Peri selectivity

Styrene, leads to [4 + 2]-adducts exclusively (251-258) 242 is also isolated in varying amounts, With methyl sorbate both [4 + 2]- and [4 + 4]-adducts (259-262) are obtained. 1,4-Dicarbomethoxybutadiene does not add to 138 242 is obtained in 90% yield. All photoadducts (251-262) are photolabile on irradiation (z — 253.7 nm) they are reconverted to 138 and the alkene. Significantly for preparative purposes, the product distributions (stereo-, regio-, and peri-selectivity) differ considerably from those obtained in the thermal Diels-Alder reactions (Section IV,B) mechanistic details are given in the original papers. 

Since these reactions are inverse electron demand cycloadditions, the relevant frontier orbital interactions are between the LUMO of the diene and the HOMO of the dienophile. The accelerating effect of a Lewis acid can be understood since complexation of the nitroalkene should decrease the HOMO—LUMO gap by lowering the LUMO of the diene. Thus, the peri-selectivity of the reaction is controlled by the complexation of the nitroalkene to the Lewis acid. 

Presset M, Coquerel Y, Rodriguez J. Peri selectivity switch of acylketenes in cycloadditions with 1-azadienes microwave-assisted diastereoselective domino three-component synthesis of a-spiro-8-lactams. Org. Lett. 2010 12 4212- 215. 

Shimo, X, Iwakiri, T. and Somekawa, K., Peri-selective photocycloadditions of methyl 2-pyrone-5-carboxylate with unsaturated cyclic ethers, J. Heterocyclic Chem., 29, 199, 1992. 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

Amongst the secretions of specialised exocrine complexes, the ancillary products which act as sticky compounds are large, often proteinaceous, molecules. Their primary, secondary and tertiary structures being inherently complex are now seen as ideal informational vehicles — alone or in combination with volatile molecules. Much recent work (Sec. 3.2, below) has identified them as the key components involved in close range transmission, and in intra-nasal peri-receptor events. Proteins are semiochemically implicated when their selective removal or presentation alters responsiveness (Belcher et al., 1990 Mucignat-Caretta et al, 1995). 

Even more pronounced steric effects have been observed for the free radical alkylation of protonated N-heterocyclic bases by the procedure of Minisci69, b d. Quinoline is attacked selectively in the 2- and 4-position by nucleophilic alkyl radicals in sulfuric acid. The largest radicals, t.-butyl, react exclusively in the 2-position because of steric hindrance by the peri-hydrogen when attack occurs at the 4-position. 

Urapidil is a selective ai-adrenoceptor antagonist with an additional central antihypertensive mechanism, mediated by the stimulation of serotonergic (5-HTia) receptors in the brain. It may be used in the treatment of essential, but also acute, peri-operative hypertension. The intravenous administration in the treatment of acute, peri-operative hypertension is not associated with a rise of intracranial pressure, in contrast to various other vasodilators. For this reason, urapidil may be used in neuro-surgical interventions. 

Prazosin and doxazosin are selective al blockers with similar properties. They are competitive antagonists that can be displaced from the al receptor by increases in catecholamine concentrations. This makes them unsuitable for use in the peri-operative management of phaeochromocytoma. Urapidil is an agonist at both central a2 and serotoninergic 5-HTlA receptors, and also has antagonist actions at cardiac pi adrenoceptors. 

M. Peri, Hydrophobic solvent type charged membranes for selective electrodialysis, Ph.D. thesis, The Weizmann Institute of Science, Rehovot, Israel, 1980. 

Substituted derivatives with bulky groups, however, are reduced at higher rates and with opposite selectivity on palladium [Eq. (11.18)]. This was explained by the partial release of peri strain in the transition state during hydrogenation. 

However, the reactivity of phenolic hydroxy groups can be modified by a fused heterocyclic ring. Thus, hydroxy groups peri to a carbonyl group, e.g. (600), are hydrogen bonded they do not react with diazomethane, and are difficult to acylate. This allows selective reactions in polyhydroxychromones. 

A number of electrocyclic reactions under PET conditions have been reported. In this way, A-benzyl-2.3-diphcnylaziridinc (40) underwent a 3 + 2-cycloaddition with alkene and alkyne dipolarophiles to afford substituted pyrrole cycloadducts (41) via the radical cation intermediate (42) see Scheme 7.80 Elsewhere, novel arylallenes have been used as dienophiles in a radical cation-catalysed Diels-Alder cycloaddition reaction with 1,2,3,4,5-pentafluromethylcyclopentadiene, which often occurred with peri-, chemo-, facial- and stereo-selectivity.81... 

The importance of the steric effect accounts for the spread of the data for lf-N in the substitution reactions. Nitration and non-catalytic chlorination, reactions of modest steric requirements, define points which fall above the arbitrary reference line. Bromination, a reaction of somewhat greater steric requirements, is not accelerated to the extent anticipated on the basis of the results for nitration or chlorination. The benzoylation reaction with large steric requirements is two orders of magnitude slower than the equally selective chlorination reaction. The unusually small ratio for lf-N/2f-N for the acylation reaction is a further indication of the steric effects. Apparently, the direct substitution reactions of naphthalene respond to the retarding steric influence of the peri hydrogen in much the same way as for other ortho substituents. 

If a benzenoid excised internal structure is 1-factorable, 2-factorable, strictly peri-condensed, has one or more bay regions, and/or has one or more selective... 

Simple 2-phenylphenol with less steric restriction can undergo diarylation to give a sterically congested 1,2,3-triphenylbenzene derivative (Eq. 1). 1-Naphthol is arylated at the peri position selectively (Eq. 2). Interestingly, 2-naphthol undergoes successive diarylation on treatment with bromobenzenes in the presence of PPh3... 

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