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The following reaction mechanism of the acetonation of l-sorbose is proposed:(Remark: Graphics omitted.)2,3:4,6-di-O-acetal (furanose) (II). The pyranose-to-furanose ring conversion (I→II) proceeds viathe keto-form intermediates Vk and VIk; VIk is formed by the preferable attack of acetonating reagents on the primary hydroxyl group at the C6of Vk followed by cyclization with the hydroxyl group at C4. The isolation of such keto-form intermediates as were successfully obtained from the acetalations of 1-O-methyl- (X) and 1,6-di-O-methyl-l-sorboses (XVIII) gave strong support to the above mechanism. This acetonation mechanism can also be used in the other acetalations of I; however, in the case of the benzylidenation there is a possibility that the reaction might be viathe pathway of 1,3-O-acetal (pyranose)\ightleftarrows1,3-O-acetal (furanose) (Vb)→1,3: 4,6-di-O-acetal (furanose).

The favored conformations of the pyranose rings of several acetonated α-l-sorbo- and β-d-fructopyranoses have been revealed by proton magnetic resonance spectroscopy. The conformations of the rings of the pyranoses are unchanged by the introduction of 1,2-O- or 1,3-O-isopropylidene groups in α-orientation of l-sorbose, but they are affected by the introduction of the 2,3-O-isopropylidene group. These findings support the fact that 2,3-O-isopropylidene-α-l-sorbopyranose was not obtained by the acetonation of l-sorbose. The rings of diacetonated d-fructopyranoses with a cis-fused acetal ring adopt a distorted conformation; however, those of diacetonated l-sorbopyranoses with a trans-fused acetal ring are only slightly distorted from the stable 1C conformation. Angyal’s anticipation of the difficult acetalation of the trans-glycol of a pyranose has been further experimentally supported.

The ethylidenation of l-sorbose (I) with acetaldehyde dimethylacetal in the presence of an acid was studied. When the reaction was interrupted at the initial stage, an equilibrium mixture of the two possible cyclic acetals, 1,2-O-(III) and 1,3-O-ethylidene-α-l-sorbopyranose (V), was obtained. When the reaction was allowed to proceed for longer periods of time, 1,3:4,6-di-O-ethylidene-β-l-sorbofuranose (IX) was mainly isolated; the transformation of this to two diastereoisomeric 2,3:4,6-di-O-ethylidene-α-l-sorbofuranoses (XV and XVIII) required the use of highly acidic conditions. A pathway of I→(III\ightleftarrowsV)→IX→(XV+XVIII) was tentatively proposed for the ethylidenation of I.

The acetonation reaction of l-sorbose with acetone dimethyl ketal was investigated. The products and the yields were greatly influenced by the reaction conditions. The reaction at 45°C gave 1, 3 : 4, 6-di-O-isopropylidene-β- (VIIa), 1, 2 : 4, 6-di-O-isopropylidene-α- (Va) and 2, 3 : 4, 6-di-O-isopropylidene-α-l-sorbofuranose (IVa) and the β-anomer of Va (VIa). At the refluxing temperature, the reaction afforded IVa and the 2-O-methyl derivative of VIIa (VIIe), while at room temperature it yielded 1, 2 : 3, 4-di-O-isopropylidene-α-l-sorbopyranose (VIIIa), 1, 2 : 4, 5-di-O-isopropylidene-α-l-sorbopyranose (IXa), and 5-O-α-methoxyisopropyl-1, 2 : 3, 4-di-O-isopropylidene-α-l-sorbopyranose (VIIIc).

It has been proposed that 1,3-O-benzylidene-l-sorbose (III) exists as an equilibrium mixture of a pyranose form (IIIp), a keto-form (IIIk) and a furanose form (IIIf) in solutions. The acetylation of III in pyridine afforded the acetates of IIIf; one of them (IV) is in a keto-form (IVk) in a crystalline state, and at a low temperature, while the acetates of IIIf and IIIp at higher temperatures. When a pyridine solution of III was allowed to stand for some time before acetylation, the yield of the acetate of IIIp increased. The recrystallization of III usually gave crystals of IIIf, however, while the addition of petroleum ether to the concentrated pyridine solution of III afforded a powder which was consisted mainly of IIIp. These results suggest that III exists as IIIf in a crystalline state and as an equilibrium mixture of IIIf, IIIk, and IIIp in solution; the existence of this equilibrium was also confirmed by proton magnetic resonance spectroscopy.

The PMR spectra of 2,2′-anhydro-2-C-hydroxymethyl-l,3 : 4,6-di-O-isopropylidene-l-sorbitol were examined in several solvents to confirm its structure. The conformation of this molecule is also discussed.

The title reaction was investigated over a range of pH 8 ∼ H−18.2. The rate-pH profile fits with an equation kobs= k(H+)/(K + (H+)). The decomposition proceeds directly via IV but not via its conjugate base V. The rate dependence on the initial concentration of IV is also discussed.

A series of l-sorbofuranose derivatives have been studied by PMR spectroscopy in order to determine their favored conformations from the coupling constants obtained by applying the spin-decoupling techniques and the solvent effects of benzene at 100 MHz. The coupling constants J3,4, have disclosed that the furanose ring of methyl 1,3,4,6-tetra-O-acetyl-α-l-sorbofuranoside (IIa) in solutions has a conformation close to a C3-endo-C4-exohalf-chair, while that of its β-anomer (IVa) has a C3-exo-C4-endohalf-chair conformation. Their conformations are unchanged over the temperature range of 32–74°C, but they are affected by the introduction of a 4,6-O-or 2,3-O-isopropylidene group, either of which locks the furanose ring to a C3-exo-C4-endohalf-chair conformation. This effect is thought to play an important role in the exclusive formation of 2,3:4,6-di-O-isopropylidene-α-l-sorbofuranose (XII) during the acetonation of l-sorbose.

The acid-catalyzed benzylidenation of l-sorbose (I) was studied. The reaction by Reichstein’s procedure gave a mixture of two diastereomeric 2,3:4,6-di-O-benzylidene-α-l-sorbofuranoses (III and IV), which were then successfully separated by preparative thin-layer chromatography. The use of slightly acidic conditions yielded l,3:4,6-di-O-benzylidene-β-l-sorbofuranose (XI), l,2:4,6-di-O-benzylidene-α- (XVII and XVIII) and-β-l-sorbofuranoses (XIX and XX), 1,2-O-benzylidene-α-l-sorbopyranoses (XXI and XXII), and 1,3-O-benzylidene-l-sorbose (XXIII). The absolute configurations at the acetal center of the five-membered benzylidene rings were assigned. A reaction sequence (I→(XXI, XXII\ightleftarrowsXXIII)→XI→IV→III) comparable with that of the ethylidenation of I was established.

The UV spectral changes and decomposition rates of ethyldithio-carbamic acid (I) were investigated in sulfuric acid solutions. The presence of the S-protonated form (IV) was established. The observed first-order rate constants fit with the rate expression, kobs= kK/(h0+ K), derivable from the proposed mechanism, IV\oversetK⇔I \oversetk→ EtNH2+ CS2.