Lactams 1,3-dipolar cycloaddition

Other approaches to (36) make use of (37, R = CH ) and reaction with a tributylstannyl allene (60) or 3-siloxypentadiene (61). A chemicoen2ymatic synthesis for both thienamycia (2) and 1 -methyl analogues starts from the chiral monoester (38), derived by enzymatic hydrolysis of the dimethyl ester, and proceeding by way of the P-lactam (39, R = H or CH ) (62,63). (3)-Methyl-3-hydroxy-2-methylpropanoate [80657-57-4] (40), C H qO, has also been used as starting material for (36) (64), whereas 1,3-dipolar cycloaddition of a chiral nitrone with a crotonate ester affords the oxa2ohdine (41) which again can be converted to a suitable P-lactam precursor (65). 

Whereas there are numerous examples of the application of the products from diastereoselective 1,3-dipolar cycloaddition reaction in synthesis [7, 8], there are only very few examples on the application of the products from metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction in the synthesis of potential target molecules. The reason for this may be due to the fact that most metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction have been carried out on model systems that have not been optimized for further derivatization. One exception of this is the synthesis of a / -lactam by Kobayashi and Kawamura [84]. The isoxazoli-dine endo-21h, which was obtained in 96% ee from the Yb(OTf)3-BINOL-catalyzed... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Dipolar cycloaddition of azides with olefins provides a convenient access to triazolines, cyclic imines, and aziridines and hence is a valuable technique in heterocyclic synthesis. For instance, tricyclic -lactams 273 - 276 have been synthesized using the intramolecular azide-olefin cycloaddition (lAOC) methodology (Scheme 30) [71]. 

Dimethyl-3-methylenepyrrolidine-2-thione, which reacts with nitrones regio- and stereoselectively at its exocyclic C=C bond to give only spirocy-cloadducts 116, behaves more complicatedly with nitrile oxides. The latter undergo 1,3-dipolar cycloaddition both to the exocyclic C=C and C=S double bonds with subsequent cycloreversion and formation of spiro-lactams 117 (281). 

Diastereoselective intramolecular 1,3-dipolar cycloadditions of alkylidene-cyclopropyl nitrones provide spirocyclopropylisoxazolidines. These compounds have been shown to undergo either thermally induced ring expansion to octahydro[l]pyrindin-4-ones or to acid induced ring contraction into fS-lactams with concomitant loss of ethylene (Scheme 2.218) (710-716). Use of chiral auxiliaries, that is (L)-2-acetoxylactate can lead to enantiomerically enriched heterocycles (715). 

Recent research deals with stereoselective 1,3-dipolar cycloadditions of nitrones for the syntheses of alkaloids and aza heterocycles asymmetric synthesis of biologically active compounds such as glycosidase inhibitors, sugar mimetics, /3-lactams, and amino acids synthesis of peptido-mimetics and peptides chemistry of spirocyclopropane heterocycles synthesis of organic materials for molecular recognition and photochemical applications. 

Dideoxyhex-2-enono-1,5-lactone derivatives (penten-5-olides) have been prepared (255-258) and employed as starting compounds in synthesis. Thus, Michael addition of benzylhydroxylamine to racemic 6-0-acetyl-2,3,4-trideoxy-D,L-g/ycerohex-2-enono-1,5-lactone (267) took place ster-eoselectively to give the unstable benzyloxyamino-2-pyrone 268, which was readily converted into the /Mactam derivative 269, a precursor of thienamy-cin (259). / -Lactams were also obtained (260) by 1,3-dipolar cycloaddition of nitrone 270 to the unsaturated 1,5-lactone 267, followed by hydrogenoly-sis and subsequent cyclization to the /Mactam 271, having a polyol side-chain at the C-3 position. 

Alcaide et al. (68,69) recently published their smdies of the intramolecular 1,3-dipolar cycloaddition reactions of alkynyl-p-lactams in which they found that the desired cycloaddition was in competition with a reverse-Cope elimination. The reaction of alkynyl aldehydes 37a-c with Al-methylhydroxylamine afforded a mixture of products depending on the reaction conditions and the chain length separating the alkyne and the lactam (Scheme 1.8). Thus, up to three separate... 

Keshava et al. (10) reported a facile synthesis of the fused tricyclic (3-lactams 44 and 45 via an intramolecular 1,3-dipolar cycloaddition of an azide with an alkene (Scheme 9.10). 1,3-Dipolar cycloaddition of the azides 43 in benzene at reflux gave a mixture of cis and trans tricyclic (3-lactams 44 and 45. As the ring size increased... 

Sha et al. (45) reported an intramolecular cycloaddition of an alkyl azide with an enone in an approach to a cephalotaxine analogue (Scheme 9.45). Treatment of the bromide 205 with NaN3 in refluxing methanol enabled the isolation of compounds 213 and 214 in 24 and 63% yields, respectively. The azide intermediate 206 underwent 1,3-dipolar cycloaddition to produce the unstable triazoline 207. On thermolysis of 207 coupled with rearrangement and extrusion of nitrogen, compounds 213 and 214 were formed. The lactam 214 was subsequently converted to the tert-butoxycarbonyl (t-Boc)-protected sprrocyclic amine 215. The exocyclic double bond in compound 215 was cleaved by ozonolysis to give the spirocyclic ketone 216, which was used for the synthesis of the cephalotaxine analogue 217. 

Similarly, both acyclic and cyclic allyl amine derivatives have been applied in 1,3-dipolar cycloadditions (134-138). Langlois et al. (139) used a,()-unsaturated-y-lactams derived from (5)-pyroglutaminol, such as 91 and 92, in the 1,3-dipolar cycloaddition with the A -benzylnitrone derived from formaldehyde (Scheme 12.30). For compound 91, one of the 1,3-dipolar cycloaddition product isomers obtained... 

Kobayashi and Kawamura (374) used the catalytic enantioselective 1,3-dipolar cycloaddition for the synthesis of an optically active p-lactam (Scheme 12.85). The... 

The formal total synthesis of the novel /3-lactam antibiotic thienamycin has been accomplished from an isoxazoline derivative generated by [3 + 2] dipolar cycloaddition <79H(l2)l 183). Reaction of the nitrile oxide derived from 3-nitropropanal dimethyl acetal with methyl crotonate gave the isoxazoline (477) regio- and stereo-selectively. The isoxazoline was converted to amino ester (478) by hydrogenation and then to /3-lactam (479) by ester saponification and ring closure with DCC. Treatment of (479) with p-nitrobenzyl chloroformate and reaction of the derived acetal (480) with excess N-p-nitrobenzyloxycar-bonylcysteamine gave thioacetal (481), a compound which has previously been converted into ( )-(8S )-thienamycin (Scheme 106). 

Another possible route to thienamycin (487) has utilized the dipolar cycloaddition of 1-pyrroline 1-oxide (482) with methyl crotonate (79TL4359). The reaction is highly stereoselective due to the operation of secondary orbital effects. The isoxazolidine (483), produced in 90% yield, was subjected to hydrogenolysis, and the resulting amino alcohol (484) was selectively blocked with hexamethyldisilazane to give (485). Treatment with ethylmagnesium bromide then gave /3-lactam (486 Scheme 107). 

Optically active a-methylene-(3-lactam 10 was synthesized and submitted to 1,3-dipolar cycloadditions with diazomethane, 4-methoxybenzonitrile oxide, and diphenylnitrone [51]. All cycloadditions proceed with complete regioselectivity giving products 11-13 in an anhfashion with respect to the substituent at the... 

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