GLYCOSYLIDENE CARBENES .13. SYNTHESIS AND THERMOLYSIS OF REPRESENTATIVE 1-AZI-GLYCOSES

In the context of the hypothesis postulating a heterolytic cleavage of a C-N bond during thermolysis of alkoxydiazirines (Scheme 1), we repo rt the preparation of the diazirines 4, 5, 7, and 8, the kinetic param eters for the thermolysis in MeOH of the diazirines 1 and 4-9, and the products of their thermolysis in an aprotic environment. The diazirin es 4,5,7, and 8(Schemes 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 togeth er with the cyclic hydroxylamine 44. Oxidation of 11, 35, and 43 loros uccinimide/1,8-diazabicyclo[5.4.0]undec-7-ene (NCS/DBU) or NaIO4) gave good yield's of the (Z)-hydroximolactones 12, 36, and 45, while the o xime 20 led to a mixture of the (E)- and (Z)-hydroximolactones 21 and 22, which adopt different conformations. Their configuration was assig ned, 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-sulfo nates 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 read ily. Oxidation of the diaziridines gave 4, 5, 7, and 8, respectively. Thermolysis of the diazirines 1 and 4-9 in MeOH yielded the anomeric m ethyl glycosides 50151, 16/17, 26/27,52/53, 39/40,48/49, and 54/55, re spectively. A comparison of the kinetic data of the thermolysis at fou r different temperatures shows the importance of conformational and el ectronic factors and is compatible with the hypothesis of a heterolyti c cleavage of a C-N bond. An early transition state is evidenced by th e absence of torsional strain by an annulated 1,3-dioxane ring. Thermo lysis of 1 in MeCN at 23-degrees led mostly to the diastereoisomeric ( Z,Z)-, (E,E)-, and (E,Z)-lactone azines 56, 57, and 59 (Scheme 6), whi ch convert to 56 under mild conditions, and to 59 (3%). The benzyloxyg lucal 59 was obtained in higher yields (18%), together with 44% of 56- 58, by thermolysis of solid 1. Similarly, thermolysis at higher temper atures of 4 in toluene, THF, or dioxane and of 9 in CH2Cl2 or THF yiel ded the (Z,Z)-lactone azines 60 and 61, respectively, the latter being accompanied by the dihydro-oxazole 62.