Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

In the context of the hypothesis postlating a heterolytic cleavage of a CN bond during thermolysis of alkoxydiazirines (Scheme 1), we report the preparation of the diazirines 4, 5, 7, and 8, the kinetic parameters for the thermolysis in MeOH of the diazirines 1 and 4–9, and the products of their thermolysis in an aprotic environment. The diazirines 4, 57, and 8 (Scheme 2–5) were prepared from the known hemiacetals 10, 19, 34 (prepared from 31 in an improved way), and 42 according to an established method. The oximes 11, 20, 35, and 43 were obtained from the corresponding hemiacetals as (E/Z)‐mixtures; 43 was formed together with the cyclic hydroxylamine 44. Oxidation of 11, 35, and 43 (N‐chlorosuccinimide/1,8‐diazabicyclo[5.4.0]undec‐7‐ene (NCS/DBU) or NaIO₄) gave good yields of the (Z)‐hydroximolactones 12, 36, and 45, while the oxime 20 led to a mixture of the (E)‐ and (Z)‐hydroximolactones 21 and 22, which adopt different conformations. Their configuration was assigned, inter alia, by a comparison with the enol ethers 28 and 29, which were obtained, together with 30, from the reaction of the diazirine 5 with benzaldehyde and PBu₃. Treatment of the hydroximolactone O‐sulfonates 13, 23, 37, and 46 with NH₃/MeOH afforded the diaziridines 15, 25, 38, and 47 in good yields, while the (E)‐sulfonate 24 decomposed readily. Oxidation of the diaziridines gave 4, 5, 7, and 8, respectively. Thermolysis of the diazirines 1 and 4–9 in MeOH yielded the anomeric methyl glycosides 50/51, 16/17, 26/27, 52/53, 39/40, 48/49, and 54/55, respectively. A comparison of the kinetic data of the thermolysis at four different temperatures shows the importance of conformational and electronic factors and is compatible with the hypothesis of a heterolytic cleavage of a CN bond. An early transition state is evidenced by the absence of torsional strain by an annulated 1,3‐dioxane ring. Thermolysis of 1 in MeCN at 23° led mostly to the diasteroisomeric (Z,Z)‐, (E,E)‐, and (E,Z)‐lactone azines 56, 57, and 58 (Scheme 6), which convert to 56 under mild conditions, and to 59 (3%). The benzyloxyglucal 59 was obtained in higher yields (18%), together with 44% of 56–58, by thermolysis of solid 1. Similarly, thermolysis at higher temperatures of 4 in toluene, THF, or dioxane and of 9 in CH₂Cl₂ or THF yielded the (Z,Z)‐lactone azines 60 and 61, respectively, the latter being accompanied by the dihydro‐oxazole 62.