We report the syntheses of combretastatins D-2 and D-4 as well as a formal synthesis of combretastatin D-1 by a conceptually new route harnessing a ring-contracting flash vacuum pyrolytic extrusion of sulfur dioxide from the respective 16-membered sulfone precursors. Via flash vacuum pyrolysis, even metaparacyclophanes as small and strained as the hitherto unknown oxa[1.5]metaparacyclophane could be prepared as a side product en route to combretastatin D-2 by synchronous extrusion of SO2 and CO2.

•Six different stabilised phosphorus ylides suitable for cascade cyclization have been prepared.•Pyrolysis mainly gives styrylquinolines accompanied by other fused-ring quinolines and carbazoles.•With a ring methyl present the quinoline is accompanied by the furocarbazole natural product Eustifoline D.•Eustifoline D is formed in 5 steps from 5-methylanthranilic acid.Six stabilised phosphonium ylides bearing ortho-benzylaminophenyl and cinnamoyl (or a heterocyclic analogue) groups have been prepared and upon flash vacuum pyrolysis at 800 °C were found to undergo cascade cyclization processes to give mainly 3-styrylquinolines but also in some cases ring-fused carbazoles and other fused-ring heterocyclic products. By starting with an appropriate ring-methylated precursor the natural product Eustifoline D was obtained in 19% yield in the pyrolysis in addition to the 3-(2-furylethenyl)quinoline (46%).Download high-res image (87KB)Download full-size image

A survey of 20 carbonyl-stabilised phosphonium ylides with recently reported X-ray structures shows a strong correlation between the CP to CO torsion angle and the value of 2JP–CO, with high values being associated with an anti configuration and low with syn. Seven new X-ray structural determinations are reported, several for types of ylide not crystallographically characterised before, and these also conform to this pattern. The value of 2JP–CO is then correlated with whether or not thermal extrusion of Ph3PO occurs to give alkynes for over 200 ylides and an empirical rule developed that the extrusion never occurs for ylides where this value is > 11 Hz. This is used to rationalise the anomalous behaviour of some trioxo ylides and cyclic ylides, two of which afford cycloalkynes, isolated after rearrangement as the isomeric 1,3-dienes. The rule also holds for a family of novel highly fluorinated ylides which afford fluorinated alkynes in good yield upon flash vacuum pyrolysis.Download high-res image (87KB)Download full-size image

•FVP over magnesium couples benzylidene chlorides to give mainly stilbenes.•Ortho-methoxybenzylidene chlorides unexpectedly give hydrocarbon products.•Halogenated poly(p-xylylenes) are deposited direct from pyrolysis of halogenated p-xylenes.•Dichlorodiphenylmethane gives 9-(diphenylmethylene)fluorene and triphenylene is formed in moderate yield from 1,2-dihalobenzenes.•The method offers a convenient one-step solvent-free alternative to catalytic procedures.Flash vacuum pyrolysis at 600 °C through glass wool coated with freshly sublimed magnesium is examined as a preparative method for dehalogenative coupling in organic synthesis. Substituted benzylidene chlorides give predominantly the corresponding stilbenes and in some cases these are readily isolated in pure form. With an ortho-halogen substituent, additional cyclisation gives phenanthrenes but the method is not compatible with the presence of several reactive groups. An ortho-methoxy substituent leads to unexpected formation of mono- and dimethyl products. With 1,4-bis(dihalomethyl)benzenes, halogenated polymers are deposited directly from the gas phase via generation of halogenated p-xylylenes. The 1,2- and 1,3-isomers lead respectively to benzocyclobutadiene, isolated as a dimer, and to pyrene. The 1,4-bis(trihalomethyl)benzenes give more highly halogenated polymers directly from the gas phase via halogenated p-xylylenes. While halobenzenes generally give the corresponding benzenes and biphenyls, 1,2-dihalobenzenes additionally produce triphenylene in preparatively useful yield by a process not involving intermediacy of free benzyne.Download high-res image (145KB)Download full-size image

A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2-dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. The X-ray structures confirm the PC and CO functions to be syn and all the compounds undergo thermal extrusion of Ph3PO to give the corresponding alkynes. In some cases there is also competing loss of Ph3P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction.

Cycloaddition of 3,4-dibromofuran with azo diesters proceeds by a Diels–Alder reaction followed by a novel rearrangement to give 3,5-dibromotetrahydropyridazin-4-ones. Variable-temperature NMR spectroscopy shows four separate conformations at low temperature attributed to restricted rotation about the carbamate functions. The ethyl compound decomposes upon storage with loss of the carbamate groups and aromatisation to give a simple bromohydroxypyridazinium salt.

•Four different bromination products can be prepared without using CCl4.•Benzotrifluoride is a suitable CCl4 replacement for radical bromination.•The X-ray structure of a new dibromination product is presented.•The target dibromide is converted to sulfur-containing quinone derivatives.The NBS bromination of 1,4-dimethoxy-2,3-dimethylbenzene has been examined under a variety of conditions in both 1,1,1-trichloroethane and benzotrifluoride. Four different bromination products have been isolated including the previously unknown 1-bromo-4-bromomethyl-2,5-dimethoxy-3-methylbenzene whose single crystal X-ray structure is presented. The synthetically useful 2,3-bis(bromomethyl)-1,4-dimethoxybenzene is readily prepared using either solvent and it has been converted into new sulfur-containing quinone derivatives.

•Both 1H and 13C NMR spectra confirm restricted rotation about the amide bond.•Spectra obtained in CDCl3 do not agree with published data which seems to be in error.•Variable temperature study in CD3SOCD3 gives a barrier to rotation of 75.6 kJ mol−1.N,N-bis(2-hydroxyethyl)acetamide shows restricted rotation about the amide bond in both 1H and 13C NMR spectra rendering the two hydroxyethyl groups non-equivalent. A variable temperature study in CD3SOCD3 allowed estimation of the free energy barrier to rotation as 75.6 ± 0.2 kJ mol−1. Previously published data in CDCl3 appears to be erroneous.

Treatment of 2- and 3-thienyloxazolines with butyllithium and bis(trimethylsilyl) peroxide results in ring hydroxylation to give products that exist mainly as the oxazolidinylidenethiophenones. The 3-oxazolidinylidenethiophen-2-one is a rare example of a stable heterocyclic o-quinone methide analogue that shows a varied pattern of reactivity, including both C- and O-alkylation, Michael addition via C-5 to an acetylenic ester, tetrachlorobenzannulation across positions 4 and 5, and formation of a hexacyclic fused-ring product with N-phenyltriazolinedione. Crystal structures of the products are dominated by inter- and intramolecular NH to CO hydrogen bonding.

A range of halogenated methylthiophenes undergo dehalogenative condensation upon FVP over magnesium or calcium to give products including benzodithiophenes; in some cases this involves a novel rearrangement by equilibration of alkenylthienyl radicals via thiophene-fused cyclopropylmethyl intermediates.

2,4-Dibromothiazole [orthorhombic, a = 6.700 (10), b = 16.21 (3), c = 5.516 (8) Å, space group Fmm2] shows a disordered crystal structure with molecules randomly oriented with respect to the direction of the Br–C(2)–N–C(2)–Br unit and the remaining ring atoms S(3) and C(3)H showing mixed occupancy. 2,4-Diacetyl-5-bromothiazole [triclinic, a = 4.040 (2), b = 8.254 (5), c = 13.208 (8) Å, α = 96.191 (17), β = 93.865 (16), γ = 94.067 (11)°, space group P-1] shows a structure dominated by halogen bonding, intramolecular between 4-COMe and 5-Br and intermolecular between 2-COMe of one molecule and 5-Br of the next.

N-(Acetylamino)phthalimide [monoclinic, a = 7.0228(15), b = 4.7285(8), c = 28.882(6) Å, β = 90.131(7)°, space group P21/c] shows an unusual crystal structure with marked pyramidalization at the phthalimide N and strong intermolecular NH to CO hydrogen bonding leading to the groups at the two ends of the hydrazine adopting an almost perpendicular arrangement. A survey of previous triacyl and tetraacyl hydrazine structures shows that both these features are unusual, with most examples being essentially planar at N and with torsion angles about the N–N bond of 63°–82°.

The fundamental heterocyclic compound 1,4-oxazine has been generated using FVP. It is the first parent heterocycle among all the possible isomeric oxazines, thiazines and their heavier atom analogues to be characterised spectroscopically, and is shown to exist in solution entirely as the 4H-isomer. X-ray structure determination of the N-Boc precursor shows significant deviations from theoretically predicted geometric parameters.

5-Chlorosalicylaldehyde [monoclinic, a = 3.800(2), b = 5.595(4), c = 15.174(11) Å, β = 93.221(17)°, space group P21] exhibits an unusual crystal structure with both intramolecular and intermolecular OH to CH=O hydrogen bonding, the latter leading to chains of molecules aligned along the b axis. A survey of previous substituted salicylaldehyde structures shows three main hydrogen bonding patterns: intramolecular only (14), intramolecular and intermolecular dimeric (5), and intramolecular and intermolecular polymeric (3).

While thermolysis of thiocarbonyl-stabilised phosphonium ylides generally results in extrusion of Ph3PS to give alkynes, those with a PCH function instead undergo a novel P to S transfer of a phenyl group to give (Z)-configured 1-phosphino-2-sulfenylalkenes of interest as bidentate ligands.

Flash vacuum pyrolysis of appropriate stabilised ylides, prepared in a few steps from 2-(methylthio)nicotinic acid, give products containing previously unknown naphtho-, phenanthro-, benzothieno- and benzofuro-fused thieno[2,3-b]pyridine ring systems.

Flash vacuum pyrolysis over a bed of freshly sublimed magnesium on glass wool results in efficient coupling of benzyl halides to give the corresponding bibenzyls. Where an ortho halogen substituent is present further dehalogenation gives some dihydroanthracene and anthracene. Efficient coupling is also observed for halomethylnaphthalenes and halodiphenylmethanes while chlorotriphenylmethane gives 4,4′-bis(diphenylmethyl)biphenyl. By using α,α′-dihalo-o-xylenes, benzocyclobutenes are obtained in good yield, while the isomeric α,α′-dihalo-p-xylenes give a range of high thermal stability polymers by polymerisation of the initially formed p-xylylenes. Other haloalkylbenzenes undergo largely dehydrohalogenation where this is possible, in some cases resulting in cyclisation. Deoxygenation is also observed with haloalkyl phenyl ketones to give phenylalkynes as well as other products. With simple alkyl halides there is efficient elimination of HCl or HBr to give alkenes. For aliphatic dihalides this also occurs to give dienes but there is also cyclisation to give cycloalkanes and dehalogenation with hydrogen atom transfer to give alkenes in some cases. For 5-bromopent-1-ene the products are those expected from a radical pathway but for 6-bromohex-1-ene they are clearly not. For 2,2-dichloropropane and 1,1-dichloropropane elimination of HCl occurs but for 1,1-dichlorobutane, -pentane and -hexane partial hydrolysis followed by elimination of HCl gives E,E-, E,Z- and Z,Z- isomers of the dialk-1-enyl ethers and fully assigned 13C NMR data are presented for these. With 6-chlorohex-1-yne and 7-chlorohept-1-yne there is cyclisation to give methylenecycloalkanes and -cycloalkynes. The behaviour of 1,2-dibromocyclohexane and 1,2-dichlorocyclooctane under these conditions is also examined. Various pieces of evidence are presented that suggest that these processes do not involve generation of free gas-phase radicals but rather surface-adsorbed organometallic species.

3-Methoxythiophene-2-methanol 2 undergoes condensation with elimination of water and formaldehyde under acidic conditions to give bis(3-methoxy-2-thienyl)methane 3 and a further product resulting from reaction of 3 with the thienylmethyl carbocation; the other electron-rich thiophenemethanols 8 and 10 react similarly.

While thermolysis of thiocarbonyl-stabilised phosphonium ylides generally results in extrusion of Ph3PS to give alkynes, those with a PCH function instead undergo a novel P to S transfer of a phenyl group to give (Z)-configured 1-phosphino-2-sulfenylalkenes of interest as bidentate ligands.

The fundamental heterocyclic compound 1,4-oxazine has been generated using FVP. It is the first parent heterocycle among all the possible isomeric oxazines, thiazines and their heavier atom analogues to be characterised spectroscopically, and is shown to exist in solution entirely as the 4H-isomer. X-ray structure determination of the N-Boc precursor shows significant deviations from theoretically predicted geometric parameters.

A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2-dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. The X-ray structures confirm the PC and CO functions to be syn and all the compounds undergo thermal extrusion of Ph3PO to give the corresponding alkynes. In some cases there is also competing loss of Ph3P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction.