Hemiacetal isomers

Mustafa, K., Perry, N.B., and Weavers, R.T., 2-Hydroxyflavanones from Leptospermum polyga-lifolium subsp. polygalifolium. Equilibrating sets of hemiacetal isomers. Phytochemistry, 64, 1285, 2003. 

This behavior of the aldehydo sugars is best interpreted as an equilibrium established between anomeric hemiacetal isomers, the solvent, and the aldehydo form. 

However, other structural features can provide a thermodynamic selection of one stereoisomer. Intramolecular aldolization of 2,2 -0-methylene-bis-D-glycerose (109) under weakly basic conditions affords only two of the four possible aldols, (110) and (111) (Scheme 4). The (110) (111) ratio greatly favors the former, and conditions were found under which only isomer (110) is produced. It was proposed that the aldol cyclization is under thermodynamic control, and that the initial products are trapped as their intramolecular hemiacetals (112) and (113). The other two possible aldols cannot form such hemiacetals. Isomer (112) is presumably favored over (113) because of the normal equatorial preference of a hydroxy group that is hydrogen-bonded to water. 

Moreover, compounds of type 7-127, which were obtained from 7-125 by reaction with Pb(OAc)4, can undergo a further domino process when treated with potassium carbonate in a mixture of water and methanol [57]. This includes saponification of the acetate moieties in 7-127 to provide 7-129 via the unstable cyclic hemiacetal 7-128 (Scheme 7.35). Retro-Claisen reaction and ring closure with the proposed intermediates 7-130 and 7-131 led to the bridged ring-system 7-132 as a mixture of di-astereomers with preference of the (3-isomer. 

Three possible mechanistic schemes can be suggested for this process. One involves elimination of the proton attached to the p-C atom of nitronate A or A followed by elimination of the OSi group from the intermediate anion (cf. Scheme 3.93). Another mechanism is associated with a 1,4-C,O-transfer of the proton from the p-C atom of nitronate A to the oxygen atom of the N—>0 fragment followed by elimination of silanol from hemiacetal B. The third mechanism is based on the concerted elimination of silanol from the minor cis isomer of SENA. 

The hemiacetal pheromone was first prepared in low yield as a mixture of isomers (Scheme 20) [108]. Thus, oxidative coupling of the dianion of ( )-3-hexenoic acid 127 and LiAlH4 reduction of the resulting diacid produced a mixture of diols from which the EyE-meso isomer 128 could be separated. The meso... 

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