Concetta La Rosa

Reaction of mesoionic compounds deriving from cyclic N-acyl-α-aminoacids with N-(phenylmethylene)benzenesulfonamide

Abstract We studied the behaviour of bicyclic mesoionic compounds derived from the cyclodehydration of cyclic N -acyl-α-aminoacids 1–4 with N -(phenylmethylene)benzenesulfonamide 5 . The reaction affords spirocyclic β-lactams and/or imidazo-condensed products (the 1,3-dipolar cycloaddition adducts) depending on the experimental conditions and on the nature of the substituent R on the mesoionic ring.

Stereoselective synthesis of N-phenylsulfonyl substituted spiro-β-lactams

Abstract A stereoselective synthesis of N -phenylsulfonyl substituted spiro-β-lactams obtained from the N -(phenylmethylene)benzenesulfonamide and the ketene valence tautomer of the bicyclic mesoionic compounds derived from 4-hydroxy substituted N -acyl- l -prolines in the presence of acetic anhydride as the dehydrating agent is presented. The reaction of (2 S ,4 R )-4-acetyloxy or benzoyloxy- N -acyl-proline with the above imine afforded a mixture of two diastereoisomeric spiro-β-lactams with complete stereoselectivity for the spiro carbon.

Cycloaddition reactions of 5H,7H-thiazolo[3,4-c] oxazolium-1-oxides with imines

Abstract The behaviour of mesoionic 5H,7H-thiazolo [3,4-c] oxazol-1-ones (α) towards imines (2) and (3) was studied. The cycloaddition reaction affords 7-thia-2,5-diazaspiro [3,4] octan-1-one (4–7) and lH,3H-imidazo [1,5-c] thiazole (8) derivatives. The possible mechanism involved in the formation of products as well as the unusual rearrangement showed by spirocyclic β-lactams (6,7) are discussed.

Regiochemical aspects of intramolecular cycloadditions of nitrones derived from N-(2-alkenyl)-2-pyrrolecarbaldehydes. Competitive entries to pyrrolizidine and indolizidine derivatives

Abstract Intramolecular cycloadditions of unsaturated nitrones derived from a series of N-(2-alkenyl)-2-pyrrolecarbaldehydes (2) have been systematically studied. A pronounced substituent effect has been observed as far as the competitive formation of fused- and bridged-ring regioisomers are concerned. Further elaboration of the two kinds of cycloadducts has given pyrrolizidine and indolizidine derivatives, respectively.

Stereoselective synthesis of 3-heteroaromatic-substituted alanines

Abstract An asymmetric synthesis of ( R )-3-heterocyclic-substituted alanines starting from (2 S )-(+)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine (Schollkopf reagent) and heteroaromatic halogenomethyl derivatives via hydrolysis of intermediate adducts is reported. The diastereocontrolled addition gives mainly compounds with the (2 S ,5 R ) configuration whose formation is explained on the basis of the accepted model for the alkylation reaction of the Schollkopf reagent, and structure confirmed by spectroscopic data.

Stereoselective synthesis of (1R, 4R)-N-acyl-2-oxa-5-aza-bicyclo[2.2.1]heptan-3-ones via mesoionic compounds. An improved synthesis of cis-4-hydroxy-D-proline

Abstract We report an asymmetric synthesis of (1 R ,4 R )- N -acyl-2-oxa-5-aza-bicyclo[2.2.1]heptan-3-ones, starting from the inexpensive and commercially available trans -4-hydroxy- l -proline and achieved by treating N -acyl- trans -4-hydroxy- l -prolines with acetic anhydride. The formation of intermediate mesoionic compounds may explain the formation of N -acyl-2-oxa-5-aza-bicyclo[2.2.1]heptan-3-ones with ( R )-absolute configuration at C(4). Acidic cleavage of these lactones readily affords N -acyl- cis -4-hydroxy- d -prolines or cis -4-hydroxy- d -proline in good yields.

Diels–Alder reactions of N-sulfonyl substituted aza-ortho-xylylenes generated from the corresponding 1,4-dihydro-2H-3,1-benzoxazin-2-one derivatives

N-Tosyl- and N-alkylidene-sulfonyl substituted 1,4-dihydro-2H-3,1-benzoxazin-2-ones 2c-g easily undergo thermal carbon dioxide extrusion leading to the aza-ortho-xylylenes 3c–g. The intermediates 3c,d can be trapped by electron-poor ethylenic and acetylenic dienophiles, giving tetrahydroquinoline and quinoline derivatives. The reactions of 2c with non-symmetrical dienophiles are completely regioselective. N-Alkylidenesulfonyl substituted aza-ortho-xylylenes 3f–g undergo intramolecular Diels–Alder reactions leading to the tricyclic compounds 10 and 11, while the aza-ortho-xylylene generated from 4-(hex-5-enyl)-N(4-methylphenylsulfonyl)-1,4-tetrahydro-2H-3,1-benzoxazin-2-one undergoes a [1,5] hydrogen shift leading to N-[2-(1E)-hepta-1,6-dien-1-ylphenyl]-4-methylbenzenesulfonamide.

Reaction of 2, 4- diphenyl-4, 5-dihydro-1, 3-oxazol-5-one with 4-phenyl-N-tosyl-1azabuta-1, 3-diene; CC versus CN double bond addition

The reaction of 2, 4-diphenyl-4, 5-dihydro-1, 3-oxazol-5-one 1 with 4-phenyl-N-tosyl-1-azabuta-1, 3-diene 2 has been investigated using different experimental conditions. Whereas at room temperature the kinetically controlled Michael adduct 3 was isolated, at 110 °C products 4, 5, 6 and 7 were obtained. Their formation is explained according to 1,3-dipolar cycloaddition and nucleophilic addition, both related to the dual nature of the azlactone 1. The reactivity of azlactone 1 towards imine 2, which is different from that observed with N-alkyl- and N-aryl-1-azabuta-1,3-dienes, is explained on the basis of the electronic effects exerted by the tosyl substituent.

Heterocycles from ylides. Part IX. A convenient synthesis of 1-sulfonyl-2,3-disubstituted 2,3-dihydroindoles

The reaction of sulfur ylides (1) with N-(2-bromomethylphenyl)benzenesulfonamides (4) gives 2,3-dihydroindoles (5) whose structure and stereochemistry were assigned on the basis of analytical and spectroscopic data. Some chemical transformations of 5 are reported.

Combinatorial Chemical Synthesis of 4-Heteroaryl-3-substituted Pyrroles from Nitroalkenes

A small library of 4-(2-furyl)- or 4-(2-thienyl)-3-substituted pyrroles has been prepared by reaction of 2-(2-furyl or 2-thienyl)-1-nitro-1-alkenes (1) with secondary enamines (2). The reaction gives pyrrole derivatives by means of a Michael-type addition intermediate. Cyclization is influenced by the nature of the solvent and the substituent in the enamine β position, and by enamine-nitrogen nucleophilicity.