Diols dehydration

The dehydration and deamination reactions appear to operate in a parallel fashion and will be considered together. Schrauzer and Silbert propose a base-catalyzed cleavage of the carbon-cobalt bond in the B12-catalyzed diol dehydration reaction as shown in Fig. 17 (81), based on demonstrated lability of the beta hydrogens in alkyl cobaloximes with electronegative groups in this position. 

Fig. 21. Scheme of Silverman and Dolphin for vitamin B12-catalyzed diol dehydration (87). 

Jensen, B. L. Slobodzian, S. V. A concise synthesis of 1-substituted 2-tetralones by selective diol dehydration leading to ketone transposition. Tetrahedron Lett. 2000, 41, 6029-6033. 

Figure 13 Illustration showing a possible mechanism of the coenzyme Bi2-catalyzed diol dehydration...

Pocker, Y., Ronald, B. P. Kinetics and mechanism of vic-diol dehydration. II. p-Anisyl group in pinacolic rearrangement. J. Org. Chem. 

The presence of a hydroxyl group at Cig has an enhancing effect on potency similar to that of a methyl group at the same position. Cortisone (5-7) constitutes the starting material for the synthesis of one of those 16)8,17)8-diols. Dehydration of cortisone occurs in ring D to afford the corresponding 16,17-dehydro derivative 18-1 (Scheme 7.18). The carbonyl... 

The use of secondary alcohols as reductants for DODH was first reported by Elhnan, Bergman, and coworkers, who employed Re-carbonyl compounds, e.g., Re2(CO)io, as pre-catalysts under aerobic conditions (Scheme 16) [36]. Optimized conditions used the glycol substrate with the mono-alcohol as the solvent, e.g., 3-octanol, at 150-175°C, with 1-2.5 mol% Re2(CO)io and TsOH as a co-catalyst (2-5 mol%). Good yields of the olefin (50-84%) were obtained with representative glycols. The sy -3,4-decanediol was converted highly selectively to trans-3-decene, implicating a sy -eUmination process in the diol to olefin conversion (Scheme 17). Erythritol was converted moderately efficiently to 2,5-dihydrofuran (62% yield), presumably the result of initial 1,4-diol dehydration followed by DODH of the THF-diol intermediate. The nature of the active catalyst was unknown at the time, but was speculated to be an oxidized Re species. 

---