Alder rule acidity


The Diels-Alder reaction of a diene with a substituted olefinic dienophile, e.g. 2, 4, 8, or 12, can go through two geometrically different transition states. With a diene that bears a substituent as a stereochemical marker (any substituent other than hydrogen deuterium will suffice ) at C-1 (e.g. 11a) or substituents at C-1 and C-4 (e.g. 5, 6, 7), the two different transition states lead to diastereomeric products, which differ in the relative configuration at the stereogenic centers connected by the newly formed cr-bonds. The respective transition state as well as the resulting product is termed with the prefix endo or exo. For example, when cyclopentadiene 5 is treated with acrylic acid 15, the cw fo-product 16 and the exo-product 17 can be formed. Formation of the cw fo-product 16 is kinetically favored by secondary orbital interactions (endo rule or Alder rule) Under kinetically controlled conditions it is the major product, and the thermodynamically more stable cxo-product 17 is formed in minor amounts only.  [c.91]

Nonsense McBain Those words were the first reaction of the chairman of a meeting of the Royal Society of London on the lecture of McBain, wherein he suggested the existence of assemblies of surfactant molecules in aqueous solution". However, as soon became apparent, McBain was right when he postulated the spontaneous fonnation of dynamic aggregates by fatty acid salts", although the bilayer structure he suggested was later corrected by Hartley, who introduced the spherical structures we now know as micelles". In the years that have passed, micellar solutions have proven to be an extremely versatile topic of research. The catalytic potential of micellar aggregates has received special attention. This chapter will focus on micellar catalysis of a Diels-Alder reaction and will provide, for the first time, a link between micellar catalysis and Lewis-acid catalysis. Before elaborating on this, the physical properties and catalytic potential of micellar solutions will be briefly reviewed.  [c.125]

Corticosteroids exhibit a wide range of physiological effects One important func tion IS to assist m maintaining the proper electrolyte balance m body fluids They also play a vital regulatory role m the metabolism of carbohydrates and m mediating the alter gic response  [c.1098]

Silylation in Organic Synthesis. Sdyl blocking agents are used in organic synthesis to protect sensitive functional groups, to alter reactivity and solubdity, and to increase stabdity of intermediates. Sdylation appHcations in pharmaceutical synthesis have been used to protect a wide range of OH groups, eg, alcohols in prostaglandins (qv) and steroid synthesis, enols in the synthesis of nucleosides and steroids (qv), and carboxyflc acids and sulfenic acids in the synthesis of penicillins and cephalosporins (6) (see Antibiotics). Sdylation has its broadest use in the commercial synthesis of penicillins. The blocking effect of trimethylsilyl and dimethylsilyl groups on 6-aminopenicillanic acid (6-APA) (1) has played an important role in the total synthetic production of semisynthetic penicillins.  [c.71]

This interaction between airborne acid components and the tree-soil system may alter the ability of the trees to tolerate other environmental stressors such as drought, insects, and other air pollutants like ozone. In Germany, considerable attention is focused the role of ozone and acid deposition as a cause of forest damage. Forest damage is a complex problem involving the interaction of acid deposition, other air pollutants, forestry practices, and naturally occurring soil conditions.  [c.121]

Melanotropins. Two melanotropins, a-MSH and p-MSH, result from post-translational processing of POMC in the pituitary gland. The a-MSH peptide is comprised of the first 13 amino acids of ACTH, except that serine 1 is acetylated and valine 13 is amidated (Fig. 1). The cleavage of hpotropin (LPH) generates P-MSH (Fig. 1). In lower vertebrates, these hormones induce changes in melanin-containing ceUular organelles which rapidly alter body pigmentation. While the physiologic role of MSH in mammals is questionable, MSH-secreting tumors and injections of MSH cause hyperpigmentation in humans. Darkening of the skin also occurs clinically with inadequate glucocorticoid secretion, which removes the negative feedback on POMC-derived hormone production and increases MSH secretion. a-MSH acts as a neurotransmitter within the central nervous system, where it may be involved in regulation of body temperature (9). SoHd-phase synthesis is used to produce MSH for use in biomedical research. No other commercial apphcations exist.  [c.175]

Retinoic Acid and Thyroid Hormone. The steroid hormones, vitamin D, the retinoic acids (RAs), trans-KA (275), 13-or-RA (276), and 9-17 -10 (277), and thyroid hormone (TH) alter cell function by acting as ligand-controUed transactivating factors or signal transducers and activators of transcription (STATs). They selectively interact with intracellular transcription factors to alter gene expression by interaction with hormone-responsive elements (HREs) on promotor regions of DNA (49,72). RA plays a pivotal role in development and embryogenesis however, excessive doses are teratogenic. RA induces differentiation in neuronal cells in vitro and is likely to play a role in neuronal differentiation in vivo.  [c.566]

The endo exo selectivity for the Lewis acid-catalyzed carbo-Diels-Alder reaction of butadiene and acrolein deserves a special attention. The relative stability of endo over exo in the transition state accounts for the selectivity in the Diels-Alder cycloadduct. The Lewis acid induces a strong polarization of the dienophile FMOs and change their energies (see Fig. 8.2) giving rise to better interactions with the diene, and for this reason, the role of the possible secondary-orbital interaction must be considered. Another possibility is the [4 + 3] interaction suggested by Singleton  [c.308]

The final class of reactions to be considered will be the [4 + 2]-cycloaddition reaction of nitroalkenes with alkenes which in principle can be considered as an inverse electron-demand hetero-Diels-Alder reaction. Domingo et al. have studied the influence of reactant polarity on the reaction course of this type of reactions using DFT calculation in order to understand the regio- and stereoselectivity for the reaction, and the role of Lewis acid catalysis [29]. The reaction of e.g. ni-troethene 15 with an electron-rich alkene 16 can take place in four different ways and the four different transition-state structures are depicted in Fig. 8.16.  [c.320]


See pages that mention the term Alder rule acidity : [c.9]    [c.48]    [c.220]    [c.19]    [c.309]    [c.311]   
Advanced organic chemistry Ч.1 (2000) -- [ c.410 ]