Intramolecularly hydrogen-bonded product

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. 

Oxaziridines, the products of photochemical isomerization of polymeric nitrones, are characterized by high stability. The formation of an intramolecular hydrogen bond stabilizes the nitrone group with respect to UV-irradiation... 

X-Ray analyses of azapentalenes have been used to determine the structure of products from reactions with ambiguous reaction sites or with concomitant rearrangements. Thus, the X-ray structure of the /3-D-[l,2,4]triazolo[4,3-3][l,2,4]triazole 2 establishes that glycosylation has occurred at the N-l position furthermore, it also shows that the compound is the ft-anomer. In addition to intermolecular hydrogen bonds in the crystal structure an intramolecular hydrogen bond is extended from OH-5 to N-7 <1984NN187>. The X-ray structure of the inner salt of compound 1 has been determined... 

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. 

Several cyclopropylimines have been synthesized and their reactions with a range of nucleophiles have been investigated. Mild hydrolysis of diimine (16) produces, amongst other products, the /3-ketoimine (17), stabilized by intramolecular hydrogen bonding. 

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 second function, and the one pertinent to this section, is the decarboxylation of oxalosuccinic acid to 2-oxoglutaric acid. This is simply a biochemical example of the ready decarboxylation of a P-ketoacid, involving an intramolecular hydrogen-bonded system. This reaction could occur chemically without an enzyme, but it is known that isocitric acid, the product of the dehydrogenation, is still bound to the enzyme isocitrate dehydrogenase when decarboxylation occurs. 

Arylsulfonation of perimidines 78 (R = H or CF3) has been carried out in polyphosphoric acid and found to occur at the 6(7)- and 4(9)-positions. Separation of the product sulfones 79 and 80 was easily achieved due to their different chromatographic mobilities. The 4(9)-isomers 79 are more mobile due to the intramolecular hydrogen bond and, in low-polarity solvents, exist virtually completely as the 9-tautomer <2002CHE1084>. 

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