Glynn Mitchell

Enhanced discrimination by aza dienophiles over their olefinic counterparts for the diastereotopic faces of methyl (E,E)-5-(2′,3′,4′,6′-tetra-O-acetyl-β-d-glucopyranosyloxy)penta-2,4-dienoate

Abstract The title diene displays a notably higher diastereofacial selectivity towards 4-phenyl-1,2,4-triazoline-3,5-dione, pyridazine-3,6-dione and phthalazine-1,4-dione than toward N -phenylmaleimide, 1,4-benzoquinone and 1,4 naphthoquinone.

Regio- and stereo-selective intermolecular interceptions of a conjugated N-acylhydrazonium ion

Abstract A notable stereoinduction accompanies the 1,2-addition of nucleophiles to a cyclic N-acylhydrazonium ion.

Preparation and rearrangement of 6a-methyl-6aH-benzo[a]carbazole and 11b-methyl-11bH-benzo[c]carbazole

The non-aromatic benzocarbazoles (4) and (10) are isolable stable compounds; (4) are produced by photolysis of the benzotriazoles (3), and on further irradiation undergo aza-di-π-methane arrangement to give the indenoquinolines (5), whilst as expected (10) is photostable.

Photolysis of 1-aryl-1,2,3-triazoles; rearrangement via 1H-azirines

The preparation and photolysis of a seriesof 1-(1-naphthyl)-1,2,3-triazoles, (2)–(5), and of 1-(2-methyl-1 -naphthyl)-1,2,3-triazoles, (6)–(9), with and without electron-withdrawing groups (CO2R, CONH2, CN, CHO, COPh) in the triazole ring, are described. In the first series, triazoles (3) with two electron-withdrawing groups, and triazoles (4) with one such group at C-4, mostly give good yields of the expected benz[g] indoles (10) and (11) respectively. Triazoles (5) with the electron-withdrawing group at C-5 also give benz[g]indoles but these are now mixtures of the expected (12) and rearranged indoles (11). Photolysis of the 1-phenyltriazoles (21) and (22) follows the same pattern, those [(22)] with the electron-withdrawing group at C-5 rearranging to give mixtures of indoles (23) and (24). This is explained by a mechanism (Scheme 3) in which the less stable carbene intermediate (13) rearranges to the more stable carbene (15)via the 1H-azirine (14), in competition with its direct cyclisation. This provides the first evidence for antiaromatic 1H-azirines as intermediates in a photochemical reaction. Photolysis of the 2-methylnaphthyl′compounds follows a different path: diester (7b) and dinitrile (7d) give high yields of the deeply coloured 9-methyl-1H-benzo[de]quinoline derivatives (25a) and (25b) respectively, the first stable examples of this ring system to be isolated. Photolysis of the nitrile (8c) gives 9b- methyl-9bH-benz[g] indole-3-carbonitrile (28), a rare example of a stable indole with a ring junction substituent. 1,2-Shifts in the carbenes are only observed with the bis(trimethylsilyl)triazoles (2d) and (6f), in keeping with the known rapid migration of trimethylsilyl groups to carbene centres.

Asymmetric synthesis of (3S)-2,3,4,5-tetrahydropyridazine-3-carboxylic acid

The title compound 1a is prepared by a two-step sequence from the cycloadduct 5d, derived from di(tert-butyl) azodicarboxylate and the diene 4b by a hetero Diels-Alder reaction.

Cyclo-octa[def]carbazole, a new heterocyclic paratropic ring system

Photolysis of the 1-(1-naphthyl)benzotriazoles (5a,b,g,h) was found to give the deep red cyclo-octa[def]carbazoles (6a,b,g,h), a new heterocyclic ring system formed by an unusual cyclisation onto the naphthalene ring junction (Scheme 1). The 1H n.m.r. spectra and high chemical reactivity of these cyclo-octacarbazoles (6) are consistent with antiaromatic paratropic character, associated with the 16π-electron periphery (25), comparable with the isoelectronic fluorenyl anion (4). The strained butadiene portion of the cyclo-octacarbazoles (6) is reactive towards addition and cycloaddition reactions, forming a tricarbonyliron complex (27) and Diels–Alder adducts (28). Naphthylbenzotriazoles (5c,d,e) with lone pair bearing substituents (MeO, Cl, Br) adjacent to the triazole ring do not give cyclo-octacarbazoles, but only products derived by cyclisation onto the naphthalene 2-position. On similar photolysis the quinolinyltriazoles (39) give the stable ylides (40) and the acridinylbenzotriazole (45) gives the quinoacridine (46). Mechanisms are proposed for all of these reactions.

Asymmetric synthesis of (3S)-2,3,4,5-tetrahydropyridazine-3-carboxylic acid and its methyl ester†

Methyl (2E,4E)-5-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)penta-2,4-dienoate 16a, assembled by a Wittig condensation of tributyl(methoxycarbonylmethylene)phosphorane 19a and (2E)-3-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)propenal 20, displays excellent Re-face reactivity towards diethyl azodicarboxylate 15a, bis(2,2,2-trichloroethyl) azodicarboxylate 15b, dibenzyl azodicarboxylate 15c, diisopropyl azodicarboxylate 15d and di-tert-butyl azodicarboxylate 15e in thermal hetero-Diels–Alder reactions to give the cycloadducts 17a–e. When subjected to the action of hydrogen over palladium–carbon, the cycloadducts 17a, 17b, 17d and 17e undergo hydrogenation of their olefinic bonds to give the dihydro derivatives 18a, 18b, 18d and 18e; in the case of the cycloadduct 17c, hydrogenolysis of the benzyloxycarbonyl group also occurs to give methyl (3S)-2,3,4,5-tetrahydropyridazine-3-carboxylate 1b with an ee of 98% and 2,3,4,6-tetra-O-acetyl-β-D-glucopyranose 22. Compound 1b, with an ee of 98%, is also available from the dihydro derivative 18e by the action of trifluoroacetic acid; however, under the acidic conditions, a condensation reaction between the aglycone 1b and the glycone 22 competes to give methyl (3S)-2,3,4,5-tetrahydro-2-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyl)pyridazine-3-carboxylate 25.Sodium (3S)-2,3,4,5-tetrahydropyridazine-3-carboxylate 1c, with an ee of 99%, is available from the ester 1b by a saponification reaction. The trifluoroacetic acid salt 27, with an ee of 95%, is obtained from benzyl (3S,6S)-1,2-bis(tert-butoxycarbonyl)-1,2,3,6-tetrahydro-6-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)pyridazine-3-carboxylate 17g by a hydrogenation–trifluoroacetolysis sequence. A hetero-Diels–Alder reaction involving the benzyl pentadienoate 16c and di-tert-butyl azodicarboxylate 15e provides the cycloadduct 17g.

Cyclo-octa[def]carbazole, a new paratropic ring system

Products of the photolyses of the naphthylbenzotriazoles (3) include the red cyclo-octa[def]carbazoles (4), a new ring system formed by unusual cyclisation onto the naphthalene 8a-position; from its 1H n.m.r. spectrum and chemical reactivity, (4) is considered to have antiaromatic paratropic character, associated with the 16π-electron periphery (12), comparable to the isoelectronic fluorenyl anion derived from (1).

1H-Azirines as intermediates in the photolysis of 1,2,3-triazoles

Photolytic conversion of 1-aryl-1,2,3-triazoles (5) and (14) into the ‘rearranged’ indoles (4) and (15), respectively, provides the first evidence for antiaromatic 1H-azirines, e.g.(8), as reactive intermediates in a photochemical reaction.