Hemiketal‐Keto Tautomerism in 2‐Deoxy‐δ‐lactones Mediated by 2‐Lithiothiazole in Solution State: A Formal Synthesis of DAH, Kamusol and Their C 5 Epimers

The reactions of 2‐lithiothiazole on 3,4,6‐tri‐ O ‐benzyl‐2‐deoxyglyconolactones have been studied, which resulted in the formation of two products, major (3,4,6‐tri‐ O ‐benzyl‐2‐deoxy‐1‐(2‐thiazolyl) glycal) and minor products 3,4,6‐tri‐ O ‐benzyl‐2‐deoxy‐1‐(2‐thiazolyl) glycal hemiketal. The hemiketal‐keto tautomerism in solution was observed in the minor product. The tautomerism has been studied in various organic solvents and their ratios have been determined by 1 H NMR spectroscopy. The major product from the reaction of 2‐lithiothiazole with 2‐deoxygluconolactone was utilized for the synthesis of 3‐deoxy‐D‐arabino‐2‐heptulosonic acid DAH 4 a and 3‐deoxy‐D‐arabino‐2‐heptulopyranose commonly known natural product kamusol 6 a via acid catalyzed hydration followed by stereoselective methyl glycoside bond formation. Similarly, major product from 2‐deoxygalactonolactone furnished 5‐ epi ‐DAH 4 b ( C ‐5 epimer of 4 a ) and 5‐ epi ‐kamusol 6 b ( C ‐5 epimer of 6 a ). The minor product has also been utilized for the synthesis of DAH 4 a , 5‐ epi ‐DAH 4 b , kamusol 6 a and 5‐ epi ‐kamusol 6 b through a common intermediate, α‐methyl glycosylated product.