Ring structure intermolecular cycloadditions

It has long been recognized that nitrone cycloadditions may allow access to spirocyclic ring systems. Such systems are inherently difficult to synthesize by conventional methods, yet are a structural component of a number of biologically active natural materials. Two common strategies have emerged for spirocycle generation from exocyclic or endocyclic nitrones (Scheme 1.45). In the exocyclic version, the carbon atom (arrowed) of the nitrone C=N double bond of dipole 209 carries a cyclic substitutent and thus an intermolecular cycloaddition reaction will... 

Cross-enyne metathesis has also been applied to solid-phase synthesis by immobilizing either the alkyne or the alkene (498,499). By combination with a Diels-Alder reaction, this process has been used to create a combinatorial library of various six-membered ring structures (500). Within this theme, intermolecular enyne metathesis is the integral synthetic method used to produce novel amino acid derivatives (501, 502). To this end, the aromatic ring of highly substituted phenylalanines was synthesized through enyne crossmetathesis and cycloaddition. 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate) [131, 132], They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffmann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO (HOMO) of the grotmd state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the cmicerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2 [132]. 

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