Glycosylidene Carbenes, Part 29 , Insertion into B−C and Al−C Bonds: Glycosylborinates, -boranes, and -alanes

Abstract

Insertion of the glycosylidene carbenes derived from the diazirines 1, 14, and 15 into the B−alkyl bond of the B-alkyl-9-oxa-10-borabicyclo[3.3.2]decanes 5, 6, and 7 yielded the stable glycosylborinates 8/9 (55%, 55 : 45), 10/11 (31%, 65 : 35), 12/13 (47%, 60 : 40), 16/17 (55%, 55 : 45), 18/19 (47%, 45 : 55), and 20/21 (31%, 30 : 70). Crystal-structure analysis of 17 and NOEs of 9 and 19 show that 17, 9, and 19 adopt similar conformations. The glycosylborinates are stable under acidic, basic and thermal conditions. The unprotected glycosylborinate 25 was obtained in 80% by hydrogenolysis of 12. Insertion of the glycosylidene carbene derived from the diazirine 1 into a B−C bond of BEt3, BBu3, and BPh3 led to unstable glycosylboranes that were oxidised to yield the hemiacetals 29 (55%), 31 (45%), and 33 (48%), respectively, besides the glucals 30 (13%), 32 (20%), and 34 (20%), respectively. Insertion of the glycosylidene carbenes derived from 14 and 15 into a B−C bond of BEt3 led exclusively to hemiacetals; only 15 yielding traces of the glucal 40 besides the hemiacetal 39. The glycosylidene carbene derived from 1 reacted with Al(iBu)3 and AlMe3 to generate reactive glycosylalanes that were hydrolysed, yielding the C-glycosides 46 (21%) and 49 (30%), respectively, besides the glucals 48 (26%) and 51 (30%); deuteriolysis instead of protonolysis led to the monodeuterio analogues of 46 and 49, respectively, which possess an equatorial 2H-atom at the anomeric center.