X-Ray crystal structures, and calculated structures (at B3LYP/6-31G level) are reported for seven N-arylbenzazoles (two carbazoles, indoles and benzimidazoles, and one indazole) bearing electron withdrawing groups in the 2-position of the N-aryl ring. The structures are markedly non-planar by rotation around the N-aryl bond, with the substituent in most cases lying s-E in relation to the N-aryl bond; intermolecular electrostatic interactions in the crystal rationalise the two examples in which an s-Z conformation is observed. A large interplanar angle between the benzazole and the N-aryl planes is associated with a small interplanar angle between the planes of the N-aryl group and the substituent and vice versa.

Flash vacuum pyrolysis (FVP) of 4-acetyltetrazolo[1,5-a]pyridine 5 at 400 °C provides 3-methyl isoxazolo[3,4-b]pyridine 6 whose structure was confirmed by X-ray crystallography. At higher pyrolysis temperatures, the unstable heteroindoxyl 8 was obtained, which exists as the keto form (1,2-dihydropyrrolo[2,3-b]pyridin-3-one) 8K in CDCl3 solution and the enol tautomer (3-hydroxypyrrolo[2,3-b]pyridine) 8E in DMSO. The heteroindoxyl 8 oxidatively dimerises to the heteroindigotin 9, undergoes condensation reactions at the 2-position and reacts with methoxymethylene Meldrum's acid at the 1-position. FVP of the corresponding acetyltetrazolo[1,5-a]quinoline 19 was much more complex, with 2-(cyanophenyl)acetonitrile 30 (rather than a heteroindoxyl) the major product at 750 °C. FVP of 3-acetyl-4-azidoquinoline 24 at 400 °C gave 3-methylisoxazolo[4,3-c]quinoline 33, but rearrangement to the heteroindoxyl was not observed at higher temperatures.

Pyrrolizine-1,3-dione 4 was made by oxidation of the alcohol 2 using pyridinium chlorochromate. The dione 4 shows ketone properties (e.g. formation of DNP derivative 11) and, in common with other pyrrolizinones, the lactam unit is readily ring-opened by methanol under basic conditions. The active methylene unit of 4 couples readily with diazonium salts to provide the hydrazone 15 whose structure was confirmed by X-ray crystallography. The ‘Meldrumsated’ derivative 18 exists exclusively as the tautomer 18F; flash vacuum pyrolysis (FVP) of 18 at 700 °C gives the pyronopyrrolizine 20 exclusively. Reaction of 4 with DMF acetal gives the dimethylaminomethylene derivative 22 which exists as a mixture of rotamers at room temperature.

Flash vacuum pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl 2-(allylthio)benzoates 6 at 650 °C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or S-allyl) bond, followed by ipso attack at the ester group, loss of CO2 and cyclisation of the resulting aryl radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted dibenzofurans 19b–f (73–90%) and dibenzothiophenes 20b–c (90–94%). Little selectivity is shown in the cyclisation of m-substituted substrates and competing interactions of the radical with the substituent—and ipso-attack—complicate the pyrolyses of o-substituted substrates. FVP of related radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas 2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of 4-methylphenyl 2-(allylamino)benzoate 42.

3-Hydroxythiophene 1 spontaneously dimerises to 4,5-dihydro-5-(3-hydroxythien-2-yl)thiophen-3(2H)-one 14. 3-Hydroxythiophenes 1E and 4–10E exist in solvent-dependent equilibrium with their thiophen-3(2H)-one 1K and 4–10K tautomers; the amount of hydroxy tautomer is greater than in the case of the corresponding 3-hydroxypyrroles. 3-Hydroxythiophenes are much less reactive to electrophiles than corresponding 3-hydroxypyrroles, but the 5-methylsulfanyl derivative 10 reacts at the 2-position with methoxymethylene Meldrum's acid and undergoes Vilsmeier formylation. The enolates derived from 3-hydroxythiophenes by treatment with base can be O-alkylated and O-acylated with high regioselectivity. 2,2-Disubstituted thiophen-3(2H)-ones undergo equilibrium conjugate addition with nucleophiles, but the resulting adducts could not be isolated.

Experiment and theory both suggest that the AAA−DDD pattern of hydrogen bond acceptors (A) and donors (D) is the arrangement of three contiguous hydrogen bonding centers that results in the strongest association between two species. Murray and Zimmerman prepared the first example of such a system (complex 3•2) and determined the lower limit of its association constant (Ka) in CDCl3 to be 105 M−1 by 1H NMR spectroscopy (Murray, T. J. and Zimmerman, S. C. J. Am. Chem. Soc. 1992, 114, 4010−4011). The first cationic AAA−DDD pair (3•4+) was described by Bell and Anslyn (Bell, D. A. and Anslyn, E. A. Tetrahedron 1995, 51, 7161−7172), with a Ka > 5 × 105 M−1 in CH2Cl2 as determined by UV−vis spectroscopy. We were recently able to quantify the strength of a neutral AAA−DDD arrangement using a more chemically stable AAA−DDD system, 6•2, which has an association constant of 2 × 107 M−1 in CH2Cl2 (Djurdjevic, S., Leigh, D. A., McNab, H., Parsons, S., Teobaldi, G., and Zerbetto, F. J. Am. Chem. Soc. 2007, 129, 476−477). Here we report on further AA(A) and DDD partners, together with the first precise measurement of the association constant of a cationic AAA−DDD species. Complex 6•10+[B(3,5-(CF3)2C6H3)4−] has a Ka = 3 × 1010 M−1 at RT in CH2Cl2, by far the most strongly bound triple hydrogen bonded system measured to date. The X-ray crystal structure of 6•10+ with a BPh4− counteranion shows a planar array of three short (NH···N distances 1.95−2.15 Å), parallel (but staggered rather than strictly linear; N−H···N angles 165.4−168.8°), primary hydrogen bonds. These are apparently reinforced, as theory predicts, by close electrostatic interactions (NH−·−N distances 2.78−3.29 Å) between each proton and the acceptor atoms of the adjacent primary hydrogen bonds.

Pyrrolizin-3-ones (e.g.1) can be easily hydrogenated to their hexahydro (pyrrolizidin-3-one) derivatives in the presence of heterogeneous catalysts. Good diastereoselectivity (up to >97:3, depending on catalysts and solvent) can be achieved if the pyrrolizin-3-one is substituted at the 1- (or 7-) position(s), but the selectivity is reduced if both positions are substituted. Subsequent deoxygenation of the pyrrolizidin-3-ones provides concise, diastereoselective routes to the necine bases (±)-heliotridane 5, (±)-isoretronecanol 6 and (±)retronecanol 7.

2-(Pyrrol-1-yl)phenoxyl, aminyl, thiophenoxyl and benzyl radicals 2a–2d, respectively, were generated in the gas-phase under flash vacuum pyrolysis conditions. In all cases except the phenoxyl, cyclisation took place providing acceptable synthetic routes to the fused heterocycles 11, 14 and 15, respectively. Only sigmatropic rearrangement products were isolated, in low yields, from the phenoxyl 2a. The pyrrolo[1,2-a]benzimidazole 11 adopts the 1H-tautomer exclusively in chloroform solution. Electrophilic substitution reactions of pyrrolo[2,1-b]benzothiophene 14 were studied, including protonation, deuterium exchange, Vilsmeier formylation and reaction with dimethyl acetylenedicarboxylate. 2-(2,5-Diarylpyrrol-1-yl)thiophenoxyl, phenoxyl and aminyl radicals 23a–f, were also generated in the gas-phase under similar conditions. The thiophenoxyls 23a/b gave extremely complex pyrolysate mixtures in which primary cyclisation products were formed by attack of the radical at the pyrrrole ring and attack at the ipso-, ortho- and meta- positions of the aryl ring. Secondary pyrolysis products were obtained by specific sigmatropic shifts of the N-aryl group. The 2,5-di(thien-2-yl)thiophenoxyl radical 23c gave the pyrrolobenzothiazole 31c as the only cyclisation product in low yield. FVP of the phenoxyl and aminyl radical generators 26d and 26f, respectively, gave 3-arylpyrrolo[1,2-f]phenanthridines 46d and 46f, respectively, by a hydrogen transfer-cyclisation mechanism.

Methoxymethylene Meldrum's acid 1 reacts with 5- and 6-membered lactams in refluxing acetonitrile to give the N-substituted products 9–15. If the reactions are continued for extended times, the Meldrum's acid derivatives decompose to provide enamidoesters e.g.22–24. Flash vacuum pyrolysis of the 5-membered ring products 9–13 provides reasonable yields of the fused pyrrolones 31–35. The constitution of the products is supported by X-ray crystal structures of 10, 12, 19, 32 and 34.

Flash vacuum pyrolysis (FVP) of dimethyl E- or Z-pyrrol-2-ylbut-2-enedioate 5 at 700 °C gave 1-methoxycarbonylpyrrolizin-3-one 1. The sequence involves E- to Z-isomerisation (if necessary), elimination of methanol and cyclisation; the elimination step is rate determining. The pyrrolizinone 1 is stable at low temperatures, but at room temperature dimerises spontaneously across the 1,2-bond to give a mixture of trans- and cis- cyclobutanes 2 and 3, respectively. In this process 1 behaves as a captodative alkene. The dimerisation can be reversed at >100 °C. Related pyrrolizinones such as the esters 14 and 22 and the amide 15 are stable to dimerisation. The diacid 12 and the amide 10 do not cyclise to pyrrolizinones under FVP conditions, but instead give the anhydride 19 and the maleimide 18, respectively; at high furnace temperatures, 2-ethynylpyrrole 17 is obtained from 12 and from 19.

Treatment of 1,2-di(pyrrol-1-yl)ethane 4 with oxalyl chloride gave a low yield of 5,6-dihydrodipyrrolo[1,2-d;2′,1′-g][1,4]diazepin-11-one 2, by an unexpected electrophilic substitution–decarbonylation process. The diazepinone 2 is unreactive to electrophiles, but can be deoxygenated to 5,6-dihydro-11H-dipyrrolo[1,2-d;2′,1′-g][1,4]diazepine 9 with lithium aluminium hydride. Reaction of 1,1-di(pyrrol-1-yl)methane 5 with triphosgene gives the related dipyrrolo[1,2-c;2′,1′-f]pyrimidin-10-one 3. NMR spectroscopic and X-ray crystal structure comparisons of 2 and 3 show that there is unexpectedly greater conjugation in the compound with the core seven-membered ring, 2, even though the molecule as a whole is less planar than 3. The corresponding reactions of 4 or 5 with thiophosgene did not give cyclisation products but the O-methyl thioesters 7 and 8 were obtained (89 and 27%, respectively).

Flavonoids are amongst the most commonly used natural yellow colourants in paintings, as lakes, and in historical textiles as mordant dyes. In this paper, evidence from isotopically labelled substrates is used to propose negative ion electrospray collision induced decomposition mechanisms of flavones, flavonols and an isoflavone. These mechanisms include a retro-Diels-Alder fragmentation (observed for flavones and flavonols) and an M-122 fragmentation (characteristic of 3′,4′-dihydroxyflavonols). In addition, the presence of a m/z 125 fragment ion is shown to be characteristic of 2′-hydroxyflavonols and an ion at m/z 149 is shown to be characteristic of 4′-hydroxyflavones. Applications of these methods are exemplified by the identification of a minor component of Dyer's camomile (Anthemis tinctoria L.) and the identification of the dye source in green threads sampled from an 18th Century Scottish tartan fragment.

Flash vacuum pyrolysis (FVP) of 2-acetyl-3-azidothiophene gives 3-methylthieno[3,2-c]isoxazole as the major product at a furnace temperature of 350 °C whereas at temperatures above 550 °C the new heteroindoxyl 4,5-dihydrothieno[3,2-b]pyrrol-6-one is exclusively formed. The heteroindoxyl exists predominantly as the keto tautomer. It is O-protonated by TFA, N-acetylated by acetic anhydride, N-nitrosated by nitrous acid, and provides an N-methylene Meldrum’s acid derivative on treatment with methoxymethylene Meldrum’s acid. Reactions of 4,5-dihydrothieno[3,2-b]pyrrol-6-one with diazonium salts, with isatin, and with dimethyl acetylenedicarboxylate take place at the methylene position to provide a hydrazone, an indirubin analogue, and a succinate derivative, respectively. Oxidation of 4,5-dihydrothieno[3,2-b]pyrrol-6-one gives a heteroindigotin, which shows a hypsochromic shift in the UV spectrum, relative to indigotin itself.

Isoindolo[2,1-a]indol-6-one 1 is formed by a sigmatropic shift–elimination–cyclisation cascade by flash vacuum pyrolysis (FVP) of methyl 2-(indol-1-yl)benzoate 7 at 950 °C. The dihydro compound 16 is easily obtained by catalytic reduction of 1, but the reaction is very sensitive to steric effects at the 11-position. Attempted ring-opening of 1 in basic methanol provides an equilibrium of isoindolo[2,1-a]indol-6-one 1 and the ester 19. Lithium aluminium hydride reduction of 1 provides the alcohol 22 which can be dehydrated to a mixture of 23 and 24 by FVP at 800–950 °C.

The thermal behaviour of argentilactone 1 has been studied by the use of flash vacuum pyrolysis (FVP) techniques. The E- and Z-isomers of 2-octenal 7 were found to be the major non-gaseous products of the thermal ring-opening of argentilactone at 650–850 °C; no undecatrienes 2–6 could be identified in the pyrolysate mixtures.

The electrospray ionisation mass spectra of the neoflavanoids brazilin and hematoxylin are reported in both their reduced (1 and 2, respectively) and their oxidised forms (3 and 4, respectively). In the reduced forms, breakdown pathways under collision induced decomposition (CID) conditions produce fragments characteristic of rings A and C; in their oxidised forms, the fragments are characteristic of rings B and D. The structural assignments of the fragments are substantiated by recording the spectra after deuterium exchange at the hydroxyl groups.Negative ion electrospray ionisation (ESI) mass spectra of the neoflavonoids brazilin and hematoxylin under collision induced decomposition (CID) conditions show fragments characteristic of rings A and C. In their oxidised forms, the fragments are characteristic of rings B and D.

The sources and structures of dyes used to colour Western historical textiles are described in this tutorial review. Most blue and purple colours were derived from indigo—obtained either from woad or from the indigo plant—though some other sources (e.g. shellfish and lichens) were used. Reds were often anthraquinone derivatives obtained from plants or insects. Yellows were almost always flavonoid derivatives obtained from a variety of plant species. Most other colours were produced by over-dyeing—e.g. greens were obtained by over-dyeing a blue with a yellow dye. Direct analysis of dyes isolated from artefacts allows comparison with the historical record.

Isoindolo[2,1-a]indol-6-one 1 is formed by a sigmatropic shift–elimination–cyclisation cascade by flash vacuum pyrolysis (FVP) of methyl 2-(indol-1-yl)benzoate 7 at 950 °C. The dihydro compound 16 is easily obtained by catalytic reduction of 1, but the reaction is very sensitive to steric effects at the 11-position. Attempted ring-opening of 1 in basic methanol provides an equilibrium of isoindolo[2,1-a]indol-6-one 1 and the ester 19. Lithium aluminium hydride reduction of 1 provides the alcohol 22 which can be dehydrated to a mixture of 23 and 24 by FVP at 800–950 °C.

Flash vacuum pyrolysis (FVP) of dimethyl E- or Z-pyrrol-2-ylbut-2-enedioate 5 at 700 °C gave 1-methoxycarbonylpyrrolizin-3-one 1. The sequence involves E- to Z-isomerisation (if necessary), elimination of methanol and cyclisation; the elimination step is rate determining. The pyrrolizinone 1 is stable at low temperatures, but at room temperature dimerises spontaneously across the 1,2-bond to give a mixture of trans- and cis- cyclobutanes 2 and 3, respectively. In this process 1 behaves as a captodative alkene. The dimerisation can be reversed at >100 °C. Related pyrrolizinones such as the esters 14 and 22 and the amide 15 are stable to dimerisation. The diacid 12 and the amide 10 do not cyclise to pyrrolizinones under FVP conditions, but instead give the anhydride 19 and the maleimide 18, respectively; at high furnace temperatures, 2-ethynylpyrrole 17 is obtained from 12 and from 19.

Methoxymethylene Meldrum's acid 1 reacts with 5- and 6-membered lactams in refluxing acetonitrile to give the N-substituted products 9–15. If the reactions are continued for extended times, the Meldrum's acid derivatives decompose to provide enamidoesters e.g.22–24. Flash vacuum pyrolysis of the 5-membered ring products 9–13 provides reasonable yields of the fused pyrrolones 31–35. The constitution of the products is supported by X-ray crystal structures of 10, 12, 19, 32 and 34.

Pyrrolizin-3-ones (e.g.1) can be easily hydrogenated to their hexahydro (pyrrolizidin-3-one) derivatives in the presence of heterogeneous catalysts. Good diastereoselectivity (up to >97:3, depending on catalysts and solvent) can be achieved if the pyrrolizin-3-one is substituted at the 1- (or 7-) position(s), but the selectivity is reduced if both positions are substituted. Subsequent deoxygenation of the pyrrolizidin-3-ones provides concise, diastereoselective routes to the necine bases (±)-heliotridane 5, (±)-isoretronecanol 6 and (±)retronecanol 7.

2-(Pyrrol-1-yl)phenoxyl, aminyl, thiophenoxyl and benzyl radicals 2a–2d, respectively, were generated in the gas-phase under flash vacuum pyrolysis conditions. In all cases except the phenoxyl, cyclisation took place providing acceptable synthetic routes to the fused heterocycles 11, 14 and 15, respectively. Only sigmatropic rearrangement products were isolated, in low yields, from the phenoxyl 2a. The pyrrolo[1,2-a]benzimidazole 11 adopts the 1H-tautomer exclusively in chloroform solution. Electrophilic substitution reactions of pyrrolo[2,1-b]benzothiophene 14 were studied, including protonation, deuterium exchange, Vilsmeier formylation and reaction with dimethyl acetylenedicarboxylate. 2-(2,5-Diarylpyrrol-1-yl)thiophenoxyl, phenoxyl and aminyl radicals 23a–f, were also generated in the gas-phase under similar conditions. The thiophenoxyls 23a/b gave extremely complex pyrolysate mixtures in which primary cyclisation products were formed by attack of the radical at the pyrrrole ring and attack at the ipso-, ortho- and meta- positions of the aryl ring. Secondary pyrolysis products were obtained by specific sigmatropic shifts of the N-aryl group. The 2,5-di(thien-2-yl)thiophenoxyl radical 23c gave the pyrrolobenzothiazole 31c as the only cyclisation product in low yield. FVP of the phenoxyl and aminyl radical generators 26d and 26f, respectively, gave 3-arylpyrrolo[1,2-f]phenanthridines 46d and 46f, respectively, by a hydrogen transfer-cyclisation mechanism.

X-Ray crystal structures, and calculated structures (at B3LYP/6-31G level) are reported for seven N-arylbenzazoles (two carbazoles, indoles and benzimidazoles, and one indazole) bearing electron withdrawing groups in the 2-position of the N-aryl ring. The structures are markedly non-planar by rotation around the N-aryl bond, with the substituent in most cases lying s-E in relation to the N-aryl bond; intermolecular electrostatic interactions in the crystal rationalise the two examples in which an s-Z conformation is observed. A large interplanar angle between the benzazole and the N-aryl planes is associated with a small interplanar angle between the planes of the N-aryl group and the substituent and vice versa.

Flash vacuum pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl 2-(allylthio)benzoates 6 at 650 °C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or S-allyl) bond, followed by ipso attack at the ester group, loss of CO2 and cyclisation of the resulting aryl radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted dibenzofurans 19b–f (73–90%) and dibenzothiophenes 20b–c (90–94%). Little selectivity is shown in the cyclisation of m-substituted substrates and competing interactions of the radical with the substituent—and ipso-attack—complicate the pyrolyses of o-substituted substrates. FVP of related radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas 2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of 4-methylphenyl 2-(allylamino)benzoate 42.

Flash vacuum pyrolysis (FVP) of 4-acetyltetrazolo[1,5-a]pyridine 5 at 400 °C provides 3-methyl isoxazolo[3,4-b]pyridine 6 whose structure was confirmed by X-ray crystallography. At higher pyrolysis temperatures, the unstable heteroindoxyl 8 was obtained, which exists as the keto form (1,2-dihydropyrrolo[2,3-b]pyridin-3-one) 8K in CDCl3 solution and the enol tautomer (3-hydroxypyrrolo[2,3-b]pyridine) 8E in DMSO. The heteroindoxyl 8 oxidatively dimerises to the heteroindigotin 9, undergoes condensation reactions at the 2-position and reacts with methoxymethylene Meldrum's acid at the 1-position. FVP of the corresponding acetyltetrazolo[1,5-a]quinoline 19 was much more complex, with 2-(cyanophenyl)acetonitrile 30 (rather than a heteroindoxyl) the major product at 750 °C. FVP of 3-acetyl-4-azidoquinoline 24 at 400 °C gave 3-methylisoxazolo[4,3-c]quinoline 33, but rearrangement to the heteroindoxyl was not observed at higher temperatures.

Pyrrolizine-1,3-dione 4 was made by oxidation of the alcohol 2 using pyridinium chlorochromate. The dione 4 shows ketone properties (e.g. formation of DNP derivative 11) and, in common with other pyrrolizinones, the lactam unit is readily ring-opened by methanol under basic conditions. The active methylene unit of 4 couples readily with diazonium salts to provide the hydrazone 15 whose structure was confirmed by X-ray crystallography. The ‘Meldrumsated’ derivative 18 exists exclusively as the tautomer 18F; flash vacuum pyrolysis (FVP) of 18 at 700 °C gives the pyronopyrrolizine 20 exclusively. Reaction of 4 with DMF acetal gives the dimethylaminomethylene derivative 22 which exists as a mixture of rotamers at room temperature.