π-Bond screening in benzonorbornadienes: The role of 7-Substituents in governing the facial selectivity for the Diels-Alder reaction of Benzonorbornadienes with 3,6-Di(2-pyridyl)-s-Tetrazine

Benzonorbornadiene 21, 7-spirocyclopropylbenzonorbornadiene 23, 7,7- dimethylbenzonorbornadiene 25, and 7-spirocyclopentylbenzonorbornadiene 27 have been reacted with 3,6-di(2-pyridyl)-s-tetrazine (rate: 21>23>25=27) to form symmetrical 4,5- dihydropyridazines which are stable towards fragmentation but rearrange with varying facility to their 1,4 isomers. The facial selectivity of attack on the π-bond changes from exo-attack for 21 and 23 to endo-attack for 25 and 27. The 7-spirocyclopropyl benzonorbornadiene 23 typically forms a mixture of dihydropyridazines with exo-stereochemistry, which undergo further stereochemical isomerisation to an exo-fused product upon acetylation (acetyl chloride in hot pyridine). Oxidation with DDQ of the dihydropyridazines individually or as mixtures gives the corresponding fused 3,6-di(2-pyridyl) pyridazines

The synthesis of O-bridged laticyclic polynorbornadienes

The synthesis of the parent oxabridged laticyclic diene, COC-[3]isopolynorbornadiene 5 and the bis(oxabridged) laticyclic diene, CO2C-[4]isopolynorbornadiene 7 have each been achieved in two steps from known starting materials; the hexachloro-derivatives of 5 and 7 are also reported

Ab initio and semiempirical modelling of stereoselectivities of diels-alder cycloadditions of furan and cyclopentadiene with norbornenes

Ab initio (RHF/3-21G and RHF/6* basis sets) and semiempirical (AM1 and PM3) quantum chemical calculations have been applied to a study of Diels-Alder reactions of furan and cyclopentadiene as 1,3-dienes, with norbornene and 7-oxanorbornene as dienophiles, in order to model pi-facial selectivities and stereoselectivities of these reactions. To achieve this goal, transition states for these reactions are located and activation energies estimated. The exclusive exo pi-facial selectivity exhibited in these cycloadditions are readily predicted using semiempirical or ab initio ethods. To determine stereochemical outcomes following pi-facial attack, ab initio calculations are required. Exo,endo stereoselectivities found experimentally, are successfully predicted by using RHF/6-31G*//3-21G or higher levels of calculations. Secondary orbital interactions are postulated to be responsible for the experimental stereoselectivities

Pyramidalized olefins : a DFT study of the homosesquinorbornene and sesquibicyclo[2.2.2]octene nuclei

Density functional theory (B3LYPl6-31G*) was used to study a series of homosesquinorbornenes and sesquibicyclo[2.2.2loctenes. The compounds in which the two faces of the double bond are different are predicted to have a pyramidal double bond with butterfly bendings (v) ranging from 1.8 to 17.9". The degree of pyramidalization of these central double bonds is greater in the homosesquinorbornenes than in the sesquibicyclo[2.2.2loctenes

The trapping of 6,6-dimethylisobenzofulvene by its 1,3-dipolar precursor : a rare example of dipolar[6 4] cycloaddition

A new route to isobenzofuran and isobenzofulvene is reported that is proposed to involve the 14e electrocyclic fragmentation of a transient 1,3-dipolar intermediate formed by ring-opening of a fused aziridinocyclobutane. 6,6-Dimethylisobenzofulvene generated in this way reacts with its 1,3-dipole precursor to form a [10pi + 4pi] cycloadduct, the first of this type involving the participation of a 1,3-dipolar species

The photolysis of fused cyclobutanotriazolines: Part 2 .n-2'-phthalimidoethyl and n-phenacyl 1,2,3-triazolines

Novel photochemistry of cyclobutanotriazolines with N-2’-phthalimido-ethyl and N-phenacyl substituents is reported. The main reaction path is fragmentation to an alkene and a triazole, while formation of aziridines by loss of dinitrogen is a minor reaction

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

Molecular systems that can enclose space in a controlled fashion are receiving active attention in the current literature.2 Noncovalent interactions and self-assembly protocols are emerging as alternatives superior to assemblies largely prepared by covalent synthesis. In particular, capsule systems that reversibly form have been predominantly based on hydrogen bonds. These include Rebeks' "sportsballs" which stem from the glycoluril building block; urea-functionalizedcalixarene dimers reported independently by Rebek, Bohmer, Reinhouldt and de Mendoza;2 and giant sperical arrays constructed from calizarene units.3 Metal ion coordination has also been explored as a means of host assembly and has been reviewed.

Current status of synthesis of deprotected tridents by substituent variations of aza-ACE reaction

The N-bridged [n]polynorbornanes are a new class of N-bridged norbornanes that have become available using cycloaddition protocols developed by our group. [1] The basis for the synthesis is a dipolar cycloaddition reaction involving benzonorbornadienes as the dipolarophile and the 1,3-dipole formed by the ring-opening of a 5-azabicyclo[2.1.0]pentane. This reaction (the aza-ACE reaction [2]) requires that the aziridine be N-substituted thereby leading to the corresponding N-substituted polynorbornane (the trident). There are limitations on the type of substituent that can be introduced into the trident (eg the CBZ groups fail) and it has been necessary to find alternative groups that can be removed from the trident if the NH-bridged systems are to be accessed. This paper presents the results of our recent endeavours toward this goal of preparing NH-bridged tridents, especially those containing more than one NH-bridge

A building BLOCK approach to bis-porphyrin cavity systems with convergent and divergent wall orientations

Porphyrin containing molecular building blocks have been linked together using s-tetazine or 1,3-dipolar coupling protocols to yield bis-porphyrin cavities of defined shape and size which were evaluated by molecular modelling

Preparation of the first isobenzofuran containing two ring nitrogens : a new diels-alder diene for the synthesis of molecular scaffolds containing one or more end-fused 3,6-di(2-pyridyl)pyridazine ligands

Preparation of a stable, crystalline isobenzofuran containing two ring-nitrogen atoms, 4,7-di(2-pyridyl)-5,6-diazaisobenzofuran (diaza-IBF) (12), is reported here for the first time. Diaza-IBF was prepared using the s-tetrazine-induced fragmentation of 1,4-di(2pyridyl)-5,8-epoxy-5,8-dihydrophthalazine (9), for which a synthesis is provided. Diaza-IBF was also prepared by flash vacuum pyrolysis (FVP) of the isomeric N-methyl-1,4-di(2-pyridyl)-5,8-epoxy-5,6,7,8-tetrahydrophthalazine-6,7-dicarboximides (18) and (19), formed in three steps from furan, N-methyl maleimide, and 3,6-di(2-pyridyl)-s-tetrazine. Diaza-IBF was employed in cycloaddition protocols with alkenes to form scaffold mono- or bis-3,6-di-(2pryidyl)-pyridazine ligands having novel molecular architectures