Intramolecular hydrogen influence

Spiroketals based upon such structures as l,7-dioxaspiro[5.5]undecane (18), occur frequently in natural products. Accordingly, an extensive amount of literature relates to the isolation and total synthesis of this type of compound. This literature was reviewed104 in 1989. The authors of Ref. 104 listed three factors that influence conformational preferences in these systems. They are (7) steric influences, (2) anomeric and related effects, and (3) intramolecular hydrogen bonding and other chelation effects. 

Intramolecular hydrogen-bonds can increase the stability of certain conformations. For example, dianhydrides that contain fJ-L-Sorp or ct-D-Frup in the 5C2 conformation have the C-4 hydroxyl group in a 1,3-diaxial relationship with 0-2, which permits the formation of an intramolecular hydrogen bond. This might, in part, offset the destabilizing influence of three or two axial substituents, respectively. This effect is decreased in hydrogen-bonding solvents. 

However, for most acids and bases of this review, the dominating influence of the concave environment on the acid-base properties is the formation of stabilizing intramolecular hydrogen bonds. The other factors have been found only in a few cases [microacidity for [45] (van Eerden et al.,... 

As mentioned before, the calculated energies are those of isolated molecules. To decrease the influence of intramolecular hydrogen bonds formed in vacuo but which are not often found... 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

In isopropyl alcohol, the erythro-isova t is formed predominantly. An increase in the polarity of the solvents results in a shift of the diastereoselectivity to the f/ r< o-isomcr and in methanol this becomes the main product. This example offers another possible influence of the solvent. An explanation of the determined diastereoselectivies can be different conformations preferred in the various solvents. In isopropyl alcohol the intramolecular hydrogen bonds can stabilize the conformation and the attack of hydrogen can occur only with the erythro-isovaer formation (Scheme 14.13). In a polar solvent, this possibility is much diminished and therefore the isomer is preferred. 

D. F. Howard and H. G. Kjaergaard, Influence of intramolecular hydrogen bond strength on OH stretching overtones. J. Phys. Chem. A 110, 10245 10250 (2006). 

A study of substituent influences on reaction of carbonyl compound (102) with trialkylallyltin (103) has established that high yield of allylation product (104) is obtained only when X = NO2 and R = OH (where R = Bu or Me and R = H or Me). The role of the intramolecular hydrogen bond in promoting rate enhancement and regioselective allylation (and reduction by HSnBus) of carbonyl compounds has been discussed. 

The conformational equilibria of the various substituted oxanes in Table I are strongly dependent on the solvent. The polarity of the solvent and the possibility to form inter- or intramolecular hydrogen bonds are of significant influence (69CJC4427 87CJC213). 

Electron-poor allylic alcohols, studied by Adam and coworkers, were found to react r/treo-selectively, regardless of whether the hydroxy functionality is free or masked by alkyl or silyl groups (Scheme 35). This diastereoselectivity, for which the nature of the solvent had no significant influence, was attributed to stereoelectronic effects rather than intramolecular hydrogen bonding or steric strain. 

DAMN isomerizes to diaminofumaronitrile (7) under the influence of light (Scheme 2) (68MI1 68TL4529). A trace of acid causes 7 to revert to DAMN. The stereostability of DAMN over 7 is due to intramolecular hydrogen bonding since the (Z)- and ( )-iV,A/,iV, iV -tetramethyl derivatives have nearly the same energy (74JOC2341). 

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