John M. Mellor

Stereochemistry of the Diels–Alder reaction: steric effects of the dienophile on endo-selectivity

Structures are assigned to adducts of cyclopentadiene with αβ-unsaturated nitriles, ketones, and esters. Product ratios are determined for series of dienophiles which differ in their position of alkyl substitution relative to the double bond. In each series increased size of the alkyl group enhances the endo-selectivity of that group. This effect is attributed to non-bonding repulsive interactions in the transition state.

Stereochemistry of the Diels–Alder reaction: effect of diene structure on endo-selectivity

Structures are assigned to adducts of cyclohexa-1,3-diene and spiro[2,4]hepta-1,3-diene with a number of dienophiles. Product ratios are determined, results are compared with reactions of cyclopentadiene, and it is concluded that the major steric interaction which controls endo-selectivity is that between substituents of the dienophile and the methylene hydrogens of cyclopentadiene and of cyclohexa-1,3-diene, and the methylene group of spiro[2,4]hepta-1,3-diene.

Stereochemistry of the Diels–Alder reaction: reaction of dienophiles with cyclopentadiene and methylcyclopentadiene

Structures are assigned to adducts of 1-methylcyclopentadiene and 2-methylcyclopentadiene with acrylonitrile and methyl acrylate. Reactions with 1-methylcyclopentadiene are highly regiospecific but 2-methylcyclopentadiene has lower regiospecificity. Methyl acrylate shows similar endo-selectivity with cyclopentadiene and methylcyclopentadienes but acrylonitrile shows little selectivity and in one case preferentially gives an exo-adduct. Effects of solvent on regiospecificity and endo-selectivity are examined. The relative importance of endo-selectivity of steric effects, secondary orbital overlap, and electronic effects are discussed. The importance of electronic effects are illustrated in additions of cyclopentadiene to 2-arylacrylic acids.

The chemistry of pyridine and its derivatives

Publisher Summary In this chapter the compounds of pyridine including reduced pyridines are covered for the period from 1985 into 1996, but inevitably the great level of activity is witnessed in the large number of publications. In the period there have been several hundred reviews of aspects of the chemistry of pyridine and its derivatives are covered. The different aspects of molecular recognition and many macro cyclic systems incorporating pyridine rings are investigated. Pyridinophanes are similarly much studied and their chemistry is reviewed. The literature of naturally occurring alkaloids is particularly described by regular updates in the natural product reports. The biological importance of pyridoxal and nicotinamide derivatives has resulted in vast biochemical literature of these compounds. Nicotinamides are reviewed generally and not from the standpoint of molecular biology. The commercial value of pyridines in other areas is highlighted. The topic of hydrogenated pyridine derivatives is presented.

Elaboration of sclareol by Claisen rearrangement

Transetherification of sclareol (labd-14-ene-8,13-diol)(1) with triethyl orthoacetate and subsequent rearrangement gave (E)- and (Z)-ethyl 8-hydroxylabd-13-en-15-ylacetate (2) and (3)(R = CO2Et). Products of cyclisation of these esters with tin(IV) chloride in benzene were characterised. Similar reactions with sclareol monoacetate (11) are described and stereochemical implications are discussed.

Dehydration of some 9-methyl-9-hydroxybicyclo[3.3.1]nona-3,7-dien-2-ones

Dehydration of 9-hydroxy-4,8,9-trimethylbicyclo[3.3.1]nona-3,7-dien-2-one (3) by toluene-p-sulphonic acid in benzene gives a trimethylbicyclo[4.3.0]nonatrienone (7) by rearrangement. Dehydration of 9-hydroxy-4,8,9-trimethyl-6-phenylbicyclo[3.3.1]nona-3,7-dien-2-one (4) similarly gives a bicyclo[4.3.0]nonatrienone (6) and also the nonrearranged 9-methylenebicyclo[3.3.1]nonadienone (5). The mechanism of rearrangement is discussed and contrasted with the behaviour in acid of other hydroxybicyclo[3.3.1]nonadienones.

Influence of a proximate 1,3-diene upon the photoreactivity of some αβ-unsaturated ketones

Bicyclo[3.3.1]nona-3,7-dien-2-ones (7)–(10) and the bicyclo[3.3.1]nona-3,6-dien-2-one (11) were synthesised and irradiated. Ketones (7), (10), and (11) were photostable and (8) and (9) only underwent equilibration by geometrical photoisomerisation. This ‘free rotor’ effect by the diene system dissipates energy and prevents other photorearrangements.

Mechanism of photorearrangement of bicyclo[3.3.1]nona-3,7-diene-2,6-diones

Irradiation of some 9-arylbicyclo[3.3.1]nona-3,7-diene-2,6-diones gives triasteranediones by successive 1,2-acyl migrations and 6-arylbicyclo[3.3.1]nona-3,7-diene-2,9-diones by 1,3-rearrangement. Emission, photosensitisation, and quenching studies show that both photorearrangements are triplet-derived, and these results are contrasted with other studies of βγ-unsaturated ketones.

Mechanism of photorearrangement of 6-hydroxybicyclo[3.3.1]nona-3,7-dien-2-ones

The 6-hydroxybicyclo[3.3.1]nona-3,7-dien-2-ones (1) and (2) have been synthesised and irradiated. The alcohol (2) gave only isomeric 9-hydroxy-4,8,9-trimethyl-6-phenylbicyclo[3·3.1]nona-3,7-dien-2-one (6) by 1,3-rearrangement, but (1) gave, in addition to compound (7) formed by similar 1,3-rearrangement, products of dehydration and of further rearrangement. The origin of each product is established and the observed rearrangements are contrasted with the different rearrangements found with bicyclo[3·3.1]nona-3,7-diene-2,6-diones. The differences are rationalised.

Mechanism of decarboxylation of bicyclic acids by lead tetra-acetate

Oxidative decarboxylation of bicyclo[3.2.1]octane-2-carboxylic acids and bicyclo[2.2.2]octane-2-carboxylic acid by lead tetra-acetate gives mainly acetates by both a carbonium ion and a non-carbonium ion pathway. Analysis of products in the decarboxylation of these and other acids indicates the intermediacy of organolead intermediates, which in the non-carbonium ion route give acetates with retention of stereochemistry. This duality of mechanism is used to explain product distributions reported in earlier studies of related carboxylic acids.