Acetal rings

Example 2.6 The bobbin shown in Fig. 2.16 has been manufactured by sliding the acetal ring on to the steel inner and then placing the end-plate in position. At 20°C there are no stresses in the acetal and the distance between the metal end-plates is equal to the length of the acetal ring. If the whole assembly is heated to 1(X)°C, calculate the axial stress in the acetal. It may be assumed that there is no friction between the acetal and the steel. The coefficients of thermal expansion for the acetal and the steel are 80 x 10 °C and 11 X 10 °C respectively. The modulus of the acetal at 100°C is 1.5 GN/m. ... 

If the acetal ring is considered as a thick wall cylinder, then at the inner surface there will be hoop stresses and radial stresses if it is constrained in a uniform manner ... 

Systems with substituents on the aeelal ring254 or with larger acetal rings may give double ring-opening (e.g. Scheme 4.39).202,203... 

PVA is used in the treatment of textiles and paper. PVA also acts as the starting material for the synthesis of a number of poly(vinyl acetals) with the general structure as given in Equation 6.66. The acetal rings on these random amorphous polymer chains restrict flexibility and increase the heat deflection temperature to a value higher than that of PVAc. 

Alkyl (or acyl) derivatives of the 6-amino-6-deoxy carbohydrates are examples of derivatives in which the hydrophilic and hydrophobic moieties are linked at other positions than C-1. Thus 6-amino-6-deoxy-D-galactose derivatives 34 were prepared from l,2 3,4-di-0-isopropylidene-6-0-tosyl-a-D-galacto-pyranose by the following reactions (1) substitution of the leaving group at C-6 by a phthaloyl function, (2) hydrazinolysis to afford a 6-amino-6-deoxy intermediate, (3) reaction of acyl or sulfonyl chlorides at the amino function, (4) deprotection of the acetal rings to afford the expected glycolipid 34 [56]. 

If the benzylidene acetal ring spans the oxygen atoms of two erstwhile secondary hydroxyl groups, the reaction with N-bromosuc-cinimide usually affords isomeric, bromodeoxy sugar benzoates. Thus, for example, the reaction with methyl 2,3-0-benzylidene-5-0-methyl-/3-D-ribofuranoside (106) afforded145 an approximately 1 1 mixture of the bromides 109 and 108, presumably by way of attack on C-2 and C-3 of the benzoxonium intermediate 107 by bromide ion. 

If a participating group, such as an ester or hydroxyl group, is present near to the acetal ring, a rearrangement of the benzoxonium intermediate can occur the rearranged ion can then be attacked intra-molecularly or externally by a nucleophile.145... 

The 3,4-diols thus obtained are closely related to tetroses, but no direct correlation with natural compounds has been made, apparently because of the unusual stability of the acetal ring towards acid hydrolysis.191 A number of studies have been devoted to alternative function-... 

Hydrolytic studies on 1,2.4,5-di-O-isopropylidene-D-mannitol showed56 that the two acetal rings are cleaved at similar rates, suggesting for the relative stabilities of the two rings, a = a-erythro. 

An interesting distinction between two isopropylidene a-acetal rings is provided by the acid hydrolysis of 5-0-benzoyl-4-C-(benzoyl-oxymethyl)-l,2 3,4-di-0-isopropylidene-L-ribitol (8), in which the... 

The use of complex radical donors and radical acceptors, such as carbohydrates tethered by an acetal-containing linkage, has been reported by Sinay [98]. Here, the radical addition of an anomeric radical to a 4-exo-methylene sugar derivative (compound 69) proceeds in the 8-endo mode, with formation of a eight-membered acetal ring. The end product of this reaction, after removal of the tether and acetylation, is the C-disaccharide 70 (Scheme 27). 

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