The Stability of Organic Compounds

Abstract

Scheme 3.2). Reversible reactions are, for this reason, not usually suitable for the formation of strained bonds.Strained cyclohexenes, such as norbornene derivatives, can undergo retro-Diels-Alder reactions even at relatively low temperatures, and this reaction can be used to prepare 1,3-dienes and alkenes (e.g. synthesis of cyclopentadiene by thermolysis of 36 Scheme 3.1. Thermal decomposition of strained compounds [1-8]. Strained Bondsdicyclopentadiene at approximately 160 C). In the example shown in Scheme 3.3 (first reaction) an enantiomerically pure bicyclic cyclohexanone is prepared from a readily available natural product. The radical scavenger is added to the starting material to prevent oxidation.The second example in Scheme 3.3 illustrates the reversibility of aldol additions. The starting bicyclic ketone is a vinylogous aldol which upon treatment with base undergoes retro-aldol addition by cleavage of a strained, hexasubstituted ethane sub-37 Scheme 3.2. Fragmentation via retro-Diels-Alder cycloaddition and retro-aldol addition. Scheme 3.3. Retro-Diels-Alder [9], retro-aldol [10] and retro-Mannich reaction [11] of strained substrates. 39 Scheme 3.5. Dealkylative oxidation of dihydropyridines [16] and reductive cleavage of cyclobutanes [17]. Scheme 3.6. Oxidation of a strained alkene by air [20] and relative rates of epoxidation of various cyclic alkenes [21]. 42 Scheme 3.10. Facile nucleophilic substitutions at 4-hydroxybenzyl derivatives [31-33].Scheme 3.11. Rearrangement of a-amino ketones [34].