A definitive investigation of the reaction of cyclopentadiene with cis-3,4-dichlorocyclobutene: A literature correction

Structural assignments are made to the three adducts derived from the title reaction. ¹H-n.m.r., ¹³C-n.m.r. and euroshift data are used to support the new structures.     

The application of C-N.M.R. spectroscopy to the determination of the stereochemistry of Diels-Alder adducts

Long-range (³J) ¹³C-H coupling is a reliable probe to evaluate the stereochemistry of cycloaddition products. The method is best applied to carbonyl containing adducts but not restricted to them. Several structures have been revised and new ones evaluated.     

Isoindole Cycloadditions. Part III: The Synthesis of "Windscreen Wiper" and Other N-Bridged Cavity Systems

The thermal addition of N-carbobenzyloxyisoindole (N-Z isoindole) 11a, generated by the reaction of 3,6-di(2-pyridyl)-s-tetrazine 9 with N-Z 7-azabenzonorbornadiene 8a, onto dimethyl tricyclo[4.2.1.0^2.5]nona-3,7-diene-3,4-dicarboxylate 17 occurred site selectively at the cyclobutene -bond to form a stereoisomeric mixture of 1 : 1-adducts 18 and 19, in which the bent-frame isomer 19 was dominant (ratio 5 : 1). In contrast, N-benzyl tetrafluoroisoindole 11c reacted with 17 only under high-pressure conditions (14 kbar, RT, 4 days) to afford 1 : 1-adducts at the cyclobutene site, in which the extended-frame isomer 18c was dominant and the accompanying bent-frame product 19c reverted to starting materials soon after isolation. These same stereoselectivities were used to prepare "windscreen wiper" compound 28c having two mobile N-benzyl substituents attached to a rigid scaffold by the reaction of N-benzyl tetrafluoroisoindole 11c with tetramethyl tetracyclo[4.4.1.0.^2,5.0^7.10]undeca-3,8-diene-3,4,7,8-tetracarboxylate 23. Cavity bis-(cyclobutene-1,2-diester) 6 reacted with N-benzyl tetrafluoroisoindole 11c twice over to produce cavity structure 36 with two O- and two N-benzyl bridges on the inner face, whereas the narrower cavity bis-alkene 32 stopped at the 1 : 1-addition stage. The dynamics of the Z-group in the dual adducts 26a–28a are discussed briefly and key adducts and cavity systems have been structurally evaluated by X-ray crystallography, VT NMR, and molecular modeling.     

A DFT study of pyramidalized alkenes: 7-oxasesquinorbornenes and 7,7′-dioxasesquinorbornenes

  DFT calculations of 7′–oxasesquinorbornenes and 7,7′-dioxasesquinorbornenes using the B3LYP/6–31G* method are reported. All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on the molecular and electronic structures of these molecules have also been investigated. Received: 11 November 2002 / Accepted: 6 June 2002 / Published online: 29 April 2003     

A Photochemical Route to Dibenzobarrelenes

Dibenzobarrelenes have been prepared by the cycloaddition of benzyne to anthracenes. The first method gives complex mixtures from which only poor yields of 1, 4-adducts may be obtained,¹ exceot wgeb tetragakivebztbes are enokited,^2,3 Muller and Vergne have reported⁴ that dibenzobarrelene is formed by the reaction of acetylene with anthracene, but it may be significant that later workers have chosen to use either the above-mentioned benzyne route or a modification of the cycloaddition to anthracenne involving dichlorethylene followed by dechlorination to prepare the parent compound.⁵ Our approach to the synthesis of this ring system employs our previously described 1,2–photoaramatisation reactions.⁶ A significant feature of the present work is that it demonstrates that a photochemical reaction can be employed to generate compounds, in this case dibenzobarrelenes, known to be photolabile.⁷     

Rack-mounted chemistry IV: the thermal rearrangement of 7-oxabenzonorbornadienes into benzo[b]oxepines on and off the polycyclic rack

The thermal rearrangement of [3]polynorbornane bis-imide rack-mounted 7-oxabenzonorbornadienes has been conducted using flash vacuum pyrolysis (FVP) at 520 °C and is compared with the FVP of similar 7-oxabenzonorbornadienes off the rack. The isomerisation is considered to involve (a) C–O bond cleavage to a vinylogous 1,5-dipole, (b) formation of a benzene epoxide by nucleophilic ring-closure and (c) valence-isomerisation of the benzene epoxide to the oxepine. Competing fragmentation to the isobenzofuran by ejection of acetylene and other rearrangements become prominent pathways off the rack, whereas isomerisation to the oxepine is highly favored on the rack.     

Porphyrin-containing molecular capsules: metal mediated dimerization of a bis-porphyrin cavity

[reaction] A self-complementary V-shaped bis-porphyrin cavity has been synthesized that is capable of dimerization to form a capsule structure. Self-assembly of the dimer occurs via metal ion coordination and produces an internal volume for guest encapsulation.     

Allylic vs homoallylic control of stereospecificity in the epoxidation of 3(1′-hydroxyethyl)-5,8-dimethoxy-1,2-dihydronaphthalen-1-ol: implications for the synthesis of chiral anthracyclines

The stereochemistry of the epcxidation of the title compound with t-BuOOH catalysed by VO(acac)₂ is subject to exclusive homoallylic control. Secondary allylic alcohols in the side chain regain their normal controlling influence over diastereoselection only when the homoallylic group is blocked.