analogs 51 was conducted by the reducing reagent AlH 3 giving the corresponding spirocyclic azetidines, which were investigated further in drug design and as an analog of the anesthetic drug bupivacaine. 47,48heme 13

Scheme 14
In addition, this modification also allowed for the synthesis of a diverse number of spiro-4cyclohexadienone-β-lactams 55 with cyclopentanone, γ-lactone, and γ-lactam side chains on their C3 position, starting from the corresponding amides 54 (Scheme 15). 50

Scheme 16
The cyclization of Dab-derived (Dab = 2,4-diaminobutanoic acid) chloroacetyl compound 59, which was obtained in a quantitative yield by treatment of compound 58 with chloroacetyl chloride in the presence of propylene oxide, gave rise to 1,6-diazaspiro [3.4]octane-2,5-dione 60 (Scheme 17).The construction of spirocylic β-lactam 60 could be explained by the concomitant formation of the β-lactam and pyrrolidinone ring, the latter due to a 5-exo-trig ring closure between the ZNH group and the carboxylic ester, followed by Zprotecting group removal. 52

Scheme 17
In the course of a study on proline-like compounds with potential biological interest, Ponticelli and coworkers 53 have devised an approach toward the synthesis of functionalized spiro-pyrrolidine 63 (Scheme 18).In particular, treatment of pyrrolidine aldehyde 61 with the reducing reagent NaCNBH 3 , together with methylamine, gave rise to the β-amino ester 62. Subsequently, the cyclization of compound 62 using two equivalents of the strong base LDA led to the desired spiro-β-lactam 63. 53 In independent research, Sharada and coworkers have also described an effective synthesis of a pyrrolidine-derived spiro-β-lactam starting from natural proline. 54

Scheme 18
The Grainger group 55 has published the preparation of a spirocyclic β-lactam by using a 4-exo-trig carbamoyl radical cyclization approach.Specifically, amine 64, prepared in one step by reductive amination of commercially available 1-cyclohexene-1-carboxaldehyde with p-methoxyaniline, 11 was converted into the carbamoyl radical precursor 65 (Scheme 19).Then, irradiation of compound 65 with a 500 W halogen lamp afforded spirocyclic dithiocarbamate 66 as a single diastereomer.The stereochemistry of compound 66 was determined by NOE spectral analysis, which showed the proton adjacent to sulfur to be on the same side of the cyclohexyl ring as the methylene group of the β-lactam.The reduction of 66 with H 3 PO 2 -ACCN successfully gave spirocyclic β-lactam 67. 55

Scheme 19
Spiro-β-lactam 75 has been synthesized as an intermediate compound in an effort to find drugs to improve hepatitis C virus (HCV) therapy (Scheme 20). 561,2-Bis(bromomethyl)benzene 68 was condensed with diethyl malonate 69, furnishing a crude diethyl malonate derivative.This derivative was reacted with sulfuric acid in the presence of methanol at 200 °C, giving methyl ester 70.Treatment of ester 70 with LDA afforded the corresponding enolate, which was trapped with TMSCl leading to silyl enol ether 71.Reaction of generated enol 71 with N-p-methoxyphenyl (PMP) imine 72 in the presence of TMSOTf gave compound 73 as a mixture of enantiomers.Cyclization of compound 73 with MeMgBr gave rise to spiro compound 74 in 70% yield.Removal of the PMP group of spiro-β-lactam 74 with CAN led to the desired compound 75. 56

Scheme 20
Indolenine β-lactams 77, present as key moieties in the complete structure of chartellines, were synthesized by an intramolecular nucleophilic substitution initiated by indolenine derivatives 76 using LiHMDS as a base (Scheme 21). 57

Scheme 22
The direct C-H cyclization of amides 80 bearing five-, six-and seven-membered rings proceeded via in situ iodonium ylide formation 80" without the assistance of a transition metal, delivering the corresponding spiroβ-lactams 81 in a single step (Scheme 23).From a mechanistic point of view, this transformation is consistent with the facile formation of a reactive singlet carbene.However, it circumvents the classical use of diazo substrates for C-H insertion chemistry. 64

Scheme 23
Phototransformation chemistry has demonstrated a great potential for challenging building block synthesis.In that regard, the irradiation (direct excitation at ~350 nm or triplet sensitization with thioxanthone at ~420 nm) of atropisomeric enones 82 gave rise to the corresponding spiro-β-lactam photoproducts 83 (as major products) and 84 (as minor products) (Scheme 24).Computational investigation revealed that the presence of an ortho-tert-butyl substituent on atropisomeric enones 82 prevented the 6π-photocyclization to undergo an efficient hydrogen abstraction leading to atropselective spiro-β-lactam formation. 65,66

Scheme 24
Cyclization of propiolamide 85 with triphenylphosphine as a catalyst in ethanol under reflux furnished the corresponding spiro-3-methyleneazetidin-2-one 86 in a moderate yield (Scheme 25).The rather low yield of spiro-β-lactam 86 might be rationalized by the steric hindrance experienced in the 4-exo cyclization step. 67

Transformations of substituents connected to monocyclic azetidin-2-ones
One of the most convenient methodologies for the preparation of spiro-fused β-lactams concerns the deployment of substituents on azetidin-2-ones.In respect, the Benito group recently on the synthesis of spirocyclic seleno-β-lactams 90 from azetidin-2-one-tethered allenols 89 and the selenenylating reagent N-phenylselenophthalimide (NPSP) via ring enlargement (Scheme 27). 72

Scheme 28
6-Diazopenicillanates 94 have been obtained in high yields by the combination of 6-β-aminopenicillanates 93 with ethyl nitrite at room temperature.Then, these 6-diazopenicillanates 94 were subjected to stereoselective 1,3-dipolar cycloaddition with acrylonitrile, acrylates and methyl vinyl ketone, giving the corresponding spiro-2-pyrazoline-β-lactam derivatives 95, 96 and 97 as major products (Scheme 29).It should be noted that the 1,3-dipolar cycloaddition was stereoselective, major cycloadduct being the result of the dipolarophile addition to the less sterically hindered of the 74

Scheme 34
Recently, Li and co-workers 85 have developed a facile access to a broad range of trifluoromethylcontaining spirocyclic β-lactams via cycloaddition of α-methylene-β-lactams 117 with CF 3 CHN 2 118, which was generated in situ from 2,2,2-trifluoroethylamine hydrochloride and NaNO 2 (Scheme 35).Under metal-free conditions, [3+2]-cycloaddition of 117 with CF 3 CHN 2 led to 2-pyrazoline-containing spirocyclic β-lactams 119 as single diastereomers in good to high yields (82-96%).On the other hand, the use of an iron catalyst (FeTPPCl) gave rise to cyclopropane-containing spirocyclic β-lactams 120 with good diastereoselectivity.The resulting spirocyclic β-lactams bearing a CF 3 moiety are considered to be useful scaffolds for drug discovery and their further exploration in terms of biological activities. 85

Other methods
The reductive Mannich-type reaction of α,β-unsaturated ester 121 with imine 122 using a Rh catalyst and Et 2 Zn provided a rapid access to spiro-β-lactam 123 (Scheme 36).A mechanism was proposed involving the formation of the rhodium−hydride complex (Rh-H) due to combination of Et 2 Zn and RhCl(PPh 3 ) 3 .Subsequently, this complex catalyzed the 1,4-reduction of α,β-unsaturated esters, resulting in a rhodium enolate as a Reformatsky-type reagent.In the end, this enolate reacted with imine 122, giving rise to spiro-βlactam 123. 86In addition, the Willcox group has described the synthesis of an analog of spiro-β-lactam 123 by an aliphatic amine C-H carbonylation process catalyzed by Pd. 87

Scheme 36
The Bruce group has recently published the synthesis of spirocyclic β-lactams 126 a palladiumcatalyzed multicomponent method, including participation of imines 125, ortho-iodo-substituted aryl imines 124 and CO (Scheme 37).The structure of the resulting spirocyclic β-lactams, bearing a trans orientation of the aromatic units, was confirmed by NOE and X-ray analysis. 88

Reactivity of Spiro-fused β-lactams
Along with efforts to improve synthetic methodologies, the deployment of spiro-fused β-lactams as interesting synthons to construct complex heterocycles has also been explored.The reactivity of spiro-fused β-lactams is related to the nature of these four-membered cyclic amides and the presence of substituents on the ring.In this section, the behavior of spiro-fused β-lactams toward ring-opening and ring-transformation reactions will be considered.

Scheme 41
The activation of spiro-β-lactam 141 with di-tert-butyl dicarbonate and subsequent intermolecular nucleophilic ring opening upon treatment with DBU/MeOH afforded 2-oxopiperidino-β-amino ester 142 in excellent yield (Scheme 42).The preparation of spiro-β-lactam 141 was performed starting from β-lactam 138, which was synthesized from ornithine derivative 137 via a three-step procedure.Accordingly, the removal of the Z protecting group from 138 by catalytic hydrogenation resulted in the formation of the 3,5-spiroderivative 139 in good yield, through a 6-exo-trig ring closure.The resulting spiro-β-lactam 139 reacted with benzyl bromide, furnishing the 1-benzyl derivative 140 in an almost quantitative yield.Treatment of compound 140 with CAN led to the removal of the p-methoxyphenyl group, yielding the N-deprotected spiroβ-lactam 141. 92

Scheme 42
Also relevant to this section is the fact that, during the course of preparing nylon-3 materials bearing diverse appended functionalities, Gellman and co-workers have recently reported the ring-opening polymerization of spiro-β-lactams generating nylon-3 homo-or co-polymers. 93

Ring-transformation reactions of substituents attached to the ring carbon atoms
Thermolysis of spiro-β-lactam-oxadiazolines 143 to generate β-lactam carbenes 144 has attracted the attention of several research groups because of the important synthetic applications of the resulting carbenes for the construction of novel mono-, spiro-or polyheterocyclic compounds. 946][97][98][99] Recently, Wang has reported on the use of β-lactam carbenes 144 to produce a novel series of 5-triazolo[1,5-a]pyrazinepyrrol-2-ones 146 in a one-pot mechanism (Scheme 43).These 5triazolo compounds can emit light both in solution and in a solid state, with emission peaks at 77K and in solid state, showing an obvious blue-shift. 94

Scheme 44
The reduction of spiro-β-lactams by different reducing reagents (AlH 3 , AlClH 2 , LiAlH 4 …) comprises one of the most convenient methodologies to generate the corresponding spirocyclic azetidines, which have been applied in drug design. 46,48,56For instance, the reduction of the amide group in compound 51 with AlH 3 afforded spirocyclic azetidine 149 in 92% yield (Scheme 45), which can indeed be seen as a promising building block for the preparation of complex molecules. 48Careful monitoring of the reaction is usually required to avoid reductive β-lactam ring opening.

Scheme 45
Furthermore, spiro-β-lactam 150 has been subjected to enolization using KHMDS in THF, and subsequent treatment of the enolate with Davis oxaziridine 151 or isoamyl nitrite led to the corresponding α-hydroxy lactam 152 or oxime 155 in high yields, respectively (Scheme 46).Then, the β-lactams 152 and 155 were successfully reduced using monochloroalane to give spirocyclic azetidines 153 and 156, respectively.The azetidin-3-ol 153 was further converted to the corresponding ketone 154 under Swern oxidation conditions. 100Scheme 46

Conclusions
In conclusion, the potential pharmacological activities, along with significant synthetic applications of spirocyclic β-lactams, have given a fresh impetus to synthetic chemists to design novel spiro-β-lactam structures.Synthetic approaches toward spirocyclic β-lactams, including the Staudinger ketene-imine cyclocondensation, the cyclization of β-amino acids and β-functionalised amides and transformation reactions, have been improved in recent years (e.g., increase yield and stereoselectivity of reactions as well as the use of greener reaction conditions) and have been intensively discussed in this review.In view of their interesting chemical and biological properties, spirocyclic β-lactams are expected to attract considerable attention in the future as well.